Jie-Bang Yan

UAS-Based Radar Sounding of the Polar Ice Sheets

Both the Greenland and Antarctic ice sheets are currently losing mass and contributing to global sea level rise. To predict the response of these ice sheets to a warming climate, ice-sheet models must be improved by incorporating information on the bed topography and basal conditions of fast-flowing glaciers near their grounding lines. High-sensitivity, low-frequency radars with 2-D aperture synthesis capability are needed to sound and image fast-flowing glaciers with very rough surfaces and ice that contains inclusions. In response to this need, CReSIS developed an Unmanned Aircraft System (UAS) equipped with a dual-frequency radar that operates at approximately 14 and 35 MHz. The radar transmits 100-W peak power at a pulse repetition frequency of 10 kHz, operates from 20 W of DC power, and weighs approximately 2 kg. The UAS has a take-off weight of about 38.5 kg and a range of approximately 100 km per gallon of fuel. We recently completed several successful test flights of the UAS equipped with the dual-frequency radar at a field camp in Antarctica. The radar measurements performed as a part of these test flights represent the first-ever successful sounding of glacial ice with a UAS-based radar. We also collected data for synthesizing a 2-D aperture, which is required to prevent off-vertical scatter, caused by the rough surfaces of fast-flowing glaciers, from masking bed echoes. In this article, we provide a brief overview of the need for radar soundings of fast-flowing glaciers at low-frequencies and a brief description of the UAS and radar. We also discuss our field operations and provide sample results from data collected in Antarctica. Finally, we present our future plans, which include miniaturizing the radar and collecting measurements in Greenland.

A Dual-Polarized 2–18-GHz Vivaldi Array for Airborne Radar Measurements of Snow

This communication presents the experimental results of a ultrawideband 2-18-GHz dual-polarized Vivaldi antenna array for airborne radar measurements of snow. The antenna design is based on the previously reported all-metal flared-notch array by Kindt and Pickles for operation over the frequency range 0.7-9 GHz. An antenna array prototype consisting of 8×8 active dual-polarized elements was fabricated with precise aluminum machining and tested in the anechoic chamber. Beamsteering upto 30° was experimental demonstrated from 2 to 18 GHz. The measurement results are in a good agreement with the full-wave simulation results in both polarization configurations. Preliminary sample results from data collected using the Vivaldi array are also presented. The antenna array enables full polarimetric measurements of snow-over-sea-ice for estimating the snow-water-equivalent (SWE), as well as fine-resolution mapping of snow-air and snow-ice interfaces for estimating thickness.

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.

Ultrawideband FMCW Radar for Airborne Measurements of Snow Over Sea Ice and Land

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.

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.

A Polarization Reconfigurable Low-Profile Ultrawideband VHF/UHF Airborne Array for Fine-Resolution Sounding of Polar Ice Sheets

A polarization-reconfigurable low-profile ultrawideband VHF/UHF antenna array is presented in this paper. The design is based on resistively loaded planar dipole antenna elements backed with a ground plane. The antenna array has two mechanically field-switchable polarization configurations which are designed to measure the crystal orientation of ice sheets. The nominal operating mode of the array is in the vertically polarized configuration with operating frequency ranges from 150 to 550 MHz. The planar dimension of the array is 3.6 m × 0.8 m with a ground plane height of 16 cm (0.08λlow, flow = 150 MHz). Anechoic chamber measurements show that the array performance is in very good agreement with simulated results, for both VSWR and array radiation pattern. It is also shown that the array is capable of handling 1-kW peak power per transmit channel. The antenna array is housed within a flight-qualified composite S2-glass/epoxy fairing and can readily be mounted on airborne platforms, such as Basler BT-67 and Twin Otter, for radar sounding of polar ice sheets and glaciers.

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.