Airborne Radar Sounding Research Articles

Airborne radar sounder for temperate ice: initial results from Patagonia

AbstractWe describe the development of a low-frequency airborne radar specifically designed for the sounding of temperate ice. The system operates at a central frequency of 1 MHz and consists of an impulse transmitter with an output voltage up to 5000 V and a digital receiver with a maximum gain of 80 dB. The radar was deployed on board a CASA 212 aircraft, which also carries a laser altimeter, an inertial navigation system, a digital camera and a GPS receiver. A description of the radar system is provided, as well as preliminary results obtained at Glaciar Tyndall, Campo de Hielo Sur (Southern Patagonia Icefield), where an ice depth of 670 m was reached.

Along-Track Focusing of Airborne Radar Sounding Data From West Antarctica for Improving Basal Reflection Analysis and Layer Detection

This paper presents focused synthetic aperture radar (SAR) processing of airborne radar sounding data acquired with the High-Capability Radar Sounder system at 60 MHz. The motivation is to improve basal reflection analysis for water detection and to improve layer detection and tracking. The processing and reflection analyses are applied to data from Kamb Ice Stream, West Antarctica. The SAR processor correlates the radar data with reference echoes from subsurface point targets. The references are 1-D responses limited by the pulse nadir footprint or 2-D responses that include echo tails. Unfocused SAR and incoherent integration are included for comparison. Echoes are accurately preserved from along-track slopes up to about 0.5deg for unfocused SAR, 3deg for 1-D correlations, and 10deg for 2-D correlations. The noise/clutter levels increase from unfocused SAR to 1-D and 2-D correlations, but additional gain compensates at the basal interface. The basal echo signal-to-noise ratio improvement is typically about 5 dB, and up to 10 dB for 2-D correlations in rough regions. The increased noise degrades the clarity of internal layers in the 2-D correlations, but detection of layers with slopes greater than 3deg is improved. Reflection coefficients are computed for basal water detection, and the results are compared for the different processing methods. There is a significant increase in the detected water from unfocused SAR to 1-D correlations, indicating that substantial basal water exists on moderately sloped interfaces. Very little additional water is detected from the 2-D correlations. The results from incoherent integration are close to the focused SAR results, but the noise/clutter levels are much greater.

A link between microwave extinction length, firn thermal diffusivity, and accumulation rate in West Antarctica

[1] The relationship between time series of physical surface temperature and microwave brightness temperature of polar firn depends both on the physical properties of the firn and the surface temperature history. In perennially dry firn this relationship is well characterized by a timescale, referred to as the extinction-diffusion time, which is the ratio of the microwave extinction length squared to the firn thermal diffusivity. The extinction-diffusion time is calculated over Antarctica from 1982 to 1999 by comparing thermal infrared observations of physical surface temperature from the advanced very high resolution radiometer (AVHRR) with passive microwave brightness temperatures measured by the scanning multichannel microwave radiometer (SMMR) and Special Senor Microwave Imager (SSM/I). Independent estimates of accumulation rate are derived both from ice cores and from spatially extensive ground and airborne radar echo sounding lines. The extinction-diffusion time is found to vary linearly with accumulation rate from approximately 10 to 50 cm/yr ice equivalent over a large area in West Antarctica. Although this simple relationship does not appear to hold at very low or very high accumulation rates, these results suggest that the extinction-diffusion time holds promise as a viable proxy for accumulation rate variability on polar ice sheets.

Echo source discrimination in single‐pass airborne radar sounding data from the Dry Valleys, Antarctica: Implications for orbital sounding of Mars

The interpretation of radar sounding data from Mars where significant topographic relief occurs will require echo source discrimination to avoid the misinterpretation of surface echoes as arising from the subsurface. This can be accomplished through the identification of all radar returns from the surface in order to positively identify subsurface echoes. We have developed general techniques for this using airborne radar data from the Dry Valleys of Antarctica. These data were collected in a single pass, including Taylor Glacier, ice‐covered Lake Bonney, and an ice‐free area of Taylor Valley. The pulsed radar (52.5–67.5 MHz) was coherently recorded. Our echo discrimination techniques included a radar simulator using a digital elevation model (DEM) to predict the location and shape of surface echoes in the radar data. Real and simulated echo strengths were used to calculate a signal‐to‐clutter ratio. This was complemented by the cross‐track migration of radar echoes onto the surface. These migrated echoes were superimposed on the DEM and imagery in order to correlate with surface features. Using these techniques enabled us to identify a number of echoes in the radar data as arising from the surface and to identify subsurface echoes, including a continuous reflector under the main trunk of Taylor Glacier and multiple reflectors beneath the terminus of Taylor Glacier. Surface‐based radar confirms the thickness of the glacier at three crossing points. The results illustrate the importance of using complementary techniques, the usefulness of a DEM, and the limitations of single‐pass radar sounding data.

