Minimize ISS: ACES

ISS Utilization: ACES (Atomic Clock Ensemble in Space)

ACES is an ESA ultra-stable clock experiment, a time and frequency mission to be flown on the Columbus module of the ISS (International Space Station), in support of fundamental physics tests. The mission objectives are both scientific and technological and is of great interest to two main scientific communities:

• The Time and Frequency (T&F) community; which aims to use ACES as a tool for high precision Time and Frequency metrology

• The Fundamental Physics community; which will benefit from the use of ACES data for accurate tests of general relativity.

The fundamental aspects of ACES deal with the physics of a cold atom clock. For the first time cold atoms will be operated in conditions which are not realizable on Earth in order to perform fundamental physics tests (relativity, possible drift of fundamental constants with time). At the same time, a number of new technologies needed by the science community, will be validated. Furthermore, the science community could take advantage worldwide of the ACES frequency stability by using ground stations to download the ACES time reference. These aspects will become increasingly important with future developments of navigation and positioning systems, new "matter-wave" inertial sensors and fundamental physics tests in solar orbit. 1) 2) 3) 4)

The ACES development was initiated in the 1990s. However, the decision to complete the development of the project has been achieved only at the ESA council at Ministerial Level of November 2008.

Test of new generation of space clocks: The basic question is: Do fundamental physical constants vary with time? A possible solution: Time variations of fundamental constants can be measured by comparing clocks based on different transitions or different atomic species. The microgravity environment of space is most suitable for experiment. 5) 6)

A new generation of clocks, reaching frequency instability and inaccuracy of a few parts in 1016, will be validated by ACES. The cesium clock PHARAO will combine laser cooling techniques and microgravity conditions to significantly increase the interaction time and consequently reduce the linewidth of the clock transition. Improved stability and better control of systematic effects will be demonstrated in the space environment.


Figure 1: Fractional frequency instability specified for the PHARAO, SHM, and ACES clock signals in space (image credit: ESA)

The reliability offered by active H-masers will be made available for space applications by SHM (Space Hydrogen Maser). SHM will demonstrate a fractional frequency instability of 1.5 x 10-15 after only 10000 seconds of integration time. Two servo-loops will lock together the clock signals of PHARAO and SHM generating an on-board time scale combining the short-term stability of the H-maser with the long-term stability and accuracy of the cesium clock.

Stable and accurate time and frequency transfer via a dedicated Microwave Link: The accurate and stable ACES clock signal will be distributed by a dedicated MWL (Microwave Link). Frequency transfer with time deviation better than 0.3 ps (picoseconds) at 300 seconds, 7 ps at 1 day, and 23 ps at 10 days of integration time will be demonstrated. These performances, surpassing existing techniques, namely TWSTFT (Two-Way Satellite Time and Frequency Transfer) and GPS, by one to two orders of magnitude, will enable common view and non-common view comparisons of ground clocks with 10-17 frequency resolution after few days of integration time.


Figure 2: Time instability specified for PHARAO, SHM, and MWL (image credit: ESA)

Applications: ACES will also demonstrate a new technique, called “relativistic geodesy”, to map the Earth gravitational potential. This technique uses a precision measurement of the Einstein’s gravitational red-shift between two clocks to determine the corresponding difference in the local gravitational potentials. The possibility of performing comparisons of ground clocks at the 10-17 frequency uncertainty level will allow ACES to resolve geopotential differences at 10 cm.

A dedicated GNSS receiver on-board the ACES payload will ensure orbit determination, important for comparing clocks and performing fundamental physics tests. In addition, the GNSS subsystem will be connected to the ACES clock signal, opening the possibility to use the GNSS network for clock comparisons or remote sensing applications (GNSS radio-occultation and reflectometry).

In addition to the ACES MWL, the ELT (European Laser Timing) link will allow clock comparisons, time transfer, and ranging experiments in the optical domain. The combination of ELT and MWL will provide a bench to test two different time transfer and ranging techniques, also opening the door to studies of atmospheric propagation delays.

