Minimize TDX

TDX (TanDEM-X: TerraSAR-X add-on for Digital Elevation Measurement)

TSX/TanDEM-X is a high-resolution interferometric SAR mission of DLR (German Aerospace Center), together with the partners EADS Astrium GmbH and Infoterra GmbH in a PPP (Public Private Partnership) consortium. The mission concept is based on a second TerraSAR-X (TSX) radar satellite flying in close formation to achieve the desired interferometric baselines in a highly reconfigurable constellation. A contract to build the TanDEM-X spacecraft was signed in September 2006 between DLR and EADS Astrium.

The primary goal of the innovative TanDEM-X/TerraSAR-X constellation is the generation of a global, consistent, timely and high-precision DEM (Digital Elevation Model), corresponding to the HRTI-3 (High Resolution Terrain. Information-3) model specifications (12 m posting, 2 m relative height accuracy for flat terrain). The HRTI specifications were defined by NGA (National Geospatial-Intelligence Agency), Washington, D. C. 1) 2) 3) 4) 5) 6) 7) 8) 9)

The achievable DEM height accuracy has been confirmed in Phase A by a detailed performance analysis taking into account all major system and scene parameters like the finite radiometric sensitivity of the individual radar sensors, co-registration and processing errors, range and azimuth ambiguities, baseline and Doppler decorrelation, the strength and orientation of surface and vegetation scattering, quantization errors, temporal and volume decorrelation, baseline estimation errors and the chosen independent post-spacing (horizontal resolution). 10) 11)

For generating the global DEM, roughly 300 TByte of raw data will be acquired using a network of ground receiving stations. Processing to DEM products requires advanced multi-baseline techniques and involves mosaicking and a sophisticated calibration scheme on a continental scale.

Beyond its primary mission objective of generating a global HRTI-3 DEM, TanDEM-X provides a configurable SAR interferometry platform for demonstrating new SAR techniques and applications, such as digital beamforming, single-pass polarimetric SAR interferometry, ATI (Along-Track Interferometry) with varying baseline, or super resolution. Close formation flight collision avoidance becomes a major issue and a new orbit concept based on a double helix formation has been developed to ensure a safe orbit separation.


Background: In the time frame 2007, the global coverage with topographic data at sufficiently high spatial resolution is inadequate or simply not available for scientific and governmental use. The first step to meet the requirements of the scientific community for a homogenous, highly reliable DEM with DTED-2 specifications was SRTM (Shuttle Radar Topography Mission), launch Feb. 11, 2000. SRTM, representing the first spaceborne single-pass interferometer, was built by supplementing the Shuttle Imaging Radar-C/X-Synthetic Aperture Radar system by second receive antennas mounted at the tip of a 60 m deployable mast structure. Within a ten day mission, SRTM collected interferometric data for a near global DTED-2 (Digital Terrain Elevation Data Level 2) land surface coverage. DTED-2 is the current basic high resolution elevation data source for all military activities and civil systems that require landform, slope, elevation, and/or terrain roughness in a digital format. DTED-2 is a uniform gridded matrix of terrain elevation values with post spacing of one arc second (approximately 30 m). SRTM mapped the Earth between 60 N and 56 S; however, there are still wide gaps, in particular at the lower latitudes.

The TanDEM-X/TerraSAR-X (TDX/TSX) constellation has the potential to close these gaps, to fulfil the requirements of a global homogeneous and high-resolution coverage of all land areas thereby providing the vital information for a variety of applications. The high-precision DEM models are of utmost interest for the civil and military communities, representing the basis for all modern navigation applications.



HRTI-3 definition


Relative vertical accuracy

90% linear point-to-point error over a 1º x 1º cell

2 m (slope ≤ 20%)
4 m (slope ≥ 20%)

12 m (slope < 20%)
15 m (slope > 20%)

Absolute vertical accuracy

90% linear error

10 m

18 m

Relative horizontal accuracy

90% circular error

3 m

15 m

Horizontal accuracy

90% circular error

10 m

23 m

Spatial resolution

Independent pixels

12 m (1 arcsec)

30 m (1 arcsec)

Table 1: DEM specification for HRTI level 3 standard - and comparison with DTED-2 model

Figure 1 gives an overview of DEM-level coverage estimates of various observation technologies in the different HRTI classes. It should be noted that a surface area of 150 x 106 km2 represents a global coverage of Terra Firma (i.e., all land areas).


Figure 1: DEM-level versus coverage indicating the uniqueness of the global TanDEM-X HRTI-3 product (image credit: DLR)


Mission concept:

The TanDEM-X mission concept is based on an extension the TerraSAR-X mission by a second almost identical satellite, namely TanDEM-X. Flying the two satellites in a close formation with typical cross-track distances of 300-500 m provide a flexible single-pass SAR interferometer configuration, where the baseline can be selected according to the specific needs of the application. 12) 13) 14) 15) 16) 17) 18) 19) 20) 21) 22) 23)

The SAR (Synthetic Aperture Radar) instruments of TerraSAR-X and TanDEM-X are fully compatible, both offer transmit and receive capabilities along with polarimetry. These features provide a maximum of flexibility in supporting operational services (acquisition of highly accurate cross-track and along-track interferograms without the inherent accuracy limitations imposed by repeat-pass interferometry) and in data product quality. The following basic interferometric SAR (InSAR) observational modes are available (Figures 2 and -3):

1) Bistatic mode where the SAR instruments of both spacecraft look into a common footprint thus providing different views of the observed target area (Note: bistatic InSAR is characterized by the simultaneous measurement of the same scene and overlapping Doppler spectra with 2 receivers, avoiding temporal decorrelation; PRF synchronization and relative phase referencing between the satellites are mandatory). - One satellite serves as a transmitter and both satellites record the scattered signal simultaneously. In this tandem configuration, both spacecraft fly in a close orbit formation. The baseline of this configuration can be selected according to the specific needs of the application. This enables the acquisition of highly accurate single-pass cross-track and/or along-track interferograms without the inherent accuracy limitations imposed by repeat-pass interferometry due to temporal decorrelation and atmospheric disturbances.

2) Pursuit monostatic mode where both satellites are operated independently avoiding the need for synchronization; hence, both SAR instruments look acquire data from the same swath with a short time difference of a few seconds corresponding to an along-track distance of 30-50 km. Different to conventional repeat-pass (i.e., two?pass or multi?pass) InSAR observations, the temporal decorrelation is still small for most terrain types with the exception of ocean surfaces and vegetation in the case of moderate to high wind speeds.

3) Alternating bistatic mode is similar to bistatic mode, but the transmitter is switched from pulse to pulse between the two satellites.

The baseline for operational DEM generation is the bistatic mode which minimizes temporal decorrelation and uses efficiently the transmit power. This mode uses either TSX or TDX as a transmitter to illuminate a common radar footprint on the Earth's surface. The scattered signal is then recorded by both satellites simultaneously. This simultaneous data acquisition makes dual use of the available transmit power and is mandatory to avoid possible errors from temporal decorrelation and atmospheric disturbances.

The alternating bistatic mode can be used for phase synchronization, system calibration, and to acquire interferograms with two different phase to height sensitivities; the simultaneously acquired monostatic interferogram has a higher susceptibility to ambiguities especially at high incident angles.

A mission concept has been developed which enables the acquisition/generation of a global DEM within three years. This concept includes multiple data takes with different baselines, different incidence angles, and data takes from ascending and descending orbits to deal with difficult terrain like mountains, valleys, tall vegetation, etc.


Figure 2: Concept of TanDEM-X InSAR observations in bistatic (left) and monostatic (right) modes (image credit: DLR)


Figure 3: Schematic view of the alternating bistatic mode (image credit: DLR)

The TanDEM-X mission concept allocates also sufficient acquisition time and satellite resources to secondary mission objectives which cover the following application spectrum:

• Moving target indication with a distributed four aperture displaced phase centre system

• The measurement of ocean currents and the detection of ice drift by along-track interferometry

• High resolution SAR imaging based on a baseline-induced shift of the Doppler and range spectra (super-resolution)

• The derivation of vegetation parameters with polarimetric SAR interferometry

• Large baseline bistatic SAR imaging for improved scene classification, as well as localized very high-resolution DEM generation based on spotlight interferometry.

• Demonstration of high resolution wide-swath SAR imaging with four-phase-center digital beamforming.

In short, the TanDEM-X mission concept encompasses enabling technologies in a number of ways, including the first demonstration of a bistatic interferometric satellite formation in space, as well as the first close formation flight in operational mode. Several new SAR techniques will also be demonstrated for the first time, such as digital beamforming (DBF) with two satellites, single-pass polarimetric SAR interferometry, as well as single-pass along-track interferometry with varying baseline. 24)


Figure 4: Artist's view of bistatic observation by the TanDEM-X configuration (image credit: EADS Astrium)


TanDEM orbits:

Close formation flight of TerraSAR-X and TanDEM-X. The TerraSAR-X spacecraft remains its sun-synchronous dawn-dusk orbit with the following parameters: mean altitude of 515 km, inclination = 97.44º, local equatorial crossing time at 18 hours on the ascending node, nominal revisit period of 11 days (167 orbits in the repeat, 15 2/11 orbits/day. 25) 26) 27) 28) 29) 30) 31)

For setting up the effective baseline, TanDEM-X is separated from TerraSAR-X in the right ascension of the ascending node. This will span a horizontal baseline, which will be adjusted between 200 m and 3000 m to achieve the effective baselines required for DEM-acquisition at different latitudes. An additional vertical separation at the northern and southern turns is achieved by a relative shift of the eccentricity vectors of the satellites. The result is a complete separation of the two satellite orbits called Helix-formation, which enables a safe operation of close formations with minimum collision risk. Such a Helix formation with an offset in eccentricity vectors and a separation in the right ascension of the ascending node is shown in Figure 2.

The TanDEM-X operational scenario requires a coordinated operation of two satellites flying in close formation. Several options have been investigated and the Helix satellite formation has finally been selected. The helix configuration allows maintaining a relatively small distance between both satellites while at the same time avoiding the collision risk at the poles. This formation combines an out-of-plane orbital displacement (e.g. by different ascending nodes) with a radial (vertical) separation (e.g. by different eccentricity vectors) resulting in a helix-like relative movement of the satellites along the orbit. Since there exists no crossing of the satellite orbits, it is now possible to arbitrarily shift the satellites along their orbits, e.g. to adjust very small along-track baselines at predefined latitudes and to allow safe spacecraft operation without autonomous control.

The Helix orbit for close formation flight, involving the maintenance of baselines of a cluster of spacecraft in orbit for cross-track and along-track interferometric observations, has been patented by DLR. The inventors are: Alberto Moreira, Gerhard Krieger, and Josef Mittermayer.

1) European Patent Office, Patent No: EP 1 273 518 A2 of Jan. 8, 2003. Title: “Satellitenkonfiguration zur interferometrischen und/oder tomographischen Abbildung der Erdoberfläche mittels Radar mit synthetischer Apertur.”

2) US Patent No: US 6,677,884 B2 of Jan. 13, 2004. Title: “Satellite Configuration for Interferometric and/or Tomographic Remote Sensing by Means of Synthetic Aperture Radar (SAR).”


Figure 5: Illustration of the Helix orbit configuration of both spacecraft (image credit: DLR)


Figure 6: Helical shape of interferometric baseline during one orbit (image credit: DLR)

The HELIX formation enables a complete coverage of the Earth with a stable height of ambiguity by using a small number of formations (e.g. ΔΩ ={300 m, 400 m, 500 m} and Δe ={300 m, 500 m}, where `Ω' is the right ascension of the ascending node, and `e' is the eccentricity. Baseline fine tuning can be achieved by taking advantage of the natural rotation of the eccentricity vectors due to secular disturbances and fixing the eccentricity vectors at different relative phasings. Since there exists no crossing of the satellite orbits, it is possible to arbitrarily shift the satellites along their orbits, e.g. to adjust very small along-track baselines at predefined latitudes and to allow safe spacecraft operation without autonomous control.