Subglacial geology in Coats Land, East Antarctica, revealed by airborne magnetics and radar sounding

During the austral summer of 2001/02 five thousand line kilometres of airborne radio echo sounding and aeromagnetic data were collected in the region of three tributaries of Slessor Glacier, East Antarctica, which drains into the Filchner Ice Shelf. Basal topography and roughness estimates were obtained from the echo sounding data, which have been combined, here, with the subglacial geological units inferred from an analysis of the aeromagnetic data. This analysis included Euler and Werner deconvolution to determine the location and depth of sills, dykes and faults. In addition, we high-pass filtered the magnetic anomaly profiles to obtain information about the near surface subglacial geology. This revealed a prominent area of low magnetic susceptibility lying in a deep topographic basin underneath the northern tributary of Slessor Glacier. Forward and inverse magnetic models indicate that this area is likely underlain by a 3 km thick sedimentary basin. Ice flow modelling suggests that this sedimentary basin influences the ice dynamics in the tributary. These results have important implications for understanding the dynamics of this and other East Antarctic flow features that may be underlain by a weak sedimentary bed.

Analysis techniques for coherent airborne radar sounding: Application to West Antarctic ice streams

Analysis of coherent radar sounding echoes from polar ice sheets can provide information suitable for classifying the subglacial environment. Echoes from a general interface consist of both specularly reflected and diffusely scattered contributions. Specular reflection results from smooth uniform interfaces, whereas diffuse scattering results from rough nonuniform interfaces and inhomogeneous media. This article discusses how these phenomena are important to the acquisition and analysis of coherent radar sounding data. Reflection results are presented from airborne surveys conducted in 1987 over the downstream portions of Whillans Ice Stream and Ice Stream C, West Antarctica. Additionally, reflection and scattering analyses along with new results are presented for repeat profiles flown in 2001 over Ice Stream C. Analysis methods include using echo amplitudes to compute reflection coefficients which are used for inferring the dielectric properties of the subglacial material. Echo phase analysis provides the locations of dominant scattering centers which relate to reflection or scattering from the interface as well as provide interface roughness estimates. Comparison of low‐ and high‐resolution imaging obtained from synthetic aperture radar techniques indicates a reflecting and/or scattering interface. Combining the results from these independent analyses provides classification of the subglacial environment. Classified regions include smooth seawater, smooth saturated sediments, accreted marine ice, rough bottom crevasses, mixed conditions with partial subglacial water, and dry frozen conditions.

Eastern margin of the Ross Sea Rift in western Marie Byrd Land, Antarctica: Crustal structure and tectonic development

The basement rock and structures of the Ross Sea rift are exposed in coastal western Marie Byrd Land (wMBL), West Antarctica. Thinned, extended continental crust forms wMBL and the eastern Ross Sea continental shelf, where faults control the regional basin‐and range‐type topography at ∼20 km spacing. Onshore in the Ford Ranges and Rockefeller Mountains of wMBL, basement rocks consist of Early Paleozoic metagreywacke and migmatized equivalents, intruded by Devonian‐Carboniferous and Cretaceous granitoids. Marine geophysical profiles suggest that these geological formations continue offshore to the west beneath the eastern Ross Sea, and are covered by glacial and glacial marine sediments. Airborne gravity and radar soundings over wMBL indicate a thicker crust and smoother basement inland to the north and east of the northern Ford Ranges. A migmatite complex near this transition, exhumed from mid crustal depths between 100–94 Ma, suggests a profound crustal discontinuity near the inboard limit of extended crust, ∼300 km northeast of the eastern Ross Sea margin. Near this limit, aeromagnetic mapping reveals an extensive region of high amplitude anomalies east of the Ford ranges that can be interpreted as a sub ice volcanic province. Modeling of gravity data suggests that extended crust in the eastern Ross Sea and wMBL is 8–9 km thinner than interior MBL (β = 1.35). Gravity modeling also outlines extensive regions of low‐density (2300–2500 kg m−3) buried basement rock that is lighter than rock exposed at the surface. These regions are interpreted as bounded by throughgoing east‐west faults with vertical separation. These buried low‐density rocks are possibly a low‐density facies of Early Paleozoic metagreywacke, or the low‐density epizonal facies of Cretaceous granites, or felsic volcanic rocks known from moraines. These geophysical features and structures on land in the wMBL region preserve the record of middle and Late Cretaceous development of the Ross Sea rift. Thermochronology data from basement rocks and offshore stratigraphy suggest that the wMBL rift margin formed and most extension occurred in mid‐ and Late Cretaceous time, before seafloor spreading initiated between wMBL and the Campbell Plateau. The Cretaceous tectonic record in wMBL contrasts with the Transantarctic Mountains that form the western rift margin, where significant rift‐flank relief developed in middle Tertiary time.