ACES status:

On Dec. 15, 2009, ESA and CNES signed an agreement that paves the way for the launch of a high-accuracy atomic clock to be attached to the outside of the European Columbus laboratory onboard the ISS. The ACES Payload Critical Design Review (CDR) and the Ground Segment Preliminary Design Review (PDR) have been successfully passed in 2009.

In 2010, the ACES mission is in phase C/D. All instruments and subsystems are in an advanced state of development with engineering models delivered or in final assembly. The ACES Engineering Model (EM) workbench has been integrated and thoroughly tested at the CNES facilities in Toulouse. 7) 8) 9)

Under the agreement, CNES will fund and develop the PHARAO atomic clock and deliver it to ESA for integration into ACES. CNES will also support the performance tests and deliver the ground console to be used to operate PHARAO from CADMOS. - ESA will develop ACES and the accommodation hardware needed for PHARAO and will integrate its operations into the overall European ISS and Columbus operations. ESA will also develop the SHM clock, which is funded by the European Program for Life and Physical Sciences (ELIPS). SHM was initially a Swiss project.

In the fall of 2011, the ACES mission is in the manufacturing and test phase C/D. All instruments and subsystems are in an advanced state of development with engineering models delivered and flight models manufacturing started. The ACES Payload and Ground Segment Critical Design Reviews (CDR) have been successfully passed. The ACES Engineering Model (EM) workbench has been integrated and thoroughly tested at the CNES facilities in Toulouse. 10)

The manufacturing and assembly of the flight model is now proceeding towards a launch with HTV to the International Space Station in the timeframe 2015-2016. ACES is designed for a mission duration of 1.5 years with the possibility for extension to a minimum of 3 years (Ref. 10).

The payload prime contractor, EADS Astrium, leads an industrial consortium for the design, development and delivery of the flight payload and the ground segment.


Launch: ACES is designed for launch in the unpressurized cargo bay of the Japanese H-II Transfer Vehicle (HTV) in 2016. Once in orbit, ACES will be attached to Columbus' Earth-facing external payload platform using the Station Robotic Arm.

The mission duration is planned for 18 to 36 months.

Orbit: Near-circular orbit of the ISS, mean altitude of ~ 350 km, inclination of 51.6º, period of ~ 90 minutes.


Figure 3: Artist's view of the ACES payload mounted externally onto the Columbus module (image credit: ESA)


Figure 4: Schematic view of the ACES payload on ISS (image credit: ESA)



ACES (Atomic Clock Ensemble in Space) experiment:

The ACES performances will be used to conduct a suit of fundamental physics experiments to test Einstein’s theory of general relativity with improved accuracy. With the progress recently achieved by clocks in the optical domain, accuracy levels even higher than originally foreseen will be reached.

According to Einstein’s theory, identical clocks placed at different positions in stationary gravitational fields experience a frequency shift that, in the frame of the PPN (Parameterized Post-Newtonian) approximation, depends directly on the Newtonian potential at the clock position. The comparison between the ACES onboard clocks and ground-based atomic clocks will measure the frequency variation due to the gravitational red-shift with a 35-fold improvement on previous experiments, testing Einstein’s prediction at the 2 ppm uncertainty level (Ref. 5).

The operation of the laser-cooled cesium atomic clock PHARAO opens opportunities in various fields of fundamental research and application fields. The ultra precise measurement of time will also allow relativistic measurements and tests, applications in atmospheric physics and geodesy, navigation and advanced telecommunications.


Figure 5: Illustration of the ACES payload design (image credit: ESA, EADS Astrium)

Legend to Figure 5: Front: PHARAO, GNSS antenna; Back: SHM, side panels removed for better visibility.

ACES mission objectives

ACES performances

Scientific background and recent results

Fundamental Physics tests

Measurement of the gravitational red shift

Absolute measurement of the gravitational red-shift at an uncertainty level < 50 ·10-6 after 300 s and < 2 ·10-6 after 10 days of integration time

Space-to-ground clock comparison at the 10-16 level, will yield a factor 35 improvement on previous measurements (GPA experiment).

