EnMAP (Environmental Monitoring and Analysis Program)
EnMAP is a next-generation German satellite program (an approved program as of March 2006). The program started the C phase in August 2008. The system CDR is scheduled for mid 2011.
The objective is to provide for detailed monitoring (relevant surface parameters), characterization and parameter extraction of rock/soil targets, vegetation, and inland and coastal waters on a global scale. The mission requirements call for Earth observation with a dedicated hyperspectral pushbroom imager on a polar orbiting state-of-the art small satellite platform. The mission is being developed within a national context, with an international scientific focus, and also a strong commercial component aiming at the support of GMES-related services (Global Monitoring for Environment and Security), a European initiative. 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11)
The EnMAP long-term program is based on a cooperative approach involving various German institutions, the main participants are: DLR (German Aerospace Center, project management), GFZ (Geoforschungszentrum), Potsdam, (science lead), the industrial partners are: Kayser-Threde GmbH, Munich (sensor), and OHB-System, Bremen (S/C bus). The goal is to launch eventually two hyperspectral satellites in the time frames 2013/14 and 2015, respectively, each for a design n on-orbit life of 5 years or more.
In November 2006, DLR awarded a design contract to Kayser-Threde GmbH as prime contractor of EnMAP. The platform provider, OHB System, Bremen, is a subcontractor to Kayser-Threde in this arrangement. 12)
Note: In June 2007, OHB Technology AG of Bremen acquired the company Kayser-Threde GmbH of Munich, Germany.
Figure 1: Overview of EnMAP project main partners (image credit: DLR)
The overall objectives of the mission are:
• To provide high-spectral resolution observations of biophysical, biochemical and geochemical variables in the spectral range of 420-2450 nm in continuous bands of 10-40 nm width, sampled at 5 to 20 nm intervals. The GSD (Ground Sampling Distance) will be 30 m.
• To observe and develop a wide range of ecosystem parameters encompassing agriculture, forestry, soil/geological environments, and coastal zones and inland waters
• To acquire high resolution spatial and spectral data from space that will enable/improve the retrieval of quantitative parameters needed by the users and are not provided by operating multispectral sensors
• To provide high-quality calibrated data and data products to be used as inputs for improved modelling and understanding of biospheric/geospheric processes. This will further contribute to the assimilation of data/information into such process models.
• To develop and market high-level data products, meeting the demands of the stakeholders in natural resource management.
Applications: The analysis spectrum of the EnMAP hyperspectral data covers the following disciplines: agriculture/forestry, biodiversity, ecology, wetlands, climate change, water, soils/land degradation, geology/mineralogy, arid zones, cartography, urban areas, fisheries, methodic development, calibration/validation.
The EnMAP minisatellite is conceived as a science and research mission and a pathfinder to evolve towards an operational/commercial service (the 2nd S/C in the program is targeted for support of commercial services). The spacecraft platform design is based on existing state-of-the art bus technology of OHB-System (SAR-Lupe heritage). OHB refers to its generic spacecraft bus as LEOBus-1000. The LEOBUS-1000 is designed for Earth observation missions with a launch mass of 600 - 1000 kg; its advantages are a very modular and flexible configuration for power generation, a highly accurate and agile attitude and orbit control concept and a high payload data-rate on-board processing and downlink system. 13)
Its key design features are a very modular and flexible configuration approach, a very precise attitude control concept implementation, and the provision of high-rate onboard processing and downlink transmission services of the payload data. 14) 15) 16) 17) 18)
The EnMAP spacecraft consists of two major modules: the bus or service module with the subsystems (at the bottom part of Figure 2) and the payload module, accommodated in the upper part of the spacecraft to comply with high stability and thermal requirements of the optical instrumentation. The payload module can be thermally decoupled from service subsystems to minimize heat fluxes.
Figure 2: Configuration of the EnMAP spacecraft (image credit: OHB-System)
Table 1: Overview of EnMAP satellite characteristics
The spacecraft is 3-axis stabilized. The attitude is sensed by 3 sun sensors, gyroscopes, magnetometers, and sun presence sensors. Actuation is provided by reaction wheels (4) and magnetic torquers. The required attitude accuracy is < 500 m with a knowledge of < 100 m.
The spacecraft mass is ~871 kg, the design life is 5 years. EnMAP features the capability of cross-track body pointing of ± 30º (providing an accessible target range of about ± 390 km). The pointing feature allows a target revisit within about 4 days (depending on latitude).
