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StatusCompleted
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Status date2017-02-22
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Activity Code4C.040
GNSS space receivers are commonly used in spacecraft platforms in low Earth orbit to determine and control the spacecraft absolute and relative orbit position, velocity, and time (PVT).
The topic of this work is on-board autonomous orbit determination using GNSS for spacecraft in transfer to Geostationary Orbit (GEO), focusing on electrical propulsion, with the Airbus DS LION Navigator GNSS receiver.
The goal of the activity is to enlarge the application area of the Airbus DS LION Navigator GNSS receiver to launch and early operation phases on transfer up to and including the nominal operation in geosynchronous orbit.
The opportunity of GNSS navigation in higher orbits arises from the interest of operators in onboard autonomous orbit determination. New missions emerged for geostationary satellites due to the introduction of electric propulsion.
This activity focuses on electrical transfer to GEO where the improvement of using on-board autonomous GNSS navigation is expected to have a major impact. The GNSS constellations considered are GPS and Galileo.
The development concerns the evolution of the receiver SW, most notably, including associated HW-in-the-loop tests.
The attempt to introduce on board autonomous navigation to spacecraft in transfer orbits is of high risk, because it extends the use of GNSS to an area in space it was not designed for, and for which GNSS signal levels are at the technical limits of receiver technologies.
Challenges are poor visibility due to unfavourable, changing spacecraft orientation and greatly varying signal power due to changing altitude and attitude. This requires adaptation of GNSS receiver algorithms (sensor, navigation solution and planning) to overcome the challenging signal conditions.
A critical part of the GNSS navigation performance assessment are the launch trajectories, considering spacecraft attitude as commanded by guidance laws and demanded by electrical power generation. There exist no standard scenarios for defining navigation accuracy, as in LEO (e.g. Nadir pointing) or GEO (e.g. Earth pointing) orbits.
The definition of realistic transfer scenarios for representative tests is of utmost importance in order to facilitate acceptance of on-board autonomous navigation in LEOP by potential customers.
Moreover, the transfer phase is too long to be analysed entirely (up to 6 months using electrical propulsion). This requires a selection of representative subsets of the transfer phase for real-time hard
The attempt to introduce on board autonomous navigation to spacecraft in transfer orbits is of high risk, because it extends the use of GNSS to an area in space it was not designed for, and for which GNSS signal levels are at the technical limits of receiver technologies.
Challenges are poor visibility due to unfavourable, changing spacecraft orientation and greatly varying signal power due to changing altitude and attitude. This requires adaptation of GNSS receiver algorithms (sensor, navigation solution and planning) to overcome the challenging signal conditions.
A critical part of the GNSS navigation performance assessment are the launch trajectories, considering spacecraft attitude as commanded by guidance laws and demanded by electrical power generation. There exist no standard scenarios for defining navigation accuracy, as in LEO (e.g. Nadir pointing) or GEO (e.g. Earth pointing) orbits.
The definition of realistic transfer scenarios for representative tests is of utmost importance in order to facilitate acceptance of on-board autonomous navigation in LEOP by potential customers.
Moreover, the transfer phase is too long to be analysed entirely (up to 6 months using electrical propulsion). This requires a selection of representative subsets of the transfer phase for real-time hardware-In-the-loop tests. ware-In-the-loop tests.
Successful HW-in-the-loop tests are essential for the acceptance of on-board autonomous navigation in transfer to GEO by potential customers. The main contribution of this work is to demonstrate successful on-board autonomous orbit determination using GNSS in electrical transfer to GEO and realistic transfer scenarios based on ELECTRA, as provided by OHB Sweden.
Currently, trajectory/orbit determination of launch vehicle and spacecraft in transfer to GEO is performed by S-Band or C-Band TT&C ranging, using ranging antennas on the ground and on-board TM/TC transponders. The increased transfer duration using electrical propulsion implies costly coordination and maintenance of the required ground network of antennas over the extended transfer duration (i.e. several months).
By on-board autonomous orbit determination, ground station contacts may be reduced to TM/TC traffic, avoiding the use of tracking antennas, spread all over the world. This is expected, particularly in case of electrical transfer, to result in a considerable reduction of overall operational complexity and costs.
The Airbus DS GmbH LION Navigator Product Line is a modular multi-GNSS-constellation multi-frequency receiver for spacecraft that is space qualified for operation in low Earth orbits and intended for geosynchronous Earth orbits.
Investigations and hardware tests with the Spirent RF signal generator confirmed that GNSS navigation in GEO is feasible and satisfies accuracy requirements. The first SGEO geostationary telecommunication satellite Hispasat AG1 includes an Airbus DS GmbH MosaicGNSS Receiver as part of the platform (scheduled for launch in2016/2017).
The development concerns the evolution of the receiver SW, most notably, including associated HW -in-the-loop tests:
- Sensor module: Adaptation of acquisition and tracking of very low signal strength (carrier-to-noise density C/N0) with high relative dynamics between user spacecraft and GNSS SV.
- Navigation Solution module: Adaptation/tuning of the extended Kalman filter for sparsely distributed measurements under perturbed conditions (use and/or estimation of external accelerations from thrust manoeuvres).
- Navigation Planning module: Improved prediction of measurements (e.g. pseudorange, range rate) to aid the acquisition and tracking of GNSS signals in the Sensor channels / tracking loops.
The project started in July 2015. The duration is 12 months. The project milestones comprise a Kick-Off Meeting (KOM), a Baseline Design Review (BDR), a Mit-Term Review (MTR), and a final Review (FR).
The test campaign was finalised successfully.
GNSS navigation performance was analysed by simulation and HW-in-the-loop tests, using electrical transfer scenarios. These were derived from realistic spacecraft orbit, attitude, and acceleration profiles of the ELECTRA spacecraft which were kindly provided by OHB Sweden.
The HW-in-the-loop tests were done with the Airbus DS LION Navigator GNSS receiver engineering model (EM) featuring an enhanced software and the Spirent Signal Simulator (GSS 9000).
The LION Navigator was configured in single-frequency mode using GPS L5 and Galileo E5a. Acquisition and tracking based on L5/E5a was performed with sufficient stability in all scenarios. As a baseline, only the GNSS transmit antenna main lobe is used for reliability. Side lobes are considered separately as possible yet unreliable improvement. An important finding is that navigation may be performed with one single receive antenna.
This provides a sound analysis of achievable in-orbit performance under the stringent conditions of the transfer to GEO with electrical propulsion. In particular, the navigation performance is achieved under the varying user spacecraft orientation conditions, as required by the optimal thrust vector guidance laws and solar power generation.
The performance of the orbit determination depends largely on the transfer scenario. The main drivers for performance in transfer to GEO are signal outages in combination with the imperfect knowledge of the thrust manoeuvres. This situation was improved by estimating the propulsion system behaviour in the extended Kalman filter.
