W. Cudlip

IDENTIFICATION OF SUBGLACIAL LAKES USING ERS-1 RADAR ALTIMETER

Radar-altimeter data from ERS-1 allow detailed topographic mapping of Antarctica to 82-degrees-S to be carried out, revealing several notable surface features. Among these is the large subglacial lake near Vostok which is mapped here in detail. The central part of the lake is found to have an exceptionally smooth surface with r.m.s. elevation variations of less than 0.2 m. A search for other large Antarctic lakes in the data is made based on the smoothness and low gradient of the surface. A number of other flat areas are identified with lake locations previously determined from radio-echo sounding observations. However, radar-altimeter observations show that a minimum lake size of 20 km is required for a surface above a lake to become flat. Numerous bowl-like features can be seen in the surface topography, and these may be associated with intermediate-sized subglacial lakes. It is determined that high spatial-resolution radar altimetery could be used to identify subglacial lakes greater than 10 km in lateral extent. Flat regions of the ice sheet are particularly useful as they may be used as height-reference surfaces to help fix the orbits of radar-altimeter satellites.

A comparison of the performance of the ice and ocean tracking modes of the ERS-1 radar altimeter over non-ocean surfaces

The European Space Agencys ERS-1 radar altimeter is the first to include separate operating modes to optimise performance over both ocean and non-ocean surfaces. As part of the ERS-1 commissioning activities, we have carried out a study of the tracking performance of this instrument over non-ocean surfaces. Statistics for land ice, sea ice, arid lands, and inland water are presented. Performance in both operating modes is shown to be better than that of previous missions.

Corrections for altimeter low-level processing at the Earth Observation Data Centre

Abstract Many corrections are required to turn the echo time delay measured by a satellite radar altimeter firstly into a range measurement and then into a surface height measurement. This paper describes the instrumental, satellite, atmospheric and surface corrections supplied with the ERS-1 Waveform Product available from the U.K. Earth Observation Data Centre. The mathematical and physical basis for the corrections, and the algorithms used to drive them, are described, together with an assessment of the associated errors.

An assessment of the capability of the satellite radar altimeter for measuring ice sheet topographic change

Abstract The monitoring of surface topography has frequently been cited as the most important application of the satellite radar altimeter over the ice sheets. The potential of the instrument in this respect is examined in detail using Seasat and Geosat altimeter data recorded over the Wilkes plateau in East Antarctica. The precision of the range measurements is optimized by the use of an improved retracking technique. However, although the r.m.s. precision of range measurements indicated at orbit crossovers, after orbit adjustment, is 20-30 cm, tilt and bias orbit adjustment techniques are shown to leave errors in the measurement of surface elevation change, with magnitudes of the order of tens of centimetres, depending on the location. Additional orbit error, which cannot be identified from crossover residuals, is almost certainly present in the difference measurements. Variable mispointing of the Geosat antenna is considered the most likely explanation for an apparent change in surface scattering prope...

The topography and surface characteristics of the Larsen Ice Shelf, Antarctica, using satellite altimetry

A comprehensive survey of the Larsen Ice Shelf has been conducted using precise orbit and re tracked Seasat radar-altimeter data with editing of erroneous values resulting from intrumental artefacts. Contour maps of elevation and radar back-scatter (with absolute accuracies of I m and 2 dB, respectively) have been produced and it has also been possible to map rifts, grounding points, rough terrain, and about 30% of the ice shelfs seaward margin. Ice thicknesses derived from these elevation data show broad agreement with those derived from previous airborne radio-echo surveys. The maps of parameters measured by Seasat represent a very substantial improvement over those previously available . They thus provide a reference against which comparison may be made with a view to detecting substantial climatic changes. This is of particular interest since, as the most northerly major ice shelf in Antarctica, the Larsen Ice Shelf may be more sensitive than others to global climatic trends.

Overview of altimeter data processing at the U.K. Earth Observation Data Centre

Abstract ERS-1 carries the fifth radar altimeter to be launched into Earth orbit. Although similar in concept and design to earlier altimeters, it is the first to include operation over non-ocean surfaces as one of its design requirements. The increased capabilities of the instrument allow for the generation of a variety of data products. The generation of these products calls for an extensive suite of software covering the processing of the raw data into basic ‘waveform products’, and the subsequent generation of higher level, geophysical products covering ocean, sea-ice, land-ice, land and lakes. This paper gives an overview of the ERS-1 altimeter data processing carried out at the U.K. Earth Observation Data Centre (EODC) at Farnborough. Besides processing the data from the satellite, a significant quantity of support data also have to be acquired and processed (e.g., improved orbit ephemerides, meteorological data, ionospheric data, etc.). The initial low-level processing of the raw data into the Wave...

An advanced terrain tracking altimeter

The results of a feasibility study into an advanced radar altimeter capable of operation over all global terrain types are presented. The study completed in June 1988 was undertaken on behalf of the European Space Agency by a team led by British Aerospace. Adaptive processing is used to allow the instrument to maintain maximum resolutir 1 ovcr the Ocean whilst adopting coarser resolution over topographic surfaccs. The possible data products and applications are summarised together with the operation of the tracking algorithms required. An experimental synthetic aperture mode of operation is assessed. The instrument has a built-in radiometer to estimate the wet tropospheric delay correction and a dual frequency altimeter to provide for ionospheric delay corrections.

Satellite Radar Altimetry Over Arid Regions

The response of the Seasat radar altimeter to a variety of arid surfaces in the Taklimakan Shamo in China is investigated. Comparisons with maps based on aerial photography and Landsat imagery demonstrate that the radar altimeter detects differences in surface morphology and provides topographic information complementary to the image data. Results are presented concerning water in the marginal zone of the desert, alluvial fans, small dunes in the sand sheets, intermittent drainage channels, and changes in dune type. The potential of radar altimeter for the study of arid lands is discussed.