The primary objective of the Proba-3 mission is to build a solar coronagraph composed of two satellites flying inclose formation on a high elliptical orbit and tightly controlled at apogee. Both spacecraft will embark a low-costGPS receiver, originally designed for low-Earth orbits, to support the mission operations and planning during theperigee passage, when the GPS constellation is visible. The paper demonstrates the possibility of extending theutilization range of the GPS-based navigation system to serve as sensor for formation acquisition and coarseformation keeping. The results presented in the paper aim at achieving an unprecedented degree of realism using ahigh-fidelity simulation environment with hardware-in-the-loop capabilities. A modified version of the flight-provenPRISMA navigation system, composed of two single-frequency Phoenix GPS receivers and an advanced real-timeonboard navigation filter, has been retained for this analysis. For several-day long simulations, the GPS receivers arereplaced by software emulation to accelerate the simulation process. Special attention has been paid to the receiverlink budget and to the selection of a proper attitude profile. Overall the paper demonstrates that, despite a limitedGPS tracking time, the onboard navigation filter gets enough measurements to perform a relative orbit determinationaccurate at the centimeter level at perigee. Afterwards, the orbit prediction performance depends mainly on thequality of the onboard modeling of the differential solar radiation pressure acting on the satellites. When not takeninto account, this perturbation is responsible for relative navigation errors at apogee up to 50 m. The errors can bereduced to only 10 m if the navigation filter is able to model this disturbance with 70% fidelity.
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