Revisiting the Limits of Spatial Coherence for Passive Radar Sounding Using Radio-Astronomical Sources

We revisit a potential limitation for passive radar sounders that propose to use extended, incoherent radio-astronomical sources for echo detection and ranging. Recent work has considered the spatial coherence effects of extended sources for passive sounding in terms of pulse broadening; we expand on this work by analyzing the spatial coherence limitations imposed by source size, wavelength, incidence angle, and altitude. Moreover, we derive additional bounds set by these parameters and the expected source extent to estimate the maximum orbital altitudes for a spacecraft using Jovian bursts as a source for passive sounding of the Moon and Europa. While the results of our analysis and simulations show that the coherence requirement is met for terrestrial ground-based experiments up to large incidence angles, the limited spatial coherence creates an upper bound for an orbital passive radar sounder at Europa. Our results therefore provide a richer understanding of the passive sounding technique, its feasibility, and its potential limitations when designing future missions.