Search for time drifts of fundamental constants

Time variations of the fine structure constant α at a precision level of α-1 dα/ dt < 1 x 10-17 year-1 down to 3 x 10-18 year-1 in case of a mission duration of 3 years

Optical clocks progress will allow clock-to-clock comparisons below the 10-17 level. Crossed comparisons of clocks based on different atomic elements will impose strong constraints on the time drifts of α, meQCD, and muQCD

Search for violations of special relativity

Search for anisotropies of the speed of light at the level δc / c < 10-10

ACES results will improve present limits on the RMS parameter α based on fast ions spectroscopy and GPS satellites by one and two orders of magnitudes, respectively.

Table 1: Overview of ACES mission objectives (Ref. 11)

ACES is a distributed system designed to disseminate a highly stable and accurate clock signal. It consists of a space payload generating the ACES atomic frequency reference and a network of ground terminals connected to high-performance atomic clocks on ground.

The ACES experiment consists of 2 instruments (atomic clocks) plus two complimentary service tasks: 11) 12) 13) 14) 15) 16) 17) 18) 19)

1) PHARAO (Project d'Horloge Atomique a Refroidissement d'Atomes en Orbite): a laser-cooled cesium atomic clock that is exploiting the microgravity conditions onboard ISS to reach unprecedented precision not achievable on Earth. CNES [design by LNE-SYRTE (Observatoire de Paris), LKB (Laboratoire Kastler Brossel) of CRNS (Centre National de la Recherche Scientifique), Paris, and CNES] is in charge of developing and integrating the PHARAO clock, which is the main French contribution to ACES. 20)

2) SHM (Space Hydrogen Maser): a reference clock of ESA and local oscillator for PHARAO.

3) FCDP (Frequency Comparison and Distribution Package): in support of frequency comparison and processing

4) MWL (MicroWave Link): a link for time-frequency transfer to the ground.

The ACES support subsystems are (Ref. 11):

• XPLC (External Payload Computer)

• PDU (Power Distribution Unit)

• Mechanical, thermal subsystems

• CEPA (Columbus External Payload Adapter)

• ELT (European Laser Timing), an optical link provided by ESA

• GNSS receiver - providing orbit determination as an operational function; in addition provision of support applications in the areas of:

- GNSS time and frequency transfer

- Radio-occultation experiments

- Coherent reflectometry experiments.

The ACES payload has a total mass of 227 kg, a volume of 1172 mm x 867 mm x 1246 mm, and a power consumption of 450 W.



Figure 6: The ACES payload with component designation (image credit: EADS Astrium)

PHARAO clock:

Concept: The core of the ACES project is the laser-cooled cesium atomic clock, PHARAO, that is exploiting the microgravity conditions onboard ISS to reach unprecedented precision not achievable on Earth. In fact, a cold atomic clock works more accurately under weightlessness than under Earth's gravity. This is related to the principle termed “atomic fountain” which is used in the clock. Under microgravity conditions, the cold atoms traverse a resonant cavity, with a slower speed than on Earth. In the cavity, the cold atom interact two times with a microwave field tuned on the transition between the two hyperfine levels of the cesium ground state (Ref. 5).

The interrogation method, based on two spatially separated oscillating fields (Ramsey scheme), allows the detection of an atomic line whose width is inversely proportional to the transit time between the two interaction regions. In a microgravity environment, the velocity of the atoms along the ballistic trajectories is constant and can be continuously changed over almost two orders of magnitude (5-500 cm/s) allowing the detection of atomic signals with sub-Hertz linewidth.


