HF/VHF Radar Sounding of Ice from Manned and Unmanned Airborne Platforms

Abstract

Ice thickness and bed topography of fast-flowing outlet glaciers are large sources of uncertainty for the current ice sheet models used to predict future contributions to sea-level rise. Due to a lack of coverage and difficulty in sounding and imaging with ice-penetrating radars, these regions remain poorly constrained in models. Increases in off-nadir scattering due to the highly crevassed surfaces, volumetric scattering (due to debris and/or pockets of liquid water), and signal attenuation (due to warmer ice near the bottom) are all impediments in detecting bed-echoes. A set of high-frequency (HF)/very high-frequency (VHF) radars operating at 14 MHz and 30-35 MHz were developed at the University of Kansas to sound temperate ice and outlet glaciers. We have deployed these radars on a small unmanned aircraft system (UAS) and a DHC-6 Twin Otter. For both installations, the system utilized a dipole antenna oriented in the cross-track direction, providing some performance advantages over other temperate ice sounders operating at lower frequencies. In this paper, we describe the platform-sensor systems, field operations, data-processing techniques, and preliminary results. We also compare our results with data from other ice-sounding radars that operate at frequencies both above (Center for Remote Sensing of Ice Sheets (CReSIS) Multichannel Coherent Depth Sounder (MCoRDS)) and below (Jet Propulsion Laboratory (JPL) Warm Ice Sounding Explorer (WISE)) our HF/VHF system. During field campaigns, both unmanned and manned platforms flew closely spaced parallel and repeat flight lines. We examine these data sets to determine image coherency between flight lines and discuss the feasibility of forming 2D synthetic apertures by using such a mission approach.

Five decades of radioglaciology

“…The tracing of englacial isochrones in the radar data acquired over Greenland between 1993 and 2013 by the University of Kansas Center for Remote Sensing of Ice Sheets and OIBis a vital step forward in the efforts to make englacial stratigraphic information readily available (Gogineni and others, 1998, 2001; MacGregor and others, 2015a; Arnold and others, 2018). The resulting data archive has increased the availability of traced isochrones by orders of magnitude.…”

CReSIS airborne radars and platforms for ice and snow sounding

“…The MCoRDS/I system has improved surface clutter rejection due to the electrically larger antenna aperture, while the HF Sounder has improved penetration through temperate ice. When an echogram from a single channel of the MCoRDS/I is generated, the bed in this region is completely undetectable (Arnold and others, 2018), again illustrating the MCoRDS/I's improved clutter rejection. This has motivated CReSIS researchers to explore multi-pass processing (flying the same flightline, offset by a few meters, multiple times to create a large synthetic aperture in the cross-track).…”

“…5.Comparison of echograms generated from the HF Sounder Mini (left) to the MCoRDS VHF Sounder on the P-3 (right). As can be seen from the HF Sounder echogram, the ice bottom is detected nearly 100% of the time along the flight line including in locations where MCoRDS did not detect the bottom (Arnold and others, 2018). …”

“…The HF Sounder Mini data was also compared to the Jet Propulsion Laboratory's Warm Ice Sounding Explorer (WISE) radar in this region. The WISE radar operates with a center frequency between 2 and 2.5 MHz (Rignot and others, 2013; Mouginot and others, 2014); despite this lower frequency of operation, the HF Sounder demonstrated improved performance (Arnold and others, 2018).

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Helicopter Test of a Strapdown Airborne Gravimetry System

“…This location was chosen because it has previously been surveyed using the SGL AIRGrav system within NASA’s Operation IceBridge (OIB). Since the fast-flowing ice stream is more than 1 km thick [15], the relatively fast-moving fixed-wing aircraft flown by OIB is not expected to capture the full resolution of the gravity signal in this area. The slower-moving helicopter platform may therefore add information to the gravity signal over the glacier, while the OIB flight lines provide a reference for the helicopter estimates.…”