Christopher Allen – The University of Kansas
CReSIS – The Center for Remote Sensing of Ice Sheets
|Figure 1. Illustratration of airborne radio echo sounding
showing the grayscale echogram and the a-scope plot of
received signal power vs. depth.
The application of radio-echo sounding (RES) to thickness measurements of glacial and sheet ice has been demonstrated since the early 1960s. The concept for this approach can be traced to 1933 at Admiral Byrd’s base, Little America, Antarctica where the first indication that snow and ice are transparent to high frequency radio signal was observed. An investigation by U.S. Army researchers, prompted by pilot reports of the “uselessness” of radar altimeters over ice suggesting the transparency of polar ice and snow in the VHF and UHF bands, led Amory Waite and others to demonstrate that a radar altimeter (the SCR 718 operating at 440 MHz) could measure the thickness and other features of polar glaciers in 1957 (Waite and Schmidt, 1961; Robin, 1972 ). This observation resulted in one of the most important technical advances in glaciology, namely the development and wide application of radio-echo sounding systems.
Following Waite’s demonstration, Stan Evans at Cambridge University’s Scott Polar Research Institute (SPRI) developed the first of several VHF systems specifically for radio echo sounding in 1963. Within the next few years several other research groups began developing and using RES systems including the United States Army Electronics Laboratory (USAEL), the British Antarctic Survey, the Arctic and Antarctic Scientific Research Institute in Leningrad, the Geophysical and Polar Research Center (GPRC) at the University of Wisconsin, the U.S. Army Cold Regions Research Laboratory (CRREL), the Canadian Department of Energy, the Technical University of Denmark (TUD), Stanford Research Institute,and the Institut Geografii of the U.S.S.R. Akademiya Nauk (Evans, 1967; Gudmandsen, 1969; Evans and Smith, 1969; Weber and Andreiux, 1970; Robin, 1975a; Macheret and Zhuravlev, 1982). These systems, all short pulse-type radar systems with operating frequencies ranging from 30 MHz to 600 MHz, successfully sounded ice sheets, ice caps and glaciers (both temperate and polar) in Greenland and Antarctica. Both surface-based and airborne measurements were conducted. Thickness estimates from RES systems agree with those from seismic and gravity based estimates (Drewry, 1975). Reflections from internal layers have also been observed since the first RES observations and a variety of sources for these internal reflections have been identified including layers of liquid water (Bamber, 1987; Davis, Dean, and Xin, 1990), layering involving small permittivity changes due to changes in acidity (from large volcanic events) (Millar, 1981), changes in the size or shape of air bubbles within the ice (Ackley and Keliher, 1979), and variations in ice crystal orientation and density (Harrison, 1973). Layers produced by the small permittivity changes represent isocrones and are useful in the interpretation of climate information (Gudmandsen, 1975; Jacobel and Hodge, 1995). In addition to measuring ice thickness and layers, other features and characteristics have been observed such as regions of bottom melting and freezing (Neal, 1979), ice bottom sliding velocity (Doake 1975), glacier velocity (Doake, Gorman and Paterson, 1976), sub-ice lakes formed by pressure melting (Owsald, 1975) and bottom crevasses (Jezek, Bentley and Clough, 1979). Information on other ice parameters including signal absorption (Neal, 1976), signal fading patterns (Harrison, 1971; Berry, 1975), propagation velocity (Robin, 1975b), and birefringence (Bentley, 1975; Hargreaves, 1977; Woodruff and Doake, 1979) have been obtained from RES data.
In the next generation of RES systems we see more specialization. For example, to reduce the reflections from nearby walls in sounding valley glaciers, the system frequency is increased to improve system directivity. A 620-MHz system succeeded in sounding the Rusty Glacier in the Yukon Territory where a 35-MHz system and conventional seismic systems had previously been unsuccessful due to echo obscuration by the transmitted radio and seismic pulses and due to the proximity of the valley walls (Clarke and Goodman, 1975). Also, several low frequency RES systems were developed specifically for sounding temperate glaciers where absorption losses are significant due to higher ice temperatures and the presence of liquid water. Systems with frequencies ranging from 1 to 32 MHz have successfully sounded temperate glaciers by numerous researchers (Strangway et al., 1974; Watts and England, 1976; Bjornsson et al., 1977; Sverrisson, Johannesson and Bjornsson, 1980; Watts and Wright, 1981).
