New Representation of Synthetic Aperture Radar Images for Enhanced Ice-Sounding Interpretation

The processing of Synthetic Aperture Radar (SAR) images is based on the coherent integration of Doppler frequencies. The associated Doppler spectrum is generated from the variation of the relative location between the radar and the scatterer. In geometries where the moving radar-platform follows a straight trajectory at constant velocity, the Doppler frequency depends on the angle of elevation from the radar to the scatterer, according to the electromagnetic (EM) propagation. In ice-sounding by airborne SAR, the EM path depends on the air-ice interface and the firn ice properties. For any of the scatterers under test, after integrating the received radar echoes from the multiple radar locations into a single pixel, the resulting amplitude image forgets which is the backscattering angle from each of the radar locations. However, this information is still within the Doppler spectrum of the image. We decompose the Doppler spectrum of the SAR image into three non-overlapping sub-bands; assign to each sub-band one of the primary colours red, green or blue, forming three sub-images; and finally merge the sub-images into a single one. Rather than a single full-beamwidth averaged amplitude value, the new composition now includes angular backscattering information, coded by one of the primary colours. Blue colour is assigned to scattering received from forwards, when the scatterer is ahead of the radar (positive Doppler frequencies); green approximately from the vertical (near zero-Doppler geometries); and red to scattering received from backwards (negative Doppler). Thus, heterogeneous scattering will be represented by one or two colours, whereas homogeneous scattering will be grey, with all the primary colours uniformly weighted. Features like internal layering, crevasses, SAR focussing quality and discrimination of multiple reflections from surface and bottom, can now be better interpreted. We present and discuss the results from the British Antarctic Survey (BAS) airborne radar PASIN2 for deep-ice sounding, in Recovery and Rutford ice streams, respectively in East and West Antarctica during seasons 2016/17 and 2019/20.