An appropriate reference scenario has been derived which enables one complete coverage of the Earth with baselines corresponding to a height of ambiguity of ca. 35 m within 1 year assuming a bistatic acquisition in stripmap mode with an average acquisition time of 140 s per orbit.

Both high precision orbit determination (POD) and interferometric baseline vector determination of the tandem configuration will be accomplished by means of the GPS-based TOR (Tracking, Occultation and Ranging) device, a dual-frequency receiver, which will be provided by GFZ as for TerraSAR-X.

Coarse orbit control and maintenance of the tandem configuration will be done as part of the regular maintenance maneuvers using thrusters. Fine-tuning of the Helix of the TanDEM-X satellite will be performed using additional cold gas thrusters.

TanDEM-X formation flight: The Helix formation geometry implies maximum out-of-plane (cross-track) orbit separation at the equator crossings and maximum radial separation at the poles. This is realized by small ascending node differences and by slightly different eccentricity vectors, respectively, as depicted in Figure 7. This concept of relative eccentricity / inclination vector separation results in a Helix-like relative motion of the satellites along the orbit and provides a maximum level of passive safety in case of a vanishing along-track separation. 32)


Figure 7: Formation building with relative eccentricity / inclination vector separation (image credit: DLR)

Legend to Figure 7: From left to right: (1) identical orbits, (2) maximum horizontal separation at equator crossings by a small offset in the ascending node (green arrow), (3) a small eccentricity offset causes different heights of perigee / apogee and hence yields a maximum radial separation at the poles. (4) Optional rotation of the argument of perigee to achieve larger baselines at high latitude regions.



During the development phase of the TerraSAR-X spacecraft, the TanDEM-X mission concept became a vision. However, a realization of the vision of two SAR missions in orbit could only have a chance with a necessary minimum extension of the SAR design on TerraSAR-X to support the synchronized operation of both radars. 'Minimum' meant that the TerraSAR-X schedule was not endangered and was further constrained to allow a cost-effective 1:1 rebuild approach for the SAR on TanDEM-X. 33)

For the spacecraft bus, the approach was constrained by only allowing software changes on TerraSAR-X. The bus design on TanDEM-X was extended to allow formation flight of both satellites - with TanDEM-X as the 'Master of the Constellation.' Particularly the bus hardware extensions were constrained by the tight schedule leading to strong orientation on existing hardware designs. The software changes are being verified during the TanDEM-X on-ground tests and will be uplinked to TerraSAR-X in preparation for the constellation flight.

Like the TerraSAR-X (TSX) satellite, the TanDEM-X (TDX) satellite is based on a mission-tailored AstroBus service module and a radar instrument developed according to the AstroSAR concept. Main differences to the TerraSAR-X satellite are the more sophisticated cold gas propulsion system to allow for constellation control, the additional S-band receiver to enable for reception of status and GPS position information broadcasted by TerraSAR-X, and the X-band intersatellite link for phase referencing between the TSX and TDX radars (the required modifications on the TSX spacecraft have already been implemented).


Figure 8: Artist's view of the TanDEM-X spacecraft (image credit: DLR)

The outer shape of the spacecraft is mainly driven by the accommodation of the X-band radar instrument, the body mounted solar array and the geometrical limitations given by the Dnepr-1 launcher fairing. A standard S-band TT&C system with full spherical coverage in uplink and downlink is used for satellite command reception and telemetry transmission.

An additional intersatellite S-band receiver, operating at the TerraSAR-X downlink frequency, will allow for the reception of status and GPS position information broadcasted by TerraSAR-X. It provides a 1-way link with which TanDEM-X can receive real-time position and velocity data from TerraSAR-X from its nominal 1-frequency GPS receiver. The TanDEM-X OBC (On-Board Computer) software uses such data from both satellites to generate a collision warning flag. Furthermore, this data is used by the TAFF (TanDEM-X Autonomous Formation Flying) algorithm running on the OBC (see Ref. 37).

Nominally, formation flying will be under ground control. The TAFF algorithm will be tested open-loop during the commissioning phase and could then become the standard approach for the constellation phases. The ISLR (Intersatellite Link Receiver) is laid out to receive the TerraSAR-X S-band transmissions in low power mode. It is cold redundant with each receiver/decoder cross-strapped to two patch antennas. This layout keeps contact gaps to less than 15 minutes in addition to an interruption imposed by the nominal high rate S-band contact with the ground station.

The OBC is a fully redundant unit that aims at performing the onboard data handling and the attitude and control functions on the satellites. The processor module in based on the ERC32, clocked at 40 MHz, and ensures an execution of software with a processing capability of more than 10 MIPS. The internal RAM memory comprises 6 MByte, with 4 MByte used nominally and 2 MByte reserved for the implementation of a cold redundancy.

The TanDEM-X attitude control system is based on reaction wheels for fine-pointing with magnet torquers for wheel de-saturation. A combined hydrazine/cold-gas propulsion system allows for orbit maintenance and rapid rate damping during initial acquisition. Attitude and orbit measurement is performed with a GPS/Star Tracker system during nominal operation and a CESS (Coarse Earth and Sun Sensor) in safe mode situations and during the initial acquisition. A combination of laser gyro and magnetometer allows for rate measurements in all mission phases.

CGS (Cold Gas Propulsion System): The CGS on TanDEM-X is of CryoSat-2 heritage and uses a high pressure tank of nitrogen gas. This provides small thruster impulses fitting the needs for constellation flight. There are 2 redundant branches each culminating in 2 redundant pairs of thrusters mounted on the satellite in each of the ± flight directions. A formation flight maneuver involves operation of a pair of thrusters in one of these directions.

The TanDEM-X spacecraft has a launch mass of about 1340 kg (payload mass of 400 kg); the nominal design life is five years after the end of the commissioning phase (estimated to be 3 months); the satellite consumables will last for 6.5 years after commissioning.

The Public-Private Partnership (PPP) between DLR and EADS Astrium has been extended to cover the design, build, launch, commissioning and operation of the TanDEM-X spacecraft. Like TerraSAR-X, TanDEM-X is a dual-purpose (scientific and commercial) Earth observation mission, providing its data services to the science (DLR) and to the non-science communities (Infoterra). This shared approach makes the program affordable to all parties of interest.


Figure 9: TanDEM-X in the satellite integration center at IABG (image credit: DLR, EADS Astrium)


Launch: The TanDEM-X spacecraft was launched successfully on June 21, 2010 on a Dnepr-1 launch vehicle with a 1.5 m long fairing extension. The launch provider is ISC Kosmotras, the launch site is the Baikonur Cosmodrome, Kazakhstan. 34) 35)

RF communications: A standard S-band TT&C system with 360º coverage in uplink and downlink is used for satellite command reception and housekeeping telemetry transmission. The uplink path is encrypted. Generated payload (SAR) data are stored onboard in a SSMM (Solid State Mass Memory) unit of 768 Gbit EOL capacity prior to transmission via the XDA (X-band Downlink Assembly) at a data rate of 300 Mbit/s. The X-band downlink is encrypted.

The on-board SAR raw data are compressed using the BAQ (Block Adaptive Quantization) algorithm, a standard SAR procedure. The compression factor is selectable between 8/6, 8/4, 8/3 or 8/2 (more efficient techniques can only be applied to processed SAR imagery). Both communication links are designed according to the ESA CCSDS Packet Telemetry Standard.


Rebuild of the TerraSAR-X satellite which was based on the Astrium Flexbus concept and extensive heritage from the CHAMP and GRACE missions


- X-band downlink horn antenna is mounted at the tip of a 3.3 m long boom
- SSMM (Solid State Mass Memory) data storage with a capacity of 768 Gbit (EOL)
- High-pressure nitrogen gas propulsion system for formation flying

Spacecraft launch mass

1340 kg (spacecraft: 1220 kg, fuel: 120 kg)

Spacecraft size

5 m length, 2.4 m diameter (hexagonal cross section)

Spacecraft design life

5 years nominal (after the end of the commissioning phase)

RF communications

- X-band of 300 Mbit/s link of payload data downlink with DQPSK modulation;
- S-band uplink of 4 kbit/s (2025-2110 MHz), BPSK modulation; S-band downlink of 32 kbit/s to 1 Mbit/s (2200-2400 MHz), BPSK modulation

Primary payload

Secondary payloads

- TDX-SAR instrument is identical to the TSX-SAR (TerraSAR-X SAR instrument)
in layout, operational performance and support modes.
- TOR (Tracking, Occultation and Ranging)
- LCT (Laser Communication Terminal)
- LRR (Laser Retroreflector)

Table 2: Overview of the TanDEM-X spacecraft parameters 36)

TAFF (TanDEM-X Autonomous Formation Flying)

TAFF is navigation and formation flying software package developed at DLR/GSOC. The overall objective of TAFF is to ease the ground and space operations. Its accurate orbit control performance facilitates the synchronization of the two SAR systems via dedicated horns. In fact the positions of the satellites will be known with a good precision well in advance of real operations. TAFF will enable a safe and robust formation control with minimum collision risk. 37) 38) 39)

On top of ensuring a stable and more precise baseline for SAR interferometry, TAFF will enhance the exploitation of along-track interferometry techniques. Along-track interferometry is enabled by a special configuration of the formation which provides dedicated osculating along-track separations at desired locations along the orbit. This method improves the detection, localization and the signal ambiguity resolution for ground moving targets and can be used for traffic monitoring applications. Furthermore real-time collision risk assessments will be performed by TAFF on a routine basis in order to support automated FDIR (Fault Detection Isolation and Recovery) tasks.

Two GPS receivers are installed on each spacecraft. The dual-frequency IGOR GPS receiver of BroadReach Inc., which serves exclusively scientific purposes, and the single frequency MosaicGNSS receiver of EADS Astrium, whose navigation data are used by TAFF.

A one-way intersatellite link (ISL) is being implemented between the two satellites, using the existing S-band downlink system on TSX and an additional receiver on TDX. The link is designed to function properly up to distances of a few km (ca. 2-5 km).


Figure 10: Overview of the ground and space segments and their interface to TAFF (image credit: DLR)

The TAFF software package resides in the OBC of the TDX spacecraft. TAFF gets as inputs the GPS data provided by the GPS receiver onboard TDX and, through the ISL, also from the GPS receiver data onboard the TSX. TAFF uses the CGS (Cold Gas Propulsion System) to control the formation and performs in-plane control maneuvers in the flight and anti-flight directions only.

The in-flight performance validation of the experimental autonomous formation keeping system embarked by the German TanDEM-X formation has been performed during a 12-day-long closed-loop campaign conducted in June 2012. Relative control performance better than 10 m was achieved, demonstrating that a significant gain of performance can be achieved when the control of the formation is done autonomously on-board instead of on-ground. Furthermore, the formation keeping system was shown to be operationally robust, easy to operate and fully predictable, i.e. fully suited for routine mission operations. This campaign concludes successfully a series of validation activities, opening new doors to future innovative scientific TanDEM-X experiments for which enhanced formation control is required.

TAFF is the first onboard autonomous formation keeping system ever employed on a high-cost scientific formation flying mission with routine data acquisition. As such, it has to face inherent natural fears and reluctance to rely on onboard autonomy for critical activities like formation maintenance. TAFF aims at making evolving the minds by proving that a proper design of the formation (passively safe) as well as a smart implementation of the onboard navigation software (robust navigation and control, internal safety mechanisms) can guarantee simple, accurate and safe formation keeping.

Table 3: Inflight performance test of TAFF 40)



Figure 11: Illustration of the spaceborne DGPS tracking scheme (image credit: DLR)


Figure 12: Photo of the MosaicGNSS (left) and IGOR (right) devices (image credit: DLR)

Parameter / Instrument

MosaicGNSS (EADS Astrium)

IGOR (BroadReach Inc.)

