scholarly journals Seasat Range Measurements Verified on a 3-D Ice Sheet

1986 ◽  
Vol 8 ◽  
pp. 69-72 ◽  
Author(s):  
N.S. Gundestrup ◽  
R.A. Bindschadler ◽  
H.J. Zwally
Keyword(s):  
Surface Topography ◽  
Ice Sheet ◽  
Great Distance ◽  
Surface Elevation ◽  
Radar Altimeter ◽  
Satellite Measurements ◽  
Volume Changes ◽  
Differential Measurement ◽  
Range Measurements ◽  
True Surface

The Seasat radar altimeter observations of a 100 km2 area in South Greenland are compared to a detailed, ground-based survey, using “geoceivers” and pressure altimeters. The comparison shows the Seasat measurement of distance between satellite and earth to be accurate to the level of the geoceiver determined surface (±2 m). Due to the great distance between satellite and surface, finer details of surface topography are not revealed in the satellite measurements. As the satellite tends to lock onto hills in the vicinity of the sub-satellite track, the satellite tends to overestimate the true surface elevation. However, a similar altimeter would make a similar overestimate, allowing accurate differential measurement of volume changes between the two surveys.

scholarly journals Seasat Range Measurements Verified on a 3-D Ice Sheet

1986 ◽  
Vol 8 ◽  
pp. 69-72 ◽  
Author(s):  
N.S. Gundestrup ◽  
R.A. Bindschadler ◽  
H.J. Zwally
Keyword(s):  
Surface Topography ◽  
Ice Sheet ◽  
Great Distance ◽  
Surface Elevation ◽  
Radar Altimeter ◽  
Satellite Measurements ◽  
Volume Changes ◽  
Differential Measurement ◽  
Range Measurements ◽  
True Surface

The Seasat radar altimeter observations of a 100 km2 area in South Greenland are compared to a detailed, ground-based survey, using “geoceivers” and pressure altimeters. The comparison shows the Seasat measurement of distance between satellite and earth to be accurate to the level of the geoceiver determined surface (±2 m). Due to the great distance between satellite and surface, finer details of surface topography are not revealed in the satellite measurements. As the satellite tends to lock onto hills in the vicinity of the sub-satellite track, the satellite tends to overestimate the true surface elevation. However, a similar altimeter would make a similar overestimate, allowing accurate differential measurement of volume changes between the two surveys.

scholarly journals Surface Topography of the Greenland Ice Sheet by Satellite Radar Altimetry (Abstract)

1986 ◽  
Vol 8 ◽  
pp. 196 ◽  
Author(s):  
R.A. Bindschadler ◽  
H.J. Zwally
Keyword(s):  
Surface Topography ◽  
Data Center ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Surface Elevation ◽  
Earth Tides ◽  
Altimeter Data ◽  
Atmospheric Effects ◽  
Range Data ◽  
Science Data

A map of the surface elevation for the southern half of the Greenland ice sheet has been produced from data gathered by the radar altimeter on board the SEASAT satellite. From June 1978 until September 1978, useful data were collected during most passes over the ice sheet, but data was not collected continuously along each pass. Over 85 000 separate ranges were obtained from the satellite to the surface at points spaced 662 m apart along each orbital pass. Techniques required for the reduction of the recorded return waveforms to surface elevations have previously been described in a series of papers (Martin and others, 1983; Brenner and others, 1983; and Zwally and others, 1983). Once all corrections have been applied to the range data due to atmospheric effects, ocean and earth tides, and orbital perturbations, the set of ranges at orbital crossing points (where ascending orbits crossed descending orbits) had a mean relative error of 2.9 m, with a standard deviation of ±2.9 m. Elevations over the flatter and smoother portions of the ice sheet have a precision as small as ±0.25 m, while data over sloping and rough areas are of lower quality. Along each orbital track, the data are corrected for the slope-induced error. The reduced set of surface elevations has been interpolated to assigned elevation values at the nodal points of a regular grid with a 10 km spacing (polar stereographic projection). This grid was then contoured at intervals of 50 m above 2400 m altitude and 100 m at lower elevations. Similar grids of slope-induced error corrections were contoured to provide some measure of its effect on the data. Ancillary plots of parameters of the fitting and gridding process are included to help in estimating the quality of the derived surface topography in different regions. The surface elevation contour map shows the existence of distinct drainage basins within the ice sheet — most notably in the southern and eastern areas. This detail will prove most useful in the delineation of these basins for hydrological or glaciological studies. In combination with ice-thickness data, these elevation data permit a more accurate measurement of the bedrock elevation. The corrected altimeter data in orbital-pass and map format have been provided to the National Space Science Data Center at Goddard Space Flight Center and to the World Data Center-A, Glaciology, as a source of information to be used by other scientific investigators. These data have already been used to produce detailed maps of the topography in more localized areas (e.g. Figure 2, from Zwally and others, 1983 and Figure 2 of Bindschadler, 1984).