The EPS (Electrical Power Subsystem) consists of the following elements: SG (Solar Generator), battery, and BMU (Battery Management Unit). The panel of the body-mounted SG (4.6 m2) is made of lightweight aluminium honeycomb structures selectively supported by dedicated CFRP elements. The SG uses triple-junction solar cells providing ~810 W of average power at EOL. The SG accommodates dedicated sun presence sensors, GPS and S-band antennas, and payload sun calibration can be performed through dedicated panel cutouts. A Li-ion battery is used with a capacity of 50 Ah. The battery provides an energy density of 113 Wh/kg, where voltage ranges from 20-34 V can be achieved. The BMU supervises all battery-relevant parameters and autonomously protects the battery in case of fatal software malfunction.
TCS (Thermal Control Subsystem). The regulation of the thermal environment within the spacecraft bus (service module) is primarily achieved by means of passive thermal control measures, such as multilayer insulation (MLI) blankets and heat pipe assemblies. Active thermal control of the satellite bus is only required for the OCS components. Hence, the propellant tank, valves and filters are equipped with heating foils to avoid propellant freezing in orbit during critical mission phases.
The EnMAP bus is equipped with an OCS (Orbit Control Subsystem) comprising the following functional components:
• Hydrazine blow-down system
• Propellant tank with at least 50 kg capacity
• 2 thrusters, each with a 1 N thrust
• Pressure and electrical components (such as valves, filters, transducers, etc.)
The OCS provides for nominal orbit acquisition (early orbit maneuvers) and general orbit maintenance.
RF communications: The payload data are transmitted in X-band (8.2 GHz) at data a rate of 320 Mbit/s. A high-speed onboard mass memory of 512 Gbit (EOL) is provided. The telemetry channel coding unit conforms to CCSDS standards. - The S-band with full duplex services (2.2 GHz) is being used for TT&C communications. The downlink data rate is 64 kbit/s, the uplink is 4 kbit/s. The TT&C unit consists of two S-band transmitters and receivers, each operated in hot redundancy. The S-band antennas provide an omnidirectional antenna pattern to ensure the availability of the TT&C links (independent from the actual S/C attitude).
Figure 3: Bus subsystem accommodation - front side with the payload not shown (image credit: OHB -System)
Figure 4: Bus subsystem accommodation - back side with the bus compartment shown only (image credit: OHB -System)
Figure 5: Functional block diagram of the EnMAP satellite bus including bus subsystems (image credit: OHB -System)
SMS (Satellite Management Subsystem): The bus SMS consists of two identical SMUs (Satellite Management Units) and the OBDH (On-board Data Handling software). A single SMU includes the data handling electronics boards and the power supply electronic boards. These PCBs (Printed Circuit Boards) are integrated in one electronic box with one common backplane (Figure 6).
The SMU interfaces and controls all electronic subsystems of the satellite by onboard autonomy. The power supply electronics supplies the data handling electronics and provides power distribution to the subsystems. The data handling electronics controls the power electronics and acquires the housekeeping data.
The redundancy is realized by the use of two SMUs as part of the SMS. In nominal operations, one SMU is operated and the other is in standby mode. The telecommand decoder of both SMUs are always switched on. The SMU provides one (redundant) MIL-1553B bus. This interface is used for command and telemetry transfers between the SMU and especially the payload and payload data handling subsystems.
Figure 6: Layout of the SMU (image credit: OHB -System)
The SMU hosts the OBDH S/W, which is running on a single radiation hardened DSP (Digital Signal Processor). The OBDH S/W is able to perform all required tasks to control and monitor the complete EnMAP satellite. The overall architecture of the OBDH software including the attached hardware subsystems is shown in Figure 7.
On the bus level, the following modes are implemented:
- Nominal mode
- Safety mode, including the autonomous FDIR (Failure Detection, Isolation and Recovery) functions.
The OBDH is designed in such a way that the satellite can nominally be operated from the ground with a minimum operational effort. Specific tasks such as data takes are commanded by Time-tagged Telecommands, which contain all information to execute the addressed task autonomously. State-of-the-art authentication methods are implemented for satellite commanding and control.
Figure 7: Overview of the OBDH software architecture (image credit: OHB -System)
AOCS (Attitude Orbit and Control Subsystem): The AOCS provides highly accurate and reliable attitude control capabilities as required for optical and microwave instruments performing Earth observing applications. To fulfil the requirements for various operational tasks, different AOCS modes are implemented, which are:
- Init mode: Check out of basic functionality of the AOCS elements according to a defined sequence of activities.
- Nominal (or Normal) mode: 3-axis autonomous attitude control with the solar generator pointing to the sun; AOCS standard operational mode.
- Precise mode: Most accurate attitude performance is provided; in this mode the attitude of the satellite is maneuvered according to dedicated attitude profiles in order to carry out specific tasks.
- Emergency mode: Spin-stabilized coarse sun pointing in case of critical anomalies.
The specific tasks of the spacecraft and especially of the payload are commanded by means of time-tagged telecommands (TTTC). If a specific AOCS pointing is required, the TTTC always includes a so-called AOCS guidance list, which represents the required AOCS pointing profile in form of attitude quaternions. The AOCS on-board software will then process the information contained in the uploaded list and will reconstruct (by means of interpolation) the complete guidance profile originally computed on the ground.
