Long-Distance Ranging and Velocity Measurements by REASON on Europa Clipper

One of the objectives of the Gravity and Radio Science (G/RS) instrument on NASA’s Europa Clipper mission is to improve our knowledge of Europa’s ephemerides, to trace the moon’s thermal-orbital evolution. Here, we present a complementary methodology to use the Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) instrument to enhance these measurements by tracking echoes from Europa at greater distances, beyond its primary role of subsurface sounding during close flybys. We investigate the possibility of occasional REASON measurements during the non-nadir phase at altitudes beyond 40,000 km to extend the ephemerides dataset. Through radar simulations of REASON’s echoes, we evaluate and demonstrate this approach by determining the maximum altitude for surface return detection, achieved by coherently processing hundreds to thousands of pulses within the signal’s round-trip time. The received signal’s power can be further enhanced by maximizing pulse length, targeting Europa’s most reflective (leading sub-Jovian) hemisphere, and summing many compressed pulses. We also explore a phase shift approach for velocity measurements, which uses existing ranging pulses. Using Europa Clipper’s most recent trajectory design, we identify flybys that align with the best working scenarios for this approach. Our performance estimates for range and velocity resolution demonstrate that this approach could obtain measurements relevant to Europa’s ephemerides. This method offers key insights into Europa’s orbital evolution and, ultimately, its long-term habitability.