scholarly journals Ice-Sheet Surface Elevation and Changes Observable by Satellite Radar Altimetry

1979 ◽  
Vol 24 (90) ◽  
pp. 491-493 ◽  
Author(s):  
H. Jay Zwally ◽  
R. L. Brooks ◽  
H. Ray Stanley ◽  
W. J. Campbell
Keyword(s):  
Drainage Basin ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
The State ◽  
Surface Elevation ◽  
Radar Altimeter ◽  
Major Question ◽  
The Status ◽  
Satellite Radar ◽  
Ice Sheet Dynamics

Abstract A major question in ice-sheet dynamics is the state of balance between the net mass input and ice flow. Since an imbalance produces a change in surface elevation, the state of balance can be studied by monitoring the elevation, and this has been accomplished by surface-leveling techniques in a few locations. Due to the requirement for accurate and repetitive measurements over large areas, it is not practical to determine the status of balance of an entire ice sheet or even a major drainage basin by conventional techniques. Now, recent results from satellite-borne radar altimeter measurements over the Greenland ice sheet demonstrate the feasibility of accurately measuring and monitoring the topography of large ice masses. The application of this new technique offers the possibility of making a meaningful mass-balance determination and for detecting actual or potential ice-sheet surges.

scholarly journals Satellite Altimetry, Semivariograms, and Seasonal Elevation Changes in the Ablation Zone of West Greenland

1990 ◽  
Vol 14 ◽  
pp. 158-163 ◽  
Author(s):  
Craig S. Lingle ◽  
Anita C. Brenner ◽  
H. Jay Zwally
Keyword(s):  
Noise Level ◽  
Satellite Altimetry ◽  
Ice Sheet ◽  
Surface Elevation ◽  
Altimeter Data ◽  
Ablation Zone ◽  
Radar Altimeter ◽  
West Greenland ◽  
Elevation Changes ◽  
The Mean

Seasonal mean changes in the surface elevation of the ablation zone of West Greenland to 72°N between spring 1985 and summer 1986 are measured using radar altimeter data from the 18-month Geosat Geodetic Mission. Semi-variograms are used to estimate the noise in the data as a function of position on the ice sheet. Mean elevation changes are computed by averaging the elevation differences measured at points where orbits ascending in latitude are later crossed by orbits descending in latitude (or the reverse), with each cross-over difference weighted in proportion to the inverse square of the noise level in the neighborhood of the cross-over point. The mean surface elevation of the ablation zone, relative to spring 1985, ranged from 1.5 ± 0.6 m lower during summer 1985 to 1.7 ± 0.4 m higher during spring 1986.

scholarly journals An improved elevation dataset for climate and ice-sheet modelling: validation with satellite imagery

1997 ◽  
Vol 25 ◽  
pp. 439-444 ◽  
Author(s):  
J. L. Bamber ◽  
R. A. Bindschadler
Keyword(s):  
High Resolution ◽  
General Circulation ◽  
Global Climate ◽  
Ice Sheet ◽  
General Circulation Models ◽  
Horizontal Resolution ◽  
Surface Elevation ◽  
Altimeter Data ◽  
Ice Thickness ◽  
Radar Altimeter