Using this guidance concept, the satellite bus is able to perform different precise pointing attitudes and profiles, such as the following, which are required during the EnMAP mission:
• Standard ground observation: Pushbroom attitude profile with up to ± 30º across-track pointing
• Payload calibration maneuvers: Deep space pointing or direct sun pointing (of the HSI sun calibration port)
• X-band downlink: Ground station pointing (of the high gain antenna)
• OCS maneuver: Attitude or inclination control maneuvers for accurate orbit acquisition and maintenance.
High-rate On-board Data Handling: The payload data handling comprises highly-efficient and reliable components that are capable to process optical Earth observation payload data with its typical high rates and volumes requiring minimum resources.
The following subsystems are part of the EnMAP payload data handling:
- DSHA (Data Science Handling Assembly), which includes high rate input multiplexer, mass memory and channel coding unit, conforming with CCSDS standards
- X-band transmitter, performing data modulation and providing the RF signal
- High gain antenna.
The DSHA interfaces to the HSI payload via two redundant channel links, each of which is operating at a data rate of 840 Mbit/s, i.e. the complete input data rate of the DSHA is 1680 Mbit/s. One highlight feature of the DSHA is the thematically sorting of the hyperspectral data into files, where one file includes the data of one channel per image only. This unique file handling concept significantly eases the downlink and the further processing of the payload data on ground. The channel files are stored within the mass memory of the DSHA, which has a capacity of 512 Gbit (EOL).
For data transmission a high gain antenna is used, which ensures the high transmission rate with sufficient link margin. This concept has the drawback, that the downlink requires a dedicated AOCS pointing maneuver, during which no imaging can be performed. To overcome this drawback, a lossless data compression is implemented within DSHA, which enables a compression ratio for the hyperspectral data of at least 1.6. Due to this, only the night time contacts with the X-band ground station in Neustrelitz are sufficient to dump the complete EnMAP payload data, which is sampled during one day (ca. 600 Gbit).
Launch: A launch of EnMAP is scheduled for 2014 on a PSLV vehicle of ISRO.
Orbit: Sun-synchronous orbit, altitude = 643 km, inclination = 98º, period = ~97 min, equatorial crossing time at 11:00 hours ±15 min LTDN (Local Time on Descending Node), repeat cycle = 21 days. The SSO will be installed with a frozen eccentricity which thus allows a “fixed” GSD at same latitude. The resulting GSD variation as a function of the latitude is shown in Figure 8.
Figure 8: GSD variation as a function of latitude (image credit: Kayser-Threde)
Sensor complement: (HSI)
HSI (Hyperspectral Imager):
The HSI instrument is a pushbroom type hyperspectral imager providing global coverage in two channels (VNIR, SWIR). The objective is to observe in the spectral region of 420-2450 nm (228 in total sampled at 5 nm in VNIR and at 10 nm intervals in SWIR) with a ground sampling distance of 30 m and a swath width of 30 km (96 bands are allocated for VNIR, and 136 bands are allocated for SWIR). The EnMAP concept is directly relevant to natural and anthropogenic issues of concern to the people as well as to the decision makers. 19) 20) 21) 22) 23) 24) 25) 26) 27)
An area up to 1000 km length with a swath of 30 km (nadir) can be covered per orbit; the maximum observation length per day is 5000 km. This implies that a maximum of 50 images can be acquired per day. For off-nadir acquisitions the instrument can be pointed up to ±30º in cross-track.
Figure 9: Schematic view of the delay observation technique of the HSI instrument (image credit: OHB, Kayser Threde)
Legend to Figure 9: The HSI pushbroom type instrument features a split FOV for the VNIR and SWIR spectrometers. Image data for the two channels (VNIR and SWIR) are recorded with a delay of 88 ms. For the chosen orbit and with 30 km swath width and ±30º off nadir pointing capability, global site accessibility is guaranteed within 4 days.
Table 2: Geometric and radiometric instrument requirements
The HSI instrument consists of three major subsystems, the instrument optics unit, the instrument power unit, and the instrument control & processing unit; the latter two units are configured in cold redundancy.
Figure 10: HSI optics unit (without cover) with full aperture diffuser assembly and star sensor assembly (image credit: Kayser-Threde)
Optics unit: The optics unit consists of two spectrometers, one for the VNIR and one for the SWIR range coverage. Both spectrometers make use of a common spectrometer entrance slit assembly. The field splitter features two entrance slits – one for each spectral channel range. The separation is kept as small as possible to achieve a good data co-registration. It is limited by the stability of the self-supporting beam between both slits.
