Passive radio sounding to correct for Europa’s ionospheric distortion of VHF signals

Abstract

The dispersive effect of Europa’s ionosphere on radio wave propagation is detrimental to the quality of radar observations for altimetry and geodesy of Europa’s surface, as it creates pulse broadening and signal delay. While using the relative delay between two active radar channels of different center frequencies is a common method to correct for this ionospheric distortion, the High Frequency (HF) active band for a dual-frequency radar sounder like the Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) is susceptible to being dominated by the bursts of Jupiter’s radio emissions when the radar is on the sub-Jovian side. At these times, the HF radar will be severely degraded or unusable alongside the slightly dispersed Very High Frequency (VHF) channel for performing radar sounding as well as reference-based radar corrections of ionospheric distortion for VHF altimetry. To address this, we assess the potential to use Jupiter’s radio emissions to correct for ionospheric distortion with a passive radio sounding approach that has been developed on Earth to estimate the thickness of ice sheets using the Sun. With REASON as an interesting test case for the technique, we discuss in general the abilities and requirements for a dual-frequency radar system using a passive HF radar alongside an active VHF radar to correct for ionospheric distortion, and we compare the passive mode’s performance to that of an active HF radar system. Finally, we show that even in the presence of the maximum expected total electron content (TEC) in Europa’s ionosphere, the passive radar approach, using the REASON HF as a receive-only instrument, not only corrects for dispersion but also performs at a level similar to that of an active HF system in terms of TEC estimation and VHF range error reduction.