Recent Accumulation Rate Changes in South Greenland from Internal Layering

The burial depths of three internal layers detected by airborne radar sounding in the Dye-3 region of southern Greenland are used to infer past changes in the accumulation rate in the region. Averaged over the period since the deepest layer was deposited (∽1700 to ∽2000 yr BP) the ratio of accumulation rates at the western and eastern ends of the 60 km recorded length of the layers was 0.63, with accumulation increasing to the east. According to recent maps, the present-day value of this ratio is 0.78-0.87. Thus, the distribution of accumulation appears to have become more uniform, because of a decrease in accumulation rate at the eastern end of the flightline, an increase at the western end, or a combination of both. Observed patterns of thickness change and the displacement of the ice crest westward relative to the flow divide are both consistent with a recent decrease in accumulation rate on the eastern end of the segment of flightline considered.

Balance mass flux and ice velocity across the equilibrium line in drainage systems of Greenland

Estimates of balance mass flux and depth‐averaged ice velocity through the cross section aligned with the equilibrium line are produced for each of six drainage systems in Greenland. The estimates are based on a model equilibrium line fitted to field data and on a revised distribution of surface mass balance for the conterminous ice sheet. Ice drainage divides and six major drainage systems are delineated using surface topography from ERS radar altimeter data. Ice thicknesses at the equilibrium line and throughout each drainage system are based on the latest compilation of airborne radar sounding data described elsewhere. The net accumulation rate in the area bounded by the equilibrium line is 399 Gt a−1, and net ablation rate in the remaining area is 231 Gt a−1. Excluding an east central coastal ridge reduces the net accumulation rate to 397 Gt a−1, with a range from 42 to 121 Gt a−1 for the individual drainage systems. The mean balance mass flux and depth‐averaged ice velocity at the cross‐section aligned with the modeled equilibrium line are 0.1011 Gt km−2 a−1 and 0.111 km a−1, respectively, with little variation in these values from system to system. In contrast, the mean mass discharge per unit length along the equilibrium line ranges from one half to double the overall mean rate of 0.0468 Gt km−1 a−1. The ratio of the ice mass in the area bounded by the equilibrium line to the rate of mass output implies an effective exchange time of approximately 6 ka for total mass exchange. The range of exchange times, from a low of 3 ka in the SE drainage system to 14 ka in the NE, suggests a rank as to which regions of the ice sheet may respond more rapidly to climate fluctuations.

Mass-Balance Studies of Ice Streams A, B, and C, West Antarctica, and Possible Surging Behavior of Ice Stream Β