Figure 7: Atomic clock principle in microgravity (image credit: CNES)

The PHARAO instrument is composed of the following subsystems: 21)

TC (Cesium Tube): In which atoms are captured, launched, cooled, selected and detected after undergoing a microwave interaction within a microwave cavity. The caesium tube provides ultra-vacuum conditions throughout the atomic path and applies a constant and extremely uniform magnetic field along the atomic path, especially inside the microwave interrogation chamber. It includes also the ion pump high voltage supply (CV-THT), which is mounted separately on the ACES baseplate. 22)

SL (Laser Source): SL is an optical bench which provides the various laser beams necessary for the capture, launch, cooling down, atomic selection and detection of the atoms.

SH (Microwave Source): SH supplies the signal to drive interrogation and preparation cavities.

UGB (On Board Management Unit): UGB processes the detection signal to command the frequency corrections to be applied to the microwave source in autonomous mode or transmitted to ACES-XPLC in the other operational modes. It also synchronizes the different phases of the atomic cycle, manages the measurements acquisitions and the remote control systems in order to modify the functional parameters of the instrument.

BEBA (Electronic Unit): BEBA controls the cesium tube magnetic coils and acquires analog signals issued by the cesium tube.


Mass (kg)

Size (L x W x H, mm)

Power consumption (W)



990 x 336 x 444




529 x 330 x 178




300 x 270 x 117




245 x 240 x 120




134 x 118 x 85


Total (including harness, fixations and margin)


in its greater length


Table 2: Summary of PHARAO subsystem parameters


Figure 8: Diagram of the PHARAO clock architecture (the black lines represent the electrical connections), image credit: CNES


SHM (Space Hydrogen Maser):

The clock operates on the hyperfine transition of atomic hydrogen at 1.420405751 GHz. H2 molecules are dissociated in a plasma discharge and the resulting beam of H atoms is state selected and sent in a storage bulb. The bulb is surrounded by a sapphire-loaded microwave cavity which, tuned on the atomic resonance, induces the maser action.

The SHM device is developed at Spectratime, Switzerland, under ESA contract. SHM provides ACES with a stable fly-wheel oscillator. The main challenge of SHM is the low mass and low volume requirement when compared to ground H-masers. SHM has a mass budget of 42 kg and a volume of 390 mm x 390 mm x 590 mm. A dedicated ACT (Automatic Cavity Tuning) system has been developed to maintain the microwave cavity tuned to resonance. The ACT system continuously injects two tones symmetrically placed around the H-maser signal. The two tones are coherently detected and the unbalance between their power levels is used to close a feedback loop acting on a varisator which steers the response frequency of the microwave cavity and stabilizes it against thermal drifts.

The SHM EM0 (Engineering Model 0) has already been delivered in 2009 with performances and interfaces representative to the flight model (e.g. onboard software). The manufacturing of the SHM EM1 representing the SHM FM (Flight Model) in form, fit and function has commenced.


Figure 9: Schematic view of the SHM instrument (image credit: Spectratime)


Figure 10: Internal elements of SHM (image credit: ESA)


Figure 11: SHM engineering models (image credit: ESA)


Figure 12: SHM clock stability (image credit: ESA)


FCDP (Frequency Comparison and Distribution Package):

FCDP is the central node of the ACES payload. Developed by Astrium and Timetech (Stuttgart) under ESA management, FCDP is the on-board hardware which compares the signals delivered by the two space clocks, measures and optimizes the performances of the ACES frequency reference, and finally distributes it to the microwave link.

The engineering model of the ACES-FCDP has been completed (Figure 13) and tested in 2009. The noise introduced by FCDP on the distributed clock signal rapidly averages down entering the 10-18 regime already after 104 seconds of integration time. Ultra-low phase noise electronics is extremely important to distribute and characterize the signal of high-performance atomic clocks. This technology is now available in a compact system, ready to be used for space applications.


