Xiaoqing Wu

Ice Sheet Bed Mapping With Airborne SAR Tomography

We develop and then demonstrate a 3-D tomographic ice sounding method applied to very high frequency (VHF) radar data that produces swath measurements of ice sheet surface topography, ice thickness, and radar reflectivity of both the surface and bed of the ice sheet. First, we formulate the ice sheet imaging problem as a problem of estimating signal arrival angles and illustrate how the method resolves ambiguous echoes arriving simultaneously from the left and right sides of the aircraft, as well as from the surface and base of the ice sheet. We then discuss why we chose the time-domain subaperture method for 2-D image formation for ice sounding. We apply the tomographic technique to the data that we collected in May 2006 and again in July 2008 from a multiple-phase-center VHF radar system. We present 3-D images of the upper and lower surfaces of the ice sheet and compare the estimated surface topography with Ice, Cloud, and land Elevation Satellite altimeter nadir track measurements and the measured swath ice thickness with independent nadir depth sounder tracks. We achieved a 5-m surface topography accuracy and a 14-m ice thickness accuracy.

Glaciers and Ice Sheets Mapping Orbiter concept

[1]xa0We describe a concept for a spaceborne radar system designed to measure the surface and basal topography of terrestrial ice sheets and to determine the physical properties of the glacier bed. Our primary objective is to develop this new technology for obtaining spaceborne estimates of the thickness of the polar ice sheets with an ultimate goal of providing essential information to modelers estimating the mass balance of the polar ice sheets and estimating the response of ice sheets to changing climate. Our new technology concept employs VHF and P-band interferometric radars using a novel clutter rejection technique for measuring surface and bottom topographies of polar ice sheets from aircraft and spacecraft. Our approach will enable us to reduce signal contamination from surface clutter, measure the topography of the glacier bed at better than 1 km intervals with an accuracy of 20 m, and paint a picture of variations in bed characteristics. The technology will also have applications for planetary exploration including studies of the Martian ice caps and the icy moons of the outer solar system. Through the concept developed here we believe that we can image the base and map the three-dimensional basal topography beneath an ice sheet at up to 5 km depth.

Two-Frequency Radar Experiments for Sounding Glacier Ice and Mapping the Topography of the Glacier Bed

We performed airborne experiments using 150- and 450-MHz radars to measure ice thickness on the Greenland ice sheet. Our objectives were to investigate to what degree surface clutter obscures the basal echo when airborne measurements are made at different elevations and at different frequencies. We also explored interferometric techniques for processing the data to form swath measurements of ice thickness. We found that surface clutter was minimal for either frequency when operated at low aircraft elevations (500 m above the ice sheet surface) or over benign regions of the ice sheet. Because signal-to-clutter ratios were favorable, we found that we could retrieve the swath measurements of ice thickness at both frequencies using an interferometric technique. At high elevation, surface clutter degraded the 150-MHz signal, but the nadir ice thickness was still retrievable. The basal return in high-elevation 450-MHz data was detectable only after additional beam-steering techniques were applied to the data to reduce the surface clutter signal. Results suggest that interferometric cross-track ice-thickness measurements can be successfully made given a sufficient number of antenna elements driven at either 150 or 450 MHz and flown at both high and low elevations over the interior ice sheet.

Radar images of the bed of the Greenland Ice Sheet

[1]xa0In this paper, we apply radar tomography methods to very-high-frequency, airborne synthetic-aperture radar data to measure the ice thickness field and to construct three-dimensional basal image maps of a 5 × 20 km study area located along the southern flank of the Jakobshavn Glacier, Greenland. Unlike ice radar measurements typically made at nadir, our approach uses radar-echo phase and amplitude measured across an antenna array to determine the propagation angle and signal strength of pixel elements distributed on each side of the aircraft flight path. That information, combined with knowledge of aircraft position and the assumed dielectric properties of the glacier, can be used to measure ice thickness and radar reflectivity across a 3-km wide swath. Combining ice thickness and surface topography data, we estimate basal topography and basal drag. We conclude that the glacier is sliding over the bed. We use the three-dimensional image maps of the bed to inspect the modern subglacial geomorphology and find for the first time beneath the Greenland Ice Sheet assemblages of long ridge-groove landforms that are oriented in the direction of the ice flow. Spatial dimensions (10 to 30 m depths, 150 to 500 m spacing and lengths of 10 km or more) and correlation with the current ice flow direction suggest that these are glacial erosional features similar to mega-grooves observed on deglaciated terrain.

