P. Kanagaratnam

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.

High‐resolution radar mapping of internal layers at the North Greenland Ice Core Project

Existing accumulation maps with reported errors of about 20% are determined from sparsely distributed ice cores and pits. A more accurate accumulation rate might be obtained by generating continuous profiles of dated layers from high-resolution radar mapping of near-surface internal layers in the ice sheet (isochrones). To generate such profiles we designed and developed an ultrawideband radar for high-resolution mapping of internal layers in the top 200 m of ice and tested it at the North Greenland Ice Core Project drill site. Reflection profiles of 2- and 10-km length reveal horizons that we correlate with electrical conductivity measurement (ECM) recordings. Our results show that the radar-determined depth of internal layers is within ±2 m of that in an ice core collected at a nearby location. Preliminary frequency analyses of layer reflections reveal that the reflections are strongest at the 500–1000 MHz frequency range. Long-term accumulation rate computed from radar data is within 5% of that obtained from snow pits.

A wideband radar for high-resolution mapping of near-surface internal layers in glacial ice

Snow accumulation rate is an important parameter in determining the mass balance of polar ice sheets. Accumulation rate is currently determined by analyzing ice cores and snow pits. Inadequate sampling of the spatial variations in the ice sheet accumulation has resulted in accumulation rate uncertainties as large as 24%. We designed and developed a 600-900-MHz airborne radar system for high-resolution mapping of the near-surface internal layers for estimating the accumulation rate of polar ice sheets. Our radar system can provide improved spatial and temporal coverage by mapping a continuous profile of the isochronous layers in the ice sheet. During the 2002 field season in Greenland, we successfully mapped the near-surface layers to a depth of 200 m in the dry-snow zone, 120 m in the percolation zone, and 20 m in the melt zone. We determined the water equivalent accumulation rate at the NASA-U/spl I.bar/1 site to be 34.9/spl plusmn/5.1 cm/year from 1964 to 1992. This is in close agreement with the ice-core derived accumulation rate of 34.6 cm/year for the same period.

Ultrawideband Radar Measurements of Thickness of Snow Over Sea Ice

An accurate knowledge of snow thickness and its variability over sea ice is crucial in determining the overall polar heat and freshwater budget, which influences the global climate. Recently, algorithms have been developed to extract snow thicknesses from satellite passive microwave data. However, validation of these data over the large footprint of the passive microwave sensor has been a challenge. The only method used thus far has been with meter sticks during ship cruises. To address this problem, we developed an ultrawideband frequency-modulated continuous-wave radar to measure the snow thickness over sea ice. We synthesized a very linear chirp signal by using a phase-locked loop with a digitally generated chirp signal as a reference to obtain a fine-range resolution. The radar operates over the frequency range from 2-8 GHz. We made snow-thickness measurements over the Antarctic sea ice by operating the radar from a sled in September and October 2003. We performed radar measurements over 11 stations with varying snow thicknesses between 4 and 85 cm. We observed an excellent agreement between radar estimates of snow thickness with physical measurements, achieving a correlation coefficient of 0.95 and a vertical resolution of about 3 cm. Comparison of simulated radar waveforms using a simple transmission line model with the measurements confirms our expectations that echoes from snow-covered sea ice are dominated by reflections from air-snow and snow-ice interfaces.

An ultra-wideband radar for measurements of snow thickness over sea ice

An ultra-wideband, frequency modulated, continuous wave radar working from 2.0 to 6.5 GHz was designed, built and tested at the Center for Remote Sensing of Ice Sheets (CReSIS) at the University of Kansas to measure snow thickness over sea ice. Improvements and modifications to the existing radar, compared to previous versions, allow for snow thickness measurements from fast-moving, long-range aircraft. Over the past year, the radar has recorded snow thickness measurements over sea ice in the Arctic and Antarctic oceans as part of NASAs Operation Ice Bridge.