Figure 13: Photo of the FCDP engineering model (image credit: ESA)


MWL (MicroWave Link):

The ACES clock signal distributed by FCDP is finally transmitted to ground stations by the ACES microwave link. MWL is developed by Astrium, Kayser-Threde (Munich), Timetech, TZR (Steinbeis Transferzentrum Raumfahrt, Stuttgart) and MIRAD under ESA management. The MWL concept is an upgraded version of the Vessot two-way technique used for the GP-A experiment in 1976 (Test of Relativistic Gravitation with a Spaceborne Hydrogen Maser) and the PRARE geodesy instrument. 23)

The MWL system consists of the MWL flight segment and the MWL ground terminal providing two-way communication in the Ku-band and an additional downlink communication in S-band (Figure 16). The flight segment supports communication sessions with up to four ground terminals simultaneously.

Code and carrier phase stability determine the performance levels achievable in the comparisons of distant clocks. The MWL long-term stability is ensured by the continuous calibration of the receiver channels provided by a built-in test-loop translator. For shorter durations (~300 seconds, corresponding to the ISS pass duration), the time stability is driven by the noise performance of the Ku-band transmitter and receiver and the reproducibility of each DLL channel after proper calibration of internal delays.

The 100 Mhz chip rate has already enabled a time stability better than 2 ps to be achieved with code measurements. Carrier phase stability is shown in Figure 15, where time deviations down to 80 fs at about 100 s are reported. For longer durations, time deviation remains well below the 1 ps level even in the worst conditions of signal to noise density ratio (C/N), corresponding to very low elevation angles of the ISS over a ground terminal. The thermal sensitivity of the system has been measured and used to calibrate MWL phase comparison data against temperature variations. The sensitivity to a series of key parameters such as clock input power, received signal C/N, supply voltage, Doppler, Doppler rate, etc. has been measured. The susceptibility of the system to narrowband and broadband interference, as well as to multipath effects has been characterized.

• Two-way link (Ref. 11):

- Removal of the troposphere time delay (8.3-103 ns)

- Removal of 1st order Doppler effect

- Removal of instrumental delays and common mode effects.

• Additional down-link in the S-band:

- Determination of the ionosphere TEC (Total Electron Content)

- Correction of the ionosphere time delay (0.3-40 ns in S-band, 6-810 ps in Ku-band)

• Phase PN code modulation: Removal of 2π phase ambiguity

• High chip rate (100 Mchip/s) on the code:

- Higher resolution

- Multipath suppression

• Carrier and code phase measurements (1 per second)

• Data link: 2 kbit/s on the S-band downlink to obtain clock comparison results in real time

• Up to 4 simultaneous space-to-ground clock comparisons.

The MWL engineering model of the flight segment electronic unit (Figure 14) has been completed and tested.


Figure 14: Photo of the MWL flight segment EM (image credit: ESA, EADS Astrium)


Figure 15: MWL flight segment stability of the carrier phase, expressed in time deviation, code phase stability (squares), carrier phase stability (triangles). Measurements are compared to MWL system requirements (blue line), image credit: ESA


Figure 16: Schematic view of the ACES-MWL architecture and the links (image credit: ESA, EADS Astrium)

The engineering model of the flight segment S-band and Ku-band antennas (Figure 17) have been completed and tested. The antennas exhibit a extremely flat phase pattern over the complete field of view covering 140º.


Figure 17: Engineering model of the MWL flight segment Ku-band antenna (image credit: EADS Astrium)


Figure 18: The ACES clock signal (image credit: ESA, Ref. 17)

ELT (European Laser Timing):

ELT is an optical link that will be part of the ACES mission; ELT is developed by the Czech Space Research Center. The on-board hardware of ELT consists of a CCR (Corner Cube Reflector), a SPAD (Single-Photon Avalanche Diode), and an event timer board connected to the ACES time scale. Light pulses fired towards ACES by a laser ranging ground station will be detected by the SPAD diode and time tagged in the ACES time scale. At the same time, the CCR will re-direct the laser pulse towards the ground station providing precise ranging information.