The telescope employs a standard off-axis unobscured TMA (Three Mirror Anastigmat) design without intermediate focus (of the GAIA spectro-telescope heritage) providing optimum performance and compactness. The lightweighted mirrors and structure of the telescope are made of aluminum. The field splitter is placed in the telescope focal plane.
By placing a micro mirror directly behind the entrance slit of the SWIR channel, the SWIR and the VNIR channels can be separated and fed into distinct spectrometer branches. The SWIR branch requires an additional folding mirror following the micro mirror. To prevent ghost images, back reflections from the sensors as well as back reflections from the entrance slits must be suppressed. Hence, the detector arrays are tilted by about 10º with respect to the optical axis and a high absorbing coating is attached to the back of the field splitter. Identical targets are being recorded in VNIR and subsequently in SWIR, with a time delay of 86 ms, corresponding to a small geometric shift (Figure 9). Furthermore both spectrometers are designed as prism spectrometers, thus providing highest possible optical transmission with low polarization sensitivity.
Figure 11: Schematic of the HSI instrument optics unit (image credit: Kayser-Threde)
Detectors: For both detectors, i.e. for the VNIR as well as the SWIR spectrometer, the SNR (Signal-to-Noise Ratio) and the high frame readout frequency of nearly 230 Hz are the main drivers for the detector development.
• The SWIR detector is being realized as a customized MCT (Mercury-Cadmium-Telluride) array with a 1024 x 256 pixel array. The pixel format is 24 µm (geometric) x 32 µm (spectral). Pixel input stage: CTIA (Capacitive Trans-Impedance Amplifier). A total of 8 output channels handle the high readout frame rate of 230 MHz. Instrument performance simulations have been performed demonstrating, that an SNR requirement of >150 @ 2200 nm can be obtained. To achieve this performance the MCT detector is actively cooled to 150 K by means of a pulse tube cooler with a flexure-bearing compressor (Stirling). The IR module features the IDCA (Integrated Detector Cooler Assembly) design of AIM Infrarot-Module GmbH, Heilbronn, Germany. The IDCA module reduces the complexity of the system design and costs, incorporating the focal plane array (FPA), dewar, cooler, and front-end electronics with 14 bit analog-to-digital-converters in one package. 28)
• The VNIR detector array size has 1024 (spatial) x 512 (spectral) pixels. A CMOS (Complementary Metal Oxide Semiconductor) type focal plane array technology implementation is being used. Aside from frame rate and noise characteristics, the full well capacity (integration time) is a prominent design driver - due to the high signal dynamics requirements resulting from targets ranging from extremely low reflectivity, like water, up to the highest reflectivity of snow-covered regions. A VNIR SNR requirement of 500 @ 495 nm is provided.
Figure 12: Schematic view of the HSI instrument optics unit (image credit: DLR)
Table 3: Summary of spectrometer parameters
Figure 13: Two detector hyperspectral imaging illumination (image credit: OHB/Kayser-Threde, GFZ)
Figure 14: The SWIR FPA (1024 x 256) mounted on open dewar (left) and demonstrator IDCA (right), image credit: AIM
Table 4: Overview of key instrument characteristics
Table 5: Summary of spectrometer parameters
A dedicated mass memory provides storage of the source data and related add-on information. The DSU (Data Storage Unit) is designed for modularity and scalability. It uses a 2 of 3 redundancy concept permitting graceful degradation. The DSU characteristics are:
- 512 Gbit of storage capacity @ EOL (End of Life)
- Input data rate capability of up to 1.6 Gbit/s
- Direct output interface from the DSU to the CCU (Channel Coding Unit) and X-band downlink.
The calibration approach for the HSI instrument uses a combination of on-ground and in-orbit techniques. The requirements call for a periodic in-flight calibration capability with the following performance spectrum:
- Absolute and relative radiance calibration
- Wavelength/spectral calibration
- Calibration of detectors with respect to DS&PRNU (Dark Signal & Photo Response Non-Uniformities).
- Straylight contribution
- Pointing calibration.
The suite of calibration tools and modes to perform measurements for the on-orbit calibration encompass the following items:
- Shutter / calibration mechanism for dark value and calibration measurements
- Full aperture diffuser for sun calibration for absolute radiometric calibration
- Main radiometric sphere (white Spectralon) for relative radiometric assessment
- Secondary sphere (doped Spectralon) for spectral calibration assessment.
Calibration measurements are performed periodically or with highest priority on request. The dark value measurements for all spatial and spectral pixels will be carried out at begin and end of each data take by closing the entrance slit with the shutter mechanism. Deep space looking with opened shutter serves as verification measurement especially in the SWIR channels. Sun calibration measurements are used for absolute radiometric calibration of the HSI in orbit. To this end the full aperture diffuser is moved into the entrance aperture of the telescope and the satellite is oriented for the measurement of the extraterrestrial sun irradiation via the diffuser.
