Design of Direct Path Synchronized Bistatic Radar Technique for Long Offset Glacial Temperature Tomography

Abstract:

Traditional ice-penetrating radar sounders cannot resolve local ice sheet temperature anomalies, which contributes to uncertainty in the flow regime and sea level rise projections. We demonstrate with an Alternating Direction Method of Multipliers (ADMM) inversion that bistatic bed-echo power measurements enable estimation of fine scale, horizontally varying, depth dependent ice properties. These simulations show that large antenna separations (4 times wider than the thermal anomaly) are required to accurately resolve a temperature gradient; however, existing radar systems have been unable to attain the large antenna separations necessary. These systems have not been able to exploit long offsets either due to cable losses in synchronized systems or poor signal-to-noise-ratio (SNR) of unsynchronized systems. Here, we overcome this challenge through coherent summation of phase re-aligned signals to recover the echo at large antenna separations without requiring hardware synchronization. Since the goal of this experimental design is not fine-scale imaging, the bandwidth requirements that are typically implemented to optimize range resolution are instead relaxed to keep the signal to interference ratio sufficiently high. Our direct-path synchronization approach paired with lower bandwidth requirements allows us to use an Autonomous Phase-sensitive Radio Echo Sounder (ApRES) as the transmitter and an unsynchronized Software Defined Radio (SDR) as the receiver. We evaluate the system's performance in terms of direct-path synchronization and coherent processing gain during a field deployment at Whillans Ice Stream, West Antarctica, where we attained a SNR of 44 dB after coherent summation at an antenna separation of 1.3 km. The method we introduce for high SNR signal recovery at large antenna separations solves a primary hurdle towards bistatic radar estimates of temperature distributions at the thermal and spatial scales that can influence ice-sheet flow and stability.