Exploitation of SHARAD data from a passive sounding perspective: a preliminary analysis

Passive radar sounding has been proposed as a low-cost, low-risk way to enrich the scientific return of planetary radar sounders, especially in the vicinity of bright radio sources such as Jupiter [Romero-Wolf et al. (2015), Icarus, 248:463-477], whose moons will be studied by radar sounders in the late 2020's and 2030's. To predict what passive radargrams may look like as a function of parameters such as the noise source spectrum and the surface/subsurface roughness, analytical and empirical models have been proposed in the literature [Schroeder et al. (2016), PSS, 134:52-60], and proof-of-concept hardware has been tested on Earth [Peters et al. (2018), TGRS, 56(12) 7338-7349]. To cement our understanding of passive sounding, we searched for traces of passively-acquired radar echoes in existing SHARAD radargrams. Such signals can be uncovered if the incoming noise was captured in one acquisition and its reflection by surface or subsurface features in the next one. Cross-correlating the two uncompressed rangelines could then reveal possible present Martian features using only the signals of opportunity. We started from the engineering parameters of SHARAD, such as its orbit, Rx window length, and PRF, to work out all the geometric configurations where Jovian emissions and their reflection from the surface could have been intercepted if such emissions were present. We made the assumption that waves must be specularly-reflected off the surface of Mars at a given angle, and looked for the angles at which the delay of the reflected noise matches the PRI of SHARAD. We have determined that, for the range of altitudes SHARAD operates at, the (Jupiter-Mars, Mars-SHARAD) angle must lie between 35° and 52°. Based on Friis-like arguments, we believe the SNR of such signals could reach 10 dB in the case of a smooth surface such as Elysium Planitia. We then cross-correlated this database of SHARAD radargrams with that of a model of Jovian noise occurrence at Mars using ExPRES [Hess et al. (2008), GRL, 35.13], and extracted a list of potential candidates. Preliminary analysis of these candidates shows that some of them may indeed contain passively-acquired signals that may be exploited scientifically. We have additionally conducted passive Stratton-Chu simulations [Gerekos et al. (2019), TGRS, 58(4) 2250-2265] of these cases to support interpretation.