GPS tracking capability

8 channels L1

16 x 3 channels L1/L2

Raw data

C/A: 5 m
L1: 3 mm

C/A, P(Y) 0.2 m
L1, L2: 1 mm

Power consumption

10 W

15 W

Radiation tolerance

35 krad

12 krad

Table 4: Key parameters of the onboard GPS receivers

Formation Flight and Safety measures:

The requirement of a configurable close formation between TSX and TDX arises from the need for a SAR interferometer in space. The satellites fly in almost identical orbits whereby the position of TDX describes a helix around the trajectory of TSX. This is achieved by separation of the relative eccentricity and inclination vector. The maximal radial separation is reached over the poles (vertical baseline typically between 200 - 500 m) and the maximum separation in normal direction occurs at the equator (horizontal baseline typically 200 – 500 m; see Figure 5). In this way, it can be assured that the radial and normal separation never become zero at the same time. The shape of the helix depends upon the mission phase. The formation with the smallest baseline had a minimum separation of 150 m. Orbit correction maneuvers are carried out with the hydrazine propulsion system simultaneously on both spacecraft with exactly the same ΔV. Additionally formation keeping maneuvers are needed to compensate the drift of the relative e-vector that arises from the J2-perturbation (Ref. 31). These maneuvers are made only on TDX with the cold gas system. 41) 42)

Thrusters were originally planned to be the prime actuators during non-nominal situations in AOCS safe mode. The experience with TSX showed, however, that the design with the thrusters mounted at the back of the satellite is far from ideal for flight in close formation. Analyses showed a collision risk of 1/500 due to orbit changes in case of a drop to the thruster based safe mode. 43)The reason is that just a minor part of the thrust is available for attitude control, whereas the major part is changing the orbit in an unpredictable way. - Hence, a second type of safe mode was implemented with the intention to control the attitude without changing the orbit. The so-called ASM-MTQ (Acquisition and Safe Mode-Magnetorquer) only uses the magnetic torque rods as actuators, whereas it still relies on CESS, magnetometer and IMU as sensors, just like the original ASM-RCS (Acquisition and Safe Mode-Reaction Control System).

However, the damping of the rotation rates and the recovery of the attitude takes longer in ASM-MTQ than in ASM-RCS due to the weakness of the magnetic field at 514 km altitude. The maximum overall body rate that can be handled are 0.5º/s due to the concept that the torque rods and the magnetometers are operated in alternation to allow disturbance free measurements of the Earth’s magnetic field.

The new FDIR (Fault detection, Isolation and Recovery) design intends to always use the magnetorquer based safe mode first when a severe anomaly has been detected. There are performance limitations in ASM-MTQ as mentioned above, and it might still become necessary to make use of the conventional but more powerful ASM-RCS. The latter will only be used if the continuation of the mission is seriously endangered. A possible scenario would be the battery voltage dropping below a certain value, a star tracker getting too hot or non-convergence of the attitude after three orbits. The thruster on-time is limited at first instance to make sure that the generated ΔV cannot lead to a collision of the satellites. A reboot of the on-board computer will follow in the worst case scenario when despite of limited use of the thrusters, no convergence was reached. The spacecraft will come up after the reboot in ASM-MTQ again, but this time with wider power/thermal limits. However, the described sequence will be tried only once. If there is still no convergence or the power/thermal limits are yet violated, the spacecraft will be sent by FDIR to ASM-RCS once more, but this time without limitations to the thruster on-time. 44)

The ISL (Inter-Satellite Link) is also used for surveillance, but is subject to some limitations. In the first place, the link only works in one direction and in the second, the connection is interrupted anytime the transmitter of TSX or TDX is switched to high-rate for ground station contacts. Therefore it is seen more as an extra safety rather than the part to rely on completely. The ISL is used to transmit some essential parameters of TSX (including GPS position and velocity) to TDX in order to feed TAFF algorithms (Tandem Autonomous Formation Flight).

AOCS surveillance: The most vital AOCS parameters, such as sensor performance, attitude errors, actuator commands, etc. are monitored on-board. In case of severe anomalies FDIR can react immediately and switch to the redundant hardware. During ground station contacts, a large number of parameters are checked in the mission control system against pre-defined limit settings and violations are indicated by yellow or red flags. The dump files (data covering also the time span in between ground station contacts) are screened with the same limit settings, and violations are reported by email. The events will subsequently be analyzed and it is then decided if they can be disregarded or if a threat to the satellite is developing.



Mission status:

• Feb. 2014: According to information provided by the TSX/TDX project of DLR, global data acquisition for DEM generation from TSX/TDX will be completed by mid-2014. The processing for the global TSX/TDX DEM is expected to last until early 2016. In early 2014, final DEMs for most of Australia, a large part of North America and Siberia are already available. The brandname or label selected by DLR for the final TSX/TDX DEM is actually called the global TanDEM-X DEM. 45)

• In early August (6-8, 2013), the two TanDEM-X mission satellites are reversing their formation. Until now, the TanDEM-X satellite has been circling around its twin, TerraSAR-X, in an anti-clockwise direction; after the reversal, it will circle clockwise. This complicated change to the formation in which they have been flying for almost three years is necessary to observe regions that are difficult to image, such as mountain ranges, from the opposite viewing angle.

The WorldDEMTM will be a global DEM (Digital Elevation Model) of unprecedented quality, accuracy, and coverage (Figure 13). This DEM is intended to be the replacement data set for SRTM (Shuttle Radar Topography Mission) and will be available from 2014 onwards for the Earth’s entire land surface — pole-to-pole. WorldDEM is based on data acquired by the TerraSAR-X and TanDEM-X mission. The accuracy of the WorldDEMTM will surpass that of any satellite-based global elevation model available today and have the following unique features:

• Vertical accuracy of 2m (relative) and 10m (absolute)

• 12 m x 12 m raster

• Global homogeneity

• Highly consistent dataset as a result of an initial global data collection window of 2.5 years and the opportunity to continue to collect locally beyond the initial collect period.

• High geometric precision of the sensors make ground control information redundant.

Infoterra GmbH, the German unit of EADS Astrium GEO-Information Services, holds the exclusive commercial marketing rights and is responsible for the adaptation of the elevation model to the needs of the commercial users world-wide. Astrium will refine the DEM according to customer requirements, e.g. editing of water surfaces or processing to a Digital Terrain Model (representing the bare Earth’s terrain).

Table 5: Introduction of WorldDEMTM in 2014 by EADS Astrium Geo-Information Services 46) 47)


Figure 13: Comparison between WorldDEMTM data of Death Valley with data from the SRTM missions (image credit: DLR)

Astrium provides intermediate Digital Elevation Models - for customers to perform a detailed assessment of the suitability of WorldDEMTM data for varied applications. 48)

The intermediate DEMs feature two different variants: the basic DSM (Digital Surface Model), which includes the heights of all natural and man-made objects, and the DSM hydro, where water body features derived from the radar data are extracted and edited. Five Quality Layers provide information on the data source and the production. These product variants and quality layers will constitute the core WorldDEMTM product offering. In the near future this offering will be complemented by a Digital Terrain Model and additional derived products.

• May 2013: Since December 2010, the two satellites TSX and TDX are flying as a large single-pass bistatic SAR interferometer in a close Helix formation at about 514 km of altitude (mean value across the equator). This orbit has an 11 day repeat cycle and is maintained for the entire mission within a 250 m toroidal tube around a predefined reference trajectory. TDX has a relative orbit to TSX and together they fly in a precise controlled formation. 49)

Originally designed for a nominal joint operation time of three years, the current status of the satellites resources (primarily fuel) will allow a mission extension of several years. In addition, the two satellites serve also for the TSX (TerraSAR-X) mission, where both satellites independently provide high-quality SAR products for the science community and for commercial customers.

In the first two years of operations, two global coverages of the Earth’s land masses, excluding Antarctica, have been acquired. All the acquisitions have been carried out in the nominal right-looking observation mode.

The TanDEM-X mission will deliver a global and consistent DEM (Digital Elevation Model) with unprecedented accuracy. The acquisition of the first coverage has been completed on March 27, 2012; the acquisition of the second one has been finished in April 2013. The third phase of the acquisition, starting from May 2013, includes for the first time the acquisition over Antarctica and further acquisitions of difficult terrain such as deserts and mountainous regions. Performance has been studied in order to derive a consistent and appropriate acquisition strategy over these areas.

The left-looking imaging mode is required for the acquisitions over inner Antarctica, while in north-descending and in south-ascending acquisition mode, with a shift of 180º in the phase of libration is planned for the difficult terrain and on the outer rim of Antarctica.

The two satellites (TDX and TSX) are kept in a close helix-formation with a distance of less than 1 km. In order to reach the intended DEM accuracy, the baseline vector between the two spacecraft needs to be determined with an accuracy of 1 mm. To achieve this goal, both satellites are equipped with high grade dual-frequency GPS receivers.

The baseline vector between the two satellites is determined by GFZ and DLR/GSOC ( German Space Operations Center ) using independent software packages. The GSOC baseline solution is processed with the FRNS (Filter for Relative Navigation of Spacecraft) software. The underlying concept is to achieve a higher accuracy for the relative orbit between two spacecraft by making use of differenced GPS observations, than by simply differencing two independent POD (Precise Orbit Determination) results. The use of single-differenced code and carrier phase observations rigorously eliminates the GPS clock offset uncertainties and largely reduces the impact of GPS satellite orbit and phase pattern errors. Double differences are used for the integer ambiguity resolution of the carrier phase observations.

In studies prior to the TanDEM-X mission, comparisons between independent software packages showed biases of a few millimeters. In order to ensure the highest accuracy, a baseline calibration and combination process has been installed. The baseline products are validated by dedicated baseline calibration data takes over test sites, where the DEM is well known. Using those DEMs as a reference, height differences in the TanDEM-X scenes are estimated. Taking into account the incident angle and the height of ambiguity, these height differences can then be used to infer errors of the baseline products.. The resultant offset parameters are then applied in the baseline calibration process. The analyses show that the derived offset parameters are in the range of few millimeters. Finally the different solutions are merged to a combined product.

Prior to the mission, it was not sure, if the requirements on baseline accuracy of 1 mm, 1D-sigma, could be met. However,it was shown, that the GPS data quality remains constant on a high level. The routine baseline comparison has helped to assess the precision of the three individual baseline products and to identify systematic offsets between them. Since all baseline products employ identical GPS data sets, these offsets are mainly attributed to different processing concepts (such as ambiguity resolution and reduced dynamic versus dynamic trajectory models) in the employed software packages. Systematic biases of at most 2 mm can be observed in the cross-track direction, while biases in the radial direction are about ten times lower. Compared to the use of a single baseline product, the combination of multiple baseline solutions has, furthermore, helped to reduce the overall noise and to identify erroneous contributions.

On the other hand, the GPS-derived baseline can at best deliver accurate information on the relative position of the two spacecraft but is insensitive to potential errors in the adopted SAR antenna phase centers or uncalibrated differential delays between the two instruments. The SAR calibration data takes have therefore been used to determine the effective biases of the baseline products and of the instrumental effects in the SAR processing chain. While it is still not possible to exactly quantify the accuracy of the final calibrated baseline products, its use in DEM-processing has proven, that it is of fully acceptable quality to reach the demanding mission goal.

Table 6: Two years of TanDEM-X baseline determination 50)

• In 2013, the TanDEM-X mission, consisting of two spacecraft, namely TSX-TDX, is fully operational and continuous its close formation flight.

Beyond the primary mission objective of global DEM acquisition, the TSX-TDX satellite formation provides a configurable SAR interferometry test bed for demonstrating new SAR techniques and applications. In particular, it offers a unique chance to measure very slowly moving sea ice as well as ocean currents by means of ATI (Along-Track Interferometry). Due to the importance of the DEM acquisition the first three years of the mission are executed with a formation optimized for this purpose. After finalization of the global DEM formation flying will be dedicated to a variety of scientific interferometric campaigns including ATI. However, during the first years of operation already thousands of data sets have been acquired in the course of the TanDEM-X Science Program.

The secondary mission objectives of the TSX-TDX constellation, like SAR ATI support, are expected to start by the end of 2013, when the primary objective of global DEM acquisition has been completed. Because of the fact that spaceborne SAR ATI with along-track separations in the order of 50 m has not been demonstrated before, there is a strong interest in preliminary ATI experiments with TanDEM-X to validate the methods foreseen for both SAR acquisition and processing.

