Near-Optimum Real-Time Range Estimation Algorithms for Proximity Links

The renewed interest in space exploration and cis-lunar situational awareness demands accurate ranging algorithms to enable navigation solutions for a multitude of spacecraft, rovers, and human explorers on the Moon and even on Mars, in the near future. Current state-of-the-art in ground-based ranging accuracy is on the order of 30 cm, however complicated equipment calibration and significant post-processing is required to achieve this level of ranging accuracy. This article examines ad-hoc approaches that achieve near-optimum real-time ranging performance with reduced complexity by utilizing a DPLL (digital phase-locked loop) to track the phase of the residual carrier for both direct and subcarrier modulated PN sequences, and a DCL (digital Costas loop) to obtain independent estimates of carrier phase and optimal combinations of these implementations to achieve near-optimum real-time performance with reduced computational complexity.