Figure 15: HSI on-board calibration system (image credit: Kayser-Threde)
For the relative radiometric calibration light sources inside the main sphere are operated at different currents, which illuminate the entire entrance slit. This kind of measurement only allows checking for the radiometric stability. The spectral calibration in orbit is carried out using the secondary (smaller) sphere coated by doped Spectralon, which realizes absorption features.
Linearity measurements will be performed by changing integration times, where different light sources can be used. Geometric calibration uses ground control information derived from orthorectified scenes of superior geometric quality (especially high resolution sensors), maps and other available sources. The test sites are distributed over a broad range of latitudes to account for thermal influences.
Table 6: Preliminary calibration approach using the on-board calibration means
Figure 16: Spacecraft pointing concept for event monitoring capability (image credit: DLR, GFZ)
Figure 17: Comparison of HSI sensor performance in relation to other air- and spaceborne multi- and hyperspectral systems (image credit: OHB/Kayser-Threde, GFZ)
Mission operations concept:
The mission operations concept is divided into two categories (Ref. 18) :
• nominal operation
• contingency operation.
Nominal operation covers the whole mission duration, from LEOP and Commissioning Phase, Nominal Phase to De-orbiting phase. The autonomous on-board operations concept is most advantageous for the nominal daily operations. For LEOP and commissioning, de-orbiting and contingency operations the advantages are less, because there the performed tasks and activities are less regular but more dedicated and specific.
Within the LEOP and Commissioning Phase several tasks have to be performed in a certain order to guarantee the correct start-up and testing of the satellite. These tasks include: initial activation, commissioning of the satellites subsystems and general performance tests. During LEOP and Commissioning, an intensive supervision of the satellite’s health is ensured. The tasks during this phase are specific and must be supervised continuously. The same conditions must be applied for de-orbiting and contingency operations. Individual operational tasks are necessary and a full time observation of the satellite’s health is needed. Therefore the autonomous on-board operations concept does not facilitate these activities.
During the nominal phase, routine operations are performed on a daily basis. The EnMAP ground stations are Weilheim and Neustrelitz, Germany, where Weilheim is used for the S-band contact (TMTC), and Neustrelitz for the X-band contact (payload data download). For payload data download, only the night time X-band contacts are sufficient to dump the complete EnMAP payload data. For the S-band contacts, an average of 30 minutes/day are reached with the given orbit characteristics. During this time, all commanding and receiving of telemetry must be completed.
During daily operations the most common tasks are image acquisitions, payload data download and subsequent deletion of the data.
For these tasks, especially for an image acquisition, several subsystems have to be activated, controlled and coordinated simultaneously. Using dedicated telecommands for each subsystem, or even for each operation within the different subsystems, would lead to a great amount of telecommands. In addition, the ground station would need to have a detailed knowledge of the on-board timings. The EnMAP satellite bus requires only one single telecommand per task, i.e. only one telecommand for an image acquisition, a payload data download or data deletion. This is named the one-telecommand-philosophy.
The complex sequence of events, each of these tasks consists of, will be performed autonomously by the satellite according to the detailed parameter included in the telecommand.
The EnMAP HSI payload is controlled via scripts, so that changes on the payload timings and sequences do not change the concept of image acquisitions. In addition, the payload data is autonomously compressed on-board if requested by imaging command. For the payload data download, depending on the downlink duration, a matching amount of data is transmitted to the ground station autonomously.
The applied one-telecommand philosophy results in a very low effort for on ground modelling of on-board resources and mission planning for nominal operations. It is not necessary to have knowledge of the exact on-board timings. The generation of telecommands for image acquisitions and other operational tasks can be done according to simple rules provided in the satellite’s operations manual.
One-telecommand philosophy: Performing tasks such as image acquisition or data download require activation, control and coordination of several subsystems simultaneously. The implemented advanced on-board operation concept with its one-telecommand philosophy facilitates the conduction of these tasks and reduces the complexity of ground operations and mission planning tasks (e.g. scheduling). The concept enables the on-board computer to execute and supervise these tasks and autonomously control the EnMAP subsystem equipments accordingly.
The one-telecommand philosophy will be applied for the following tasks:
• Image acquisition during standard ground observation
• HSI calibration
• Payload science data download
• Payload science data deletion
• Orbit control maneuver.
For each of these tasks only one telecommand has to be sent from the ground station. These tasks are commanded via time-tagged telecommands (TTTCs). The TTTC includes all necessary parameters for the corresponding task and a time tag to define the start of execution. The on-board execution of the TTTC starts automatically at the specified time. The on-board computer controls all necessary subsystems and commands the required subsystem operations during the execution of the TTTC.
Image acquisition and HSI calibration: Image acquisition and calibration maneuver tasks can be performed anywhere on the satellite orbit. They are initiated via time-tagged telecommands, which are uploaded to the spacecraft during a previous ground station contact.