Techniques for determining additional information on both the geography and roughness of subglacial terrain have also developed. Significant changes in the polarization of the returned echoes from land ice and an ice shelf indicate that tidal strain on crystal orientation at the hinge zone results in a large change in birefringence of the ice, indicating that polarization could be used to distinguish floating from grounded ice (Woodruff and Doake, 1979). A method for continuously monitoring the returned echo power from a sub-glacial ice/rock or ice/water interface was also developed for detecting changes in the reflection coefficient, the absorption properties of the ice, as well as identifying the changing nature of the basal reflection properties along a flow line (Neal, 1976). As part of a physics experiment investigating a theoretical fifth force affecting Newton’s inverse square law for gravitation, an extensive series of RES measurements were made around the DYE-3 complex in southern Greenland. Sounding data were collected along 124 radial lines, each about 5 km in length producing a map of bedrock topography with an uncertainty of less than 5 m over most of the survey area (Fisher et al., 1989). A precise grid pattern was flown over the summit region of Greenland with the TUD RES system to obtain both ice surface and ice bottom topography to accuracies of ±6 m and ±50 to 125 m, depending on the bottom roughness (Hodge et al., 1990). Synthetic-aperture radar (SAR) techniques were applied to RES data by researchers from the British Antarctic Survey to produce two dimensional maps of echo strength showing the grounding line of a glacier in the Antarctic Peninsula (Musil and Doake, 1987).
As enabling technologies emerged, RES systems became more capable. Digital data acquisition, signal processing and recording have significantly improved system capabilities not the least of which is dynamic range (Goodman, 1975; Sivaprasad, 1978; Wright, Bradley, and Hodge, 1989). Walford and others developed a coherent RES system permitting the measurement of both the amplitude and phase of the received (Walford, Holdorf and Oakberg, 1977; Walford and Harper, 1981). The Coherent Antarctic Radar depth sounder (CARDS), the first RES system designed completely with solid-state, computerized components, which is coherent and employs pulse compression to reduce peak transmit power requirements, was field tested in Antarctica by researchers at the University of Kansas (Raju, Xin and Moore, 1990). Subsequent upgrades to this system incorporate microwave monolithic integrated circuits (MMICs) (Gogineni, Legarsky, and Thomas, 1998), alternating transmit waveforms, and multiple independent receive channels (Lohoefener, 2006). Figure 1 illustrates the concept of airborne radio echo sounding and sample data products. The advent of systems with multiple receive antennas (each with a dedicated receive channel and digitizer) enable digital beam steering, null steering (for clutter suppression), and interferometric processing. Another such system is the ground-based, 8-channel SAR for bed imaging first applied near Summit, Greenland to produce maps of basal backscatter over an area extending 6 km x 28 km with VV polarization and spatial resolution of 30 m with 15 looks to reduce speckle (Paden et al., 2004; Allen et al., 2008), as shown in Figure 2. Interferometric processing of data from this 8-channel system also permitted discrimination of off-nadir scattering sources providing further insight regarding basal topography.