A first set of ATI experiments was already performed in February and March 2012 in the background of the on-going TerraSAR-X and TanDEM-X missions. At that time, the ground-controlled formation geometry comprised of minimum satellite distances of 150 m in the plane perpendicular to the flight direction and a favorable along-track separations over northern Europe. For the acquisition of the ocean current at the Pentland Firth on Feb. 26, 2012, the along-track separation was slightly adjusted to yield optimal observation conditions without affecting the routine DEM acquisition. The acquired data impressively illustrates the high potential of TanDEM-X for mapping water surface currents with high spatial resolution, which has applications in the field of optimal placement of renewable energy sites and to validate global circulation models which are used for climate research.

Table 7: ATI measurements of sea ice and ocean currents will follow the primary DEM mission by the end of 2013 (Ref. 32)

• July 2012: Global maps of Earth's topography: Within the last decade, three global mappings took place and especially those performed with radar brought dramatic progress: 51)

- The SRTM (Shuttle Radar Topography Mission) acquired the first global and consistent 30 m DEM between ± 60º latitude in the year 2000.

- After that the optical Terra/ASTER system (NASA) acquired another near global 30 m DEM between ±83º latitude since the year 2000.

- Since 2010, the German TanDEM-X mission is acquiring a global 12 m DEM with significantly improved resolution and accuracy (Ref. 9).

In the course of the quality control of the routine processing , TanDEM-X DEMs are continuously compared to SRTM DEMs. In this comparison not only the expected differences in resolution and accuracy have been found, but also significant changes that occurred to the Earth’s surface over 10 years since the SRTM mission. Even the differences between TanDEM-X acquisitions separated by a couple of months reveal interesting details.

Accuracy : TanDEM-X data are currently (2012) being acquired and first accuracy assessments confirm that the specification of 2 m vertical point-to-point error (90%) will be fulfilled for the final DEM composed of two coverages, in many cases even in a single coverage (Ref. 51).

• In 2012, the TanDEM-X mission, consisting of two spacecraft, namely TSX (TerraSAR-X) and TDX (TanDEM-X), is fully operational and continuous its close formation flight. Both satellites are capable of monostatic and bistatic SAR operation, supporting several imaging modes and geometries like along- and cross-track interferometry, or new bistatic SAR techniques. 52) 53)

- The primary mission goal of TanDEM-X is the systematic acquisition of a global, homogeneous DEM (Digital Elevation Model). The nominal DEM acquisition is performed in the bistatic stripmap mode (single polarization). Figure 2 (in bistatic mode, left) shows one satellite transmitting and two satellites receiving the signal.

- On a secondary level, the system supports experimental applications in order to perform flexible multistatic configurations for scientific purposes. An alternating bistatic acquisition is depicted in Figure 3, where the transmitting signal is toggled between the satellites, while both satellites are always receiving. Further configurations can be commanded, but processing is on best effort for the following modes (bistatic and alternating bistatic): all spotlight variants with all polarizations, and stripmap dual-polarization for alternating bistatic. Quad-polarization (from dual-receive antenna mode) and pursuit monostatic acquisitions for all imaging modes are possible to be acquired too, but they are restricted to dedicated mission phases.

A key role for fast SAR processing of the bistatic data channel is the exact timing knowledge of the whole system. The knowledge is provided by the evaluation of synchronization pulses - exchanged via a special in-orbit sync link system- from one to the other satellite. These pulses are interspersed into the imaging radar pulses and exchanged via the sync-horn antennas. The synchronization pulses are embedded into a synchronization sequence which contains 4 pulses: the initialization of the synchronization, sync-pulse from one satellite to the other, sync-pulse from the other satellite back to the first one, and exit of the synchronization. Each of the 4 pulses is performed with a specific PRF in order to harmonize the receiving windows of the two independent SAR instruments (Figure 14). The duration of the 4 pulses is equal to an integer multiple of the imaging PRI (Pulse Repetition Interval) = 1/PRF.


Figure 14: Insertion of synchronization sequence during bistatic image acquisition. The sync PRIs have different lengths than the nominal PRIs (image credit: DLR)

• In January 2012, after a year of formation flight of TanDEM-X with TerraSAR-X, the twin satellites have completely mapped the entire land surface of Earth for the first time. The data is being used to create the world's first single-source, high-precision, 3D digital elevation model of Earth. DLR controls both radar satellites, generates the elevation model, and is responsible for the scientific use of TanDEM-X data. 54) 55) 56)


Figure 15: On Jan. 12, 2012, the TDX/TSX mission had completely mapped all land surfaces on Earth except Antarctica (image credit: DLR)

Legend to Figure 15: The color scale of the first global coverage stereo map shows the relative height error (10 m in red to 0 m in dark green) derived from the mean coherence of each individual Raw DEM. Gray-shaded strips have been recorded, but have yet to be processed (Ref. 56). 57) 58)

• Over the course of 2011, the distance between the satellites was progressively reduced down to the minimum permitted value of 150 m.


Figure 16: The first TanDEM-X mosaic of Iceland (image credit: DLR)

• In the summer of 2011, the combined space and ground segment performs remarkably well and by now, more than half of the global land masses have been mapped. Currently the efforts concentrate on completing the processing and calibration chains. 59)

• In March 2011, the TanDEM-X and TerraSAR-X mission continue to acquire bistatic mode DEM data in their formation flight. So far, no problems in operational handling of bistatic flight configuration and related safety measures were encountered. 60) 61) 62) 63)


Figure 17: Artist's view of TDX and TSX in formation flight (image credit: DLR)

• In January 2011, the TanDEM-X mission is operational, flying in close formation with TerraSAR-X (a single pass SAR interferometry configuration) and providing stereo SAR imagery. The collection of data for a global homogeneous DEM started - as planned - in early 2011.






June 21, 2010

Drift phase to acquire target orbit

The initial separation between TDX and TSX was 15700 km and after one month of drifting a formation in pursuit monostatic configuration with an along-track distance of 20 km was reached.

June 21 - July 22, 2010

Wide formation:

TDX monostatic radar instrument commissioning. The wide formation of 20 km was maintained for 3 months to calibrate the TDX radar instruments and to perform first bistatic and interferometric experiments employing large baselines.

July 22 - Oct. 12, 2010


On October 14, both satellites were maneuvered into a close formation to start the bistatic commissioning phase. During this phase, the radial and cross-track baselines were kept constant at 360 and 400 m, respectively, and the mean along-track distance was set to 0 m. The results from both the mono- and bistatic commissioning phase already demonstrated the unique interferometric performance of TanDEM-X.

Oct. 12 - 15, 2010

Close formation:

TSX/TDX bistatic instrument commissioning; Begin of routine monostatic radar operation on both satellites

Oct. 15 - Dec. 12, 2010

Begin of routine DEM acquisition with flexible baselines: operational phase

Operational DEM acquisition started on December 12, 2010, less than 6 months after satellite launch. Since then, the total landmass of the Earth has been mapped once with a height of ambiguity ranging from 40 to 60 m. Global DEM data acquisition with varying baselines will continue until 2013, mapping difficult terrain like mountains, valleys, tall vegetation, etc., with at least two heights of ambiguity as well as from multiple incidence/aspect angles. The latter will be achieved by swapping the Helix formation. This allows for a shift of the DEM acquisition quadrants from ascending to descending orbits in the northern hemisphere and vice versa in the southern hemisphere.

The fully mosaicked DEM shall become available in 2014 for 90 % of the global landmass. Figure 18 shows two examples of TanDEM-X DEMs that have been acquired during the commissioning phase.

Dec. 12, 2010


In order to collect sufficient measurements for a global DEM (Digital Elevation Model), three years of formation flying are foreseen with flexible baselines ranging from 200 m to a few kilometers.



Current fuel consumption and battery degradation on the TerraSAR-X satellite is well below specification and will probably allow for life time extensions of two to three years, i.e. close formation flying until 2015 seems feasible. The prolonged mission time will allow for additional DEM acquisitions with improved accuracy and resolution as well as the conduction of advanced bistatic and multistatic SAR experiments in unique configurations, modes and geometries. 64)

Fall 2012

Table 8: TanDEM-X (TDX) mission timeline leading to formation flight with TerraSAR-X (TSX) 64) 65)

The achieved relative orbit determination position accuracy is better than 50 cm in cross-track (2D, rms) and about 1 m in along-track direction (rms) and therefore well suited for the purpose of formation monitoring and control. After a short commissioning phase the GSOC formation control process has been fully automated. Daily in-plane maneuver pairs for execution on-board TDX are performed to compensate the natural eccentricity vector drift and to adjust the along-track separation while out-of-plane control is used to adjust the horizontal separation at less frequent control interval (typically 11 days). The achieved relative control accuracy is nominally 5 m (rms) in cross-track direction and can be up to 10 m (rms) during phases of large horizontal separation drifts as performed in the beginning of the DEM acquisition phase. The along-track control accuracy is better than 30 m (rms). The control requirements of 28 / 200 m (rms) in cross-track /along-track direction are clearly overachieved paving the way for the acquisition and processing of a global digital elevation model within the upcoming years of TSX-TDX formation flight (Ref. 65). 66)


Figure 18: Examples of digital elevation models acquired by TanDEM-X acquired during the commissioning phase. Top: Italian volcano Mount Etna, located on the east coast of Sicily. Bottom: Chuquicamata, the biggest copper mine in the world, located in the north of Chile (image credit: DLR, Ref. 64)


Figure 19: Overview of TanDEM-X commissioning phase and its sub-phases in 2010 (image credit: DLR, Ref. 60)


Figure 20: Projected in-orbit lifetimes of TSX and TDX (image credit: DLR, Ref. 62)

• On December 14, 2010, the TanDEM-X mission reached another important milestone: the radar mission's test phase has been concluded in less than six months according to plan, paving the way for routine operations - the collection of elevation data. This meant in particular the start of systematic data acquisition for global DEM generation. 67) 68)


Figure 21: This TanDEM-X DEM image shows Salar de Uyuni, the largest salt flats in the world covering ~10,000 km2, located next to the volcanic region of the Atacama Desert (image credit: DLR)

• In-orbit performance of TSX-1 and TDX-1: During the first three month the primary goal task was to check whether the TDX-1 satellite has the same performance as TSX-1 and to calibrate it to fulfill the requirement of the TerraSAR-X mission. Over 2000 acquisitions have been performed and analyzed w.r.t. various aspects like: 69)

- SAR instrument characteristics (noise, raw data balance, signal power and ADC saturation)

- Doppler centroid

- Noise equivalent sigma zero

- Signal to noise ratio

- Peak-to-side lobe ratio

- Integrated side lobe ratio

- Geometric resolution

- Radiometric resolution

- Repeat pass acquisition performance.

It could be validated that TDX-1 acquisitions fulfill the TerraSAR-X performance requirements and that the performance is very similar to TSX-1 acquisitions. TDX-1 has a slightly better noise performance, but on the other hand, transmits with slightly less power which in total results in the same imaging performance as TSX-1.

The commissioning period for the interferometric TanDEM-X aspects was very compacted; an important goal is to start the systematic DEM acquisition as soon as possible. Hence, the first priority was to ensure the quality of the DEM acquisitions which are acquired in bistatic stripmap single polarization mode. The bistatic commanding and the bistatic synchronization performance between the two satellites worked as expected, which allowed early acquisitions for further performance evaluations. In total more than 1500 bistatic acquisitions were acquired and analyzed during the two month lasting bistatic commissioning phase period. The acquired data was evaluated in two ways:

1) Statistically, which meant that the performance parameters like coherence or phase unwrapping errors for many acquisitions had to be averaged in bigger areas (e.g. 30 km x 50 km) and then evaluated on a large scale basis.

2) Individually, which meant that smaller areas were analyzed in detail to verify certain effects and to find new effects.

