We developed a high-speed, compact, and lightweight data acquisition system (DAQ) for remote sensing applications using small Unmanned Aircraft Systems (sUAS). The DAQ is developed on the AMD Xilinx Radio-Frequency System on Chip (RF-SoC) platform. It can be used to directly digitize signals up to 2.212GHz. We used it to develop an ultra-wideband (UWB) monostatic pulse radar operating over the 0.7-2.2GHz frequency range with a pulse duration of 1µs and theoretical range resolution of 10cm. The monostatic radar system prototype weighs around 2.2kg and it was deployed on xFold Dragon-x6 multirotor sUAS to collect data in the field at a 60m altitude above the ground. This paper will provide an overview of the data acquisition subsystem design, its integration into the UWB radar, and results obtained over land in the University of Alabama Arboretum. The outcomes suggest that the developed UWB radar is capable of achieving detailed resolution in mapping vegetation and soil moisture with low transmitting power of −5dBm.

We developed a high-sensitivity airborne multi-channel ultra-wideband (UWB) frequency-modulated continuous-wave (FMCW) radar for snow depth measurements. This low-transmit power eight-channel radar has a near-ideal point target response and provides a multi-look ability. We developed a T-shape Mills-Cross antenna array to obtain a small, overlapped footprint. We integrated it with a 2DOF gimbal mechanism to correct pointing errors in the roll and pitch movements of the aircraft. We performed airborne UWB radar measurements in conjunction with in-situ characterization and dielectric measurements with a monopole dielectric probe over snow in Grand Mesa, CO, USA, from 24thJanuary to 14thFebruary 2023. The radar mapped top and bottom snow interfaces and internal density changes of 1.2-2.6 m of snow with high signal-to-noise of about ~45 dB with only 10 mW transmit power. The multi-channel system with gimbaled antenna mount allowed us to capture quasi-specular returns even with aircraft roll deviations as large as 15 deg. The comparison between radar data and in-situ measurements shows excellent agreement between the two in terms of snow depth and internal snow layers.

We developed a very high sensitivity ultra-wideband radar operating at 200 MHz and optimized signal processor to map internal layers close to the bed. We used the radar to perform measurements with a large antenna array to collect data during the 2019–2020 field season in Antarctica. We processed and analyzed these data to generate detailed maps of internal layers to assist with selecting a drill site. The paper provides a brief summary of the radar system and field program. We also provide an overview of the algorithm, results, and their interpretation.

We developed and deployed a high sensitivity and low transmit power airborne UWB FMCW radar for snow depth measurements. The radar has a near-ideal point target response so that we can produce near-real-time snow thickness maps after each survey flight. The improved performance is achieved by carefully designing the radar hardware to reduce internal reflections between various components, third-order products generated by mixers, higher-order harmonics generated in multipliers and non-linear devices, and amplitude and phase errors in transmitted chirp signals. In addition, we performed extensive linear and non-linear system simulations to predict degradations in the radar hardware in advance and applied the remedies to correct them. These improvements allowed for near-real-time data products to be generated by reducing the need for advanced signal processing techniques. We also developed a T-shape Mills-Cross antenna array to obtain a small overlapped footprint of transmit and receive antennas. We performed measurements over snow in Grand Mesa, Colorado, from March to April 2022, and the radar mapped the top and bottom interfaces and density changes of 1.2-2.1m of snow. We generated a snow thickness map from the data collected over the grid flown and compared results with in-situ measurements. The comparison between radar estimates and in-situ measurements shows that the average snow depths obtained from the radar data are within a standard deviation from the mean of in-situ measurements.

We describe a CubeSat constellation integrated with ultrawideband (UWB) multiple input and multiple output (MIMO) radars operating over the frequency range of 50–600 MHz for remote sensing of large ice sheets on the Earth, as well as distant planets and moons. It can be used for measurements of ice thickness and basal conditions of Antarctica and Greenland ice sheets, the mapping of lunar lava tubes, and sub-surface detection/characterization of volatiles on Mars. In this paper we provide a short discussion of the science requirements and conceptual design of a constellation of CubeSats with UWB MIMO radars for sounding ice and mapping of lava tubes.

Ice sounding and mapping internal layers with radars over large areas is an effective way to obtain data to understand and model ice dynamics. To map the bottom most layers and determine the ice-bed topography of more than 3-km Greenland ice with fine resolution, we developed a 180 - 340 MHz ultrawide band (UWB) dual-polarized ice sounding radar. The radar was deployed as a part of the East Greenland Ice-core Project (EGRIP) in summer 2019. It is configured as a 12-channel multiple-input-multiple-output (MIMO) system. We designed and built a dual-polarized tightly coupled antenna-array. This paper presents the radar design, antenna performance and sample results from the data collected in the field.

Polar regions are among the least known and most challenging areas of Earth in terms of their topography, water mass and transport, and accessibility. This paper shows a solution for advanced SAR capabilities on a unique light sport aircraft (LSA) platform suitable for widespread radar measurements of Polar regions. The LSA is integrated with an 8 channel 170–470 MHz ultra-wideband (UWB) radar capable of sounding Polar ice sheets and ice layers down to the bed. We designed, constructed, and tested the sub-systems, including a test of the RF electronics with a surface vehicle on the Greenland ice sheet.

An ultrawide-band radar system was developed to measure snow thickness from an airborne platform. It is a second-generation device, improving upon a system that was deployed to Antarctica in 2018-2019. The radar operates from 2.7 -10.7 GHz and from 10-18 GHz for a theoretical resolution of less than 2 cm, It was deployed to the Grand Mesa National Forest in Colorado on a Twin Otter DHC-6 aircraft in March of 2019. The system successfully mapped the air-snow and snow-ground interfaces as well as some internal layering over both vegetative and non-vegetative areas. In this paper, we discuss the design, development and deployment of this radar and show some preliminary results.

A surface-based L-band radar sounding system was developed to measure ice sheet thickness, basal conditions, and ice-shelf melt rate. The radar system operates in a pulse-chirped mode with a peak transmit power of 4 kW through eight channels of 500 W each. The system is equipped with an array of high gain Yagi antennas to achieve the required sensitivity. We have deployed the radar to the East Greenland Ice-Core Project (EGRIP) site in Greenland for field tests during the 2019 Summer field season. We have successfully demonstrated, for the first time, radar sounding of kilometers-thick polar ice sheet at L-band. In this paper, we will present the radar system design, field operation, as well as field results.