Recent studies by several groups have indicated that the performance of general circulation models (GCMs) over the ice sheets is severely limited by the relatively low resolution of the models at the margins, where surface slopes are greatest. To provide accurate energy-budget estimates, resolutions of better than 0.5° are desirable, requiring nested or multiple gridding and accurate, high-resolution boundary conditions. Here we present a new, high-resolution (5 km) digital elevation model for the Antarctic ice sheet, derived from radar-altimeter data obtained from the geodetic phase of the satellite, ERS-1. These data have been combined with the revised ice-thickness grid reported in Bamber and Huybrechts (1996) to produce a bed- and surface-elevation dataset for use in regional and global climate and paleo-climaie modelling applications. The real level of spatial detail in the datasets has been examined with the aid of Landsat Thematic Mapper data. Imagery around Ice Stream D, West Antarctica, shows that the revised ice-thickness grid is accurately geolocated, and contains valuable fine-scale topographic detail beyond that available from the cartographic version of the data (Drewry, 1983). The surface topography in the region of the Ross Ice Shelf has been used to illustrate the level of detail in both the vertical and horizontal resolution of (he surface dataset. Laudsat data has also been used to examine features in the surface-elevation data. In particular, the location of the grounding zone, for Ice Streams D and E, derived from the two data sources shows good agreement. The results of this validation underscore the utility of the new datasets for high-resolution modelling, and highlight the limitations of the Folio maps for such applications.

scholarly journals Boundary conditions of an active West Antarctic subglacial lake: implications for storage of water beneath the ice sheet

2013 ◽  
Vol 7 (3) ◽  
pp. 2979-2999 ◽  
Author(s):  
M. J. Siegert ◽  
N. Ross ◽  
H. Corr ◽  
B. Smith ◽  
T. Jordan ◽  
...  
Keyword(s):  
Ice Sheet ◽  
Surface Elevation ◽  
Altimeter Data ◽  
Radar Altimeter ◽  
Local Flow ◽  
Antarctic Ice Sheet ◽  
Subglacial Lake ◽  
West Antarctic ◽  
Ice Surface ◽  
The Antarctic

Abstract. Repeat-pass IceSat altimetry has revealed 124 discrete surface height changes across the Antarctic Ice Sheet, interpreted to be caused by subglacial lake discharges (surface lowering) and inputs (surface uplift). Few of these active lakes have been confirmed by radio-echo sounding (RES) despite several attempts (notable exceptions are Lake Whillans and three in the Adventure Subglacial Trench). Here we present targeted RES and radar altimeter data from an "active lake" location within the upstream Institute Ice Stream, into which 0.12 km3 of water is calculated to have flowed between October 2003 and February 2008. We use a series of transects to establish an accurate appreciation of the influences of bed topography and ice-surface elevation on water storage potential. The location of surface height change is over the downslope flank of a distinct topographic hollow, where RES reveals no obvious evidence for deep (> 10 m) water. The regional hydropotential reveals a sink coincident with the surface change, however. Governed by the location of the hydrological sink, basal water will likely "drape" over existing topography in a manner dissimilar to subglacial lakes where flat strong specular RES reflections are measured. The inability of RES to detect the active lake means that more of the Antarctic ice sheet bed may contain stored water than is currently appreciated. Variation in ice surface elevation datasets leads to significant alteration in calculations of the local flow of basal water indicating the value of, and need for, high resolution RES datasets in both space and time to establish and characterise subglacial hydrological processes.

scholarly journals Boundary conditions of an active West Antarctic subglacial lake: implications for storage of water beneath the ice sheet

2014 ◽  
Vol 8 (1) ◽  
pp. 15-24 ◽  
Author(s):  
M. J. Siegert ◽  
N. Ross ◽  
H. Corr ◽  
B. Smith ◽  
T. Jordan ◽  
...  
Keyword(s):  
Ice Sheet ◽  
Surface Elevation ◽  
Altimeter Data ◽  
Radar Altimeter ◽  
Local Flow ◽  
Antarctic Ice Sheet ◽  
Subglacial Lake ◽  
West Antarctic ◽  
Ice Surface ◽  
The Antarctic

