T. L. Akins

Coherent radar ice thickness measurements over the Greenland ice sheet

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.

Substantial thinning of a major east Greenland outlet glacier

Aircraft laser-altimeter surveys in 1993 and 1998 over Kangerdlugssuaq Glacier in east Greenland reveal thinning, over the 5-year interim, of several meters for all surveyed areas within 70 km of the seaward ice front, rising to 50 meters in the final 5 km. Such rapid thinning is best explained by increased discharge velocities and associated creep thinning, most probably caused by enhanced lubrication of the glacier bed. The calving ice front over the past decade has occupied approximately the same location as in 1966. Velocity estimates for 1995/96 are about the same as those for 1966 and 1988, but significantly less than for 1999, suggesting that major thinning began after 1995.

Ice-sheet bed 3-D tomography

Information on bed topography and basal conditions is essential to developing the next- generation ice-sheet models needed to generate a more accurate estimate of ice-sheet contribution to sea-level rise. Synthetic aperture radar (SAR) images of the ice-bed can be analyzed to obtain information on bed topography and basal conditions. We developed a wideband SAR, which was used during July 2005 to perform measurements over a series of tracks between the GISP2 and GRIP cores near Summit Camp, Greenland. The wideband SAR included an eight-element receive-antenna array with multiple-phase centers. We applied the MUltiple SIgnal Classification (MUSIC) algorithm, which estimates direction of arrival signals, to single-pass multichannel data collected as part of this experiment to obtain fine-resolution bed topography. This information is used for producing fine- resolution estimates of bed topography over a large swath of 1600 m, with a 25 m posting and a relative accuracy of approximately 10 m. The algorithm-derived estimate of ice thickness is within 10 m of the GRIP ice-core length. Data collected on two parallel tracks separated by 500 m and a perpendicular track are compared and found to have difference standard deviations of 9.1 and 10.3 m for the parallel and perpendicular tracks, respectively.

Focused synthetic aperture radar processing of ice-sounder data collected over the Greenland ice sheet

The authors developed a synthetic aperture radar (SAR) processing algorithm for airborne/spaceborne ice-sounding radar systems and applied it to data collected in Greenland. By using focused SAR (phase-corrected coherent averaging), they improved along-track resolution by a factor of four and provided a 6-dB processing gain over unfocused SAR (coherent averaging without phase correction) based on a point-target analysis for a Greenland ice-sounding data set. Also, They demonstrated that the focused-SAR processing reduced clutter and enabled them to identify bedrock-interface returns buried in clutter. Using focused-SAR technique, they processed data collected over a key 360-km-long portion of the 2000-m contour line of southwest Greenland. To the best of their knowledge, these are the first high-quality radar ice thickness measurements over this key location. Moreover, these ice-thickness measurements have been used for improving mass-balance estimates of the Greenland ice sheet.

Suppressing coherent noise in radar applications with long dwell times

A method for suppressing internally produced coherent noise in radar applications is presented and experimentally demonstrated. The technique enhances conventional coherent averaging and involves interpulse phase modulation that is introduced digitally in the transmit waveform and removed digitally following digitization in the receiver. Experimental demonstration of this concept resulted in an additional noise floor reduction of 15 to 20 dB (compared to the conventional coherent averaging alone) when the number of coherent averages was between 1000 to 100 000, beyond which no significant suppression was observed.

Mobile Robots for Harsh Environments: Lessons Learned from Field Experiments

Mobile robots for harsh environments provide useful means for automating the collection of research data in the field by reducing human involvement. MARVIN II has been designed and constructed to autonomously collect radar measurements to determine properties of the polar ice sheets. This paper discusses the lessons learned from a number of field experiments with its predecessor MARVIN, and how these lessons influenced the new design of MARVIN II.