Search for proxy indicators of young sea ice thickness

The determination of young sea ice thickness from space remains an elusive goal for those interested in the interaction of the oceans and the atmosphere, the thermal and chemical state of the ocean, and sea ice dynamics. Recent experiments and models have shown relationships between active and passive microwave signatures of new, growing ice and ice thickness. The two processes that dominate in determining the microwave signature are changes in dielectric properties and changes in surface roughness. In this paper we investigate the competition between these two processes in determining radar backscatter, the usefulness of surface roughness as an indicator of young ice thickness, and the optimum sensor parameters for observing changes in scattering linked to ice thickness. We present simulations that are based on radar observations made on laboratory-grown saline ice. These observations confirm that surface scattering dominates over volume scattering for 13.9 GHz radar backscatter from young, rough ice at most angles and for young, smooth ice below 30°. Although rms roughness and backscatter (at 5.3 and 13.9 GHz, 23° incidence, and VV polarization) increase together after about 10 cm of ice growth under quiet conditions, it is unlikely that surface roughness and ice thickness are simply connected in real sea ice, where surface roughness can change rapidly due to the action of wind, waves, and snow. Simulations show, however, that formation of frost flowers is detectable by spaceborne radar and can serve to classify ice of roughly 5–20 cm thickness since it is a distinct, transient event that occurs under physical conditions that constrain the thickness of the ice. Our experimental data show that future sensors operating near 12° incidence may offer potential for probing the relationship between near-surface dielectric properties and ice thickness, since the effects of variability in roughness and snowfall are minimized near this angle.

High-resolution radar mapping of internal layers at NGRIP

A major goal of NASAs Office of Earth Science Polar Program is to determine the mass balance of the Greenland and Antarctic ice sheets. A key variable in assessing the mass balance of an ice sheet is accumulation rate. Currently, accumulation rate is determined from ice cores and pits. There are large uncertainties in existing accumulation rate maps derived from sparely distributed ice cores and pits. There is an urgent need for developing remote sensing techniques for determining the accumulation rate. A prototype Frequency Modulated Continuous Wave (FMCW) radar system has been developed for mapping internal layers from known volcanic events in the ice. The prototype system has been designed and developed using the latest RF technologies. The system was operated from 100 to 2000 MHz, for imaging the top 200 meters of ice with high resolution. We tested this system during the 1998 and 1999 surface experiments at the North GReenland Ice core Project (NGRIP) ice camp. Our results show that internal layers were successfully mapped with high resolution down to 200 m.

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.

An airborne radar system for high-resolution mapping of internal layers

Accumulation rate is a key variable in assessing the mass balance of polar ice sheets. An improved knowledge of the mass balance of polar ice sheets is needed to determine their role in current and future sea level rise. Existing accumulation maps, derived from sparsely distributed ice cores and pits, contain accumulation rate errors as large as 20% in certain areas. Remote sensing methods to complement and supplement in situ measurements are required to generate improved accumulation maps. For this reason we have been investigating the use of high-resolution radars for mapping of near-surface internal layers and generating continuous profiles of the dated layers in the ice sheet (isochrones). We successfully mapped isochrones within 2 m of those in an ice core up to a depth of 280 m at the North GReenland Ice core Project (NGRIP) ice camp during the 1998 and 1999 field seasons using a 170-2,000 MHz frequency modulated continuous wave (FM-CW) radar. We described the system and reported results of our experiments in previous IGARSS meetings [Kanagaratnam et al. 1999, 2000]. Recently we performed detailed analysis of the data collected from these experiments and determined the frequency response of reflections from the internal layers. The results showed the optimum frequency range for monitoring near-surface layers is between 500 and 1000 MHz. Based on these results we are developing a 600-900 MHz coherent airborne radar for high-resolution mapping of the internal layers in the Greenland ice sheet. We designed this system to operate in different modes: simple pulse, chirped pulse, step-frequency, and FM-CW. We have also developed a digital data acquisition system to collect and process data. We designed this system such that it can operate in an undersampling mode to digitize the received signal directly without down conversion. In this paper we present results of data analysis and detailed design of the airborne radar.

Target simulator to calibrate wideband radar in measuring the internal layers of the Greenland ice sheet

We developed a hardware target simulator for measuring the system response and testing of an airborne wideband radar that operates over the frequency range 600-900 MHz to map the near-surface internal layers in glacial ice. It uses optical and microwave delay lines for evaluating and optimizing the performance of the wideband radar in the continental United States without expensive field trips to polar regions and can be used to test the radar without interfering with commercial wireless devices and television stations. The target simulator replicates the feed-through signal between the transmit and receive antenna, as well as reflections from the air-firn interface and the internal layers of the ice sheet, which are spaced about 50 cm apart. This is because the measured radar resolution is about 60 cm in free space and can map layers with about 50-cm resolution in firn. We simulated the internal layers by incorporating a feedback loop with a short delay line. The target simulator is being used for testing and evaluation of the radar in the laboratory and for in-flight testing of the radar.