Dielectric loss of lunar regolith estimated from LRP data. (a) Average LPR trace amplitudes from 100 m of CE-3 data and 100 m of CE-4 data, used to assess signal decay in the lunar subsurface. The position of time-zero is corrected according to the largest amplitude. (b) The loss tangent of lunar regolith as a function of maximum penetration time. (c) The density normalized loss tangent as a function of TiO₂ weight percentage. The Apollo data points only represent measurements at 450 MHz. (d) The loss tangent at CE-3 and CE-4 landing sites as a function of frequency. The data at 3, 7.8, 19.35, and 37 GHz are taken from Siegler et al. (2020).