|M.Sc Student||Elisha Yossef|
|Subject||Relative Satellite Attitude Determination Using GPS with|
Applications to Astronomical Observations
|Department||Department of Physics||Supervisors||Professor Emeritus Stephen Lipson|
|Professor Pinchas Gurfil|
|Full Thesis text|
Over the past few years, GPS has been demonstrated as a successful method for precise orbit determination of a single satellite. CHAMP L1 GPS data was determined using the orbit determination algorithm and a typical position accuracy of less than 20 cm 1-sigma has been achieved.
GPS methods for relative positioning have been also applied to problems pertaining to satellite formation flying, a concept wherein multiple satellites can work as a group to accomplish the objective of a larger, usually more expensive, satellite. Estimation of the relative position and attitude between the satellites within the formation is an important issue that is essential for many missions. The main objective of this research was to demonstrate that carrier-phase differential GPS techniques can be employed as the primary means for sensing both the relative position and the relative attitude of two satellites.
Based on the L1 GPS receiver measurements of the Gravity Recovery and Climate Experiment (GRACE) satellites, 1 cm relative positioning accuracy was obtained after fixing the double-difference carrier-phase ambiguities to integer values.
Two methods for relative attitude determination have been developed, implemented, tested and compared. The first one estimates the absolute satellite attitude and then derives the relative attitude between the satellites. The relative attitude accuracy achieved by using this method was about 0.4 degrees. The second method uses a differential solution between all the GPS antennas and therefore yields better results. This method’s general idea is to find minimal differences between the entire GPS baseline measurements and the satellite model. The relative attitude determined by using the improved method was about 0.1 degrees - four times more accurate than the first method.
The main objective of this research was to demonstrate that carrier-phase differential GPS techniques can be employed as the primary means of sensing both the relative position and the relative attitude of two satellites. Based on the research results, it can be concluded that this objective has been fulfilled. The algorithms developed in this study would enable determination of precise relative attitude of the spacecraft in the formation by tracking the relative carrier phase received in each of three antennas mounted on a satellite. Consequently, the relative positions of each spacecraft in the formation can be precisely determined from the satellite-to-satellite phase and pseudo-range data. Such antennas can be quite small and inexpensive.