A wideband radar for mapping internal layers in the polar icesheets for estimating accumulation rate

摘要

Determination of the mass balance of the polar ice sheets requires information on the accumulation rate. Remote sensing methods to determine the accumulation rate are essential in reducing the uncertainty associated with interpolating in situ measurements that are obtained from ice cores and pits. This is essential to reducing the 20% of uncertainty in current accumulation rate maps. Using data from surface-based radar experiments we determined optimum parameters for an airborne radar. We developed an airborne prototype and successfully demonstrated that we can map internal layers with about 1 m resolution to a depth of about 120 m over the Greenland ice sheet. We reported the system design, construction and preliminary experimental results at the 2001 IGARSS meeting. We have developed an operational radar system for routine measurement. This system operates in FM-CW and stepped-frequency pulse modes and it has 20-dB more sensitivity than the prototype radar. We also developed a radar target simulator for testing and evaluating system performance. The target simulator was constructed using fiber optic cables, microwave delay lines and RF/optical transceivers to simulate reflections from the air/snow interface, internal layers and the antenna reflection, which degrades the system's sensitivity. The simulator serves a dual purpose of optimizing the system performance in the laboratory and for internal calibration in the field. We also used a CAD package to design and simulate overall radar performance. The use of CAD package and target simulator reduced cost and time associated with the radar development. In addition, we are also able to obtain an accurate system model to deconvolve the system effects from the received signal. We discuss detailed design, construction and performance of the target simulator and operational radar. We show a comparison of the simulation results and laboratory measurements of the radar system. Also we present analysis of the results from measurements made during the 2001 experiments and preliminary results from planned measurements in May 2002.