In this letter, we report on the design, development, and field operation of a surface-based multi-channel ultrawideband (UWB) ultrahigh frequency (UHF) radar to measure ice thickness, basal conditions, and ice-shelf bottom melt rates. The radar concept is based on the recent success in sounding shallow low-loss ice (~1 km) and measuring the ice-shelf melt rates with a 600-900-MHz low-power radar, referred to as the accumulation radar. Our proposed radar system operates over the same frequency band, from 600 to 900 MHz, with a peak transmit power of 800 W. We used a large and lightweight 16 m × 17 m antenna array arranged in a Mills cross-configuration to obtain the required radar sensitivity to sound more than 3-kmthick ice and image the internal layers at a fine vertical resolution of about 60 cm. We used the system at the East Greenland Ice-coring Project (EGRIP) site in Summer 2018 to collect data over ~100 km of lines. We sounded about 2.8-km-thick ice with more than 40-dB signal-to-noise ratio and mapped the internal layers to a depth of almost 2.5 km. Our results show that an airborne or spaceborne radar operating at frequencies as high as 900 MHz with a large antenna array can be used to map large ice sheets in Greenland and Antarctica.

We present a compact, multi-channel, wideband VHF radar system for fine-resolution measurements of the base and interior of large ice sheets. Data from this radar will be used in the identification of potential drill locations to retrieve ice core samples more than 1 million years old in East Antarctica. The radar is a lightweight instrument equipped with four >1-kW peak power transmit channels, eight independent digital receivers, and an array of high-gain antennas. We developed the system as part of a collaborative effort between the United States of America, Japan and Norway. The instrument was used for surface-based surveys near Dome-Fuji onboard a tracked vehicle during the 2018/2019 Austral Summer, covering 2,000 line-km. This paper presents an overview of the radar system, accompanied by laboratory and field test results that demonstrate the system's ability to map the internal ice sheet structure and basal conditions with outstanding detail.

An ultra-wideband frequency-modulated-continuous-wave radar was designed and developed at the Remote Sensing Center at the University of Alabama. The radar was deployed to Dome Fuji in East Antarctica for the measurement of near-surface snow accumulation as a part of the ice core drilling site selection effort by the National Institute of Polar Research (Japan) and the Norwegian Polar Institute. The oldest ice is expected to be located in low-accumulation areas of the East Antarctica. The UWB radar is to map near-surface internal layers for estimating snow accumulation over areas surveyed to find an optimum site. The radar operates from 2 to 8 GHz with theoretical range resolution of 2.5 cm. In this paper, the design and development of this radar, as well as some preliminary results, are presented.

Two surface-based multi-channel radar systems, operating in the VHF and UHF bands, were designed, developed and deployed to the East Greenland Ice-coring Project (EGRIP) site in Summer 2018 for ice sheet sounding and imaging. Both radar systems, sharing the same digital waveform generator and digitizer, were installed and operated inside a tracked vehicle. The VHF system operates over the frequency range of 170-230 MHz with a peak transmit power of 600 W. The radar is equipped with an 8-m long monopole array. The UHF system operates from 600 to 900 MHz with a peak transmit power of 800 W, and is equipped with a large and lightweight 16 m × 17 m antenna array arranged in a Mill's Cross configuration. In this paper, we will present the design of the two radar systems and the radar data collected from EGRIP.

In spite of more than 50 years of airborne radar soundings of Antarctic ice by the international community, there are still large gaps in ice thickness data. We propose a CubeSat satellite mission for complete sounding and imaging of Antarctica with 50 CubeSats integrated with a VHF radar system to sound the ice and image the ice-bed. One of the major challenges in orbital sounding of ice is off-vertical surface clutter that masks weak ice-bed echoes. We must obtain fine resolution both in the along track and cross track directions to reduce surface clutter. We can obtain fine resolution in the along track direction by synthesizing a large aperture by taking advantage of the forward motion of a satellite. However, we need a large antenna-array to obtain fine resolution in the cross track direction. We propose a train of 50 CubeSats with optimized offset position to obtain a 500-m long aperture and also coherently combine data from multiple passes of the train to obtain a very large aperture of 1–2 km in the cross track direction. Our initial analysis shows that we can obtain measurements with horizontal resolution of about 200 m and vertical resolution of about 20 m. The CubeSat will carry a transmitter and receiver with peak transmit power of about 50 W. We will synchronize all transmitters and receivers with a Ka-band system that serves as a communication link between the earth and Cubesats to downlink data and as command and control for the CubeSats.

A new L-band radar system is proposed to measure ice thickness, basal conditions and ice-shelves bottom melt rates. The concept for ice measurements with an L-band radar is based on the recent success in sounding shallow low-loss ice (<; 1 km) and measuring ice-self melt rates with a 600-900 MHz low-power radar, referred to as accumulation radar [1]. A surface-based radar operating over 1.2-1.4 GHz with a peak transmit power of 2 kW is proposed to sound and image more than 4 km thick ice. The higher frequency at 1.3 GHz will provide the sensitivity required to detect basal water film as thin as 0.5 mm as compared to radars operating now at frequencies less than 600 MHz. These radars need a minimum film thickness of 4 mm or more for reliable detection. The proposed L-band radar will also measure the bottom melt rate of ice shelves. The current plan is to deploy the proposed radar for field test in Antarctica during the 2018-2019 field season.

In this paper, we analyze the direction-of-arrival (DOA) of the ice-sheet data collected over Jakobshavn Glacier with the airborne Multichannel Radar Depth Sounder (MRDS) during the 2006 field season. We extracted weak ice-bed echoes buried in signals scattered by the rough surface of the fast-flowing Jakobshavn Glacier by analyzing the DOA of signals received with a five-element receive-antenna array. This allowed us to obtain ice thickness information, which is a key parameter when generating bed topography of glaciers. We also estimated ice-bed roughness and bed slope from the combined analysis of the DOA and radar waveforms. The bed slope is about 8° and the roughness in terms of rms slope is about 16°.

In the above paper[1], there is an error inTable I. The value “30” in the bottom row, fifth column should be “350.” The corrected table is provided here.

We present an ultrawideband frequency-modulated continuous-wave radar for airborne measurements of snow thickness. The radar operates over a frequency range of 2-18 GHz and is capable of about 1.4-cm range resolution at a nominal survey altitude of 500 m. The system was installed on a Twin Otter and used to collect data to demonstrate the capability of fine-resolution measurements of snow thickness over both sea ice and land near Barrow, AK. Data collected over a relatively smooth water surface, a lead, were used to deconvolve system effects to reduce range sidelobes and obtain close-to-ideal range resolution. Radar data collected over snow covered sea ice and land from the field campaign showed that we can map air-snow and snow-ice interfaces of thin and thick snow. The radar-derived snow thickness data are in a very good agreement with the in situ measured data with a correlation of 0.88.