The laser link will perform comparison of distant clocks, both space-to-ground and ground-to-ground, to frequency uncertainty levels well below 1 x 10-16 after a few days of integration time. Because of the high stability of the ACES clock signal, non-common view comparisons of clocks across intercontinental distances will be possible with ELT. The optical link also finds interesting applications in the distribution of the ACES time reference and in the synchronization of geodetic observatories. Combined with MWL performance, ELT will contribute to the characterization and cross-comparison of two different time transfer and ranging systems. Optical versus dual-frequency microwave measurements also provide useful data for the study of atmospheric propagation delays and for the construction of mapping functions at three different wavelengths.


Figure 19: ELT detector engineering model (image credit: ESA, EADS Astrium)

Tests on breadboard level were conducted at the Geodetic Observatory Wettzell (WLRS) and at the University of Graz to assess the feasibility of reaching the specified performances. By ranging satellites equipped with laser reflectors and providing a second independent detection port and laser-pulse timing unit with independent time scale, it has been possible to evaluate the performance of the hardware proposed for ACES.

GNNS receiver:

A GALILEO/GLONASS/GPS receiver will be part of the ACES payload and directly connected to the ACES clock signal. The GNSS system is developed by EREMS, JAVAD and the Geoforschungszentrum Potsdam. The system is designed to provide orbit determination and payload positioning for evaluating relativistic corrections in the space-to-ground clock comparison measurements. Additionally, it offers the potential for remote sensing applications from space in the field of radio-occultation and reflectometry exploring the use of the new GNSS signals.

The GNSS receiver will be accommodated on the ACES payload, with the GNSS antenna looking along the velocity vector of the ISS, with a tilt angle of about 35º in the zenith direction. The GNSS receiver/antenna subsystem is designed to provide ACES with a completely autonomous system for orbit determination, at the same time avoiding extravehicular activity for its installation. Even if not optimal in terms of visibility towards the GNSS constellation, the selected accommodation is able to fulfil ACES requirements for orbit determination. In addition, this antenna geometry turns out to be particularly favorable for supplementary GNSS science such as radio-occultation and reflectometry.

The current (2010) ACES GNSS subsystem baseline consists of a redundant set of commercial-of-the-shelf (COTS) JAVAD GNSS Triumph TRE-G3TH receiver boards and a power and data interface board. The interface board protects the GNSS receiver against latch-up effects, handles the receiver data flow and converts the 100 MHz ACES clock signal down to 10 MHz. The 10 MHz clock signal can be optionally used by the receiver as clock reference signal.



Ground segment:

The ACES GS will provide the monitoring and control facilities for the utilization of the ACES system and the services to the different categories of ACES users: 24)

• Investigator Working Group (IWG)

• Scientific users

• Payload and instrument developers, for engineering support during the mission lifetime.

The ACES ground segment will be integrated within the overall ISS ground architecture providing the communication links between ground and space through the Columbus Control Centre and the NASA ground segment.

The main ground segment components are:

- ACES-USOC (User Support and Operations Center) providing all functions related to online monitoring and control of the ACES payload and the MWL Ground Terminal network, the ACES operational utilization, the ACES operations planning and the provision of the ACES scientific data to the ACES user community.

- MWL (MicroWave Link) ground terminals distributed worldwide and connected to the ACES-USOC

- Externally distributed facilities for Users, Payload Instrument developers and the POD (Precise Orbit Determination) function.

The ACES ground segment builds on existing infrastructure: ISS and Columbus Laboratory, the Japanese HTV (H-2 Transfer Vehicle), NASA and Columbus ground segments and CC (Control Centers), and the ACES USOC (User Support and Operations Center) infrastructure collocated at CADMOS (Centre d'Aide au Développement des activités en Micro-pesanteur et des Opérations Spatiales) of CNES in Toulouse, France.

The main functions of the ACES ground segment are the monitoring and control of the ACES payload and of the MWL ground terminals as well as the generation, archiving and distribution of the data products based on the measurements performed in space and on ground.