Recent airborne radar sounding has made it possible to map accurately three of the West Antarctic ice streams that flow into Ross Ice Shelf. In previous work we have shown that ice streams A and Β have negative mass balances, whereas inactive Ice Stream C has a strongly positive balance. In this paper we examine in more detail the balance of ice streams A and Β by constructing several gates across them where velocities and ice thicknesses have been measured. We then examine the net fluxes in blocks of the ice streams delimited by successive pairs of gates.Ice Stream A as a whole is apparently discharging more ice than is being accumulated in the catchment area, and currently thinning at the rate of 0.08 ± 0.03 m a−1. The situation on Ice Stream Β is more complex. We have calculated separately the fluxes from tributary ice streams Bl and B2, and examined their individual fluxes within Ice Stream Β by tracing the suture zone between them down-stream of their confluence. The flow band that is the farthest up-stream (girdle), encompassing both Ice Stream Bl and Ice Stream B2, shows a strongly negative net flux that we attribute to lateral and headward expansion of the ice streams within the band. Such expansion can occur by lateral movement of an ice-stream boundary, by temporally accelerating ice flow at the head of the ice stream, or by activation of formerly slowly moving “island” or “peninsula” ice.The imbalance in this flow band, 8 ± 2 km3 a−1 (equivalent mean rate of change in ice thickness, is nearly half of the total excess outflow for the Ice Stream Β system (20 ± 4 km3 a−1), — the remainder is mostly the difference between flow through the uppermost gate and mass input to the catchment area .When for the whole of Ice Stream Β is plotted against the distance along the entire Ice Stream B, the overall pattern appears to be of mild thinning in the catchment, intense thinning in the girdle, and thickening in the main body of the ice stream, which decreases with distance from the girdle. This global behavior is suggestive of a major transient response, resulting from either a change in the internal dynamics or an internal adjustment to a change in the external forcings. We argue that there are a number of conditions which could lead to this type of response pattern. One possibility is a surge. Although the distribution of the changes in thickness is one characteristic of a surge, we caution that this alone is not sufficient to classify the behavior as a surge. Several other possibilities that support a picture of Ice Stream Β as a system in the process of dynamic change and in unsteady state are discussed.At present, Ice Stream C and its catchment area are thickening over their entire area The present surface elevation does not suggest that Ice Stream Β has captured part of Ice Stream C. Moreover, the shut-down of Ice Stream C and the large mass imbalance of Ice Stream Β are not related.

Mass-Balance Studies of Ice Streams A, B, and C, West Antarctica, and Possible Surging Behavior of Ice Stream Β

Recent airborne radar sounding has made it possible to map accurately three of the West Antarctic ice streams that flow into Ross Ice Shelf. In previous work we have shown that ice streams A and Β have negative mass balances, whereas inactive Ice Stream C has a strongly positive balance. In this paper we examine in more detail the balance of ice streams A and Β by constructing several gates across them where velocities and ice thicknesses have been measured. We then examine the net fluxes in blocks of the ice streams delimited by successive pairs of gates. Ice Stream A as a whole is apparently discharging more ice than is being accumulated in the catchment area, and currently thinning at the rate of 0.08 ± 0.03 m a−1. The situation on Ice Stream Β is more complex. We have calculated separately the fluxes from tributary ice streams Bl and B2, and examined their individual fluxes within Ice Stream Β by tracing the suture zone between them down-stream of their confluence. The flow band that is the farthest up-stream (girdle), encompassing both Ice Stream Bl and Ice Stream B2, shows a strongly negative net flux that we attribute to lateral and headward expansion of the ice streams within the band. Such expansion can occur by lateral movement of an ice-stream boundary, by temporally accelerating ice flow at the head of the ice stream, or by activation of formerly slowly moving “island” or “peninsula” ice. The imbalance in this flow band, 8 ± 2 km3 a−1 (equivalent mean rate of change in ice thickness, is nearly half of the total excess outflow for the Ice Stream Β system (20 ± 4 km3 a−1), — the remainder is mostly the difference between flow through the uppermost gate and mass input to the catchment area . When for the whole of Ice Stream Β is plotted against the distance along the entire Ice Stream B, the overall pattern appears to be of mild thinning in the catchment, intense thinning in the girdle, and thickening in the main body of the ice stream, which decreases with distance from the girdle. This global behavior is suggestive of a major transient response, resulting from either a change in the internal dynamics or an internal adjustment to a change in the external forcings. We argue that there are a number of conditions which could lead to this type of response pattern. One possibility is a surge. Although the distribution of the changes in thickness is one characteristic of a surge, we caution that this alone is not sufficient to classify the behavior as a surge. Several other possibilities that support a picture of Ice Stream Β as a system in the process of dynamic change and in unsteady state are discussed. At present, Ice Stream C and its catchment area are thickening over their entire area The present surface elevation does not suggest that Ice Stream Β has captured part of Ice Stream C. Moreover, the shut-down of Ice Stream C and the large mass imbalance of Ice Stream Β are not related.