The main activities dealt with the topics of “height error analysis” and “polarization analysis” — resulting in the following conclusions: (Ref. 69)

The TanDEM-X (i.e., TSX+TDX) global DEM will be generated by merging at least two different acquisitions from the same area and thus increasing the performance. The height error requirement for 90% of the points is 2 m for slopes < 20º and 6 m for slopes > 20º.

All global DEM acquisitions will be performed in HH polarization since the performance prediction indicated a slightly better performance in comparison to VV. During the commissioning phase this prediction has been verified by real acquisitions. The slightly better performance for HH has been confirmed. - Please note that the absolute performance difference between HH and VV is very small.


• Calibration of the TanDEM-X SAR system- all items completed in early December 2010: The main part of all calibration activities, especially of measurements executed against precise reference targets, was concentrated on the monostatic CP (Commissioning Phase) for deriving all calibration parameters (see Figure 19). The activities executed during the interferometric CP were concentrated on the verification of all these parameters, i.e. whether they are still valid in bistatic constellation and enable consequently a precise synchronization of both systems. 70) 71)

The calibration effort and consequently the duration for commissioning the whole TanDEM-X (TDX) system could be optimized by the experience and the results which had been achieved for TSX since launch in 2007. This was the baseline for executing the TDX commissioning phase as fast as possible. Thus, a maximum overlap of the lifetime of both satellites required for the global DEM acquisition could be achieved.

- On top, most of the acquisitions against precise reference targets could be applied twice during one pass, i.e. once for TDX and once for TSX. These results approve once again that the stability and the accuracy of the TSX system is still of unprecedented quality, three years after launch.

- Internal calibration

- Geometric calibration

- Antenna pointing

- Antenna model verification

- Radiometric calibration

Calibration procedure




Internal calibration

0.25 dB
< 1.0º

< 0.1 dB
< 1.0º

< 0.1 dB
< 1.0º

Antenna pointing knowledge


< 0.002º
< 0.002º

< 0.004º
< 0.001º

Pixel localization accuracy

2 m
2 m

0.30 m
0.50 m

0.29 m
0.48 m

Antenna model verification
Elevation pattern
Azimuth pattern ††
Beam-to-beam gain offset

±0.2 dB
±0.1 dB
±0.2 dB

< ±0.2 dB
< ±0.1 dB
< ± 0.2 dB

< ±0.2 dB
< ±0.1 dB
< ± 0.2 dB

Radiometric calibration
Radiometric stability
Relative accuracy
Absolute accuracy

0.5 dB *
0.68 dB
1.1 dB ‡

0.15 dB **
0.17 dB ‡
0.48 dB ‡

0.15 dB **
0.16 dB ‡
0.39 dB ‡

Table 9: Results of the calibration procedure

Legend to Table 9: two-way, †† one-way, * requirement defined over a period of 6 months, ** measured by TSX after 2 years (2009), ‡ StripMap mode.

After the launch of the second satellite TDX, the whole TanDEM-X system could be successfully commissioned and consequently calibrated in 2010. Furthermore, the performance of the first satellite TSX is still of unprecedented quality, three years after launch.

Never before have two independent spaceborne SAR systems - at an altitude of > 500 km - been more accurately calibrated and consequently precisely matched to each other than TSX and TDX. The geometric offset between both SAR systems is smaller than half the wavelength. The radiometric offset is essentially < 0.1 dB. All requirements and/or goals have been achieved, even to a better extend than predicted. The results are listed in Table 9 (Ref. 70).

• Baseline calibration of TanDEM-X. Typical baseline lengths of the TDX/TSX constellation are in the order of 500-1500 m. The challenging baseline determination relative accuracy requirement is 1 mm, since the precise knowledge of this magnitude is essential for achieving the required height accuracy of the DEM (Digital Elevation Model). Previous baseline determination experiments did not exclude the existence of residual baseline biases in the order of 2 to 8 mm. Therefore, baseline calibration became one of the main TanDEM-X commissioning phase activities. 72)

The orbit positions are calculated based on the measurements of the on-board GPS navigation receivers, namely the TOR/IGOR system (Tracking, Occultation and Ranging / Integrated GPS Occultation Receiver). An absolute accuracy of 5 cm (1σ) has been achieved, as verified by the in-orbit tests of the TerraSAR-X satellite.

However, it is possible to track the relative changes between both satellites (baseline) with a much higher accuracy. This is performed through processing the navigation information derived from the DDGPS (Double Differential GPS) carrier phase measurements between both satellites and applying a Kalman filter method to the data. The use of the differential information even eliminates ionospheric errors and other characteristic GPS perturbations. The resulting relative baseline determination accuracy is expected to be in the order of 1 mm, based on the performance of the DDGPS method in similar missions like GRACE.

The calibration activities during the TanDEM-X satellite commissioning phase have proven their efficacy to characterize the baseline error in LOS over time and latitude location, as well as to identify its systematic. The first in-orbit results of these characterization activities show baseline accuracies of around 1.5 mm (1σ) for the calibrated baseline product (Ref. 72).

Legend to Figure 21: The blue to dark blue areas show the lowest lying parts of the salt flats. A trained eye can see the boundaries of rock deposits in the three-dimensional model. This information about landscape features helps the project to draw important conclusions about the origins and development of the area.

• From Oct. 14, 2010 onwards, the TanDEM-X (TDX) and TerraSAR-X (TSX) spacecraft are flying in a close flight formation starting the bistatic commissioning phase. During this phase, the radial and cross-track baselines were kept constant at 360 and 400 m, respectively, and the mean along-track distance was set to 0 m. - Bistatic DEMs are being acquired since then. With TDX delivering identical single SAR product quality as TSX, the TerraSAR-X mission is running operationally on both satellites since October 25, 2010.

• On October 11, 2010, the thrusters of TanDEM-X were fired to reduce the separation between TanDEM-X and TerraSAR-X from 20 km to 500 m. TerraSAR-X remained in its original, circular orbit, while TanDEM-X, moved on a slightly eccentric orbit in a plane that is rotated by a small angle with respect to that of its partner.

At the North Pole, TerraSAR-X overtakes TanDEM-X, as the latter, because of its eccentric orbit, is slightly higher and thus orbiting more slowly. At the South Pole, the situation is reversed; TanDEM-X orbits lower and faster, and overtakes TerraSAR-X. Of course, a collision must be prevented – 'touching' is not allowed! Viewed from side, the two orbits can be imagined as being like two links in a chain – one link round, the other elliptical – intertwined but not touching one another. 73)


Figure 22: The circular orbit of TerraSAR-X (red) and the eccentric orbit of TanDEM-X (green) never cross, preventing a collision between the two satellites (image credit: DLR)

In this “narrow” formation flight configuration - flying over the same target area reduces the time difference into the region of ms (milliseconds), a condition most suitable for interferometric observations. Full commissioning of TanDEM-X and TerraSAR-X in formation flight is expected in December 2010.

• In early August 2010, the TanDEM-X and TerraSAR-X spacecraft of DLR are in their “wide” formation flight configuration - flying over the same target area in a time difference of ~ 3 seconds. In this formation flight configuration, interferometric imagery may be obtained automatically. 74) 75)

Already on July 20, 2010, TanDEM-X had achieved a “wide” formation with TerraSAR-X. The nominal distance to TerraSAR-X was 20 km in flight direction (300 m radial and 1305 m horizontal direction). The wide formation will be kept throughout the monostatic commissioning phase of TanDEM-X, planned to last to the end of September 2010. 76)

• During its monostatic commissioning phase, the system has been mainly operated in pursuit monostatic mode. However, some pioneering bistatic SAR experiments with both satellites commanded in non-nominal modes have been conducted with the main purpose of testing the performance of both space and ground segments in very demanding scenarios. - Two sets of innovative bistatic experiments have been carried out during the monostatic commissioning phase.

1) the first bistatic acquisition, complemented with a repeat-pass interferometric processing of consecutive bistatic surveys

2) the first single-pass bistatic interferometric acquisition.

In both cases, the geometrical configuration coincided with the one depicted in Figure 23: the beams used in pursuit monostatic operation are represented by solid lines, corresponding the dashed ones to a bistatic operation with symmetric azimuth steering. 77) 78) 79)

- Table 10 lists the main parameters of the bistatic acquisitions. The column ’Experiment 1’ refers to the first bistatic imaging acquisition, as well as to the repeat-pass bistatic interferometric results; the column ’Experiment 2’ refers to the first single-pass bistatic interferometric acquisition. All data-takes were acquired using the regular stripmap modes of the satellites. The data have been processed using the experimental TanDEM-X interferometric processor (TAXI), a flexible and versatile processing suite especially developed for the evaluation of TanDEM-X experimental data products.


Experiment 1

Experiment 2

PRF (Pulse Repetition Frequency)

3182.52 Hz

2991.24 x 2 Hz

Incident angle



Squint angle TSX/TDX



Cross-track baseline

253 m

43 m

Table 10: Acquisition parameters


Figure 23: Formation of the TSX and TDX satellites during the TDX monostatic commissioning phase (image credit: DLR)

The acquisition, carried out on Aug. 8, 2010, was planned over the city of Brasilia. For this first bistatic experiment, TSX operated monostatically with a squint of -0.8º, whereas TDX was set in receive-only mode with a squint of 0.8º. Due to the small bistatic angle, no relevant modifications of the timing schemes were required. Synchronization pulses were exchanged during the data-take, from which the clock phase error could be measured. A squinted monostatic image and a non-squinted bistatic one were obtained, but with no spectral overlap between them. Similar acquisitions were conducted in consecutive passes of the system over the same area to produce bistatic repeat-pass interferograms, i.e., after 11 days.

The first bistatic image acquired by TanDEM-X is shown in Figure 24 (green channel), overlaid with the simultaneous TSX monostatic image (magenta channel). Two different aspects of the previous images can be outlined. In the city center, the dominant scattering mechanism seems to be monostatic, but there exist distinct building areas near the lake, where the bistatic scattering dominates. The conclusion is that, even for the small bistatic angle of the experiment, about 1.6º, significant changes in target reflectivity, especially in man-made structures, can be expected between monostatic and bistatic observations. This feature might be very helpful to enhance the performance of existing identification or classification algorithms. The second one is the distribution of the azimuth ambiguities. Considering the azimuth ambiguities of the point target of opportunity in the center of Figure 24, which are mapped on the lake and zoomed within the ochre rectangle on the bottom right corner of the figure, a range difference in the position of the monostatic and bistatic ambiguities appear. This happens because the monostatic image is squinted, whereas the bistatic is not, and therefore the 2D monostatic impulse response, unlike the monostatic, is skewed. This feature might be exploited to develop ambiguity identification/suppression strategies.


Figure 24: TDX first bistatic image (green channel) over Brasilia. TSX monostatic image (magenta channel) overlaid. Radar coordinates (horizontal range, vertical azimuth time), image credit: DLR

Figure 25 shows the repeat-pass interferogram using the two bistatic images after eleven days. Note that the images are rotated 90º with respect to Figure 24. The figure on the left shows the non-synchronized bistatic interferogram. The azimuthal fringes of the interferogram are due to the differential clock frequency change between the two passes, of about 1 Hz. The right figure shows the synchronized interferogram; note that the SRTM DEM has been used in Figure 25 to remove topography. The residual fringes correspond mainly to a priori DEM errors and (possibly) marginally to unaccounted atmospheric effects. The mean value of the coherence is 0.35; in urban areas, this value increases to about 0.5. There are no significant differences between the values obtained from the monostatic repeat-pass and the bistatic interferograms.
Concerning the interferometric performance, the baselines are practically the same, as is the SNR of both acquisitions. Note that the monostatic image has a squint, but since no significant changes in target reflectivity other than for certain man-made structures have been observed, the results are definitely consistent. Besides its novelty, the relevant conclusion of this experiment was that we could obtain with the new system bistatic images and interferograms of similar quality to the monostatic (more mature) TerraSAR-X counterparts, a quite relevant information at the time.