Figure 20: Overview of the ACES mission architecture (image credit: ESA)

MWL GT (Ground Terminal: The MWL GT electronics is similar to the MWL flight hardware, symmetry being important in a two-way system to reduce instrumental errors. The ACES MWL GT is a microwave station interfacing the local clock on ground to the ACES payload allowing space-to-ground clock comparisons. To reduce phase instabilities due to the tracking motion, the electronic unit of the MWL GT has been rigidly attached to the antenna unit. The Ku-band signal is delivered to the antenna feeder via a waveguide, a high stability RF cable is used for the S-band. The antenna is a 60 cm offset reflector with a dual-band feed system automatically pointed in azimuth and elevation by a steering mechanism. A computer controls the steering unit based on ISS orbit prediction files, collects telemetry and science data both from the local clock and the MWL GT electronics, and interfaces directly with the ACES USOC (User Support and Operation Center).

The antenna system is housed in a protective radome cupola, which also allows to stabilize the temperature of the enclosed volume by an air conditioning system, part of a separate service pallet. The MWL ground terminal is presently being assembled (Figure 22). After dedicated tests on the ground terminal, a MWL system level test will be conducted. The test will be performed using dedicated test equipment that will allow to reproduce the signal dynamics both in terms of amplitude and frequency (Doppler) variations.


Figure 21: Illustration of MWL ground terminals with protective radom (image credit: EADS Astrium)


Figure 22: The MWL GT under assembly (image credit: EADS Astrium)


Figure 23: The ACES ground segment components (image credit: ESA)


ACES-OC (ACES Orbitography Center):

The ACES-OC is implemented at GSOC (German Space Operations Center), where also the Col-CC (Columbus Control Center) is located. The ACES-OC is responsible for producing three different orbit products based on the observations of the JAVAD GNSS receiver on a routine basis: 25)

• a combined orbit fit and predicted orbit product, which is delivered in near real-time

• a final high precision orbit product, which is delivered daily with a latency of one or two days

• a mid-term prediction product, which is delivered twice a day in the form of TLEs (Two Line Elements).

Although the POD concept using GPS observations is routinely operated on several LEO missions, with accuracies much better than required for ACES, the POD for the ISS poses new challenges:

- Due to construction reasons the GNSS antenna will be attached to the ACES module with a tilt of 60° from the zenith direction and of 50° in azimuth direction. Hence the field of view is limited and asymmetric w.r.t. the flight direction, which results in a suboptimal geometrical distribution of the observed GPS satellites.

- The solar panels of the ISS further obstruct the field of view of the GNSS antenna limiting the number of observable GPS satellites to five in the worst case.

- The orbit height of the ISS is with ~350 km significantly lower than that of most LEO satellites. Combined with the large surface of the rotating solar panels, this makes modelling of air drag and thus the orbit prediction more difficult than for relatively compact satellites.

Operational orbit determination: The ACES-OC will provide three types of orbit products. A combined orbit fit and predicted orbit product, a final orbit product and a medium term orbit prediction product.

The combined orbit fit and prediction product will be generated in near real-time and updated every 90 min and serves two purposes. The first one is to generate accurate position and velocity vectors for the computation of quick-look relativistic clock corrections. The second is to provide a short term prediction for SLR stations and MWL terminals. Hence the product will contain 6 h orbit fit to the observed data, and 6 h prediction.

To generate the near real-time products, the ACES-OC receives all relevant observations from the ACES data stream via the Col-CC. The data will be provided in 5 min batches and assembled at the ACES-OC to cumulative products in order to reduce the transmission latency. The data package contains raw observations of the GPS, GLONASS and Galileo constellations recorded by the JAVAD GNSS receiver. As a backup for data gaps in the raw observations, the navigation solutions of the JAVAD GNSS receiver and the SIGI (Space Integrated GPS/INS) receiver are provided. The ISS attitude information generated by the SIGI receiver is necessary for orbit determination as well.

The adjusted part of the combined product is computed using the most recent 6 h of raw observation data of the JAVAD receiver with a reduced dynamic orbit determination (RDOD). As a baseline, the IGS (International GNSS Service) ultra-rapid ephemeris product will be used for the near-real time processing. As an alternative, use of DLR’s RETICLE (Real-Time Clock Estimation) product or the upcoming IGS real-time products are considered to further increase the quality of near real-time orbit determination.

