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  • 1
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    Comparisons between Geosat and Seasat tracking over non-ocean surfaces (1990)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: The differences in both the quantity and quality of the nonocean measurements of the Geosat and Seasat altimeters due to the tracker modifications and satellite and antenna design changes are presented. The Geosat tracker exhibits more agility over the rougher surfaces and therefore is able to maintain lock better than Seasat. Large off-nadir attitude excursions created differences in Geosat tracking between cycles. The Geosat altimeter was at times able to track over surfaces Seasat could not, but sometimes the acquisition problem was worse. It is observed that Geosat yields denser coverage, but over land Seasat measurements yield a better overall picture of the general topography. It is concluded that the Geosat elevation data set over land can best be utilized to augment the information obtained from the Seasat data and is most useful when investigating topography at scales less than 100 km.
    Keywords: GEOPHYSICS
    Type: Geophysical Research Letters (ISSN 0094-8276); 17; 1537-154
    Format: text
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  • 2
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    Slope-induced errors in radar altimetry over continental ice sheets (1983)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: Altimeter measurements of ranges to modelled irregular surfaces are simulated and two correction schemes are used to reconstruct the modelled surfaces from the simulated data. When the cross-track slope is negligible, the problem can be treated as two-dimensional and the relocation method is preferred to the slope correction method. In a two-dimensional test case, 85 percent of the slope-induced rms error was removed by the relocation method. The mean error along profiles of about 75 km or longer is usually reduced more than the rms error. An alternative slope correction scheme, which uses the local slope to calculate the expected error, is less effective in the two-dimensional case. Over a simulated three-dimensional surface, where groundtracks are widely spaced and cross-track slopes are significant, the slope-correction method must be used in at least the cross-track direction.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research; 88; Feb. 28
    Format: text
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  • 3
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    Analysis and retracking of continental ice sheet radar altimeter waveforms (1983)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: The Seasat-1 radar altimeter data set acquired over both the Antarctic and Greenland continental ice sheets is analyzed to obtain corrected ranges to the ice surface. The radar altimeter functional response over the continental ice sheets is considerably more complex than over the oceans. Causal factors identified in this complicated response include sloping surfaces, undulating ice surfaces with characteristic wavelengths on the same spatial scale as the altimeter beam-limited footprint, off-track reflections, and dynamic lag of the altimeter tracking circuit. Retracking methods using the altimeter return pulse waveforms give range corrections that are typically several meters. The entire set of Seasat-1 altimetry over the continental ice sheets is being retracked by fitting a multi-parameter function to each waveform. Many waveforms have double ramps indicating near-normal reflections from two distinct portions of the ice surface within the altimeter beam. Two independent range measurements differing by less than 25 m are obtained from retracking the double-ramp waveforms.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research; 88; Feb. 28
    Format: text
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  • 4
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    Surface elevation contours of Greenland and Antarctic ice sheets (1983)
    In:  Other Sources
    Details
    Publication Date: 2011-08-18
    Description: Preliminary results from Seasat radar altimetry over Antarctica north of 72 deg S and Greenland south of 72 deg N are presented. Surface elevations of the ice sheets, obtained from computer retracking of the radar altimeter waveforms, are contoured at 50-m intervals for Greenland and at 100-m intervals for Antarctica. Elevation differences at orbital crossover points are analyzed to obtain a precision of 1.9 m; this figure is partly determined by radial errors of approximately 1.0 m in orbital determination and partly by noise due to ice surface irregularities. Adjustment of the radial components of the orbits to minimize the differences in elevations at crossovers over a small, relatively flat region reduces the rms difference to 0.25 m, which is indicative of the optimum precision obtainable over the ice sheets. However, the precision degrades as the slope of the surface or amplitude of the undulations increases, yielding an overall precision of + or - 1.6 m.
    Keywords: GEOPHYSICS
    Type: Journal of Geophysical Research; 88; Feb. 28
    Format: text
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