A Wideband Radar for Mapping Near-Surface Layers in Snow

We developed a wideband radar to map near- surface internal layers in firn with fine resolution of about 3 cm to a depth of about 10 m. It is a frequency-modulated continuous-wave (FM-CW) radar that operates over the frequency range of 12-18 GHz with an antenna operated in the near field to obtain plane-wave illumination. The plane-wave illumination reduces off-vertical scattered signals from masking reflections caused by internal layers. To operate the radar on the snow surface, we designed and built a sled that includes a gimbaled mount and control system to ensure that the antenna points at nadir. In addition, the antenna is offset from the sled so it points at undisturbed snow. The radar features a fast transmit waveform synthesizer implemented using a voltage-controlled oscillator (VCO) and a phase-locked loop (PLL) with a linear digital chirp as a reference. The highly linear reference chirp is compared against the instantaneous VCO output to generate a highly linear 12 to 18 GHz transmit signal. The waveform synthesizer can be swept from 12 to 18 GHz in 1 millisecond. We tested the radar at both Summit, Greenland, and a field camp in West Antarctica in July 2005 and January 2006, respectively. We collected a large amount of data at both sites, and we have been able to follow internal layers over distances exceeding 10 km. We verified radar data by comparing radar echoes to visible wind crust and depth hoar layers observed in 2-m deep snow pits. We also measured snow and firn density with a resolution of 5 cm to determine the dielectric constant for estimating propagation velocity of the wave in snow and firn. We collected more than 200 sample traces at each pit location for comparison with visual observations. Each sample trace uses 10 sweeps that are coherently integrated to improve signal-to-noise ratio (SNR). We made measurements in stationary mode and by dragging the sled behind a snowmobile driven at a speed of about 2.5 km/hr. Results show an excellent agreement between the snow pit stratigraphy and echoes from our plane-wave radar.

Ice thickness measurements of the southwest Greenland 2000-m contour line

During 1998, the University of Kansas Radar Systems and Remote Sensing Laboratory performed ice thickness measurements along several flights in the southern part of Greenland. The authors used an improved coherent radar depth sounder operating at 150 MHz to collect these data. They obtained ice thickness data along flight lines for which no reliable thickness information was available. They present results from these experiments along a 2000-m contour line in the southwestern part of the Greenland.

High-resolution monitoring of internal layers at NGRIP

A key variable in assessing the mass balance of an ice sheet is accumulation rate. Currently, accumulation rate is determined from sparsely distributed ice cores and pits. There are uncertainties in existing accumulation rates derived from these cores and pits. The authors developed an ultra wideband frequency modulated continuous wave (FMCW) radar for mapping internal layers, from known volcanic events, in the ice for estimating accumulation rate from high-resolution radar data. The authors tested the radar system during the 1998 surface experiment at the North Greenland Ice core Project (NGRIP) ice camp. Their results show that internal layers were mapped with high resolution down to 200 m. In this paper, they present the results of the 1998 NGRIP surface experiment.

Remote sensing of sea ice thickness by a combined spatial and frequency domain interferometer : formulations, instrument design & development

The thickness of Arctic sea ice plays a critical role in Earths climate and ocean circulation. An accurate measurement of this parameter on synoptic scales at regular intervals would enable characterization of this important component for the understanding of ocean circulation and the global heat balance. Presented in this paper is a low frequency VHF interferometer technique and associated radar instrument design to measure sea ice thickness based on the use of backscatter correlation functions. The sea ice medium is represented as a multi-layered medium consisting of snow, sea-ice and sea water, with the interfaces between layers characterized as rough surfaces. This technique utilizes the correlation of two radar waves of different frequencies and incident and observation angles, scattered from the sea ice medium. The correlation functions relate information about the sea ice thickness. Inversion techniques such as the genetic algorithm, gradient descent, and least square methods, are used to derive sea ice thickness from the phase information related by the correlation functions. The radar instrument is designed to be implemented on a spacecraft and the initial test-bed will be on a Twin Otter aircraft. Radar system and instrument design and development parameters as well as some measurement requirements are reviewed. The ability to obtain reliable phase information for successful ice thickness retrieval for various thickness and surface interface geometries is examined.