GNSS ISL Based Navigation of Satellites in Geotransfer Orbits

GNSS space receivers are widely used for onboard autonomous navigation of spacecraft platforms in low earth orbit. However, at a higher orbit, the situation changes dramatically as most of the directional GPS transmit antennas no longer point toward the user. Besides, the deficient power levels seen at the receiver's antenna can worsen the already poor relative geometry of the GNSS receiver to the GNSS satellites. Because inter-satellite link (ISL) is designed for inter-satellite ranging and communication between the navigation satellites, the space coverage of ISL is better than the normal GNSS signal, which means we can obtain more strong enough signals for navigation. To evaluate the performance of navigation based ISLs, the GNSS satellite visibility for an ISL receiver are proposed, considering the user aperture angle, the Earth tangent horizon mask, the GNSS satellite antenna pattern and the signal power threshold of the receiver. Then, we design an Extended Kalman Filter (EKF) based algorithm to estimate the orbit parameter of user spacecraft and different measurement strategies are analyzed. Simulation results show a nadir-pointing antenna is enough to determine the orbit parameter of user spacecraft in most of the case and two antennas oriented in the nadir direction, and zenith direction respectively is recommended to ensure the high estimation precision at the perigee of the early phases of the geotransfer orbit (GTO). Besides, different measurement strategies have a significant influence on the performance of the estimation.