In this paper, we present the development of a multi-channel VHF/UHF ultra-wideband airborne radar sounder and imager for measurements of polar ice sheets. The radar was developed at the Center for Remote Sensing of Ice Sheets (CReSIS) for operation onboard the German Alfred Wegener Institute (AWI) Basler BT-67 aircraft. The system operates from 150 to 600 MHz corresponding to a vertical resolution of 33 cm in free space. The radar is equipped with three 4-m long 8-element antenna subarrays installed under the fuselage and both wings to support 8 transmit and 24 receive channels. The radar waveform from each transmit channel can be configured individually to enable real-time transmit beamforming for wide-swath ice bed imaging of up to 10 km wide. The radar system was deployed to Greenland in the spring of 2016 as a part of the joint AWI/CReSIS test campaign to conduct measurements over glaciers. Sample radar data from this field campaign are presented to illustrate the capability of the radar.

In this paper, we present the development of an ultra-wideband microwave radar for fine-resolution measurements of snow. The radar was developed for operation on a Twin Otter aircraft with a nominal survey altitude of 500 m. The radar is designed to operate in frequency modulated continuous wave (FM-CW) mode from 2 to 18 GHz. The radar has a total transmit power is of 1.5 W and a vertical resolution of 1.6 cm as demonstrated experimentally. The radar has two modes of operation: snow sounding mode for snow thickness measurements and accumulation mapping, and side-looking mode for snow backscatter measurements and imaging. The radar system was flown from Barrow, Alaska in spring 2015 and 2016 as a part of the Naval Research Laboratory sea ice survey campaign to characterize sea ice and snow on sea ice. Between February and March 2016, we also took advantage of radar test flights in Colorado to conduct terrestrial snow measurements over the alpine areas. Sample radar data from the field campaigns will be presented.

Prof. Richard Moore introduced me to FM-CW radars on my first day at the University of Kansas as a graduate student in 1979 and asked me to put together a radar using laboratory test equipment. I put it together, but it did not provide the results we wanted for detecting buried pipes. This was mainly because of the lack of suitable inexpensive RF and digital technologies at that time. Prof. Moore was a strong advocate for using ultra-wideband FM-CW radars. We are able to implement what he taught me because of recent advances in RF microwave and digital technologies, allowing us to develop the ultra-wideband radars Prof. Moore envisioned for remote sensing of snow and ice. We developed ultra-wideband radars for airborne measurements over ice and snow. One of these radars operates over a frequency range of 150-600 MHz for sounding ice sheets, imaging the ice-bed interface, and mapping internal layers in polar firn and ice; additional radars operate over the frequency ranges of 2-8 and 12-18 GHz for airborne measurements of the thickness of snow over sea ice and land and surface elevation measurements, respectively.

Satellite radar altimeter (RA) measurements are important for continued monitoring of rapidly changing polar regions. In 2010, the European Space Agency launched CryoSat-2 carrying SIRAL, a Ku-band RA with objectives of determining the thickness and extent of sea ice and the topography of the ice sheets. One difficulty with Ku-band radar surveys over snow and ice is unknown penetration of RA signal into snow cover. Improving our understanding of the interactions of RA signals with snow and ice is needed to produce accurate elevation products. To this end, we developed a low-power, ultrawideband (12-18 GHz) RA for airborne surveys to provide fine resolution measurements capable of detecting both scattering from the surface and layers within sea ice and ice sheets. These measurements provide a means of identifying the dominant scattering location of lower resolution RA measurements comparable to satellite-based instruments. We generated two products: a full-bandwidth waveform (FBW) to identify scattering targets at fine resolution and a reduced-bandwidth waveform (RBW) to represent conventional RA measurements. Retrackers are used to generate height estimates over various surface conditions for comparisons. Over ice sheets, the leading-edge tracker provided consistent ice-surface elevation measurements between the FBW and RBW results; however, there were significant differences between the results from the centroid tracker. Over sea ice, the location of the dominant return between the results from snow-covered sea ice is highly variable. This paper provides an overview of RA surveys in polar regions, a description of the CReSIS system, and a discussion of the results.

A wideband multi-channel airborne sounding and imaging radar for cryospheric remote sensing applications has been recently developed by the Center for Remote Sensing of Ice Sheets (CReSIS). The radar is designed to measure ice thickness, image the ice-bed interface, and map internal layers in ice sheets and glaciers. This newly-developed radar uses the wide bandwidth for high-resolution imaging and cross-track array processing for suppression of surface clutter. The radar was integrated onto a BT-67 aircraft and completed its first field deployment in Antarctica during the 2013/2014 Austral Summer season. This paper focuses on the development and deployment of the radar. A few sample results from the field survey in Antarctica are also presented to demonstrate the high resolution features of the radar.

This paper presents a radar sensor package specifically developed for wide-coverage sounding and imaging of polar ice sheets from a variety of aircraft. Our instruments address the need for a reliable remote sensing solution well-suited for extensive surveys at low and high altitudes and capable of making measurements with fine spatial and temporal resolution. The sensor package that we are presenting consists of four primary instruments and ancillary systems with all the associated antennas integrated into the aircraft to maintain aerodynamic performance. The instruments operate simultaneously over different frequency bands within the 160 MHz-18 GHz range. The sensor package has allowed us to sound the most challenging areas of the polar ice sheets, ice sheet margins, and outlet glaciers; to map near-surface internal layers with fine resolution; and to detect the snow-air and snow-ice interfaces of snow cover over sea ice to generate estimates of snow thickness. In this paper, we provide a succinct description of each radar and associated antenna structures and present sample results to document their performance. We also give a brief overview of our field measurement programs and demonstrate the unique capability of the sensor package to perform multifrequency coincidental measurements from a single airborne platform. Finally, we illustrate the relevance of using multispectral radar data as a tool to characterize the entire ice column and to reveal important subglacial features.

The National Aeronautics and Space Administration (NASA) initiated a program called Operation IceBridge for monitoring critical parts of Greenland and Antarctica with airborne LIDARs until ICESat-II is launched in 2016. We have been operating radar instrumentation on the NASA DC-8 and P-3 aircraft used for LIDAR measurements over Antarctica and Greenland, respectively. The radar package on both aircraft includes a radar depth sounder/imager operating at the center frequency of 195 MHz. During high-altitude missions flown to perform surface-elevation measurements, we also collected radar depth sounder data. We obtained good ice thickness information and mapped internal layers for both thicker and thinner ice. We successfully sounded 3.2-km-thick low-loss ice with a smooth surface and also sounded about 1-km or less thick shallow ice with a moderately rough surface. The successful sounding required processing of data with an algorithm to obtain 56-dB or lower range sidelobes and array processing with a minimum variance distortionless response algorithm to reduce cross-track surface clutter. In this paper, we provide a brief description of the radar system, discuss range-sidelobe reduction and array processing algorithms, and provide sample results to demonstrate the successful sounding of the ice bottom interface from high altitudes over the Antarctic and Greenland ice sheets.

