Real-time navigation for Mars final approach using the Mars Network
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Real-time navigation during the final approach phase of an interplanetary mission can significantly increase the accuracy of aerocapture and pin-point landing. The Mars Network is a versatile telecommunications network that is ideally situated to provide spacecraft-to-spacecraft radiometric navigation during Mars final approach and entry, descent, and landing via the Electra UHF transceiver, which is capable of providing autonomous, on-orbit, real-time trajectory determination using two-way Doppler measurements between a Mars approach vehicle and a Mars Network orbiter. A detailed dynamic analysis and link analysis of the final approach problem is presented, which seeks to determine the expected operating conditions of the Electra transceiver. In particular, the maximum Doppler shift and Doppler rate, which determine the transceiver tracking loop requirements, and the total received signal power and signal-to-noise ratio, which determine the range at which the communications link can be closed, are investigated for a range of Mars Network orbital geometries. A model of the Electra signal is developed on the basis of the results of the dynamic analysis and link analysis and is used as input to a high-fidelity simulation of the Electra transceiver. A Monte Carlo analysis is performed to determine the performance of the Electra transceiver for a range of signal and tracking loop parameters. In particular, the performance analysis focuses on the maximum range at which the link can be closed and on the acquisition and tracking performance of the second-order tracking loop. The analysis of the tracking performance is used to characterize and model the error in the Doppler measurement of the Electra transceiver. The error model is incorporated into the design of an extended Kalman filter, in order to improve the fidelity of the navigation filter design. The information content in the Doppler measurement and the observability of the estimated states are investigated for various orbital geometries and the accuracy of the navigation solution is analyzed.