Subsystems involved are mainly the HSI payload, the mass memory and coding unit (DSHA) and the AOCS (Attitude and Orbit Control Subsystem). Concerning the handling of the subsystems, there is no difference for the on-board computer between the image acquisitions and the payload calibrations (in case an AOCS precise maneuver is required). Both tasks, image acquisition and calibration, require the same management of the AOCS and the DSHA. The only difference is that the instrument will be commanded to the image acquisition or calibration command sequence.
Depending on the imaging type (standard ground observation or HSI calibration), a different number of spectral channels will be gathered by the HSI and transferred to the DSHA for recording. The type of imaging and the spectral channel configuration to be used will be defined in the parameters of the TTTC. According to these parameters, the DSHA and the HSI will be setup, and the DSHA will be configured so it is ready for the storage of payload data at the defined start time.
For an image acquisition a specific AOCS pointing to the target is required. Therefore the AOCS will be set to the so-called precise mode, where the satellite will be moved according to a specific guidance profile. The TTTC includes an AOCS guidance list, which represents the required AOCS pointing profile in form of attitude quaternions. The AOCS on-board software will process the information contained in the uploaded list and will reconstruct (by means of interpolation) the complete guidance profile originally computed on ground. By the utilization of such a ground computed AOCS profile, which is included in each data take TTTC, the satellite can be “guided” to autonomously perform different pointing attitudes and profiles.
The instrument will be preheated a specific time before the data take starts. The according command and the imaging time tag will be sent from the board computer to the instrument so that the data take can take place at the required time, controlled by the HSI. In case there are two image acquisitions with a short time offset (less than the time required to pre-heat the HSI camera), the camera will not enter a stand-by mode after the first image acquisition end and be kept on the preheating temperature. In the case that the second image acquisition is not performed (e.g. deleted), the HSI will autonomously switch the camera to standby mode after a defined time if no further imaging sequence is started.
An example of a simplified imaging profile is depicted in Figure 18, showing a fictive sequence of different ACS (i.e. AOCS) precise modes to perform a sequence of two of subsequent image acquisitions at different pointing angles. One can see that the duration of the complete TTTC is longer than the data take itself. Before the data take sufficient time must be considered for the AOCS pointing and the initialization of the involved subsystems (mainly HSI and DSHA). After the data take some additional time must be considered as well for the re-initialization of the subsystems. Consequently, there has to be a timely gap between two consecutive data takes, which shall be performed at spacecraft attitudes. This gap must be considered when scheduling the data takes and preparing the adequate time-tagged telecommands.
Figure 18: Simplified imaging profile (image credit: OHB/Kayser-Threde)
The TTTC also includes information whether the data shall be compressed or not. If compression is requested, the process is started autonomously as soon as the acquisition of data is stopped.
The compression process can be interrupted by an execution of a new image acquisition TTTC. If there is a new image acquisition, the DSHA will automatically suspend the compression process if it is still running. Once the new files are recorded, the file compression process will be continued. The compression process works autonomously supervised by the DSHA.
The different time parameters will be calculated on ground, according to simple rules defined in the operations manual, enabling the configuration of the complete Image Acquisition TTTC on the basis of the Image Acquisition start time. But once having sent the one telecommand to the S/C all necessary tasks for the image acquisition are performed autonomously on-board. Combining all the times in one telecommand enables the on-board S/W to supervise the complete sequence belonging to one complex task. The execution reply represents the status of the complete task, and not only of one dedicated command to one element or subsystem. Due to this, the operator is able to judge about a successful or erroneous sequence by means of one reply code. In case of any failure during the sequence, an entry is generated in a log file, which indicates the error reason.
Payload science data download: A payload file download maneuver is also initiated via one time-tagged telecommand. In this telecommand all necessary information is given so that the downlink can be performed without any further ground contact via S-band.
Within the DSHA the image data is sorted in such way, that for each channel of an image acquisition a separate file is generated. These files are marked with the according image ID and channel ID. The image ID is given by the ground operator. The channel identifier is given autonomously during the image acquisition. Each file is addressed via its image ID and its channel ID. This guarantees an explicit allocation of the image files to the commanded image acquisition.
The TTTC for the file download maneuver contains information of the start and end image Ids and channel IDs and the duration of the X-band contact. The files within the DSHA are marked with a so-called download flag. This flag indicates whether a file has already been downloaded or not. Using this information the DSHA autonomously detects the files to be downloaded and transmits them, via the X-band transmitter, during the contact period to ground.
All downloaded files are marked as downloaded after the completion of the download. In the case that not all of the commanded files can be downloaded, the remaining files will not be marked as downloaded. These files can be downloaded in a following contact.