This work addresses the use of compressive sensing to compress real Synthetic Aperture Radar (SAR) raw data. Due to the low computational resources of the acquisition platforms and the steadily increasing resolution of SAR systems, huge amounts of data are collected and stored, which cannot generally be processed on board and must be transmitted to the ground to be processed and archived. Although compressive sensing (CS) has been proposed and studied by a lot of researchers, almost none of them touches the real application of it. While, in this paper, we test the sparsity of the real SAR raw data (obtained by University of Kansas in Greenland, 2010), compress it using compressive sensing, and then recover the original signal using several CS recovery algorithms (Basis Pursuit, Matching Pursuit and Orthogonal Matching Pursuit), and compare these methods' performance. Simulation results are presented to prove the successful application of CS to real SAR raw data. When proper sparsity matrix is chosen, the real SAR data could be transformed to sparse signal. Using our designed algorithm, the positions and the exact values of the SAR raw data can be almost perfectly recovered with a very low MSE at a compression ratio of 1/8. This is of great significance to help us perform further research in the applications of CS to real SAR raw data.

A Ku-band ultra-wideband radar altimeter with 6 GHz of bandwidth has been developed for surface elevation measurements in polar ice sheets. The radar is equipped with a newly-designed chirp generator with sufficient linearity to resolve adjacent targets at ranges of 500 m or more. This capability allows the airborne radar to resolve closely spaced sub-surface reflectors and internal layers in polar firn. In this paper, we discuss the design and development of the radar and present sample results from recent field measurements over Byrd Glacier in Antarctica.

Antarctic sea ice and its snow cover are integral components of the global climate system, yet many aspects of their vertical dimensions are poorly understood, making their representation in global climate models poor. Remote sensing is the key to monitoring the dynamic nature of sea ice and its snow cover. Reliable and accurate snow thickness data are currently a highly sought after data product. Remotely sensed snow thickness measurements can provide an indication of precipitation levels, predicted to increase with effects of climate change in the polar regions. Airborne techniques provide a means for regional-scale estimation of snow depth and distribution. Accurate regional-scale snow thickness data will also facilitate an increase in the accuracy of sea ice thickness retrieval from satellite altimeter freeboard estimates. The airborne data sets are easier to validate with in situ measurements and are better suited to validating satellite algorithms when compared with in situ techniques. This is primarily due to two factors: better chance of getting coincident in situ and airborne data sets and the tractability of comparison between an in situ data set and the airborne data set averaged over the footprint of the antennas. A 2–8-GHz frequency modulated continuous wave (FMCW) radar loaned by the Center for Remote Sensing of Ice Sheets to the Australian Antarctic Division is used to measure snow thickness over sea ice in East Antarctica. Provided with the radar design parameters, the expected performance parameters of the radar are summarized. The necessary conditions for unambiguous identification of the air/snow and snow/ice layers for the radar are presented. Roughnesses of the snow and ice surfaces are found to be dominant determinants in the effectiveness of layer identification for this radar. Finally, this paper presents the first in situ validated snow thickness estimates over sea ice in Antarctica derived from an FMCW radar on a helicopterborne platform.

Ice sheets in Greenland and Antarctica are based on continental rock and are coupled strongly to changes in global sea level. Their beds may be frozen or thawed, but it has not been known how much or where basal melt occurs, except in special cases where a borehole has reached the bed or where airborne radar has revealed subglacial lakes and reflective ice stream beds in Antarctica. We have used a previously published technique to detect subglacial melt water in the general case and have here applied it to radar-sounder data collected over northern and central Greenland. We have found extensive subglacial water along between 13% and 20% of the flight paths. This paper provides maps of the measured locations and probable extent of subglacial water.

We performed airborne experiments using 150- and 450-MHz radars to measure ice thickness on the Greenland ice sheet. Our objectives were to investigate to what degree surface clutter obscures the basal echo when airborne measurements are made at different elevations and at different frequencies. We also explored interferometric techniques for processing the data to form swath measurements of ice thickness. We found that surface clutter was minimal for either frequency when operated at low aircraft elevations (500 m above the ice sheet surface) or over benign regions of the ice sheet. Because signal-to-clutter ratios were favorable, we found that we could retrieve the swath measurements of ice thickness at both frequencies using an interferometric technique. At high elevation, surface clutter degraded the 150-MHz signal, but the nadir ice thickness was still retrievable. The basal return in high-elevation 450-MHz data was detectable only after additional beam-steering techniques were applied to the data to reduce the surface clutter signal. Results suggest that interferometric cross-track ice-thickness measurements can be successfully made given a sufficient number of antenna elements driven at either 150 or 450 MHz and flown at both high and low elevations over the interior ice sheet.

We develop and then demonstrate a 3-D tomographic ice sounding method applied to very high frequency (VHF) radar data that produces swath measurements of ice sheet surface topography, ice thickness, and radar reflectivity of both the surface and bed of the ice sheet. First, we formulate the ice sheet imaging problem as a problem of estimating signal arrival angles and illustrate how the method resolves ambiguous echoes arriving simultaneously from the left and right sides of the aircraft, as well as from the surface and base of the ice sheet. We then discuss why we chose the time-domain subaperture method for 2-D image formation for ice sounding. We apply the tomographic technique to the data that we collected in May 2006 and again in July 2008 from a multiple-phase-center VHF radar system. We present 3-D images of the upper and lower surfaces of the ice sheet and compare the estimated surface topography with Ice, Cloud, and land Elevation Satellite altimeter nadir track measurements and the measured swath ice thickness with independent nadir depth sounder tracks. We achieved a 5-m surface topography accuracy and a 14-m ice thickness accuracy.

During NASA's Operation Ice Bridge a gridded survey was flown over Pine Island Glacier (PIG). This survey was a finer grid than previously flown over this area. The data collected confirm that the majority of the ice at the bottom of PIG is below sea level which could be a major cause in the speed-up of the ice flow in that area. These data can be used in flow rate calculations and to the mass balance in that area.

