Impact of 1-Bit Sampling of Radar Sounding Data for Deep Space Applications: Demonstration at Mars and Implications for Europa

Abstract:

Stringent control of data volume size is a critical factor in the success of all deep-space planetary science missions. Measurements must be carefully planned such that the resulting datasets fit on the on-board data storage system and can be downlinked prior to the end of the mission based on predicted network availability. The Radar for Europa Assessment and Sounding: Ocean to Near-surface (REASON) instrument on-board NASA’s Europa Clipper spacecraft is not immune to these types of data volume constraints. An approach in place for investigating the full-depth (down to 30 km) of Europa’s ice shell with the 15 m free-space range resolution REASON VHF band (10 MHz bandwidth, 12 MHz sampling), as a complement to the 150 m free-space range resolution HF band (1 MHz bandwidth, 1.2 MHz sampling), while not exceeding REASON data allocation limits is to record the full-depth VHF echoes with a severely reduced radiometric fidelity. To prepare for these data, we investigate the implications of single bit (i.e., the sign bit) data acquisition and processing (range compression followed by SAR focusing) using minimally processed SHARAD EDR data. The most noticeable effect of reducing the radiometric fidelity of the SHARAD EDR data prior to data processing is a significant reduction in both mean echo power (by 10 to 15 dB) as well as signal-to-noise ratio (SNR) (by 2 to 5 dB). The reduction in echo strength does not appear to be related to the level of on-board data processing (i.e., stacking) that occurred prior to radiometric fidelity reduction. Due to the reduction in echo strength, weak reflections visible in the full radiometric fidelity radargrams, such those from deep layers within the Martian Northern Polar Layered Deposits (NPLD), can be lost in noise of the corresponding single bit radargrams. However, regional-scale structure, such as the base of the NPLD, remains interpretable. Finally, the variability in the echo strength reduction due to reduced radiometric fidelity appears inversely proportional to the strength of the full radiometric fidelity echo. That is to say, as the strength of the full radiometric fidelity echo decreases, the echo strength loss due to reduced radiometric fidelity results in greater deviation (both positively and negatively) around the mean loss level.