A Wideband Radar for Mapping Near-Surface Layers in Snow

We developed a wideband radar to map near- surface internal layers in firn with fine resolution of about 3 cm to a depth of about 10 m. It is a frequency-modulated continuous-wave (FM-CW) radar that operates over the frequency range of 12-18 GHz with an antenna operated in the near field to obtain plane-wave illumination. The plane-wave illumination reduces off-vertical scattered signals from masking reflections caused by internal layers. To operate the radar on the snow surface, we designed and built a sled that includes a gimbaled mount and control system to ensure that the antenna points at nadir. In addition, the antenna is offset from the sled so it points at undisturbed snow. The radar features a fast transmit waveform synthesizer implemented using a voltage-controlled oscillator (VCO) and a phase-locked loop (PLL) with a linear digital chirp as a reference. The highly linear reference chirp is compared against the instantaneous VCO output to generate a highly linear 12 to 18 GHz transmit signal. The waveform synthesizer can be swept from 12 to 18 GHz in 1 millisecond. We tested the radar at both Summit, Greenland, and a field camp in West Antarctica in July 2005 and January 2006, respectively. We collected a large amount of data at both sites, and we have been able to follow internal layers over distances exceeding 10 km. We verified radar data by comparing radar echoes to visible wind crust and depth hoar layers observed in 2-m deep snow pits. We also measured snow and firn density with a resolution of 5 cm to determine the dielectric constant for estimating propagation velocity of the wave in snow and firn. We collected more than 200 sample traces at each pit location for comparison with visual observations. Each sample trace uses 10 sweeps that are coherently integrated to improve signal-to-noise ratio (SNR). We made measurements in stationary mode and by dragging the sled behind a snowmobile driven at a speed of about 2.5 km/hr. Results show an excellent agreement between the snow pit stratigraphy and echoes from our plane-wave radar.