We developed and deployed, in July 2005, a wideband 8-channel synthetic aperture radar (SAR) at Summit Camp, Greenland (72.5783° N and 38.4596° W). The radar was designed to map internal layers, measure topography, and generate backscatter maps - all in a single pass. Information on ice thickness, bed topography and basal conditions is essential to the refinement of glaciological models of ice sheets, which are used to predict ice-sheet behavior (especially mass balance) and to select deep ice-core sites. This work focuses on the use of fine-resolution 3D imaging algorithms for combining all 8-channels to form cross-track image slices through the ice. As compared with traditional 2D depth sounding, these 3D images allow for the characterization of bed topography with very fine resolution. They also allow for the generation of strip-map SAR images with absolute geocoding without ground control points (these are unavailable at the bottom of the ice), and the ability to analyze the ice-sheet volume in 3D. Topography, backscattering, and 3D ice volume results are illustrated here.

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 NASA's Operation Ice Bridge.

The Multichannel Coherent Radar Depth Sounder (MCoRDS) system was developed by the Center for Remote Sensing of Ice Sheets (CReSIS) to map the thickness of ice sheets. This radar system was used in Antarctica as one of the primary sensors for NASA's Operation Ice Bridge (OIB) during the fall of 2009. Compared to its predecessors, MCoRDS features several new capabilities which enabled it to successfully capture ice thickness measurements over multiple glaciers on an aerial platform. This paper will focus on the capabilities of MCoRDS and also provide a sample of the processed radar results.

Information on the bedrock topography below the Greenland and Antarctic ice sheets is vital to developing models of future sea-level rise. To measure the topography, advanced data acquisition and processing techniques, including Synthetic Aperture Radar (SAR), are required. This work investigates the optimal beamwidth that would enable SAR processing to maximize the signal to noise ratio of the target. Platform height above the ice surface and bedrock roughness determine the optimal beamwidth. We found that for data collected at a “typical” altitude of 867 m, the optimal beamwidth is about 8°. In the high-altitude case, we found that beamwidth did not have a significant effect on the signal-to-noise ratio. This is probably related to scattering from the ice surface.

We discuss results from a high-sensitivity, multichannel, very high frequency, and surface-based radar depth sounder/imager. The instrument was used to map deep internal layers and characterize basal conditions over a 240- km2grid in the vicinity of the West Antarctic Ice Sheet Divide ice core site. The ice thickness at the core site was found to be about 3470 m, and we detected internal layers to within 350 m of the ice/bed interface. Radar-detected layer stratigraphy does not show evidence of flow-induced disturbances that might complicate the depth-age relationship and the interpretation of climate history preserved in the ice. We also found that bed reflectivity over the region varies by more than 30 dB. Approximately 15 dB of this variability appears to be the result of transitions from a frozen to a thawed bed in a number of places. The remainder probably results from changes in bed roughness. Our data are important for planning drilling to the bed, as well as providing constraints and boundary conditions for regional ice-flow models.

Many satellite, airborne, and in situ observations have been made to better understand the mass balance of the ice sheets. Satellite missions such as GRACE, ICESat, and Cryosat provide broad coverage, but are only capable of collecting data at relatively coarse temporal and spatial resolutions. Satellite observations alone are not sufficient to fully understand all mechanisms responsible for changes in the overall ice sheet mass balance. While these are sufficient over much of the interior of the ice sheet, to understand and model the dynamics of fast flowing glaciers and the margins of the ice sheet, finer resolution data are required. Airborne platforms, especially autonomous platforms, allow for key regions of the ice sheets to be measured with fine-resolution remote sensing instruments. These platforms provide more accurate ice thickness estimates, internal layer mapping, and ice-bedrock interface imaging. To address this gap in the observations, we are designing and developing an instrumentation suite to be deployed on crewed and uncrewed aircrafts. Here we will focus on two radars in the instrumentation suite: an accumulation radar and a radar altimeter. The altimeter will be capable of measuring surface elevation and near-surface internal layers to a depth of about 10 m. The accumulation radar will be capable of measuring internal layers to a depth of about 100 m. A previously developed 150 MHz radar depth sounder/imager will be used to map layers below 100 m, as well as the ice-bedrock interface. This radar is beyond the scope of this paper. Field data collection using these systems will be performed simultaneously, providing a fine-resolution characterization of the ice sheet from surface to bedrock. The altimeter provides annual and short-term information on the accumulation, while the accumulation radar provides information on the decadal scale variability. The depth sounder provides information on the century scale variability. Initial data collection occurred during the early spring 2009 Greenland field season; additional data collection will continue during future campaigns both in Greenland and Antarctica. System refinements will allow for this suite to be deployed on uncrewed aerial vehicles (UAVs), also being developed at the Center for Remote Sensing (CReSIS) at the University of Kansas.

Sea-level rise will affect populations worldwide with considerable and lasting consequences in the not-too-distant future. Accurate measurement of fast flowing outlet glaciers in Greenland and Antarctica are of vital importance to ice sheet models that predict the course of sea-level rise. The Center for the Remote Sensing of Ice Sheets (CReSIS) has developed a suite of tools designed for use with data collected by CReSIS depth sounding radar platforms. This suite includes algorithms for removing clutter and noise from coherent radar data, and the results show successful sounding of some of these fast-flowing glaciers for the first time.

A VHF-band radar is being developed to characterize polar ice sheets and their basal conditions from a UAV developed specifically for low-altitude polar operation. The radar's 195-MHz center frequency, 30-MHz bandwidth, and 200-W transmit power will map internal layers and ice thickness with a depth resolution of less than 3 m in ice, and image the ice-bed interface. To satisfy the mass and volume constraints of the UAV, a distributed architecture was developed employing transmit/receive modules mounted on each of the eight wing-mounted wide-bandwidth Vivaldi antennas. An eight-channel digital waveform generator will be used to create transmit waveforms to simultaneously measure ice thickness and map internal layers along the nadir track, and image ice-bed interface on the left and right sides of the platform in strip-map mode.

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.

We are developing a multifrequency multistatic synthetic aperture radar (SAR) for determining polar ice sheet basal conditions. To obtain data for designing and optimizing radar performance, we performed field measurements with a network-analyzer-based system during the 2003 field season at the North Greenland Ice Core Project camp (75.1 N and 42.3 W). From the measurements, we determine the ice sheet complex transfer function over the frequency range from 110-500 MHz by deconvolving out the system transfer function. Over this frequency range, we observe an increase in total loss of 8/spl plusmn/2.5 dB using a linear regression to the log-scale data. With the ice sheet transfer function and an ice extinction model, we estimate the return loss from the basal surface to be approximately 37 dB. These measurements have broad applicability to interpreting radar-sounding data, which are widely used in glaciological studies of the polar ice sheets. These data have also been used in the link budget for the design considerations of the multifrequency multistatic SAR system.

