Unfocused SAR using a Next-Generation COoherent Radar Depth Sounder for measurement of Greenland ice sheet thickness

The authors have been participating in the Program in Arctic Regional Climate Assessment (PARCA) by performing airborne ice-thickness measurements using coherent radar depth sounders. The authors have two complete coherent radar systems, which operate as unfocused SARs. The Improved COherent Radar Depth Sounder (ICORDS) is the prototype system, which the authors have used annually to collect ice-thickness data successfully since 1993. The Next-Generation COherent Radar Depth Sounder (NG-CORDS) is a refined, compact version of ICORDS. NG-CORDS uses radio frequency integrated circuits and microwave monolithic integrated circuits. The complete RF module is easily contained within an 8"/spl times/10.17"/spl times/4" enclosure. In 1997, the authors successfully collected ice-thickness data using the fully operational NG-CORDS. The radar systems operate at the center frequency of 150 MHz with peak transmit power of about 200 W. They use complementary SAW devices for pulse expansion and compression. The I and Q signals are digitized by two 12-bit A/D converters sampling at 18.75 MHz. These sampled data are coherently and/or incoherently integrated. These, plus the display of the post-integration results in A-scope form and 64-level gray-scale radio echogram, are carried out in real time. GPS information and the digitized backscattered data are stored on the hard disk of the digital section for subsequent processing. The unfocused SAR images collected in a portion of southwestern Greenland where the ice thickness is around 2000 m show very weak bottom returns and require extra processing.