Figure 25: Results of bistatic 11-day repeat-pass interferometry with TanDEM-X (image credit: DLR, Ref. 77)

- Bistatic single-pass interferometry: Following the success of the bistatic imaging acquisitions, a natural next experiment consisted of performing a single-pass bistatic interferometric experiment with a large along-track baseline before the end of the pursuit monostatic commissioning phase. However, a way to overcome the spectral decorrelation of the previous bistatic configuration was needed. Because of the small bistatic angle, simultaneous monostatic and bistatic images with similar equivalent squint angles have Doppler spectral overlap, i.e., the images are coherent. This equivalency is depicted in Figure 23.

To achieve this, an imaginative commanding of the satellites was designed, with a switch of the azimuth antenna patterns of TSX and TDX on a pulse-to-pulse basis. Both satellites transmitted one pulse using the non-squinted beams (solid lines) in Figure 23; in the next pulse TSX transmitted with a squint of -0.9º and TDX only received with a squint of +0.9º (depicted with dashed lines in Figure 23). All things considered, one pursuit monostatic interferogram with full baseline, plus two symmetric bistatic interferograms with half baseline could be computed.

However, the acquisition had a couple of drawbacks: firstly, the PRF needed to be doubled, i.e., the swath was halved; secondly, due to the specifics of the commanding, no calibration nor synchronization pulses were available. The acquisition was carried out over the Parque nacional del volcán Turrialba, in Costa Rica, a gracefully mountainous area. Note that this experiment was conducted early October 2010, about a week before the first official bistatic TanDEM-X interferograms in close formation were obtained, and are therefore the first bistatic single-pass spaceborne SAR interferometric acquisitions.

TDX uses a direct X-band link to measure the differential phase between the two radar master clocks. For the cases where this synchronization information is missing, TAXI has an automatic synchronization module which is capable of estimating the synchronization error using the bistatic data, which definitely substantiates the scientific character of the experiment. The estimated clock carrier frequency difference of about 124 Hz is also consistent with available contemporary SyncLink samples.

Figure 26 shows the DEM generated using one of the bistatic interferograms. The validity of the approach can be shown by cross-checking the results obtained using the single-pass bistatic interferogram of Figure 26 and the one resulting from the conventional pursuit monostatic one.


Figure 26: Geocoded (North points rightwards) DEM of the are surrounding Turrialba volcano, the first single-pass bistatic interferometric TanDEM-X acquisition (image credit: DLR)

Figure 27 shows the height difference between the two DEMs. A mask has been used to avoid including values with low coherence. No trends in range or azimuth can be identified, which qualitatively validates the automatic synchronization approach. The standard deviation in the height error of the DEMs computed using single-look interferograms corresponds to 23.3 m, which results in an effective averaging factor of about 20 for the DEM error of the figure.


Figure 27: Difference between pursuit monostatic and single-pass bistatic DEMs. Effective averaging factor is roughly 20, (image credit: DLR, Ref. 77)

• On July 22, 2010, DLR published the first 3D images from the TanDEM-X satellite mission. A digital elevation model test image was created of an ice cap located on one of a group of Russian islands in the Arctic Ocean, referred to as “October Revolution Island.” The image of Figure 28 represents a large ice cap in the center of the island, it is mapped accurately to a few cm in elevation.

Close formation flying of TanDEM-X and TerraSAR-X is yet to be achieved. Nevertheless, DLR researchers were able to generate the first 3D images by waiting for the optimum time when the two satellites – in their near-polar orbits – were fairly close together. Such a close approach incident of the two spacecraft occurred on July 16, 2010 when the interferometric imagery was observed.

The DLR project started the orbital correction maneuvers on July 12, 2010 - a deceleration maneuver, to prevent TanDEM-X from overtaking TerraSAR-X. On July 16, the distance between the satellites had shrunk to 370 km. This distance corresponded to a time gap of 48 seconds, during which the Earth continues to revolve, so that the trailing satellite did not cover the identical target area. Although the orbits are nearly identical for both satellites, Earth’s rotation caused the ground tracks at the equator to be separated from one another by more than 20 km. This distance is too great for interferometric observations. At the poles, the ground tracks approach one another and eventually intersect. This provided a unique opportunity to observe the same area with two satellites, one after another, from similar positions and with a reduced distance between them. However, because of the 48 second time delay and Earth's rotation, adjustments had to be made in the analysis of the imagery for each different imaging location. 80)

• The initial separation between TDX and TSX was 15700 km and after one month of drifting a formation with an along-track distance of 20 km was reached (20 km correspond to ~ 3 seconds). This formation was maintained for 3 months to calibrate the TanDEM-X radar instruments and to perform first bistatic and interferometric experiments employing large baselines.


Figure 28: Digital elevation model of an ice cap in the middle of October Revolution Island (image credit: DLR)


Figure 29: Ice floes on the coast of October Revolution Island (image credit: DLR)

• On June 24, 2010, only 3 days into the mission, TanDEM-X sent its first imagery back to Earth. The data transmission was received and automatically processed by the DLR ground station in Neustrelitz, Germany. This ended the LEOP (Launch and Early Orbit Phase) and started the commissioning phase which is expected to last for about 3 months. 81) 82)


Figure 30: First downlinked image of the TanDEM-X mission showing northern Madagascar (image credit: DLR)

Legend of Figure 30: The colored pale yellow on the right side of the image are the waves of the Indian Ocean. The change in the waves at the entrance to the Diego Suarez Bay is clearly visible. The water in the bay itself, on the shore of which lies the provincial capital city, Antsiranana, can be recognized. The water in the bay is very flat – in contrast to the undulating ocean – and reflects the radar signals from TanDEM-X more uniformly. The area of valleys to the south drains the volcanic cone of Ambre-Bobaomby into the Indian Ocean.

• A first signal of the TanDEM-X spacecraft was received via the Troll ground station in Antarctica. TrollSat is Norway's 7.5 m X-band ground station of KSAT (Kongsberg Satellite Services AS) in Antarctica (72º S, 2º E, since 2007) operated from Tromsø, Norway.



Sensor complement and support modes:

The TDX-SAR instrument is identical to the TSX-SAR (TerraSAR-X SAR instrument) in layout, operational performance and support modes. Hence, the reader is referred to the TSX-SAR instrument description of the TerraSAR-X mission.

Each spacecraft in the formation is equipped with 6 small horn antennas that are used to exchange X-band synchronization pulses with a frequency of about 10 Hz. This allows for a phase synchronization with an accuracy in the order of 1º.


Figure 31: Accommodation of the synchronization horn antennas with the beams shown in red (image credit: DLR)

The intersatellite X-band synchronization is established by a mutual exchange of radar pulses between the two satellites. For this information exchange, the nominal bistatic SAR data acquisition is shortly interrupted, and a radar pulse is redirected from the main SAR antenna to one of six dedicated synchronization horn antennas mounted on each spacecraft. The pulse is then recorded by the other satellite which in turn transmits a short synchronization pulse. This trigger establishes the bidirectional link between the two radar instruments which allows for mutual phase referencing without the exact knowledge of the actual distance between the satellites.

The TanDEM-specific SAR instrument features provide a scheme for transmission and reception of USO (Ultra Stable Oscillator) phase information between the SAR instruments with adequate SNR (at X-band for low atmospheric sensitivity). The synchronization horn antennas are connected to the unused polarization inputs/outputs of the leaf amplifier assembly TRMs. The major driver was to find a suitable accommodation (see Figure 31) and RF-design of the horns to give full solid-angle coverage with low phase disturbance. The data take start time accuracy has to be improved (relative to onboard GPS time) to allow for synchronization of data takes of both SARs. These modifications have already been implemented on the TerraSAR-X spacecraft.

Note: The TSX-SAR is already built for repeat pass interferometry and thus provides several essential features for the TanDEM-X mission, like precise orbit control, a single-frequency GPS for orbit determination, and excellent RF phase stability.





Satellite orbit altitude

514 km

Antenna length

4.8 m

Nominal swath width

30 km

Antenna width

0.7 m

Swath overlap

6 km

Antenna elements

32 x 12

Carrier frequency

9.65 GHz

Antenna tapering

Linear phase

Chirp bandwidth

100 MHz

Antenna mounting


Sampling frequency

110 MHz

Data quantization

3 bit/sample

Peak Tx power

2.26 kW

Image pair misregistration

< 0.1 pixel

Duty cycle


Proc. azimuth bandwidth

2266 Hz

Noise figure TRM

4.3 dB


0.1 pixel

Losses (proc., atm., taper, degradation)

3.1 dB

Sigma nought (σο) model (Ulaby, 90%, X-band)

Soil and rock, VV

Independent post spacing

12 m x 12 m

Along-track baseline

< 1 km

Table 11: X-SAR instrument parameters of TanDEM-X (TDX)

RF center frequency

9.65 GHz (X-band, 3.1 cm wavelength)


up to 300 MHz

Incidence angle range

20º to 55º


single, dual, quad

SAR modes

Stripmap, ScanSAR, Spotlight,
Dual-phase (split-antenna) center mode

Table 12: Main characteristics of the X-SAR instrument on TSX and TDX satellites

Regarding the X-SAR instrument on TDX, several optimizations were performed to further improve the producibility of the TRM (T/R Module) and to guarantee the excellent quality reached with the TSX TRMs. 83)



Secondary payloads of TanDEM-X (TOR, LRR, LCT)

TOR (Tracking, Occultation and Ranging)

TOR, a dual-frequency GPS flight receiver with two zenith antennas for POD (Precise Orbit Determination) applications support, is of TerraSAR-X heritage provided by GFZ Potsdam and CSR (Center of Space Research) of the University of Texas at Austin. - The excellent data provided by the TOR device flown on the TerraSAR-X mission led to the decision to duplicate this instrument for TanDEM-X. The additional TanDEM-X requirement of intersatellite baseline determination between both satellites with mm accuracy is essential for the success of the anticipated generation of global DEMs (Digital Elevation Models). 84)

The TOR payload consists of the dual-frequency GPS receiver IGOR (Integrated GPS Occultation Receiver), developed and built by Broad Reach Engineering Company of Tempe, AZ, and LRR (Laser Retro Reflector) for evaluation of GPS-based orbit data as an independent tracking technique. - The general description of TOR and LRR is provided under the TerraSAR-X mission.

LCT (Laser Communication Terminal)

LCT is again of Tesat-Spacecom GmbH manufacture and a contribution in kind of DLR. The objective of LCT on TanDEM-X is to demonstrate and verify the performance of a 2 Gbit/s optical LEO-to-GEO link as part of an experimental broadband data relay transmitting a 300 Mbit/s user data stream from TanDEM-X to ground. 85) 86)


Figure 32: Future application scenario of the LCT payload on various spacecraft (image credit: ESA, Tesat)

Compared to the TerraSAR-X mission, the enhanced TanDEM-X LCT terminal allows optical laser links to a relay satellite in the geostationary orbit. The technology demonstration of optical high data rate intersatellite links and the subsequent Ka-band downlink is important to cope with the increased demand of data downlink capacity in future EO missions (LCTs are being planned to support the European GMES program missions). 87)

Homodyne binary phase shift keying (BPSK) is based on coherent detection, i.e. the signal to be detected is superposed to a local oscillator laser running on the same frequency as the signal's carrier. - The general description of LCT is provided under the TerraSAR-X mission.

TDX/TSX applications:

Beyond the global HRTI-3 DEM as the primary mission objective, TanDEM-X will demonstrate several enabling technologies like: VLBI (Very Large Baseline Interferometry), along-track interferometry, polarimetric SAR interferometry, four phase center moving target indication, bistatic SAR imaging, and digital beamforming.

• The VLBI concept takes advantage of the large bandwidth of the TSX-SAR instrument to significantly improve the height accuracy for local areas by combining multiple interferograms with different baseline lengths. This can e.g. be used to acquire DEMs with HRTI-4 like quality on a local or even regional scale. A temporal comparison of multiple large baseline TanDEM-X interferograms (either phase or coherence) provides furthermore a very sensitive measure for vertical scene and structure changes. 88)

• Along-track SAR interferometry can either be performed by the so-called dual receive antenna mode with a baseline of 2.4 m from each of the satellites or by adjusting the along-track distance of the two satellites to the desired size (Figure 33). The HELIX orbit concept allows this distance (called along-track baseline) to be adjusted from zero to several kilometers. This technical feature is essential as this application requires velocity measurements of different fast and slow objects. Mainly four scientific application areas are identified to explore the innovative along-track mode: oceanography, traffic monitoring, glaciology and hydrology.