The PRISM project at the University of Kansas has developed the necessary tools and infrastructure to bring the daily activities of polar ice sheet research to students and the general public via the Web. PRISM is developing advanced intelligent remote sensing technology to measure key ice sheet characteristics, and PRISM investigators travel to remote ice sheet sites annually. We have developed "Virtual PRISM" as part of the PRISM web page, to allow the public to follow the field activities of the PRISM team in Greenland. "Virtual PRISM" includes a daily field log, digital images, video clips, access to weather data on the ice sheet, and a Java-based applet called the "Virtual Dashboard" that simulates sitting in the driver's seat of the PRISM robotic rover.

We developed a wideband coherent radar depth sounder (WCORDS) system developed to measure glacial ice thickness and map internal layers with high resolution. The radar operates over a frequency range of 50-200 MHz, providing a resolution of about 1 m in ice. A high-speed arbitrary waveform generator (AWG) is used to generate a chirp from 50-200 MHz over a very small pulse width to obtain high sensitivity needed to sound 5-km thick cold ice and map internal layers high resolution. It also consists of a two-channel receiver to obtain very high dynamic range. The low-gain channel is used to map the shallow internal layers and the high-gain channel provides ice sheet thickness and bedrock properties up to a depth of 5000 m. The gain in both the channels can be adjusted to obtain optimum performance. A high-speed data acquisition system is used to digitize and perform necessary real time processing on the data before transferring it to the storage device. The radar and data-acquisition systems have been significantly miniaturized using the latest RF and fabrication technologies. The entire system is designed to fit into multiple compact-PCI cards. Laboratory tests show that the radar system has the required sensitivity to map 5000-m-thick ice. Considerable improvement in sidelobe performance was achieved. The radar system will be tested during the summer 2004 field experiment at the Summit Camp, Greenland.

Ice sheet models are necessary to understand ice sheet dynamics and to predict their behavior. Of the primary inputs to these models, basal conditions are the least understood. By observing the forward and backscatter across a wide frequency range (over two octaves) the basal conditions can be established with a high level of confidence. For this purpose, we developed a multistatic synthetic aperture radar system that operates on three frequency bands (75-85 MHz, 140-160 MHz, and 330-370 MHz). The radar system is designed to use pulse compression techniques and coherent integration to obtain high loop sensitivity (203 dB) necessary to overcome radio frequency losses in ice. The system will be tested at Summit, Greenland (72deg34'N, 38deg29'W) during July 2004

Rising sea level has important humanitarian and economic implications. Scientists are rigorously investigating the contribution of glacial ice sheets to sea-level rise. Knowledge of the mass balance of the ice sheets is important in understanding their dynamics. The accumulation rate of snow on ice sheets is an important variable in determining this mass balance. The accumulation rate is currently determined using ice cores and pits. This is a tedious method to obtain coverage over the entire ice sheet due to the limited number of samples that can be acquired. The only practical means of obtaining coverage over a large area would be by means of remote sensing. We have developed a wideband radar to map the isochronous layers in the ice sheet This will help reduce the uncertainty associated with sparse sampling of the ice sheet. We built a compact FM-CW radar that operates from 500 to 2000 MHz with range resolution of about 10 cm. Both the transmitter and receiver were housed in a single Compact PCI chassis. We used a YIG oscillator to generate the FM signal. The performance of an FM-CW is usually degraded by the nonlinearity of the source. We linearized the sweep of the YIG oscillator by means of a phase-locked loop (PLL). We have successfully tested the radar in the lab and we will be performing tests during the 2004 summer field experiments at the Summit camp in Greenland. We successfully tested a connectorized version of this radar during the 2003 field experiments at the North Greenland Ice Core Project (NGRIP) camp. We were able to map the internal layers up to a depth of about 150 m over a 5-km transect. We also conducted detailed snow pit studies at several spots over the transect for correlating the visually determined layers with the radar determined layers. We will present the radar design, laboratory test results of its performance, results from the experiments at Summit and a comparison of the radar data with information derived from ice cores and snow pits.

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.

The PRISM project at the University of Kansas has developed a multifaceted outreach program designed to interest students and the general public in research conducted in harsh polar environments. This outreach program involves K-12 students and teachers, undergraduate students from a minority institution, undergraduate journalism students with an interest in science and technology reporting, and the general public.

We report on the PRISM research project currently underway at the University of Kansas – a multidisciplinary effort sponsored by the National Science Foundation and NASA. The objective of this project is to develop and deploy an innovative sensor system for measuring key glaciological parameters and studying the acquired data to understand the contributions of polar ice sheets to sea level rise. This project involves research in autonomous vehicles (i.e., rovers), intelligent systems, sounding radars and ice sheet modeling. When completed, we anticipate that the data collected and experience gained will make a substantial contribution to the long-term science goal of predicting ice sheet response to climate change, while advancing fundamental knowledge in the design, implementation and operation of distributed autonomous systems and self-sufficient computing. Keywords-component; polar ice sheets, SAR, sea-level rise; sounding radar, rovers, autonomous systems.

Snow cover of variable thickness exists on sea ice with thickness fluctuations in the range from less than a few centimeters to several meters depending on snow drifts and ice type. Snow largely controls the thermal and electrical properties of a sea ice cover. Because of its low thermal conductivity, it effectively insulates the sea ice surface from cold polar air and modifies the heat flux between the atmosphere and ocean. It also changes the sea ice albedo. Additionally, thick snow cover acts as a mechanical load and can depress the ice surface below sea level, causing the ice floe to be flooded with sea water. Thus accurate knowledge of snow thickness on sea ice is essential for determining the overall heat budget in the polar regions, which in turn can impact global ocean circulation and climate. We developed an ultra-wideband radar for measuring snow thickness. It operates over the frequency range from 2-8 GHz in FM-CW mode. We used a phase-locked YIG oscillator to generate a very linear 2-8 GHz chirp by using a low-frequency (5-20 MHz) digital chirp generator as a reference signal for the phase detector. We also constructed a receiver with a large dynamic range and fast settling time. The received signal was digitized using a 12-bit A/D converter and stored for further processing. We also developed simple models to simulate radar performance. We modeled snow as a multi-layered media and sea ice as dielectric half-space, and performed extensive simulations using snow geophysical data collected during Antarctic cruises to optimize radar performance. We evaluated the radar's performance by measuring its response to point targets such as a delay line and corner reflectors. With the Hanning window function, the measured radar range resolution is about 3.75 cm. We collected data on snow-covered ground in conjunction with measurements of snow parameters such as density, particle size, and roughness. The results from these measurements show that we can clearly delineate returns of the snow-air and snow-ground interfaces for 6-cm-thick dry snow.

