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Precise relative navigation is a critical enabler for distributed satellitesto achieve new mission objectives impossible for a monolithic spacecraft.Carrier phase differential GPS (CDGPS) with integer ambiguity resolution (IAR)is a promising means of achieving cm-level accuracy for high-precisionRendezvous, Proximity-Operations and Docking (RPOD), In-Space Servicing,Assembly and Manufacturing (ISAM) as well as satellite formation flying andswarming. However, IAR is sensitive to received GPS signal noise, especiallyunder severe multi-path or high thermal noise. This paper proposes asensor-fusion approach to achieve IAR under such conditions in two couplingstages. A loose coupling stage fuses through an Extended Kalman Filter theCDGPS measurements with on-board sensor measurements such as range fromcross-links, and vision-based bearing angles. A second tight-coupling stageaugments the cost function of the integer weighted least-squares minimizationwith a soft constraint function using noise-weighted observed-minus-computedresiduals from these external sensor measurements. Integer acceptance tests areempirically modified to reflect added constraints. Partial IAR is applied tograduate integer fixing. These proposed techniques are packaged intoflight-capable software, with ground truths simulated by the Stanford SpaceRendezvous Laboratory's S3 library using state-of-the-art force modelling withrelevant sources of errors, and validated in two scenarios: (1) a highmulti-path scenario involving rendezvous and docking in low Earth orbit, and(2) a high thermal noise scenario relying only on GPS side-lobe signals duringproximity operations in geostationary orbit. This study demonstrates successfulIAR in both cases, using the proposed sensor-fusion approach, thusdemonstrating potential for high-precision state estimation under adversesignal-to-noise conditions.

Precise Distributed Satellite Navigation: Differential GPS with Sensor-Coupling for Integer Ambiguity Resolution