It can be configured whether all files (already downloaded and not yet downloaded) or only not yet downloaded files should be transmitted. Thus, the ground operator does not need to specify exactly which files should be downloaded. He can just define a start and an end image ID and all files that have not been downloaded before will be transmitted. In case a download of a dedicated file should be repeated, the download flag can easily be set back to “not downloaded”.
Due to this, the ground station can schedule a download maneuver with always the same set of parameters for the start image identifier, start channel and end image identifier and end channel. Only the start time for the X-band contact, the duration of the data downlink and the AOCS parameters must be adjusted for the specific maneuver. The size of the files must not be checked and no calculation which files can be downloaded during the X-band contact must be made on ground. Due to the incrementing ID order, which is used for the image acquisition and during the download, the first acquired data is also downloaded first.
Within the mass memory, no file will be autonomously deleted. To delete any files, a specific telecommand initiated by the ground station must be executed.
The execution of the payload data download maneuver is similar to the image acquisition maneuver. The ACS subsystem needs to perform a rotation of the S/C to point the X-band antenna towards the ground station. During the AOCS preparation, the DSHA will be configured and switched to its transmission mode, to enable the file download. Other than for image acquisition, the HSI payload is not involved for the file download process.
The duration of downlink phase is equivalent to the X-band contact time. At the start of the downlink phase, a set of commands are sent by the on-board computer to the affected subsystems to start the transmission and the file download process. The DSHA will be set to its transmission mode, so that the transmission process can be initiated and prepared within the DSHA. After contact establishment the DSHA will be commanded to start the download of the payload data. The DSHA transmits autonomously a matching amount of files based on the information (downlink duration, files to be downloaded) from the TTTC. Only complete files are downloaded. If no image data is available the DSHA autonomously generates idle frames.
In addition to the DSHA the X-Band transmitter is controlled by the on-board computer. It will be switched on and off according to the X-band contact time given in the TTTC. The control of the AOCS subsystem is identical to an image acquisition maneuver.
As for the image acquisition the file download process is performed completely autonomously on board the S/C having sent only one telecommand.
Payload science data deletion: Files in the mass memory of the DSHA will be deleted only by a telecommand from ground and not autonomously. The deletion telecommand, similar to the file download command, can be executed for all files or only for already downloaded files in a specific range.
This concept foresees that a general delete command can be executed, which does not consider the specific file number for the actual deletion process. Instead of adjusting the file number for every single file deletion, it is recommended to setup the telecommand that a range of files can be deleted.
With the use of the telecommand parameter, which defines that only already downloaded files will be deleted, the not downloaded files are still available for the next download maneuver.
With this concept of the file deletion process, the ground station does not necessarily have to know which files are already downloaded when scheduling and uploading this command. But it is recommended to check which of the files are successfully downloaded to avoid the deletion of the files that were not successfully downloaded. Note that each file is marked with a download flag. This flag can be easily configured, and hence set back from “downloaded” to “not downloaded”. Thus, the designated file will not be deleted and the download of the file can be repeated during the next contact. - Again, only one telecommand is needed to delete all files or all already downloaded files within the mass memory.
Nominal operation scenario: During the EnMAP mission quite complex daily operations scenarios of multiple, target driven imaging tasks are going to be performed. In Figure 19, an example of a nominal daily operations scenario including image acquisitions and data downloads is shown.
Regardless of the complexity of this scenario, the number of telecommands, which are necessary to perform the different tasks, is reduced to a minimum. In the depicted example, 21 imaging tasks, some of which with different pointing profiles, and 3 data downloads are performed, i.e., a total number of just 24 time-tagged telecommands have to be uploaded, plus the telecommands for the deletion of the files in the DSHA (Data Science Handling Assembly).
This minimized number of telecommands, which is enabled by usage of the one-telecommand philosophy of EnMAP, also supports the long-term scheduling and commanding of tasks, e.g. over the weekend, during which all scheduled tasks can be performed by the spacecraft autonomously without any mandatory ground intervention in-between.
Figure 19: Example of a daily operations scenario (image credit: OHB/Kayser-Threde)
Orbit control maneuver: Orbit control maneuvers are not a part of daily operations. Nevertheless the one-telecommand philosophy is applied here as well.
For an orbit control maneuver mainly the ACS (Attitude Control Subsystem) and the OCS (Orbit Control Subsystem) are involved. Again all necessary tasks are controlled by the on-board computer according to the telecommand being sent to the S/C.
The ACS needs to perform a precise maneuver similar to the maneuvers for image acquisition or payload data download. The S/C’s attitude has to be changed so that the thrusters will be fired in the correct direction. For the OCS, there are three main tasks that have to be controlled during the maneuver: thruster catalyst preheating, thruster activation and deactivation.
The TTTC for an orbit control maneuver includes the time to start the telecommand, time tags for activation/ deactivation of thrusters, and the necessary ACS guidance information.