The morphology of ice streams A, B, and C, west Antarctica, and their environs

Airborne radar soundings of the ice sheet surface made in 1984–1985, together with elevations measured by oversnow traverses between 1957 and 1964 have been used to produce a surface elevation map of ice streams A, B, and C and much of the region around them. The flight positions and elevations were tied to many ground control stations whose positions in three dimensions were fixed by satellite tracking. Ice thicknesses were also measured along the flight lines but have not yet been mapped. The surfaces of active ice streams A and B exhibit a longitudinal ridgetrough topography of uncertain origin. Prominent surface valleys are associated with most, but not all, of their marginal shear zones. There is a deep subglacial trough beneath the grid northeastern side of ice stream A that connects to the subglacial trough beneath Reedy Glacier. The boundary between ice streams Bl and B2, the two tributaries of ice stream B, persists as a suture zone for 250 km downstream from their merger. Ice stream Bl overlies a relatively deep subglacial trough that continued under the corresponding part of ice stream B until it disappears close to the grounding line. Between ice streams Bl and B2 there is a complex zone containing several regions of undisturbed ice separated by bands of disturbed ice, which suggest to us that “rafts” of ice are being incorporated into the ice streams. The previously reported negative mass balance for the drainage system of ice stream B may arise from the expansion of the ice stream into the interior of the West Antarctic ice sheet. Crevasse bands at some places on ridge AB, together with high glacial velocities reported elsewhere, also suggest this expansion. Close to the Ross Ice Shelf, ice stream B widens into a nearly flat area where the mean surface slope is only 3.5 × 10−4, and the surface elevations are only a few tens of meters above hydrostatic equilibrium. Inactive ice stream C differs from active ice streams A and B in surficial and basal characteristics. No elongated ridges and troughs are observed; instead, the ice stream surface exhibits several terraces, including some maxima and minima in elevation. Radar sounding reveals areas where basal echoes are strong and steady, indicating subglacial water, alternating with areas of weaker echoes with short fading lengths. Calculations indicate that the strong reflections require a water layer at least several centimeters to several meters thick, depending on the electrical conductivity of the water. Due to reversals in the surface slope, the hydrologic potential gradient changes sign several times along ice stream C, which implies that subglacial water should collect in some locations.

West Antarctic ice streams draining into the Ross Ice Shelf: Configuration and mass balance

Extensive airborne radar soundings, closely tied to accurately located ground stations made over a portion of the West Antarctic inland ice sheet and the Ross Ice Shelf, have enabled us to map the boundaries of the ice streams and their flow bands on the Ross Ice Shelf. The surfaces of the active ice streams, A and B, are heavily crevassed, but there are no visible crevasses on ice stream C. However, the existence of numerous crevasses at a depth of about 35 m implies that ice stream C ceased to be active about 250 years ago. There is a complex zone at the head of ice stream B that suggests that ice stream B is currently widening and advancing toward the interior of the ice sheet. The grounding lines between the inland ice and the ice shelf were mapped from the surface elevations using the requirement of hydrostatic balance, and the character of the radar echoes. The inland ice is more extensive than previously mapped, and includes a large, weakly grounded area downstream of ice streams A and B, where the surface elevations are barely above hydrostatic equilibrium. Crary Ice Rise extends 70 km upstream of the previously mapped position. To relate surface elevation data from satellite observations to sea level, three geoidal models were tested. We conclude that the GEM 10C model is the best of the three for this section of Antarctica. A new map of surface elevation shows deep marginal troughs and longitudinal ridges along ice streams A and B, but not on ice stream C, whose surface contours show signs of stagnation. Ice stream C shows a strongly positive and ice stream B a significantly negative net balance rate. Ice streams A and D also show a significantly negative net flux, whereas ice streams E and F do not. The overall net balance of this part of the West Antarctic inland ice is suggestively negative (−23 ± 15 km3 yr−1). From the measured flux into the Ross Ice Shelf and previous measurements we calculate an average basal melt rate from beneath the ice shelf of 0.12 ± 0.03 m yr−1.

Ice Streams and Grounding Zones of West Antarctica and the Ross Ice Shelf (Abstract)

In 1984-85, airborne radar soundings were carried out over West Antarctic ice streams A, B, and C, the neighboring parts of the Ross Ice Shelf, and Crary Ice Rise. Here we use the radar data to map the boundaries of the ice streams, to calculate surface elevations, and to measure ice thicknesses. Lee thicknesses and surface elevations have been used together to map the grounding zones and ice rises (Figure 1).

Ice Streams and Grounding Zones of West Antarctica and the Ross Ice Shelf (Abstract)

In 1984-85, airborne radar soundings were carried out over West Antarctic ice streams A, B, and C, the neighboring parts of the Ross Ice Shelf, and Crary Ice Rise. Here we use the radar data to map the boundaries of the ice streams, to calculate surface elevations, and to measure ice thicknesses. Lee thicknesses and surface elevations have been used together to map the grounding zones and ice rises (Figure 1).

