In 1991, NASA initiated an airborne remote sensing program in conjunction with coordinated surface measurements for determining the mass balance of the Greenland ice sheet, which plays in important role in the rise of global sea level. Starting in 1995, NASA combined various efforts on the mass-balance studies into a coordinated effort called Program in Arctic Regional Climate Assessment (PARCA). The University of Kansas has been participating in this program to make airborne ice thickness measurements using coherent radar depth sounders. Since 1993, the authors have collected a large volume of ice-thickness data over the ice sheet. They have demonstrated that coherent radars can acquire ice thickness and internal structure data over the thickest part of the ice sheet and outlet glaciers located around the ice margin.
Radar
Fine depth resolution profiling of the top 100 m of the ice column is achieved with this radar designed to map variations in the snow accumulation rate. When operated from aircraft, it operates from 600 to 900 MHz providing 28-cm depth resolution in ice and when operated on the ground (500 MHz to 2 GHz) a 5.6-cm depth resolution in ice is achieved. This fine depth resolution enables area extensive spatial mapping of the annual accumulation layers.
The Airborne Synthetic Aperture Radar (AIRSAR) was an all-weather imaging tool able to penetrate through clouds and collect data at night. The longer wavelengths could also penetrate into the forest canopy and in extremely dry areas, through thin sand cover and dry snow pack. AIRSAR was designed and built by the Jet Propulsion Laboratory (JPL) which also manages the AIRSAR project. AIRSAR served as a NASA radar technology testbed for demonstrating new radar technology and acquiring data for the development of radar processing techniques and applications. As part of NASA’s Earth Science Enterprise, AIRSAR first flew in 1988, and flew its last mission in 2004.
The utility of millimeter-wave radars have been successfully used for cloud sensing and cloud microphysical studies. Studies of the impact of cloud feedbacks on the earth's radiation budget have underscored the importance of having a means of measuring the vertical distribution of clouds. Millimeter-wave radars can provide this information under most conditions, with high resolution, using a relatively compact system.
The Airborne Cloud Radar (ACR) for profiling cloud vertical structure was developed by the Jet Propulsion Laboratory and the University of Massachusetts in 1996. It is a W-band (95 GHz) polarimetric Doppler radar designed as a prototype airborne facility for the development of the 94 GHz Cloud Profiling Radar (CPR) for NASA CloudSat mission.
The ACR is a third-generation millimeter-wave cloud radar. While adopting the well tested techniques used by its predecessors, ACR also has a number of new features including an internal calibration loop, frequency agility, digital I and Q demodulation, digital matched filtering, and a W-band low-noise amplifier.