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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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    Ice sheet topography by satellite altimetry (1978)
    In:  Other Sources
    Details
    Publication Date: 2011-08-17
    Description: The measured time between the transmission and rzturn of 13.9 GHz radar pulses from the GEOS 3 satellite (at a mean altitude of 844.5 km and an inclination of 114 deg 52 min) is used to determine the thickness of the Greenland ice cap, with an accuracy in surface elevation on the order of 2 m. Attention is given to changes in ice thickness as an indicator of climatic change in general, and change in mean sea level in particular. Each elevation data point obtained by the satellite represents an average along 0.67 km of ground track, and three-dimensional maps are presented to illustrate the data.
    Keywords: EARTH RESOURCES AND REMOTE SENSING
    Type: Nature; 274; Aug. 10
    Format: text
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  • 3
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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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  • 4
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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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  • 5
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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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  • 6
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    Satellite passive-microwave observations and the distribution of net surface accumulation in the interior of Greenland (1991)
    In:  Other Sources
    Details
    Publication Date: 2019-01-25
    Description: The rate of net accumulation at the surface and its distribution in ice sheet areas bounded by the dry snow line may be approximated by a determination of either firn external emissivity or various other emissivity terms. A compilation of mean annual brightness temperature for the Greenland ice sheet based on Nimbus-5 Electrically Scanning Microwave Radiometer (19 GHz) data is used to approximate the distribution of the accumulation rate in the area bounded by the dry snow line, applying a simple conversion involving a polynomial function. This is compared with a compilation produced elsewhere using Nimbus-6 SCAMS (31 GHz) data, applying a complex conversion involving the temperature dependence of the absorption coefficient, and with the latest compilation based on surface measurements of the accumulation rate. The findings indicate that in Greenland, the use of polynomial functions to convert for net surface accumulation is a reliable and simple method for estimating both the rate of accumulation and its distribution in areas where there is no melting at the surface.
    Keywords: EARTH RESOURCES AND REMOTE SENSING
    Type: Calgary Univ., Proceedings of the 14th Canadian Symposium on Remote Sensing; p 502-507
    Format: text
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  • 7
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    Matrix partitioning and EOF/principal component analysis of Antarctic Sea ice brightness temperatures (1984)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: A field of measured anomalies of some physical variable relative to their time averages, is partitioned in either the space domain or the time domain. Eigenvectors and corresponding principal components of the smaller dimensioned covariance matrices associated with the partitioned data sets are calculated independently, then joined to approximate the eigenstructure of the larger covariance matrix associated with the unpartitioned data set. The accuracy of the approximation (fraction of the total variance in the field) and the magnitudes of the largest eigenvalues from the partitioned covariance matrices together determine the number of local EOF's and principal components to be joined by any particular level. The space-time distribution of Nimbus-5 ESMR sea ice measurement is analyzed.
    Keywords: EARTH RESOURCES AND REMOTE SENSING
    Type: NASA-TM-83916 , NAS 1.15:83916
    Format: application/pdf
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  • 8
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    Corrections of anomalous time dependent shifts in the brightness temperature from the Nimbus 5 ESMR (1980)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Temporal variations of the brightness temperature of selected regions representing ocean, sea ice, and ice sheets were studied. Shifts that could not be ascribed to geophysical effects were found to occur during the same periods in the various regions. The procedures employed to correct for these shifts are described.
    Keywords: EARTH RESOURCES AND REMOTE SENSING
    Type: NASA-TM-82055
    Format: application/pdf
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  • 9
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    Satellite-derived ice data sets no. 1: Antarctic monthly average microwave brightness temperatures and sea-ice concentrations, 1973 - 1976 (1981)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: A summary data set concerning 4 years of Antarctic sea-ice conditions was created and is available on magnetic tape. The data were derived from electrically scanning microwave radiometer brightness temperatures and were mapped into a polar stereographic grid enclosing the 50 deg S latitude circle. The grid size varies from about 32 by 32 sq km at the poles to about 28 by 28 sq km at 50 deg S. The microwave brightness temperatures of Antarctic sea ice are predominantly characteristic of first-year ice with an emissivity of 0.92 at 19 GHz frequency. Sea ice concentrations were calculated from the brightness temperature data for each grid element with an algorithm that uses an emissivity value of 0.92 and an ice physical temperature estimate from climatological surface air temperatures. Monthly, multiyear monthly, and yearly maps of brightness temperatures and sea ice concentrations were created for the 4 years, except for 7 months for which useable data were insufficient.
    Keywords: EARTH RESOURCES AND REMOTE SENSING
    Type: NASA-TM-83812
    Format: application/pdf
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  • 10
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    Microwave emission from snow and glacier ice (1975)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: The microwave brightness temperature for snow fields was studied assuming that the snow cover consists of closely packed scattering spheres which do not interact coherently. The Mie scattering theory was used to compute the volume scattering albedo. It is shown that in the wavelength range from 0.8 to 2.8 cm, most of the micro-radiation emanates from a layer 10 meters or less in thickness. It is concluded that it is possible to determine snow accumulation rates as well as near-surface temperature.
    Keywords: EARTH RESOURCES AND REMOTE SENSING
    Type: NASA-TM-X-70871 , X-910-75-36
    Format: application/pdf
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