An improved coherent radar depth sounder

Self Cite

156

129

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...

Section: System Descriptionmentioning

confidence: 99%

Coherent radar ice thickness measurements over the Greenland ice sheet

Gogineni¹,

Tammana²,

Braaten³

et al. 2001

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156

129

“…The instrument used to acquire the data was a pulsecompression coherent radar mounted on a NASA P-3 aircraft and is described in detail by Gogineni et al [1998]. With a center frequency of 150 MHz and a 17 MHz bandwidth, the radar is capable of full penetration to the bedrock over most of the ice sheet area.…”

Section: Data Setmentioning

confidence: 99%

Internal layer tracing and age‐depth‐accumulation relationships for the northern Greenland ice sheet

Fahnestock¹,

Abdalati²,

Luo³

et al. 2001

Self Cite

Abstract. Clues to previous ice sheet structure and long-term glaciological processes are preserved in the internal layering configuration of the Greenland ice sheet. Information about these internal layers has been retrieved over many parts of the ice sheet with the University of Kansas ice-penetrating radar. We report on the coherence of these layers over very large distances, describe a method of tracing these layers along thousands of kilometers of flight line, and do so for one flight during the 1999 Program for Arctic Regional Climate Assessment (PARCA) aircraft campaign. We determine the ages of these layers, based on information at the GRIP ice core site, and extend these ages along the flight line to Camp Century, where they are compared to modeled-derived age estimates. These ages agree with each other to between 2 and 15%, differences that can be substantially reduced with minor changes to the model parameters (accumulation rate and shear layer depth). Finally, we are able to derive estimates of accumulation rates along the flight line by fitting the age-depth data from layer tracing to a Dansgaard-Johnsen model with a minimization technique, providing estimates that match recent accumulation patterns within a few centimeters per year.

“…Because of the maneuverability of aircraft platforms, such airborne surveys can be made along and across many glaciers at various time intervals and spatial separations. Moreover, critically important information on ice thickness can be acquired at the same time by a low-frequency ice-penetrating radar that is flown simultaneously on the same platform [Gogineni et al, 1998]. …”

Section: Introductionmentioning

confidence: 99%

Outlet glacier and margin elevation changes: Near‐coastal thinning of the Greenland ice sheet

Abdalati¹,

Krabill²,

Frederick³

et al. 2001

116

124

Abstract. Repeat surveys by aircraft laser altimeter in 1993/1994 and 1998/1999 have revealed significant thinning along many parts of the Greenland ice sheet at elevations below about 2000 m. In this paper we examine elevation changes from 29 repeat aircraft surveys over the lower portions of some of the larger outlet glaciers and parts of the ice sheet margin. Here thinning rates in excess of 1 m/yr are common in the lower sections of the flight lines, but in some cases, this rate is measured at elevations as high as 1500 m. Warmer summers along parts of the coast may have caused a few tens of cm/yr additional melting, but the magnitudes and character of the elevation changes suggest that in many cases they are more likely a result of glacier dynamics and creep thinning. The most extreme thinning was observed near the terminus of the Kangerdlugssuaq Glacier in southeastern Greenland where rates as high as 10 m/yr were measured. There are a few areas of significant thickening (over 1 m/yr), which is probably related to higher than normal accumulation rates during the observation period; but one location L, Bistrup Brae, had local regions of thickening of 8 to 9 m/yr. Three glaciers in the northeast show patterns of thickness change that may suggest surging behavior, and one has been independently documented as a surging glacier. Overall, the lowest reaches of the outlet glaciers and ice sheet edges appear to be changing significantly, with thinning observed more frequently than thickening. IntroductionOutlet glaciers are particularly important to the mass balance of the Greenland ice sheet because they are the means by which the ice is discharged into the surrounding seas. Such discharge comprises nearly half of the ice sheet mass loss [Weidick, 1985]. An understanding of the balance of these glaciers and the mechanisms that affect this balance is essential to understanding the present and future states of the ice sheet. The factors affecting outlet glacier mass balance are complex, because this balance is not simply a result of accumulation and surface ablation, but rather it is largely influenced by flow characteristics, which are often nonlinear.Until recently, large-scale assessment of glacier and ice sheet mass balance was not possible by conventional means. While in situ measurements can provide detailed information about specific locations, large-scale coverage is often not feasible because of the costs, difficulties, and dangers associated with work on these remote glaciers. Satellite radar altimetry facilitated study of the higher portions of the Greenland ice sheet through elevation-change measurements from which mass balance of the surveyed areas can be estimated. This technique, however, is limited to shallow-sloped and relatively smooth areas and provides no useful information about steeper areas