Figure 33: Along-track interferometry modes in TDX/TSX configuration (image credit: DLR)

• Polarimetric SAR Interferometry combines interferometric with polarimetric measurements. This allows e.g. for the extraction of vegetation density and vegetation height. Fully polarimetric operation uses the split antenna and is susceptible to ambiguities which limit the swath width. This could be avoided by a restriction to dual polarized measurements and/or an acquisition of multiple polarizations in successive passes.

• Bistatic Imaging provides additional observables for the extraction of new scene and target parameters. A combination of bistatic and monostatic images can e.g. be used to improve segmentation, classification and detection. Data takes with large bistatic angles are planned at the beginning and end of the TanDEM-X mission.

• The TerraSAR-X/TanDEM-X mission will be the first operational mission requiring a post-facto baseline reconstruction with an accuracy of 1 mm. The feasibility of achieving this goal using GPS dual-frequency measurements of the IGOR GPS receiver has earlier been demonstrated in the GRACE mission.

• Digital beamforming (DBF) and super resolution techniques can be used to suppress ambiguities and to enhance the ground resolution. The combination of the four independent phase centers in TSX and TDX enables also a first demonstration of high resolution wide swath (HRWS) SAR imaging.

With TanDEM-X, innovative SAR techniques will be demonstrated and exploited, which open up new perspectives for future SAR systems. The focus will be on the following application areas and the associated application topics:

Application area

Application topics

Cross-track SAR interferometry

Land environment

Navigation, Crisis and security management, Urban areas,


Ice and snow, Sea-ice, Morphology/hydrology


Geological mapping/morphology, Earthquakes/volcanoes, Landslides, Subsidence (land and urban areas)

Land cover and vegetation

Land cover/surface parameters, Forestry,


Wind and waves/ocean dynamics, Ship detection, Coastal zones

Along-track SAR interferometry


Ocean currents, Coastal zones, Ship detection, Wind and waves


Traffic flow monitoring, Development of new SAR techniques


Ice flow monitoring


River flow monitoring, Development of new SAR techniques

New SAR techniques

Multistatic processing

Bistatic/multistatic processing, Land cover, Development of new SAR techniques

Pol-InSAR (polarimetric SAR interferometry)

Forest, Agriculture, Snow and ice, Ship detection, Wind and waves, Geological mapping, Urban areas, Landslides/subsidence, Earthquakes/volcanoes, Development of new SAR techniques

Digital beam forming (DBF)

Wide swath imaging and ambiguity suppression, Development of new SAR techniques

Super resolution

Development of new SAR techniques

InSAR processing

Development of new SAR techniques, Ultra high resolution DEM with multiple baselines

Formation flying

Development of new SAR techniques, Precise baseline determination and orbit control

Table 13: Overview of application areas and topics for the various science teams


Figure 34: General outline of the data acquisition plan (image credit: DLR)


Figure 35: The TanDEM-X timeline (image credit: DLR, outdated image)



TanDEM-X ground segment:

The TanDEM-X and TerraSAR-X missions are closely linked and share resources of the space and ground segments. Consequently, for the overall TanDEM-X system, besides the additional TanDEM-X satellite, the TerraSAR-X ground segment has to be extended. The ground segment consists of three major parts all covered by DLR: 89) 90)

• MOS (Mission Operations Segment) to simultaneously operate two satellites in close formation and to optimally merge the acquisition plans for both missions. The MOS service is provided by the German Space Operation Center (GSOC).

• PGS (Payload Ground Segment) to handle the increased data volume, to include a network of receiving stations, to adapt the processing chain for new data products and to generate the global HRTI-3 DEM. The PGS service is provided by the German Remote Sensing Data Center (DFD) and the Remote Sensing Technology Institute (IMF).

• IOCS (Instrument Operations and Calibration Segment) to operate the two SAR sensors in bistatic mode including synchronization, to calibrate and validate interferometric products. The IOCS service is being provided by the Microwaves and Radar Institute (IHR).

The GFZ (Geoforschungszentrum) Potsdam is responsible for the IGOR data analysis for precise baseline determination.

The scientific exploitation of the TanDEM-X data is coordinated by the DLR Science Service Segment. Commercial customers are served by Infoterra GmbH.


Figure 36: Overview of the TanDEM-X overall system architecture (image credit: DLR)

The ground stations for data reception are: 91)

- Neustrelitz, Germany (DLR station)

- Weilheim, Germany (DLR station)

- GARS (German Antarctic Receiving Station) at O’Higgins in Antarctica with 9 m L/S/X-band antenna

- Inuvik, NWT (Northwest Territories), Canada: DLR station with 13 m L/S/X-band antenna. Inuvik is a CCRS-controlled facility located just outside the town of Inuvik (68º 19’ N; 133º 32’ W). On August 10, 2010, DLR inaugurated the new Inuvik station for TanDEM-X use. 92) 93) 94) 95) 96)

The Inuvik station is fully automated, from antenna control, through the complete reception chain, to the recording of the encrypted raw data onto tape. These tapes are then sent to DLR for processing. Monitoring of the station operations is performed at the Earth Observation Center in Oberpfaffenhofen, where all the data from the various TanDEM-X ground stations comes together and is assessed for completeness and quality.

- ERIS Chetumal, Mexico (Yucatan Peninsula): DLR transportable ground station with L/S/X-band 9 m antenna (ERIS = Estacion para la Reception de Imagenes Satelitales).

- Kiruna, Sweden, a partner ground station of SSC (Swedish Space Corporation).


Figure 37: Structure of the twin SAR missions on an organizational level (image credit: DLR)


Figure 38: Overview of the German spaceborne SAR development line (image credit: DLR)

SAR products

DEM products

Experimental products from operational modes (co-registered complex images –“CoSSCs”)

HRTI-3 specified DEM

Experimental mode raw data (processing with help from DLR contact scientist)

Intermediate DEM: close to HRTI-3 specified DEM

TS-X mission basic products* from selected TanDEM-X raw data sets
* : TS-X basic product performance parameter specification does not apply

FDEMs: DEMs processed to finer pixel spacing and higher random height error

“Byproduct“ of operational DEM processing chain: archive of CoSSCs from all acquisitions for DEM generation (multi-temporal global coverage)

HDEMs: HRTI-4 like DEMs (high resolution DEM, were additional acquisitions are needed)

Table 14: TanDEM-X data products

Dual Satellite and Dual Mission Operations Concept: 97)

The basic operations concept is to handle TSX and TDX operations independently as far as possible. TSX and TDX spacecraft are nearly identical from hard- and soft-ware point of view. There is no master/slave configuration between both spacecraft but an equal level operations approach. Figure 39 shows a simplified work flow for TSX and TDX operations in the control center. Each satellite, TSX and TDX, has a dedicated TT&C link from ground to space. There is no telecommand routing from one spacecraft to the other and also no complete telemetry routing.


Figure 39: Schematic, simplified workflow for dual satellite/dual mission operations (image credit: DLR)

Legend to Figure 39: The green box summarizes elements being part of the Control Center. TDX and TSX space craft are each controlled by a satellite specific monitoring & control system. The elements Mission Planning and Flight Dynamics are combined systems for both satellites, generating output to both monitoring & control systems. The Mission Planning System receives external orders from the TerraSAR-X mission and the TanDEM-X mission. Relative orbit control performed by Flight Dynamics is based on the required geometry of the TanDEM-X mission phase.


GSCDA (GMES Space Component Data Access)

The GMES Space Component (GSC) includes the Sentinel satellites and the coordinated access to ESA and European EO missions.

In 2007, ESA and the EC (European Commission) signed an agreement to allow ESA to ensure the coordinated and timely supply of satellite-based Earth Observation data for the preoperational phase of GMES from 2008 to 2010.

ESA is managing the GSCDA project in the frame of the FP7 space program as part of the European Space Policy focusing on coordinating the access to space-based observation data to support GMES services.

ESA targets the introduction of the following capabilities to achieve a coordinated access to data from current and future missions:

HMA (Heterogeneous Mission Access). GMES data access implies also a coherent data access to ~40 different EO missions (inside and outside of ESA). Aside from the current and future ESA missions (Envisat, GOCE, SMOS, CryoSat-2, MSG-3, Swarm, ADM/Aeolus, GMES Sentinels, etc.), the European space agencies are also cooperating with their EO missions to make HMA become possible for a global EO community. 98)

QA4EO (Quality Assurance Framework for Earth Observation data). 99)

LTDP (Long Term Data Preservation). 100)

DLR is a cooperative member of the GSCDA initiative. The ground segment of the missions TerraSAR-X (launch June 15, 2007), TanDEM-X (launch planned for Oct. 2009), and EnMAP (launch 2013) are part of GSCDA (implementation of HMA customization). The GSCDA/HMA feature will be made compatible through the DLR DIMS-based (Data and Information Management System) catalog/archive. 101)


Figure 40: German GMES EO data interfaces (image credit: DLR)

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41) Daniel Schulze, Jaap Herman, Sebastian Löw, “Formation Flight in Low-Earth-Orbit at 150 m Distance - AOCS In-Orbit Experience,” Proceedings of SpaceOps 2012, The 12th International Conference on Space Operations, Stockholm, Sweden, June 11-15, 2012, URL:

42) A. Schwab, Ch. Giese, D. Ulrich, “TDX-TSX - On-board autonomy and FDIR of whispering brothers,” Proceedings of SpaceOps 2012, The 12th International Conference on Space Operations, Stockholm, Sweden, June 11-15, 2012, URL:

43) R. Kahle, “TSX/TDX Formation Collision & Illumination Aspects,” TD-MOS-TN-4060; GSOC, 2008

44) J. Herman, D. Fischer, D. Schulze, S. Loew, M. Licht, “AOCS for TanDEM-X – Formation Flight at 200 m Separation in low-Earth Orbit”; SpaceOps conference, Huntsville, Al, USA, April 25-30, 2010, URL:

45) Information provided by Manfred Zink of DLR

46) Thomas Schrage, Juergen Janoth, Alexander Kaptein, Noemie Bernede, Steffen Gantert, Ralf Duering, “TerraSAR-X Next Generation – Mission Overview,” Proceedings of the 64th International Astronautical Congress (IAC 2013), Beijing, China, Sept. 23-27, 2013, paper: IAC-13-B1.2.8

47) “WorldDEMTM Reaching New Heights,” Infoterra GmbH, 2012, URL:

48) “Experience the Quality and Accuracy of the WorldDEMTM,” Astrium, URL:

49) Daniela Borla Tridon, M. Bachmann, D. Schulze, C. J. Ortega Miguez, M. D. Polimeni, M. Martone, “TanDEM-X: DEM acquisition in the third year era,” Proceedings of the 5th International Conference on Spacecraft Formation Flying Missions and Technologies (SFFMT), Munich, Germany, May 29-31, 2013, URL of paper : , URL of presentation:

50) Martin Wermuth, Rolf König, Yongjin Moon, John Mohan Walter Antony, Oliver Montenbruck, “Two years of TanDEM-X baseline determination,” Proceedings of the 5th International Conference on Spacecraft Formation Flying Missions and Technologies (SFFMT), Munich, Germany, May 29-31, 2013, URL of paper: , URL of presentation:

51) Michael Eineder, Thomas Fritz, Wael Abdel Jaber, Cristian Rossi, Helko Breit, “Decadal Earth Topography Dynamics Measured with TanDEM-X and SRTM,” Proceedings of IGARSS (International Geoscience and Remote Sensing Symposium), Munich, Germany, July 22-27, 2012

