Ionospheric Delays Compensation for On-The-Fly Integer Ambiguity Resolution in Long Baseline LEO Formations

This paper deals with the real-time onboard accurate relative positioning by Carrier-phase Differential GPS (CDGPS) of LEO formations with baselines of hundreds of kilometers. On long baselines, high accuracy can be achieved only using dual-frequency measurements and exploiting the integer nature of Double Difference (DD) carrier-phase ambiguities. However, large differential ionospheric delays and broadcast ephemeris errors complicate the integer resolution task. The present paper is concerned with analyzing possible approaches to DD ionospheric delays compensation in such applications. Two different strategies are implemented to deal with DD ionospheric delays. The first formulation models differential ionospheric delays as a function of the vertical total electron content above the receivers, whereas the second one is based on combining the DD measurements for removing ionospheric delays from the observation model. The effectiveness of the developed solutions is assessed by comparing the relative positioning accuracy that can be obtained on actual flight data from the Gravity Recovery and Climate Experiment mission. This is done using a recently developed, common relative positioning approach. Results show that ionospheric activity plays a major role in determining the relative positioning performance. Modeling the delays is advantageous for relative positioning in mild ionospheric conditions, but the solution without ionospheric delays becomes preferable as the ionosphere�s electron content increases.