Sea ice plays an important role in the Earth climate system. Techniques used in the past to determine sea ice thickness had fundamental limitations in terms of spatial and temporal coverage. We performed extensive simulations using published ice-core data from both polar regions, and our results suggested the use of a UHF radar system (300-1300 MHz) for sounding thin Antarctic sea ice and thin ice in the Arctic and a VHP radar system (50-250 MHz) for sounding thick Arctic sea ice. Based on the simulation results, we designed a prototype radar system that operates on both the frequency ranges. We have successfully used the system to collect data over sea ice at Barrow, Alaska. The experiments show that with our system, we can measure sea ice thickness with less than 20 cm uncertainty in thickness.

We report the results of a three-year, NASA-funded project at The University of Kansas Radar Systems and Remote Sensing Laboratory on the development of a laser radar that uses RF pulse compression to significantly improve system performance. Receiver sensitivities of less than -90 dBm have been demonstrated by applying heterodyne optical downconversion and RF pulse compression. With the improved sensitivity, the required transmit power is significantly reduced. This system approach also permits multi-kilohertz pulse-repetition frequencies that enable spatially dense range measurements. Compared to lidars like GLAS and MOLA, this sensor requires a lower peak transmit power while providing orders of magnitude more measurements per second. In the receiver design, we have evaluated two detection schemes: envelope detection and direct downconversion. Envelope detection provides the benefit of discarding the effects of optical phase variations on the detected signal consequently avoiding many temporal correlation issues, however it is less efficient in terms of the resulting signal-to-noise ratio (SNR). Direct downconversion to baseband is more SNR efficient, however the baseband signal contains the effects optical phase variations, which include laser phase noise, effects of atmospheric turbulence, and frequency shifting due to Doppler effects. We have demonstrated the feasibility of using a linear array of optical fibers in the telescope's focal plane to launch and receive the optical signals. Using separate fibers for transmit and receive while sharing telescope optics, we have achieved the required transmitter-receiver isolation of a bistatic system without the accompanying alignment difficulties. With our breadboard system ranging measurements from both man-made and natural extended targets have been made and the results are presented. These results support the feasibility of a satellite-based altimeter (600 km altitude), capable of making more than 4000 range measurements per second with 10 cm accuracy using less than 10 W peak transmit power. While the present breadboard operates at 1319 nm, the overall concept is wavelength independent. Benefits of this development may include increased system reliability, reduced power requirements, smaller sensor mass and volume, improved eye-safety, and lower probability of signal detection.

We have conducted airborne measurements over the Greenland ice sheet from the NASA P-3B aircraft using a 150-MHz coherent radar depth sounder to obtain extensive ice sheet thickness measurements. Simultaneous measurements of ice sheet elevation were also made using a laser altimeter. In outlet glacier areas along the ice sheet margin, the ice surface is characterized by a very rough, crevassed surface near the calving front. The rough ice surface generates signal clutter, which can mask the bottom echo. We are developing a technique to remove the surface clutter component from the signal to reveal the bottom echo, providing a complete ice thickness survey to the calving front of outlet glaciers. This technique makes use of a dense array of ice surface elevation measurements provided by the laser altimeter to characterize surface roughness. Surface roughness parameters are then used in a backscatter model to characterize the clutter waveform, which can be subsequently removed from the radar signal.

Determination of the mass balance of the polar ice sheets requires information on the accumulation rate. Remote sensing methods to determine the accumulation rate are essential in reducing the uncertainty associated with interpolating in situ measurements that are obtained from ice cores and pits. This is essential to reducing the 20% of uncertainty in current accumulation rate maps. Using data from surface-based radar experiments we determined optimum parameters for an airborne radar. We developed an airborne prototype and successfully demonstrated that we can map internal layers with about 1 m resolution to a depth of about 120 m over the Greenland ice sheet. We reported the system design, construction and preliminary experimental results at the 2001 IGARSS meeting. We have developed an operational radar system for routine measurement. This system operates in FM-CW and stepped-frequency pulse modes and it has 20-dB more sensitivity than the prototype radar. We also developed a radar target simulator for testing and evaluating system performance. The target simulator was constructed using fiber optic cables, microwave delay lines and RF/optical transceivers to simulate reflections from the air/snow interface, internal layers and the antenna reflection, which degrades the system's sensitivity. The simulator serves a dual purpose of optimizing the system performance in the laboratory and for internal calibration in the field. We also used a CAD package to design and simulate overall radar performance. The use of CAD package and target simulator reduced cost and time associated with the radar development. In addition, we are also able to obtain an accurate system model to deconvolve the system effects from the received signal. We discuss detailed design, construction and performance of the target simulator and operational radar. We show a comparison of the simulation results and laboratory measurements of the radar system. Also we present analysis of the results from measurements made during the 2001 experiments and preliminary results from planned measurements in May 2002.

Using ground-penetrating radars to investigate the subsurface of Mars will be a key scientific objective over the next several years, especially in light of the large possibility that water could exist within the planet. Radars operating from a few megahertz up to a gigahertz will be able to provide valuable information concerning the subsurface electrical structure at resolutions ranging from a few centimeters near the surface to a few tens of meters at greater depths. One of the major goals of the work presented was to develop a lightweight, low-power, frequency-modulated radar system that could be used to detect subsurface deposits of ice and water. An inexpensive prototype system was developed using off-the-shelf connectorized components and evaluation boards. To verify the operation of this prototype system, a preliminary experiment was conducted in Lawrence, Kansas. Next, experiments were conducted over locations containing permafrost and ice in Fairbanks, Alaska. Results from these experiments are presented.

This study examines differences in the normalized radar cross section, derived from ground-based versus spaceborne radar data. A simple homogeneous half-space model, indicates that agreement between the two improves as 1) the distance from the scatterer is increased; and/or 2) the extinction coefficient increases.

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.