52) José Luis Bueso Bello, Christo Grigorov, Ulrich Steinbrecher, Thomas Kraus, Carolina González, Daniel Schulze, Benjamin Bräutigam, “System Commanding and Performance of TanDEM-X Scientific Modes,” Proceedings of EUSAR 2012 (9th European Conference on Synthetic Aperture Radar), Nuremberg, Germany, April 23-26, 2012

53) Manfred Zink, “TanDEM-X Mission Status,” Proceedings of IGARSS (International Geoscience and Remote Sensing Symposium), Munich, Germany, July 22-27, 2012

54) Elisabeth Mittelbach, Manfred Zink, “A step closer to mapping the Earth in 3D,” DLR, Jan. 13, 2012, URL:

55) “A Step Closer to Mapping the Earth in 3D - First TanDEM-X Coverage of the World Completed,” Astrium, Feb. 2012, URL:

56) M. Zink, Michael Bartusch, Dieter Ulrich, “TanDEM-X Mission Status,” Proceedings of EUSAR 2012 (9th European Conference on Synthetic Aperture Radar), Nuremberg, Germany, April 23-26, 2012

57) Benjamin Bräutigam, Michele Martone, Paola Rizzoli, Markus Bachmann, Gerhard Krieger, “Interferometric Performance of TanDEM-X Global DEM Acquisitions,” Proceedings of EUSAR 2012 (9th European Conference on Synthetic Aperture Radar), Nuremberg, Germany, April 23-26, 2012

58) Helko Breit, Marie Lachaise, Ulrich Balss, Cristian Rossi, Thomas Fritz, Andreas Niedermeier, “Bistatic and Interferometric Processing of TanDEM-X Data,” Proceedings of EUSAR 2012 (9th European Conference on Synthetic Aperture Radar), Nuremberg, Germany, April 23-26, 2012

59) M. Zink, Michael Bartusch, David Miller, “TanDEM-X Mission Status,” Proceedings of IGARSS (International Geoscience and Remote Sensing Symposium), Vancouver, Canada, July 24-29, 2011

60) Manfred Zink, “TanDEM-X Mission Status & Commissioning Phase Overview,” 3rd TanDEM-X Science Team Meeting, Feb. 17, 2011, URL:

61) Agenda of the TanDEM-X Science Meeting along with all presentation, Feb. 17, 2011, Oberpfaffenhofen, Germany, URL:

62) Christoph Giese, “The TanDEM-X Space Segment,” 3rd TanDEM-X Science Team Meeting, Feb. 17, 2011, URL:

63) Rolf Scheiber, “Die Dynamik der Eisbewegung,” March 1, 2011, URL:

64) Gerhard Krieger, Manfred Zink, Alberto Moreira, “TanDEM-X: A Radar Interferometer with two Formation Flying Satellites,” Invited paper, Proceedings of the 63rd IAC (International Astronautical Congress), Naples, Italy, Oct. 1-5, 2012, paper: IAC-12-B4.7B.3

65) Ralph Kahle, Benjamin Schlepp, Michael Kirschner, “TerraSAR-X / TanDEM-X Formation Control - First Results from Commissioning and Routine Operations,” Proceedings of the 22nd International Symposium on Space Flight Dynamics (ISSFD), Feb. 28 - March 4, 2011, Sao Jose dos Campos, SP, Brazil, URL:

66) Martin Wermuth, Oliver Montenbruck, Anna Wendleder, “Relative navigation for the TanDEM-X mission and evaluation with DEM calibration results,” Proceedings of the 22nd International Symposium on Space Flight Dynamics (ISSFD), Feb. 28 - March 4, 2011, Sao Jose dos Campos, SP, Brazil, URL:

67) “TanDEM-X: ready for routine operations in 2011,” DLR, Dec. 15, 2010, URL:

68) “TanDEM-X: Mapping the world in 3D,” Astrium, Dec. 16, 2010, URL:

69) Daniel Schulze, Paola Rizzoli, Benjamin Bräutigam, Gerhard Krieger, “In-orbit Performance of TSX-1 and TDX-1,” Proceedings of IGARSS (International Geoscience and Remote Sensing Symposium), Vancouver, Canada, July 24-29, 2011

70) Marco Schwerdt, Jaime Hueso Gonzalez, Markus Bachmann, Dirk Schrank, Björn Döring, Nuria Tous Ramon, John Mohan Walter Antony, “In-orbit Calibration of the TanDEM-X System,” Proceedings of IGARSS (International Geoscience and Remote Sensing Symposium), Vancouver, Canada, July 24-29, 2011

71) Marco Schwerdt, Dirk Schrank, Markus Bachmann, Jaime Hueso Gonzalez, Björn J. Döring, Nuria Tous-Ramon, John Walter Antony, “Calibration of the TerraSAR-X and the TanDEM-X Satellite for the TerraSAR-X Mission,” Proceedings of EUSAR 2012 (9th European Conference on Synthetic Aperture Radar), Nuremberg, Germany, April 23-26, 2012

72) Jaime Hueso González, John Walter Antony, Gerhard Krieger, Marco Schwerdt, “Baseline Calibration in TanDEM-X,” Proceedings of IGARSS (International Geoscience and Remote Sensing Symposium), Vancouver, Canada, July 24-29, 2011

73) Stefan Buckreuss, “Everybody waltz,” October 12, 2010, URL:

74) Birgit Schättler, “TanDEM-X ground segment kicks off,” DLR, Aug. 2, 2010, URL:


76) Ralph Kahle, “Die 20-Kilometer Formation ist eingestellt,” DLR, July 20, 2010, URL:

77) Marc Rodriguez-Cassola, Pau Prats, Daniel Schulze, Nuria Tous-Ramon, Ulrich Steinbrecher, Luca Marotti, Matteo Nannini, Marwan Younis, Paco Lopez-Dekker, Manfred Zink, Andreas Reigber, Gerhard Krieger, Alberto Moreira, “First Bistatic Spaceborne SAR Experiments with TanDEM-X,” Proceedings of IGARSS (International Geoscience and Remote Sensing Symposium), Vancouver, Canada, July 24-29, 2011

78) Marc Rodriguez-Cassola, Pau Prats, Daniel Schulze, Ulrich Steinbrecher, Nuria Tous-Ramon, Marwan Younis, Paco López-Dekker,Manfred Zink, Andreas Reigber, Alberto Moreira, Gerhard Krieger, “Non-nominal Experimental Bistatic SAR Acquisitions with TanDEM-X,” Proceedings of EUSAR 2012 (9th European Conference on Synthetic Aperture Radar), Nuremberg, Germany, April 23-26, 2012

79) Pau Prats, Rolf Scheiber, Josef Mittermayer, Steffen Wollstadt, Stefan V. Baumgartner, Paco López-Dekker, Daniel Schulze, Ulrich Steinbrecher, Marc Rodríguez-Cassolà, Andreas Reigber, Gerhard Krieger, Manfred Zink, Alberto Moreira, “TanDEM-X Experiments in Pursuit Monostatic Configuration,” Proceedings of EUSAR 2012 (9th European Conference on Synthetic Aperture Radar), Nuremberg, Germany, April 23-26, 2012

80) Gerhard Krieger, “The first 3D experiment,” July 22, 2010, URL:

81) “TanDEM-X sends its first images in record time,” DLR, June 25, 2010, URL:

82) Space Daily, June 28, 2010, URL:

83) K. Biller, M. Adolph, H. Dreher, A. Fleckenstein, U. Hackenberg, G. Hoefer, R. Rieger, M. Wahl, R. Zahn, “Design and Performance of the TanDEM-X T/R Modules,” Proceedings of EUSAR 2008, 7th European Conference on Synthetic Aperture Radar, June 2-5, 2008, Friedrichshafen, Germany

84) L. Grunwaldt, “The TOR Payload on TanDEM-X,” Proceedings of EUSAR 2008, 7th European Conference on Synthetic Aperture Radar, June 2-5, 2008, Friedrichshafen, Germany


86) Robert Lange, Frank Heine, Hartmut Kämpfer, Rolf Meyer, "High Data Rate Optical Inter-Satellite Links," 35th ECOC (European Conference on Optical Communication) Sept. 20-24, 2009, Vienna, Austria

87) Berry Smutny, Hartmut Kaempfner, Gerd Muehlnikel, Uwe Sterr, Bernhard Wandernoth, Frank Heine, Ulrich Hildebrand, Daniel Dallmann, Martin Reinhardt, Axel Freier, Robert Lange, Knut Boehmer, Thomas Feldhaus, Juergen Mueller, Andreas Weichert, Peter Greulich, Stefan Seel, Rolf Meyer, Reinhard Czichy, “5.6 Gbps optical intersatellite communication link,” Free-Space Laser Communication Technologies XXI,. Edited by Hemmati, Hamid, Proceedings of the SPIE, Volume 7199., pp. 719906-719906-8, 2009

88) Irena Hajnsek, Thomas Busche, Alberto Moreira & TanDEM-X Team, “Mission Status and Data Availability: TanDEM-X,” Proceedings of the 4th International POLinSAR 2009 Workshop, Jan. 26-30, 2009, ESA/ESRIN, Frascati, Italy, URL:

89) Information provided by Irina Hajnsek of DLR, Oberpfaffenhofen, Germany

90) B. Schättler, R. Kahle, R. Metzig, U. Steinbrecher, M. Zink, “The Joint TerraSAR-X / TanDEM-X Ground Segment,” Proceedings of IGARSS (International Geoscience and Remote Sensing Symposium), Vancouver, Canada, July 24-29, 2011

91) Erhard Diedrich, Norbert Bauer, Robert Metzig, Max Schwinger, “Ground Station Network for Payload Data Reception of German TanDEM-X Mission,” Proceedings of the SpaceOps 2010 Conference, Huntsville, ALA, USA, April 25-30, 2010, paper: AIAA 2010-1976

92) Mikael Stern, Erhard Diedrich, Jean-Marc Soula, “The Inuvik Station in Canada: An Example on how Space Agencies and Industry Share Risks and Benefits,” Proceedings of the SpaceOps 2010 Conference, Huntsville, ALA, USA, April 25-30, 2010, paper: AIAA 2010-1901

93) “Inauguration Of First DLR Ground Station In Canada,” Space Daily, Aug. 12, 2010, URL:

94) “Set-up of DLR-receiving antennas for the TanDEM-X mission in INUVIK (North Canada) has been successfully completed,” Aug. 12, 2010, URL:

95) Jan Wörner, Inauguration of the DLR ground station in Inuvik“ Aug. 12, 2010, URL:

96) R. Metzig, E. Diedrich, R. Reissig, M. Schwinger, F. Riffel, H. Henniger, B. Schättler, “The TanDEM-X Ground Station Network,” Proceedings of IGARSS (International Geoscience and Remote Sensing Symposium), Vancouver, Canada, July 24-29, 2011

97) E. Maurer, S. Zimmermann , F. Mrowka, H. Hofmann, “Dual Satellite Operations in Close Formation Flight,” Proceedings of SpaceOps 2012, The 12th International Conference on Space Operations, Stockholm, Sweden, June 11-15, 2012

98) M. Eugenia Forcada, H. Laur, B. Hoersch, J. Martin, P. Goryl, G. Ottavianelli G. Buscemi, S. Badessi, “ESA Missions and Sentinels ground segment interoperability,” GSCB (Ground Segment Coordination Body) Workshop, ESA/ESRIN, Frascati, Italy, June 18-19, 2009, URL:

99) Pascal Lecomte, Greg Stensaas, “Overview of progress towards a data quality assurance strategy to facilitate interoperability,” GSCB (Ground Segment Coordination Body) Workshop, ESA/ESRIN, Frascati, Italy, June 18-19, 2009, URL:

100) V. Beruti, M. Albani, “European framework for the long term preservation of Earth Observation space data,” GSCB (Ground Segment Coordination Body) Workshop, ESA/ESRIN, Frascati, Italy, June 18-19, 2009, URL:

101) Gunter Schreier, Jürgen Janoth, “TerraSAR-X, TanDEM-X, EnMAP,” GSCB (Ground Segment Coordination Body) Workshop, ESA/ESRIN, Frascati, Italy, June 18-19, 2009, URL:

The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.