Beginning in the decade of the 1990s, a new generation of specialized impulsive RES systems emerged to address specific glaciological questions. A miniature impulse RES system capable of operating from 1 to 200 MHz was developed and field tested by Canadian researchers for sounding glaciers and ice caps (Narod and Clarke, 1994). Similarly the British Antarctic Survey (King, Woodward, and Smith, 2007) has used the Deep Look Radio Echo Sounder (DELORES) system (1 to 20 MHz tunable) for sounding the 3-km thick Rutford Ice Stream in West Antarctica. Researchers from the University of Alaska at Fairbanks have used a 1.7 MHz impulse-type RES system to measure a cross-section of Taku Glacier, Alaska and estimate the mass-balance flux (Nolan et al., 1995). University of Washington researchers probed the bed reflectivity in an active ice stream in West Antarctica using a 2-MHz impulsive system (Raymond et al., 2006). A 3-MHz impulsive radar was used to collect 1850 km of RES data in 2001 along the U.S. leg of the International Trans-Antarctic Scientific Expedition traverse (US-ITASE) (Welch and Jacobel, 2003). Backpack-portable impulsive radar operating with a 5-MHz center frequency was used to map 300-m deep topography in temperate valley glaciers (Matsuoka, Saito, and Naruse, 2004). Researchers from the University of Munster developed and fielded two ground-based 35 MHz RES systems, the first a single pulse system intended to measure the reflections from internal layering with high resolution while the second system uses a burst transmitter designed to penetrate the sheet ice and observe the underlying bedrock (Hempel and Thyssen, 1992).
Commercial ground penetrating radars (GPRs) have also been used for ice probing. Units from Geophysical Survey Systems, Inc. (GSSI) have been used by the U.S. Army Cold Regions Research and Engineering Laboratory (CRREL) to study the firn in Alaska at 135 MHz (Arcone, 2002), in Antarctica at 400 MHz (Arcone, Spikes, and Hamilton, 2005) and at 80 MHz (Clarke et al. 2000), and in Svalbard at 900 MHz (Wadham et al., 2006). A GSSI system was also used to map ground ice in Canada’s Northwest Territory at 200 and 400 MHz (De Pascale, Pollard, and Williams, 2008). Products from Sensors and Software Inc. have been operated at 100 MHz to sound the internal and basal characteristics of Svalbard glaciers (Murray et al., 2000) and at 100 and 200 MHz to study crevasse formation in Antarctica (Nath and Vaughan, 2003). Equipment from Mala Geoscience was used in Svalbard at 50 MHz to map snow accumulation variability on the Nordenskjöldbreen glacier (Palli et al., 2002) and at 50 and 200 MHz to probe the firn-ice transition-zone (Palli, Moore, and Rolstad, 2003), and at the Amundsenisen plateau in Antarctica at 200 and 250 MHz to map variations in the ice sheet’s top 100 m (Rotschky et al. 2004).
Network analyzers have been configured to operate as continuous-wave, step-frequency radars by various researchers to study ice sheets and glaciers. Norwegian researchers have applied synthetic-aperture radar techniques to image the internal structure of the subpolar glacier Slakbreen in Spitsbergen, Svalbard operating from 5 to 20 MHz and from 320 to 370 MHz (Hamran and Aarholt, 1993) and to characterize the Riiser–Larsenisen ice shelf in Antarctica from 10 to 30 MHz, 155 to 170 MHz, and 330 to 360 MHz (Hamran et al., 1998). British researchers used a vector network analyzer configured as a polarimetric radar was used to study the birefringence of Antarctic ice shelves from 200 to 400 MHz (Doake et al. 2003) and to study the basal reflection at the grounding line of the Rutford Ice Stream, Antarctica from 270 to 321 MHz (Jenkins et al., 2006). Researchers from Sweden (Pettersson, Jansson, and Holmlund, 2003) studied the cold temperate transition on Storglaciären in Sweden from 320 to 360 MHz and 700 to 900 MHz. A bistatic basal sounder was fashioned around a network analyzer to characterize the basal scattering and ice attenuation parameters over the 110 to 500 MHz frequency range (Paden et al., 2005).