This paper highlights the interrelationship of research completed by a team of investigators and presented in the several individual papers comprising this Special Section on the Office of Naval Research (ONR), Arlington, VA, Sponsored Sea Ice Electromagnetics Accelerated Research Initiative (ARI). The objectives of the initiative were the following: (1) understand the mechanisms and processes that link the morphological and physical properties of sea ice to its electromagnetic (EM) characteristics; (2) develop and verify predictive models for the interaction of visible, infrared, and microwave radiation with sea ice; (3) develop and verify inverse scattering techniques applicable to problems involving the interaction of EM radiation with sea ice. Guiding principles for the program were that all EM data be taken with concurrent physical property data (salinity, density, roughness, etc.) and that broad spectral data be acquired in as nearly a simultaneous fashion as possible. Over 30 investigators participated in laboratory, field, and modeling studies that spanned the EM spectrum from radio to ultraviolet wavelengths. An interdisciplinary approach that brought together sea ice physicists, remote-sensing experts (in fill measurements), and forward and inverse modelers (primarily mathematicians and EM theorists) was a hallmark of the program. Along with describing results from experiments and modeling efforts, possible paradigms for using broad spectral data in developing algorithms for analyzing remote-sensing data in terms of ice concentration, age, type, and possibly thickness are briefly discussed.

Estimating the magnitude of brine flux to the upper ocean requires an ability to assess the dynamics of the formation of sea ice in a region. Brine storage and rate of expulsion is determined by the environmental conditions under which the sea ice forms. In this paper, the physical and electromagnetic properties of sea ice, formed under wave-agitated conditions, are studied and compared with results obtained from ice formed under quiescent conditions. Wave agitation is known to have a profound effect on the air-ice interface and internal ice structure. A variety of sensors, both active and passive, optical and microwave, were used to perform this characterization. Measured electromagnetic parameters included radar backscatter, microwave emission, and spectral albedo in the visible and infrared. Measured physical properties included ice structure, brine and temperature distribution, profiles of the vertical height of the air-ice interface, and ice formation processes. Results showed that emission, backscatter, and albedo all take different signature paths during the transformation from saline water to young sea ice and that the paths depend on sea surface state during ice formation.

Young first-year sea ice is nearly as important as open water in modulating heat flux between the ocean and atmosphere in the Arctic. Just after the onset of freeze-up, first-year ice is in the early stages of growth and will consist of young first-year and thin ice. The distribution of sea ice in this thickness range impacts heat transfer in the Arctic. Therefore, improving the estimates of ice concentrations in this thickness range is significant. The NASA Team Algorithm (NTA) for passive microwave data inaccurately classifies sea ice during the melt and freeze-up seasons because it misclassifies multiyear ice as first-year ice. We developed a hybrid fusion technique for incorporating multiyear ice information derived from synthetic aperture radar (SAR) images into a passive microwave algorithm to improve ice type concentration estimates. First, we classified SAR images using a dynamic thresholding technique and estimated the multiyear ice concentration. Then we used the SAR-derived multiyear ice concentration to constrain the NTA and obtained an improved first-year ice concentration estimate. We computed multiyear and first-year ice concentration estimates over a region in the eastern-central Arctic in which field observations of ice and in situ radar backscatter measurements were performed. The fused estimates of first-year and multiyear ice concentration appear to be more accurate than NTA, based on ice observations that were logged aboard the US Coast Guard Icebreaker Polar Star in the study area during 1991.

Experiments were performed at the U.S. Army Cold Regions Research and Engineering Laboratory (CRREL) in Hanover, NH, to precisely determine the relative contributions of surface and volume scattering from saline ice that has well-known surface roughness characteristics. The ice growth phase of the experiment made use of two 6-ft diameter tanks and a 6-ft diameter mold with known roughness statistical parameters of rms height=0.25 cm and Gaussian correlation (correlation length=2.0 cm). One tank was used for growing a moderately thick saline ice sheet with very smooth surface, and the other was used for growing a thin layer of freshwater ice over the surface mold. The latter resulted in a layer with one statistically known rough boundary and one smooth boundary. Wide-bandwidth, multiple incidence angle backscattering measurements were performed, first on the bare saline ice sheet and then on the same sheet after the thin freshwater ice sheet was placed on top of it. Results indicate that the surface scattering dominates over saline ice volume scattering at all frequencies for low incidence angles for both the very smooth and Gaussian rough surfaces. The significance of volume scattering depends strongly on angle of incidence, frequency, volume scattering albedo, surface roughness, and surface correlation function.>

The authors utilized the concept of a compact antenna range to obtain plane-wave illumination to accurately measure scattering properties of simulated sea ice. They also made simultaneous measurements using conventional antennas. Measured scattering coefficients obtained with the plane-wave system at 10 GHz decreased by about 35 dB when the incidence angle increased from 0/spl deg/ to 10/spl deg/. Scattering coefficients derived from data collected with the radar system at 13.5 GHz using conventional far-field antennas decreased by about 20 dB over the same angular region. This demonstrates that the far-field properties of a widebeam antenna are inadequate for measuring the angular scattering response of smooth surfaces. They believe that application of the compact antenna range concept for scattering measurements has a wide range of applications and is the solution to the long-standing problem of how to directly measure scattering consisting of coherent and incoherent components.>

Indoor laboratory facilities were used to measure radar backscatter at Ku band (13.9 GHz) over urea ice, which has been shown to be structurally similar to sea ice. Data were collected at angles of incidence from normal to 55 degrees , over very thin (0 to 9 cm) ice, snow-covered ice, and ice with a hooded snow cover. The laboratory proved to be useful in creating and controlling specific physical properties of ice while keeping all other variables constant, a difficulty with measurements collected in the field. It was found that surface scattering and the dielectric constant are the dominant factors that cause variations (up to 15 dB) in the measured backscatter. The addition of a snow cover increased the surface roughness of the smooth ice, increasing the backscatter at 20 degrees incidence angle by about 11 dB and decreasing the backscatter at normal incidence by about 6 dB. The subsequent flooding of this snow layer increased the backscatter at all angles of incidence due to the increased dielectric constant of the wet slush layer. These results indicate the importance of the snow layer in influencing the surface characteristics of the ice sheet, which in turn modifies the backscattered signal.>

A numerical procedure for estimating true scattering coefficients, sigma /sup 0/, from measurements made using wide-beam antennas is discussed. The use of wide-beam antennas results in an inaccurate estimate of sigma /sup 0/. To reduce this error, the authors propose a correction procedure that estimates the error resulting from the use of narrow-beam approximation and utilizes the error to obtain a more accurate estimate of sigma /sup 0/. An exponential model is assumed to take into account the variation of sigma /sup 0/ with incidence angles, and the model parameters are estimated from measured data. Based on the model and knowledge of the antenna pattern, the procedure calculates the error due to the narrow-beam approximation. The procedure is shown to provide a significant improvement in the estimation of sigma /sup 0/ obtained in wide-beam antennas. The proposed procedure is also shown to be insensitive to the assumed sigma /sup 0/ model.>