Field Studies of Bottom Crevasses in the Ross Ice Shelf, Antarctica

AbstractSurface and airborne radar sounding data were used to identify and map fields of bottom crevasses on the Ross Ice Shelf. Two major concentrations of crevasses were found, one along the grid-eastern grounding line and another, made up of eight smaller sites, grid west of Crary Ice Rise. Based upon an analysis of bottom crevasse heights and locations, and of the strength of radar waves diffracted from the apex and bottom corners of the crevasses, we conclude that the crevasses are formed at discrete locations on the ice shelf. By comparing the locations of crevasse formation with ice thickness and bottom topography, we conclude that most of the crevasse sites are associated with ice rises. Hence we have postulated that six ice rises, in addition to Crary Ice Rise and Roosevelt Island, exist in the grid-western sector of the ice shelf. These “pinning points” may be important for interpreting the dynamics of the West Antarctic ice sheet.

Field Studies of Bottom Crevasses in the Ross Ice Shelf, Antarctica

AbstractSurface and airborne radar sounding data were used to identify and map fields of bottom crevasses on the Ross Ice Shelf. Two major concentrations of crevasses were found, one along the grid-eastern grounding line and another, made up of eight smaller sites, grid west of Crary Ice Rise. Based upon an analysis of bottom crevasse heights and locations, and of the strength of radar waves diffracted from the apex and bottom corners of the crevasses, we conclude that the crevasses are formed at discrete locations on the ice shelf. By comparing the locations of crevasse formation with ice thickness and bottom topography, we conclude that most of the crevasse sites are associated with ice rises. Hence we have postulated that six ice rises, in addition to Crary Ice Rise and Roosevelt Island, exist in the grid-western sector of the ice shelf. These “pinning points” may be important for interpreting the dynamics of the West Antarctic ice sheet.

Field Studies of Bottom Crevasses in the Ross Ice Shelf, Antarctica (Abstract only)

Surface and airborne radar sounding data were used to identify and map fields of bottom crevasses on the Ross Ice Shelf. Two major concentrations of crevasses were found, one along the grid-eastern grounding line and another, made up of eight smaller sites, grid-west of Crary Ice Rise.Based upon an analysis of bottom crevasse heights and locations, and of the strength of radar waves diffracted from the apex and bottom corners of the gridcrevasses, we conclude that the crevasses are formed at discrete locations on the ice shelf. By comparing the locations of crevasse formation with ice thickness and bottom topography, we conclude that most of the crevasse sites are associated with grounding. Hence we have postulated that six grounded areas, in addition to Crary Ice Rise and Roosevelt Island, exist in the grid-western sector of the ice shelf. These pinning points may be important for interpreting the dynamics of the West Antarctic ice sheet.

Field Studies of Bottom Crevasses in the Ross Ice Shelf, Antarctica (Abstract only)

Surface and airborne radar sounding data were used to identify and map fields of bottom crevasses on the Ross Ice Shelf. Two major concentrations of crevasses were found, one along the grid-eastern grounding line and another, made up of eight smaller sites, grid-west of Crary Ice Rise. Based upon an analysis of bottom crevasse heights and locations, and of the strength of radar waves diffracted from the apex and bottom corners of the gridcrevasses, we conclude that the crevasses are formed at discrete locations on the ice shelf. By comparing the locations of crevasse formation with ice thickness and bottom topography, we conclude that most of the crevasse sites are associated with grounding. Hence we have postulated that six grounded areas, in addition to Crary Ice Rise and Roosevelt Island, exist in the grid-western sector of the ice shelf. These pinning points may be important for interpreting the dynamics of the West Antarctic ice sheet.

Airborne Radar Sounding of the Greenland Ice Cap: Flight 1

The dielectric properties of cold, dry ice in the polar ice caps are favorable for the use of a radar technique in the rapid and continuous measurement of ice thickness from an aircraft. Feasibility and applicability to extended traverses have been demonstrated by a series of flights over the Greenland ice cap, in July 1966. Pulsed 30-MHz signals at a 20-kHz repetition frequency, 2.5-msec duration, 400-W peak power, 7-W average power, with A-scope presentation, were photographically recorded on 35-mm film with frequency-stabilized time and yielded very good results over four flights, comprising 9600 km total traverse. Maximum ice thickness recorded was approximately 3300 ± 50 m. The aircraft used was an EC-121-P (Super Constellation), flying 600 m above the air-ice interface at approximately 300 km/hr. Special problems of warm (wet) snow, antenna design, recording, and navigation are discussed.