Radar-Based Range Estimation in Cis-Lunar Space via Fade-Resistant Signals and Algorithms

There is renewed interest in human and robotic exploration of the Moon, as exemplified by NASA’s Artemis mission, Russia’s M1 lunar flyby, and China's Crewed Lunar Mission 1. In addition, robotic exploration has produced a profusion of lunar orbiters, landers and other spacecraft of multi-national origin that need to be detected, monitored and characterized to ensure the safety of future missions, as well as for general cis-lunar situational awareness. In this paper we describe two promising radar-based ranging algorithms that were tested using 34 meter Beam-Waveguide antennas of the Deep Space Network at the NASA Goldstone facility in California. These algorithms rely on week reflected radar echoes that do not require a co-operative transponder on the spacecraft, hence can be used to detect and characterize non-cooperative or lost spacecraft, as well as near-earth asteroids and other objects of interest in the cis-lunar environment. The algorithms described here utilize weak radar echoes received from decommissioned geo-synchronous satellites, utilizing precisely timed phase-modulated frequency ramps to determine start-time, Doppler and Doppler-rate via zone-filtered inverse FFTs to minimize noise and reconstruct the received time-sequence. The radar echo is analyzed via maximum likelihood and "beat-note" algorithms to estimate round-trip delay and obtain one-way range estimates, which are then compared to the predicted range of the satellite in geo-synchronous orbit.