Emily J. Arnold

Advanced Multifrequency Radar Instrumentation for Polar Research

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.

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.

A Modified Wideband Dipole Antenna for an Airborne VHF Ice-Penetrating Radar

A 15-element wideband dipole antenna array was developed for operation with the Multichannel Coherent Radar Depth Sounder/Imager on board the National Aeronautics and Space Administration P-3B aircraft. The array, aligned in the cross-track direction, was designed for applying digital beam forming and direction of arrival estimation algorithms to improve clutter suppression and for 3-D imaging of ice sheets. The antenna array is embedded inside an aerodynamic fairing structure designed for airborne operation. While the fairing meets all the structural and aircraft requirements, initial measurements performed on the original prototype array revealed the adverse impact of the fairing structure on antenna performance. The materials used for the construction of the fairing produced electrical loading effects on the radiating structure, which adversely impacted the bandwidth and return loss characteristics of individual antenna elements. This paper describes a set of modifications to the original antenna design based on computer simulations and laboratory measurements, aimed at optimizing antenna return loss and bandwidth while reducing mutual coupling. The final antenna and fairing structure achieved a fractional bandwidth of 40% at a center frequency of 195 MHz with a demonstrated peak power handling capability of 150 W. We were able to reduce the mutual coupling between antenna elements by a factor of two through modification of the dipole ends.

Identifying and Compensating for Phase Center Errors in Wing-Mounted Phased Arrays for Ice Sheet Sounding

Highly crevassed ice surfaces at ice-sheet margins and fast-flowing glaciers significantly scatter radar signals. The scattered signals, often known as surface clutter, mask weak echoes from the ice-bed interface. Large wing-mounted antenna arrays are essential to synthesizing low-sidelobe patterns to reduce surface clutter. However, wing-mounted arrays are susceptible to structural flexure, which causes amplitude and phase errors that result in shifting and filling of desired array pattern nulls. In this communication, we characterize the effects of wing flexure on array beamformation using a scaled array model, and we present a compensation method to mitigate phase center errors caused by wing flexure. The compensation greatly improves clutter suppression through improved null formation. Experimental results show that we obtain an average of 7.5 dB improvement in the signal-to-interference noise ratio.

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.

Effects of Vibration on a Wing-Mounted Ice-Sounding Antenna Array

Airborne sounding of ice sheets requires large, wing-mounted antenna arrays to effectively filter and suppress the surface clutter that often masks weak bed echoes. However, when a high-sensitivity antenna array is mounted to the wings of an aircraft, the array is subjected to structural dynamics and subsequent deformation. We measured the response of a scaled wing-mounted array when excited at four different vibration frequencies to characterize the effects of airframe vibration on array beamforming and received radar signals. We determined that phase and amplitude errors caused by the expected vibration from the aircraft do not significantly degrade the radiation pattern when the Chebyshev or minimum-variance distortionless response (MVDR) beamformers are used. In the case of the Chebyshev-weighted array, vibrations did not cause pattern sidelobes to vary by more than 1.5 dB. In the case of the minimum-variance-distortionless-response-weighted array, vibrations did cause pattern nulls to shift and decrease in depth, but these pattern distortions were negligible, and did not significantly degrade clutter suppression. In addition, we were able to identify the frequency of vibration as well as the frequency of local structural modes by taking the FFT of the signals phase.

Ultra-wideband radars operating over the frequency range of 2-18 GHZ for measurements on terrestrial snow and ice

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.

Method for Design and Analysis of Externally Mounted Antenna Fairings in Support of Cryospheric Surveying

Snow and ice penetrating radar antenna arrays have been developed and flown on NASA DC-8 and P-3 aircraft as part of the NASA Operation Ice-Bridge program. These arrays required custom, externally-mounted fairings to house the antennas. This paper documents the method for generating project requirements, load development, and subsequent structural design and analysis. Static load and modal tests were also performed to verify the physics based simulations. Involvement in such interdisciplinary work provides the basis for large numbers of graduate and undergraduate students to gain practical experience in designing, fabricating, ground testing and flight testing aerospace structures.

Sensor-Driven Preliminary Wing Ground Plane Sizing Approach and Applications

Structurally integrated antenna arrays provide synergies allowing the integration of large apertures onto airborne platforms. However, the surrounding airframe can greatly impact the performance of the antenna array. This paper presents a sensor-driven preliminary wing ground plane sizing approach to provide insight into the implications of design decisions on payload performance. The improvement of a wing-integrated antenna array that utilizes the wing as a ground plane motivated this study. Relationships for wing span, wing chord, and thickness are derived from extensive parametric electromagnetic simulations based on optimum antenna performance. It is expected that these equations would be used after an initial wing-loading design point has been selected to provide the designer guidance into how various wing parameters might affect the integrated antenna performance.