1PRECISE A POSTERIORI GEOMETRIC TRACKING OF LOW EARTH ORBITERS WITH GPS

Precise, a posteriori orbit determination is required for a wide variety of spaceborne scientific applications. Current Global Positioning System (GPS)-based orbit determination (OD) methods combine spaceborne GPS (SGPS) receiver tracking measurements with the classical orbit determination technique in an optimal estimation process. The research we have carried out investigated the class of low earth orbiters (LEOs) that are at altitudes below approximately 700 km and/or that have complex dynamic behaviour, making precise orbit modelling difficult. Dynamic OD strategies would require presently unattainable high-fidelity force and spacecraft models to ensure precise tracking estimates. To obviate the need for such elaborate yet insufficient models for use with the classical orbit determination process, a completely geometric approach that follows the traditional geodetic relative positioning technique is proposed. This approach combines SGPS receiver data with freely-available data from a network of terrestrial GPS receivers to estimate the SGPS receiver positions over time. Error propagation studies with synthetic data show that decimetre-level precision of position components is possible with only tens of minutes of data processing. Future work includes augmenting the developed software to process actual SGPS data and developing interpolation algorithms to estimate LEO positions between receiver measurements