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Improvements in autonomous GPS navigation of Low Earth Orbit satellites

Universitat Politècnica de Catalunya, October 24th. 2008
Satellites in low Earth orbits (LEO) are generally defined to be up to an altitude of 2000 km above Earth’s surface and given the rapid decay of objects on the lower altitude range due to atmospheric drag, it is commonly accepted that a typical LEO height lies between 200 and 2000 km. This altitude range makes LEO satellites useful for a wide range of applications such as communication transponders, remote sensing, gravimetric and magnetometric sounding, ocean altimetry, atmospheric retrieval and Search and Rescue alarm operations. Its accurate positioning is of great importance in the successful accomplishment of their objectives.
In this sense, most LEO satellites have a GPS receiver, which allows to collect GPS measurements in its full revolution around the Earth. These measures can be used to precisely estimate the trajectory of the spacecraft. This operation is normally done on ground, after the satellite was able to downlink all the data it collected. The capacity to do this positioning in real-time onboard the satellite is a necessity for some of the applications, and would also allow a faster science product delivery.
This autonomous positioning is very different that the one that can be done on ground, as the satellite processor has large limitations in computational resources, so the complex models and calculus done in a normal computer on ground are completely unaffordable for the onboard processor. Besides, some of the models used in the trajectory estimation need some additional data (such as solar activity, or Earth rotation parameters) that are not available in real-time, so some approximations must be done to cope with these lack of data.
This thesis will deepen into the study of autonomous GPS navigation of LEO satellites, a trend that is increasing its importance for their applications in both science and technological fields. It will develop new algorithms and methods in order to provide accurate and continuous positions for the satellites. Different aspects have been covered:
  • Multipath and interference mitigation. Reflections of GPS signals in the spacecraft structure cause a distress that affects the measured distance. On the other hand, some spacecraft have more than one GPS antenna on its payload. This creates a cross-talk interference that also affects the measures. The repeatability of these effects in relation to the attitude of the spacecraft can be used to mitigate its impact into the final navigation solution. Multipath mitigation techniques have been developed for both single- and dual-frequency receivers.
  • Dynamic force models. The high predictability of the trajectory of Earth orbiters is used in conjunction to GPS measurements to provide a more accurate solution than GPS standalone positions. This is a widely used technique in postprocessing strategies, but has high computational requirements and needs parameters not available in real-time. The simplifications of these models, along with the
  • suppression of the parameters not available in an onboard environment is necessary to use these kind of positioning by a satelliteprocessing in real-time conditions.
  • Maneuver handling. Earth orbiters do not follow a fully predictable orbit, some low-order perturbations modifies its trajectory on the long term, and atmospheric drag slowly brakes the satellite, decreasing its altitude. This makes necessary a periodic correction of its trajectory. This is done by short impulses produced by the satellite propulsion systems in what is called a maneuver. When a spacecraft is in a maneuver, it no longer follows the free-flight dynamic models, so this should be taken into account in the estimation filter.
All the algorithms and methods have been tested with real data from different missions: SAC-C, CHAMP, JASON-1 and GRACE. Several test cases covering a wide range of days and parametrization options have been done in order to assess its performance.

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