Abstract. Repeat-pass ICESat altimetry has revealed 124 discrete surface height changes across the Antarctic Ice Sheet, interpreted to be caused by subglacial lake discharges (surface lowering) and inputs (surface uplift). Few of these active lakes have been confirmed by radio-echo sounding (RES) despite several attempts (notable exceptions are Lake Whillans and three in the Adventure Subglacial Trench). Here we present targeted RES and radar altimeter data from an "active lake" location within the upstream Institute Ice Stream, into which at least 0.12 km3 of water was previously calculated to have flowed between October 2003 and February 2008. We use a series of transects to establish an accurate depiction of the influences of bed topography and ice surface elevation on water storage potential. The location of surface height change is downstream of a subglacial hill on the flank of a distinct topographic hollow, where RES reveals no obvious evidence for deep (> 10 m) water. The regional hydropotential reveals a sink coincident with the surface change, however. Governed by the location of the hydrological sink, basal water will likely "drape" over topography in a manner dissimilar to subglacial lakes where flat strong specular RES reflections are measured. The inability of RES to detect the active lake means that more of the Antarctic ice sheet bed may contain stored water than is currently appreciated. Variation in ice surface elevation data sets leads to significant alteration in calculations of the local flow of basal water indicating the value of, and need for, high-resolution altimetry data in both space and time to establish and characterise subglacial hydrological processes.

scholarly journals Determination of the Surface and Bed Topography in Central Greenland

1990 ◽  
Vol 36 (122) ◽  
pp. 17-30 ◽  
Author(s):  
Steven M. Hodge ◽  
David L. Wright ◽  
Jerry A. Bradley ◽  
Robert W. Jacobel ◽  
Neils Skou ◽  
...  
Keyword(s):  
Surface Topography ◽  
Sea Level ◽  
Bottom Topography ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Ice Thickness ◽  
Radar Altimeter ◽  
The South ◽  
South West ◽  
Ice Radar

AbstractThe surface and bottom topography of the central Greenland ice sheet was determined from airborne ice-radar soundings over a 180 km by 180 km grid centered on the 1974 “Summit” site (lat. 72°18′N., long. 37°55′W.), using the Technical University of Denmark 60 MHz ice radar. Over 6100 km of high-quality radar data were obtained, covering over 99'% of the grid, along lines spaced 12.5 km apart in both north-south and east-west directions. Aircraft location was done with an inertial navigation system (INS) and a pressure altimeter, with control provided by periodically flying over a known point at the center of the grid. The ice radar was used to determine ice thickness; the surface topography was determined independently using height-above-terrain measurements from the aircraft’s radar altimeter. The calculated surface topography is accurate to about ±6 m, with this error arising mostly from radar-altimeter errors. The ice thickness and bottom topography are accurate to about ±50 m, with this error dominated by the horizontal navigation uncertainties due to INS drift; this error increases to about ±125 m in areas of rough bottom relief (about 12% of the grid).The highest point on Greenland is at lat. 72°34′ N., long. 37°38′W., at an altitude of 3233 ± 6 m a.s.l. The ice surface at this point divides into three sectors, one facing north, one east-south-east, and one west-south-west, with each having a roughly uniform slope. The ice divide between the last two sectors is a well-defined ridge running almost due south. The ice is about 3025 m thick at the summit. Excluding the mountainous north-east corner of the grid, where the ice locally reaches a thickness of about 3470 m and the bed dips to about 370 m below sea-level, the maximum ice thickness, approximately 3375 m, occurs about 97 km south-south-west of the summit. The average bed altitude over the entire grid is 180 m and the average ice thickness is 2975 ± 235 m. The ice in most of the south-west quadrant of the grid is over 3200 m thick, and overlies a relatively smooth, flat basin with altitudes mostly below sea-level. There is no predominant direction to the basal topography over most of the grid; it appears to be undulating, rolling terrain with no obvious ridge/valley structure. The summit of the ice sheet is above the eastern end of a relatively large, smooth, flat plateau, about 10–15 km wide and extending about 50 km to the west. If the basal topography were the sole criterion, then a site somewhere on this plateau or in the south-west basin would be suitable for the drilling of a new deep ice core.