Subsequently a position fit (PosFit) is performed over the 6 h of precise orbit and propagated for 6 h into the future. The RDOD and PosFit software tools are part of the GHOST package (GPS High precision Orbit determination Software Tools), developed at GSOC. Usually orbit determination products refer to the center of mass (CoM) of the spacecraft. In case of ACES, the orbit trajectory is transformed to the ACES reference point, which is located in the center of the microwave cavity of the PHARAO clock. This facilitates use of the resulting orbit product for the science processing and minimizes the impact of attitude uncertainties in the overall processing chain. The combined orbit fit and prediction product will be delivered in SP3 (Standard Product 3) orbit format and in the CPF (Consolidated laser ranging Prediction Format).

Twice a day, TLEs will be generated based on the results of the latest near real-time POD. These TLEs serve for mid-term planning of MWL and SLR station contacts.

With a latency of a few days, the final orbit product is computed. This period is used to fill potential downlink data gaps and to ensure the availability of high-quality GPS ephemerides. For the final orbit product, rapid GPS ephemerides generated by the CODE (Center for Orbit Determination in Europe) will be used. Similar to the near real-time POD, the final POD is computed with the RDOD software - with the difference, that it is computed over predefined daily 30 h arcs with 3 h overlap to the previous and following days. The overlap periods enable an internal consistency check: if the difference between two overlapping arcs is too large, the automated processing is stopped and an operator has to generate the product manually.

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11) L. Cacciapuoti, “ACES: Atomic Clock Ensemble in Space,” Proceedings of the GPhyS (Gravitation and Fundamental Physics in Space) Kick-Off Colloquium, Les Houches, France, Oct. 20-22, 2009, URL:

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14) Steve Feltham, Francisco Reina, Giacinto Gianfiglio, “ACES - A Time & Frequency Mission for the ISS,” 18th European Frequency and Time Forum EFTF 2004., April 5-7, 2004, Guildford, UK

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17) L. Cacciapuoti, “Atomic Clock Ensemble in Space, Mission Concept and Status,” International Workshop on ACES and Future GNSS-based Earth Observation and Navigation, Munich, Germany, May 26-27, 2008, URL:

18) Wolfgang Schäfer, “Advanced Concepts for Ranging and Time Transfer Applications and Mission Support,” GGOS/IGCP 565 Workshop, Graz, Austria, Sept. 30 - Oct. 2, 2009

19) Luigi Cacciapuoti, Christophe Salomon, “ACES - Mission Concept and Scientific Objectives,” ESA, March 28, 2007, URL:

20) C. Salomon, “Cold Atom Clocks and Fundamental Tests,” TAM (Time and Matter International Conference), Bled, Slovenia, August 27, 2007, URL:


22) Stéphane Thomin, Olivier Grosjean, Philippe Laurent, “PHARAO’s Cesium Tube,” Proceedings of the 63rd IAC (International Astronautical Congress), Naples, Italy, Oct. 1-5, 2012, paper: IAC-12-A2.1.4

23) Jürgen Schmolke, Michael Zähringer, “Antenna design and testing for the ACES microwave link,” URL:

24) E. Daganzo, S. Feltham, R. Much, R. Nasca, R. Stalford M. P. Hess, L. Stringhetti, “ACES Ground Segment functionality and preliminary operational concept,” EFTF-IFCS 2009 (European Frequency and Time Forum - International Frequency Control Symposium), April 20-24, 2009, Besançon, France

25) Martin Wermuth, Oliver Montenbruck, Achim Helm, Luigi Cacciapuoti, “Precise Orbit Determination and prediction of the ISS in the frame of the ACES Mission,” Proceedings of NAVITEC 2012, 6th ESA Workshop on Satellite Navigation Technologies, ESA/ESTEC, Noordwijk, The Netherlands,Dec. 5-7, 2012

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.