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Dr. Jie-Bang Stephen Yan Co-PI Two Papers Dr. Jie-Bang Stephen Yan Co-PI Two Papers: Direction-of-Arrival Analysis of Airborne Ice Depth Sounder Data UAS-Based Radar Sounding of the Polar Ice Sheets IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 55, NO. 4, APRIL 2017 2239 Direction-of-Arrival Analysis of Airborne Ice Depth Sounder Data Ulrik Nielsen, Jie-Bang Yan, Member, IEEE, Sivaprasad Gogineni, Fellow, IEEE, and Jørgen Dall, Member, IEEE Abstract— In this paper, we analyze the direction-of- determine the boundary conditions of the ice-sheet models. arrival (DOA) of the ice-sheet data collected over Jakobshavn Basal conditions largely impact on the ice flow velocity, and Glacier with the airborne Multichannel Radar Depth Sounder therefore, precise knowledge of them is especially important (MCRDS) during the 2006 field season. We extracted weak ice– bed echoes buried in signals scattered by the rough surface of the for the estimation of the mass balance [6]. fast-flowing Jakobshavn Glacier by analyzing the DOA of signals received with a five-element receive-antenna array. This allowed A. Multiphase-Center-Based Radar Ice Sounding us to obtain ice thickness information, which is a key parameter when generating bed topography of glaciers. We also estimated The weak nadir radar signals from the ice–bed interface ice–bed roughness and bed slope from the combined analysis of are often masked by off-nadir surface clutter, signals scattered the DOA and radar waveforms. The bed slope is about 8° and from extremely rough crevassed surfaces in ice-sheet margins. the roughness in terms of rms slope is about 16°. Synthetic aperture radar (SAR) processing can be used to Index Terms— Airborne radar, direction-of-arrival (DOA) suppress surface clutter in the along-track direction, but it is estimation, glacier, ice sounding, radar remote sensing, surface ineffective in reducing the across-track clutter. Large across- scattering. track antenna arrays can be used to obtain a narrow across- track antenna beam to suppress surface clutter in this direction. I. INTRODUCTION At the same time, to avoid excessive attenuation of the ATELLITE observations show that both the Greenland and signals reflected within the ice, radars are normally operated SAntarctic ice sheets are losing mass [1], [2]. Most of the in the very high frequency (VHF) part of the electromag- ice loss is occurring around ice-sheet margins and through fast- netic spectrum. The long wavelengths in this band require flowing glaciers [3]. Although satellites provide much-needed large antenna dimensions to obtain an antenna beam that information on ice-surface elevation, surface velocity, and total is sufficiently narrow to reduce across-track surface clutter. mass, there is currently no satellite-based sensor that is able to Such large antenna dimensions cannot be accommodated on measure ice thickness. Bed topography and basal conditions airborne platforms, and additional clutter suppression is, there- for areas losing ice are needed to improve ice-sheet models. fore, needed to compensate for these limitations. The current These models are essential to predicting the response of the research in this field is based on multichannel systems com- ice sheets to a warming climate. One of the key parameters bined with advanced coherent postprocessing of data. By using needed is ice sheet thickness, which can be extracted using multichannel-receivers to sample array elements individually, radar depth-sounding techniques [4], [5]. In addition, we are beamforming techniques can be utilized to synthesize adaptive interested in the basal conditions of the ice sheets as they antenna patterns that suppress the surface clutter from specific off-nadir angles, while a high gain is maintained in the nadir Manuscript received January 14, 2015; revised July 29, 2015, direction [7]. February 28, 2016 and September 10, 2016; accepted October 9, 2016. Date of publication January 16, 2017; date of current version February 24, B. DOA Estimation in Radar Ice Sounding 2017. This work was supported by the National Science Foundation under Grant ANT0424589. In addition to beamforming, the multiphase-center sys- U. Nielsen was with the Department of Microwaves and Remote Sensing, tems also provide the opportunity to perform direction-of- National Space Institute, Technical University of Denmark, DK-2800 Kongens Lyngby, Denmark. He is now with IHFood A/S, DK-1577 Copenhagen, arrival (DOA) estimation of the different signal components Denmark (e-mail: [email protected]). within the received returns. In relation to ice sounding, early J.