Daniel Gomez-Garcia

High-Altitude Radar Measurements of Ice Thickness Over the Antarctic and Greenland Ice Sheets as a Part of Operation IceBridge

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.

Measurements of In-Flight Cross-Track Antenna Patterns of Radar Depth Sounder/Imager

Antenna arrays with low sidelobes in the cross-track direction are needed for sounding and imaging ice-sheets margins including outlet glaciers. Weak radar signals from the ice-bed interface are often masked by off-vertical surface clutter from extremely rough crevassed surfaces in ice-sheet margins. Synthetic aperture radar (SAR) processing can be used to synthesize a large array for reducing clutter in the along-track direction. Low-side-lobe transmit- and receive-antenna patterns must be generated from a limited size array in the cross-track direction. Airborne antenna pattern measurements are critical to verifying pattern characteristics in the presence of a non-ideal ground plane and neighboring aircraft structures, as well as in-flight operational dynamics. In this paper, we describe a set of airborne measurements performed to determine and optimize antenna patterns for the very high frequency (VHF) array used to sound and image polar ice sheets. We measured antenna patterns by flying over a relatively smooth ice surface at an altitude of about 2700 m. The pattern data were obtained by processing the surface echoes with aircraft rolled from left to right over more than five cycles. We also simulated antenna patterns using a three-dimensional computer model of the entire airborne platform and compared with experimental results. The discrepancies between the measured and simulated results are less than 2.7 dB for 85% of the data samples. The measured pattern data will be used to optimize our array processing algorithms.

Linear chirp generator based on direct digital synthesis and frequency multiplication for airborne FMCW snow probing radar

This paper presents a linear chirp generator for synthesizing ultra-wideband signals for use in an FM-CW radar being used for airborne snow thickness measurements. Ultra-wideband chirp generators with rigorous linearity requirements are needed for long-range FMCW radars. The chirp generator is composed of a direct digital synthesizer and a frequency multiplier chain. The implementation approach combines recently available high-speed digital, mixed signal, and microwave components along with a frequency pre-distortion technique to synthesize a 6-GHz chirp signal over 240 μs with a <;0.02 MHz/μs deviation from linearity.

Fine-Resolution Radar Altimeter Measurements on Land and Sea Ice

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.

KU-Band radar altimeter for surface elevation measurements in polar regions using a wideband chirp generator with improved linearity

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.

Multichannel Wideband Synthetic Aperture Radar for Ice Sheet Remote Sensing: Development and the First Deployment in Antarctica

We developed a multichannel wideband synthetic aperture radar (SAR) that operates over a frequency range of 190-450 MHz for measurements over the ice sheets in Antarctica and Greenland. The antenna-array, which consists of eight elements housed in a certified external structure for a BASLER aircraft, was used for measurements during the 2013-2014 Antarctic field season. We performed measurements with this system in conjunction with two ultra-wideband radars operating over a frequency range of 2-8 GHz and 12-18 GHz on Siple Coast ice streams in West Antarctica during December 2013 and January 2014. We sounded ice thicker than 2 km with a signal-to-noise ratio (SNR) of more than 20 dB in an area with two-way ice loss of about 27 dB/km. The same system also simultaneously mapped near-surface internal layers with submeter resolution from the ice-surface to a depth of about 1100 for 1200 m thick ice. In this paper, we provide a detailed overview of the radar instrumentation and signal processing algorithms and present a few sample results. The radar will be operated over a frequency range of 150-550 MHz with a 24-element antenna-array for wide-ranging measurements over the Greenland and Antarctic ice sheets, starting around August 2015.

Ultra-wideband radars for measurements over ICE and SNOW

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.

Corrections to “Fine-Resolution Radar Altimeter Measurements on Land and Sea Ice”

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

Wideband imaging radar for cryospheric remote sensing

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.

Multichannel ultra-wideband airborne radar for snow backscattering measurements

An airborne 2–18 GHz frequency-modulated continuous wave (FM-CW) radar was designed and deployed to Barrow, AK for fine-resolution measurements of snow. In addition to conventional snow thickness measurements as demonstrated in the past, another mission objective is to demonstrate the radars capability to perform ultra-wideband measurement of snow backscatter at off-nadir angles. The backscatter data will be used to estimate snow-water-equivalent (SWE), which is a key parameter in sea ice models. In this paper, we will describe the design of the radar and antenna array that enable the measurement of snow backscatter. We will present some sample data collected in the field.