Near nadir Ka-band sar interferometry: SWOT airborne experiment

To better prepare data processing system of the Ka-band Radar Interferometer (KaRIn) for the Surface Water and Ocean Topography (SWOT) [1] mission, we opportunistically collected data over several diverse fresh-water targets in the Van Hook Arm areas of North Dakota, USA, using a Ka-band interferometric radar [2] developed at JPL. To make the collection relevant to SWOT, the aircraft was rolled to direct the antenna boresight toward nadir to mimic the SWOT geometry. Using a modified airborne interferometric SAR processor [3] developed at JPL, we were able to process the collected Ka-band airborne data and produced the height and magnitude image products. These results are the first of the kind for Ka-band interferometric synthethic aperture radar over water surfaces with near nadir looking geometry. These initial results will help to characterize the power returns from water surface and land, and provide guidance for the design of post-processing algorithms including methods for water and land classification. The height accuracy we get from the water surface height images can be used to project the water surface height accuracy we would get from SWOT mission. This airborne experiment will help us verify the SWOT data processing chain and make us better prepared for SWOT data processing task.

Global Ice Sheet Mapping Observatory: Airborne experiments

We describe tests of a radar-system concept envisioned to measure the spatial reflectivity and the 3-dimensional surface and basal topography of terrestrial ice sheets and to determine the physical properties of the glacier bed. We conducted experiments during May 2006 over northwestern Greenland using a 150 MHz radar operated from a Twin Otter aircraft. We conducted more extensive experiments in September 2007 over northern and central Greenland when we operated 150 and 450 MHz radars installed on the NASA P-3 aircraft. We performed a final set of Twin Otter supported 150 MHz experiments in July 2008 near Jacobshavn Glacier in western Greenland. Here we show 150 and 450 MHz interferograms of the ice sheet base. We use both radar interferometry and tomography to construct three-dimensional bottom topography.

Ka-Band Mapping and Measurements of Interferometric Penetration of the Greenland Ice Sheets by the GLISTIN Radar

Measuring ice surface topography over the major ice caps of Greenland and Antarctica is crucial to quantifying and understanding the effect of climate change on the Earths environment. Multiple sensors including radars, lidars, and optical systems have been utilized in making these measurements. To integrate data from these multiple sensors into a coherent and self-consistent history of ice cap topography requires knowledge of where vertically within the snow volume the elevation measurement corresponds. This paper examines the penetration of a Ka-band cross-track interferometric radar into the dry firn at Greenlands summit using the NASA GLISTIN Ka-band interferometric radar. GLISTIN elevation measurements are compared to NASA Wallops Airborne Terrain Mapper lidar and kinematic GPS survey measurements to assess the amount of relative penetration with GPS-surveyed corner reflectors deployed to establish the absolute vertical positioning of the radar data. We found an interferometric penetration depth estimate of 27

A virtual mission to estimate discharge using assimilation of high-resolution simulated SWOT Data: initial results over the Ohio river

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Uavsar program: Recent upgrades to support vegetation structure studies and land ICE topography mapping

0.3 cm. Moreover, we compare these penetration measurements to model derived estimates of the amount of interferometric penetration and provide sensitivity analysis of the amount of penetration to various ice properties. Interferometric radar mapping systems also have the ability to make wide swath topographic measurements over a wide range of weather conditions either day or night making them ideal instruments for wide area mapping. We illustrate this aspect of interferometric radar mapping with a mosaic of 24 passes of the GLISTIN instrument of the Jakobshavn Glacier area.

Interferometric penetration into dry snow and sea ice at Ka-band

In this paper we present a framework for a virtual mission (VM) that 1) models the basin-wide physical and hydraulic processes of a chosen study region, namely the Ohio river basin, 2) samples those processes with a high fidelity SWOT simulation to deliver realistic high-resolution temporally subsampled, spatially dense interferograms, 3) reconstructs the water surface elevation from the synthetic interferometric measurement. This is part of an ongoing effort where the elevation measurements, as during the SWOT mission, will be integrated into a data assimilation framework to estimate discharge. We present results from the key elements of the VM Ohio basin configuration including the study region extent and hydrodynamics model setup. Furthermore, an example of georeferenced reconstructed heights derived from the SWOT instrument simulator over the Ohio modeled region is given. These results illustrate the effects of random (thermal and fading/speckle) noise of the height estimates as well as explicitly evaluating the effects of topographic layover. In the near future the hydrodynamic modeling and synthetic SWOT acquisitions over the region will be run for several virtual months to provide data to support the discharge estimation algorithm development in preparation for SWOT.