Determination of the Surface and Bed Topography in Central Greenland

1990 ◽  
Vol 36 (122) ◽  
pp. 17-30 ◽  
Author(s):  
Steven M. Hodge ◽  
David L. Wright ◽  
Jerry A. Bradley ◽  
Robert W. Jacobel ◽  
Neils Skou ◽  
...  
Keyword(s):  
Surface Topography ◽  
Sea Level ◽  
Bottom Topography ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Ice Thickness ◽  
Radar Altimeter ◽  
The South ◽  
South West ◽  
Ice Radar

AbstractThe surface and bottom topography of the central Greenland ice sheet was determined from airborne ice-radar soundings over a 180 km by 180 km grid centered on the 1974 “Summit” site (lat. 72°18′N., long. 37°55′W.), using the Technical University of Denmark 60 MHz ice radar. Over 6100 km of high-quality radar data were obtained, covering over 99'% of the grid, along lines spaced 12.5 km apart in both north-south and east-west directions. Aircraft location was done with an inertial navigation system (INS) and a pressure altimeter, with control provided by periodically flying over a known point at the center of the grid. The ice radar was used to determine ice thickness; the surface topography was determined independently using height-above-terrain measurements from the aircraft’s radar altimeter. The calculated surface topography is accurate to about ±6 m, with this error arising mostly from radar-altimeter errors. The ice thickness and bottom topography are accurate to about ±50 m, with this error dominated by the horizontal navigation uncertainties due to INS drift; this error increases to about ±125 m in areas of rough bottom relief (about 12% of the grid).The highest point on Greenland is at lat. 72°34′ N., long. 37°38′W., at an altitude of 3233 ± 6 m a.s.l. The ice surface at this point divides into three sectors, one facing north, one east-south-east, and one west-south-west, with each having a roughly uniform slope. The ice divide between the last two sectors is a well-defined ridge running almost due south. The ice is about 3025 m thick at the summit. Excluding the mountainous north-east corner of the grid, where the ice locally reaches a thickness of about 3470 m and the bed dips to about 370 m below sea-level, the maximum ice thickness, approximately 3375 m, occurs about 97 km south-south-west of the summit. The average bed altitude over the entire grid is 180 m and the average ice thickness is 2975 ± 235 m. The ice in most of the south-west quadrant of the grid is over 3200 m thick, and overlies a relatively smooth, flat basin with altitudes mostly below sea-level. There is no predominant direction to the basal topography over most of the grid; it appears to be undulating, rolling terrain with no obvious ridge/valley structure. The summit of the ice sheet is above the eastern end of a relatively large, smooth, flat plateau, about 10–15 km wide and extending about 50 km to the west. If the basal topography were the sole criterion, then a site somewhere on this plateau or in the south-west basin would be suitable for the drilling of a new deep ice core.

scholarly journals Satellite Altimetry, Semivariograms, and Seasonal Elevation Changes in the Ablation Zone of West Greenland

1990 ◽  
Vol 14 ◽  
pp. 158-163 ◽  
Author(s):  
Craig S. Lingle ◽  
Anita C. Brenner ◽  
H. Jay Zwally
Keyword(s):  
Noise Level ◽  
Satellite Altimetry ◽  
Ice Sheet ◽  
Surface Elevation ◽  
Altimeter Data ◽  
Ablation Zone ◽  
Radar Altimeter ◽  
West Greenland ◽  
Elevation Changes ◽  
The Mean

Seasonal mean changes in the surface elevation of the ablation zone of West Greenland to 72°N between spring 1985 and summer 1986 are measured using radar altimeter data from the 18-month Geosat Geodetic Mission. Semi-variograms are used to estimate the noise in the data as a function of position on the ice sheet. Mean elevation changes are computed by averaging the elevation differences measured at points where orbits ascending in latitude are later crossed by orbits descending in latitude (or the reverse), with each cross-over difference weighted in proportion to the inverse square of the noise level in the neighborhood of the cross-over point. The mean surface elevation of the ablation zone, relative to spring 1985, ranged from 1.5 ± 0.6 m lower during summer 1985 to 1.7 ± 0.4 m higher during spring 1986.

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