-B. Yan was with the Center for Remote Sensing of Ice Sheets, The Univer- studies on airborne InSAR in [8] can be seen as a precursor sity of Kansas, Lawrence, KS 66045 USA. He is now with the Department of Electrical and Computer Engineering, The University of Alabama, Tuscaloosa, to DOA estimation. A ground-based radar configuration was AL 35487 USA (e-mail: [email protected]). used in [9] to perform actual DOA estimates of the bed return. S. Gogineni is with the Center for Remote Sensing of Ice Sheets, The In [10], DOA data are used as the primary data product to University of Kansas, Lawrence, KS 66045 USA (e-mail: [email protected]). J. Dall is with the Department of Microwaves and Remote Sensing, National produce swath measurements of both the ice surface and the Space Institute, Technical University of Denmark, DK-2800 Kongens Lyngby, bedrock topography. This paper is the first published work on Denmark (e-mail: [email protected]). DOA estimation applied to airborne ice sounding data. The Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. results reported in [10] are based on the data acquired by Digital Object Identifier 10.1109/TGRS.2016.2639510 the Multichannel Radar Depth Sounder (MCRDS) developed 0196-2892 © 2017 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. 2240 IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 55, NO. 4, APRIL 2017 by the Center for Remote Sensing of Ice Sheets at the University of Kansas. The radar system is, in this experiment, operated in ping-pong mode to provide 12 effective receive phase centers. Estimation of the DOA angles of the surface clutter and bed return is used to compute relative elevations in slant-range geometry, followed by a mapping to ground range to obtain the topographic map in Cartesian coordinates. DOA estimation based on the data acquired with an upgraded version of the system, Multichannel Coherent Radar Depth Sounder/Imager (MCoRDS/I) [11], has been used to support the investigation of the bed topography of more glaciers, including Jakobshavn [12]. In [13], DOA estimation has been applied to data acquired with the four-channel POLarimetric Airborne Radar Ice Fig. 1. Photograph showing the five-element subarray of folded dipole Sounder (POLARIS) [14] developed by the Technical Univer- elements mounted under the right wing of the Twin-Otter aircraft. sity of Denmark, to improve the performance of surface clutter suppression techniques. The DOA angles of the surface clutter are estimated and used to optimize the synthesis of the antenna patterns for improving clutter suppression. Recently, DOA estimation based on POLARIS data is used to show an along-track variation of the effective scattering center of the surface return caused by a varying penetration depth [15], which directly provides glaciological information. In this paper, we present further applications of the DOA estimation technique for radar ice sounding. We used MCRDS multiphase-center data collected over Jakobshavn Glacier during the 2006 Greenland field season to convert radar echograms into a DOA representation. With this representation of the radar data, we were able to detect some of the most challenging parts of the bed along the channel of the fastest Fig. 2. Flight track (red) over the Jakobshavn Glacier at the west coast of flowing glacier on the earth. A model-based approach was then Greenland in the 2006 field season. The blue line corresponds to the location of the glacier channel. The flight track corresponds to frame 5, segment 4 in used to interpret the DOA estimation of the bed return. Further the data set acquired May 30, 2006. analysis showed that the backscattering characteristics of the ice–bed could be estimated by combining the DOA data and folded dipoles mounted in the across-track direction. The the radar waveform data. Based on the data, the across-track array was divided into two five-element subarrays installed slope of the bed was estimated as a fitted model parameter. under each wing, as shown in Fig. 1. The left wing subarray Finally, information on the bed roughness in terms of the rms was used for transmission and the right for reception. All slope was obtained by forward modeling using the incoherent elements in the transmit array were excited with uniform μ Kirchhoff model (IKM). weights during transmission. The pulse length was 10 s with a total transmit power of 800 W. A multichannel receiver was C. Paper Outline used to sample signals from each receive-antenna element This paper is organized as follows. Section II provides individually. The spacing of the effective phase centers was . λ λ details on the MCRDS system and the associated data set. approximately 0 3 ,where is the wavelength in the free A signal model is presented in Section III along with algo- space of the center frequency. rithms for DOA estimation. In Section IV, the algorithms are Data acquired with the MCRDS system in 2006 at the applied to data and used to provide an alternative representa- Jakobshavn Glacier were used for the DOA analysis. The tion based on DOA. This representation is used for detection data were acquired according to the flight track shown in of the bed in Section V and for retrieval of its backscat- Fig.
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