In the case two orbit control maneuvers should be performed a minimum time offset is necessary. A determination of the changed orbit should be performed by the ground segment to ensure the maneuver has been performed correctly and the changes are as expected. In addition, the ACS subsystem requires a specific time to autonomously de-saturate the reaction wheels after one maneuver. Compared to the time needed for the orbit determination the time needed for reaction wheel desaturation is relatively small.
The attitude accuracy of the satellite during thruster firing is autonomously supervised onboard the satellite. The ground station can configure a limit value for the attitude deviation by a configuration command. If the satellite attitude deviation exceeds the configured limit value at any time during thruster firing, a current orbit control maneuver is autonomously aborted. Otherwise deviations of the satellite attitude during thruster firing would lead to deviations of all orbit parameters from the calculated target values. - As for the other maneuvers the orbit control maneuver is performed completely autonomously on board the S/C having sent only one telecommand (Ref. 18).
• The MOS (Mission Operations System) responsible for commanding and controlling the satellite and instrument.
• The PGS (Payload Ground System) responsible for data reception, handling, archiving and delivery as well as for the user interfaces for observation and product orders.
• The PCV (Processor, Calibration and Validation segment) responsible for processing of raw image data and identification of clouds in satellite images (L0 processor). Hence, PCV results form the basis for MOS inputs concerning the results of image acquisition planning and the resulting cloud contamination of images.
The EnMAP space segment will rely on common RF equipment (standard S-band and X-band links), and will be completely compliant with the existing German ground segment infrastructure at DLR, i.e. mission control at GSOC in Oberpfaffenhofen (Weilheim station support for TT&C services), and imagery data reception via the Neustrelitz, and other DLR X-band ground stations. The processing and archiving functions are allocated to DFD (German Remote Sensing Data Center).
EnMAP is conceived as a science and research mission and a pathfinder to evolve towards an operational/commercial service. The primary and immediate targets are the science community with its specific needs for research and development and the value adding companies offering information of great interest and use by public and commercial sectors.
Figure 20: Overview of the EnMAP ground segment elements (image credit: DLR)
Figure 21: Illustration of EnMAP ground system subsystems (image credit: DLR)
In addition, there are value-adding companies in the EnMAP team on the commercial applications side (GAF AG, Vista, Definiens, etc.). RapidEye AG is contributing the synergy of EnMAP with RapidEye's 5 satellite constellation of high resolution, multispectral imagers (launch of RapidEye constellation on August 29, 2008).
EnMAP product generation:
The EnMAP products are derived from tiled data takes of size 1000 x 1024 pixels (~30 km x 30 km), which are generated by the processing system on demand and delivered to the user community. Different product definitions (e.g. Committee on Earth Observation Satellites CEOS, European Space Agency ESA or from the different satellite data providers) are in common use, but a coherent assignment of the foreseen EnMAP product types to these definitions is not possible. Therefore the EnMAP standard product definitions are specified as follows:
• At-Sensor-Radiance Product (L1): The Level 1 product is radiometrically calibrated, spectrally characterized, geometrically characterized, quality controlled and annotated with preliminary pixel classification (usability mask). The auxiliary information (e.g. position and pointing values, interior orientation parameters, gain and offset) necessary for further processing is attached, but not applied.
• Orthorectified Product (L2 geo): The Level 2geo product is derived from the L1 product and geometrically corrected (correction of sensor, satellite motion and terrain related distortions) and re-sampled to a specified grid (orthorectified). Auxiliary data for further processing are attached, but not applied.
• Atmospheric Corrected Product (L2 atm): The Level 2atm product is derived from the L1 product, atmospherically corrected and the data converted to ground surface reflectance values. Auxiliary data for further processing are attached, but not applied.
• Orthorectified and Atmospheric Corrected Product (L2): The Level 2 product is derived from the Level 2 geo product, atmospherically corrected and the data converted to ground surface reflectance values.
Table 7: Overview of EnMAP data products
Figure 22: EnMAP processing chain and product generation (image credit: DLR)
EnMAP user interface:
The user interface consists of two hierarchically structured online portals interfaced with several subsystems of the EnMAP ground segment (Figure 23). The EnMAP portal (www.enmap.org) is the central entry point for all international users interested to learn about the EnMAP mission, its objectives, status, data products and processing chains. Additionally, this platform informs about the conditions and requirements for the EnMAP data access and the ongoing scientific programs and activities that are initiated by the PI (Principle Investigator) at the GFZ (German Research Center for Geosciences). No prior registration is required for the portal in order to access the information. 35) 36)
EDAP ((EnMAP Data Access Portal): EDAP is the entry point for any EnMAP data request comprising functions for observation planning, submission of observation requests, the proposal process and catalog orders.
Figure 23: The EnMAP ground segment user interfaces (image credit: DLR)
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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.