Abstract. We developed two 150-MHz coherent radar depth sounders for ice thickness measurements over the Greenland ice sheet. We developed one of these using connectorized components and the other using radio frequency integrated circuits (RFICs). Both systems are designed to use pulse compression techniques and coherent integration to obtain the high sensitivity required to measure the thickness of more than 4 km of cold ice. We used these systems to collect radar data over the interior and margins of the ice sheet and several outlet glaciers. We operated both radar systems on the NASA P-3B aircraft equipped with GPS receivers. Radar data are tagged with GPS-derived location information and are collected in conjunction with laser altimeter measurements. We have reduced all data collected since 1993 and derived ice thickness along all flight lines flown in support of Program for Regional Climate Assessment (PARCA) investigations and the North Greenland Ice Core Project. Radar echograms and derived ice thickness data are placed on a server at the University of Kansas (http://tornado. rsl.ukans.edu/Greenlanddata.htm) for easy access by the scientific community. We obtained good ice thickness information with an accuracy of _+ 10 m over 90% of the flight lines flown as a part of the PARCA initiative. In this paper we provide a brief description of the system along with samples of data over the interior, along the 2000-m contour line in the south and from a few selected outlet glaciers. IntroductionIn 1991, NASA started a polar research initiative aimed at determining the mass balance of the Greenland ice sheet. This program consisted of coordinated surface, airborne, and spaceborne measurements for determining the mass balance of the ice sheet. The initial airborne program consisted of a laser altimeter and a Ku-band radar altimeter for measuring surface elevation of the ice sheet along selected flight lines. In 1993 the airborne instrumentation suite was expanded to include a radar depth sounder to collect ice thickness data along the same flight lines. Ice thickness is a key variable in the timedependent equation of continuity and is essential to any study of ice sheet dynamics.Raju et al.[1990] developed a coherent radar sounder for measurements in the Antarctic. We used this system to collect ice thickness data during the 1993 field season. Although the system collected good quality data in certain areas in the north and central parts of the ice sheet, its performance was less than optimum for obtaining ice thickness data over a few parts of the ice sheet in southern Greenland. These are in temperate areas of the ice sheet with thick, warm ice. To overcome its limitations and improve its performance, we developed two new systems: one using connectorized components and the other using radio frequency integrated circuits (RFICs). The transmitter and receiver prototypes for the system using RFICs were developed by senior undergraduate students as part of a capstone design project. We used these prototypes to devel...

In 1991, NASA initiated a research program for testing airborne laser and radar altimeters for measuring surface elevation of the Greenland ice sheet in conjunction with a coordinated set of surface measurements for validating and interpreting satellite data sets. In 1993, the airborne program was expanded to include the University of Kansas airborne radio echo sounder for acquiring ice thickness along with laser elevation measurements. We have collected a large volume of data with our radio echo sounder and supplied these data to the glaciological community worldwide, and we improved the digital system for collecting coherent data for further processing. We developed a SAR processing algorithm based on the f-k migration technique and used this algorithm to process a subset of data collected over the 2000-m contour line and several outlet glaciers. The results show a 4-10 dB improvement in signal-to-noise ratio and improved along-track resolution. In this paper we will provide a brief description of the algorithm and results of the processing over selected areas INTRODUCTION

ABSTRACT. Extensive aircraft-based radar ice-thickness measurements over the interior and outlet-glacier regions of the Greenland ice sheet have been obtained by the University of Kansas since 1993, with the latest airborne surveys conducted in May 2001. The radar has evolved during this period to a highly versatile system capable of characterizing ice thickness over a wide variety of ice-sheet conditions. Before 1997, the digital system was limited, only capable of storing incoherent data or coherent data with a very large number of presumed signals at a low pulse-repetition frequency. In 1998, the radar was upgraded with modern components allowing coherent data to be stored with a small number of presumed returns for 1024 range cells at a high pulse-repetition frequency. The new data on ice thickness of Greenland outlet glaciers are archived and made available to the scientific community in the form of radar echograms and derived ice thickness at http://tornado.rsl.ukans.edu/Greenlanddata.htm. The U.S. National Snow and Ice Data Center (NSIDC) also provides a link to these data, and NSIDC will eventually serve as the permanent archive of these data. Improvements in radar sensitivity in outlet-glacier regions have been achieved by collecting coherent radar data and applying various signal-processing techniques. Deep outlet-glacier channels that were previously unresolved with incoherent data can now be mapped using a coherent signal, signal conditioning and synthetic aperture radar (SAR) processing.