An airborne radar system for high-resolution mapping of internal layers

Accumulation rate is a key variable in assessing the mass balance of polar ice sheets. An improved knowledge of the mass balance of polar ice sheets is needed to determine their role in current and future sea level rise. Existing accumulation maps, derived from sparsely distributed ice cores and pits, contain accumulation rate errors as large as 20% in certain areas. Remote sensing methods to complement and supplement in situ measurements are required to generate improved accumulation maps. For this reason we have been investigating the use of high-resolution radars for mapping of near-surface internal layers and generating continuous profiles of the dated layers in the ice sheet (isochrones). We successfully mapped isochrones within 2 m of those in an ice core up to a depth of 280 m at the North GReenland Ice core Project (NGRIP) ice camp during the 1998 and 1999 field seasons using a 170-2,000 MHz frequency modulated continuous wave (FM-CW) radar. We described the system and reported results of our experiments in previous IGARSS meetings [Kanagaratnam et al. 1999, 2000]. Recently we performed detailed analysis of the data collected from these experiments and determined the frequency response of reflections from the internal layers. The results showed the optimum frequency range for monitoring near-surface layers is between 500 and 1000 MHz. Based on these results we are developing a 600-900 MHz coherent airborne radar for high-resolution mapping of the internal layers in the Greenland ice sheet. We designed this system to operate in different modes: simple pulse, chirped pulse, step-frequency, and FM-CW. We have also developed a digital data acquisition system to collect and process data. We designed this system such that it can operate in an undersampling mode to digitize the received signal directly without down conversion. In this paper we present results of data analysis and detailed design of the airborne radar