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

Abstract — There is renewed interest in human and robotic exploration of the Moon, as exemplified with NASA’s Artemis mission starting in 2024, Russia’s M1 crewed lunar flyby in 2029, and China’s Crewed Lunar Mission 1 in 2030. 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 several promising radar-based ranging algorithms that are currently being 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 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 and evaluated here utilize radar echoes received from the decommissioned geo-synchronous NOAA satellite GOES-11, and rely on precisely timed amplitude-modulated frequency ramps that are less sensitive to deep fades in the received radar signals to first detect the presence of the echo via short-term FFTs, then determine the start-time via the application of zone-filtered inverse FFTs to minimize noise and reconstruct the received time-sequence. The reconstructed frequency-ramp is further analyzed by extracting the beat-note generated by the round-trip delay with frequency proportional to the round-trip light time, and compared with PN-coded ranging techniques to evaluate the potential improvement over conventional correlation-based ranging techniques.