FM-CW (frequency modulated, continuous wave) radars have been designed specifically for cryospheric research. CRREL researchers used ground-based C-band (3.95 to 5.89 GHz) and X-band (8.2 to 12.4 GHz) FM-CW radars to profile frozen lakes in Alaska (Arcone, Yankielun, and Chacho, 1997) and an airborne L-band (1.12 to 1.76 GHz) FM-CW radar to sound the temperate Black Rapids Glacier in Alaska from a helicopter (Arcone and Yankielun, 2000). Using airborne UHF (600 to 900 MHz) FM-CW radar researchers at Kansas University have mapped the accumulation rate variability from a NASA P-3 aircraft over the Greenland ice sheet (Kanagaratnam et al., 2004).
Antenna technologies used for RES have changed as well over the years. For ground-based applications, resistively-loaded, half-wave antennas have been used (Watts and Wright, 1981). The stepped FM system (Strangway et al., 1974) that operates at discrete frequencies between 1 and 32 MHz uses two orthogonal, horizontal electric dipoles for transmit and a set of three orthogonal receiving coils on a vehicle. For airborne applications, two resistively loaded wire antennas have been towed from the aircraft wing tips, one for transmit, the other for receive (Watts and Wright, 1981).
A variety of antennas have been used with the 35-MHz SPRI system. For ground-based applications, a simple trailing wire (end-fed) has been towed behind the vehicle has been used resulting in a bandwidth of 6 MHz, a rigid, two-conductor half-wave dipole arrangement has been used yielding a bandwidth of 10 MHz, and a loaded dipole with a tank circuit balun has been used (bandwidth not reported), primarily as it preserves the pulse shape (Robin, Evans, and Bailey, 1969). The 35-MHz TUD system has used two folded dipoles about 4-m long suspended about 1 m beneath each wing, one for transmit and the other for receive (Gudmandsen, 1969).
|Table 1 – Parameters of time-domain radio depth sounder
systems reported in the literature (excludes GPR, impulse, and
At 60 MHz, Danish researchers at TUD use linear array of four dipoles suspended a quarter wavelength beneath the wing resulting in a narrow 22º beamwidth in the cross-track plane and 110º in the along-track plane (Gudmandsen, 1976; Drewry and Meldrum, 1978). An antenna system composed of two half-wave dipoles mounted beneath opposite wings (one for transmit, the other for receive) has also been used at 60 MHz. By using the wings as reflectors, a gain of about 8 dB is obtained (Gorman and Cooper, 1987).
Finally, the value of radio-echo sounding data is reduced without accurate knowledge of where the data were collected. Therefore, navigation and position measurement techniques are of great significance to RES systems. When RES systems were first field tested, navigation relied on observation by system operators, which is significantly hampered in regions without discernible landmarks. In 1967 navigation records including a galvanometer showing aircraft heading, air temperature, static pressure and airspeed were recorded along with airborne radio echo sounding data to obtain position information. By 1971 an inertial navigation system (INS), the Litton 51C, was used to annotate the RES data with position information (Evans, Drewry and Robin, 1972; Robin, 1975a). Position measurement systems employing microwave signals and transponders at known geographic points have been used to obtain position data within limited areas with an accuracy of ±10 m in ground based RES experiments (Goodman, 1975). By the late 1970s, LORAN and satellite navigation were available providing less accurate yet affordable position information on a global basis suitable for RES applications (Sverrisson, Johannesson and Bjornsson, 1980). A Doppler navigator linked through a Tactical Air Navigation System (TANS) navigation computer was used in 1983 to provide a continuous read-out of latitude and longitude with sub-kilometer accuracy (Gorman and Cooper, 1987; Drewry and Liestol, 1985). Currently differential GPS data is routinely collected with RES measurements reducing positional uncertainties to ±2 to 5 m (Nolan et al., 1995) and with post-processing to within ±10 cm (Krabill et al., 1995).
Table 1 is an updated summary of the characteristics of the various time-domain radar sets first tabulated by Goodman (Goodman, 1975) and later augmented (Gogineni et al., 1998).
This work was supported by the National Science Foundation (ANT-0424589 & OPP0122520), NASA (NAG5-12659), the Kansas Technology Enterprise Corporation (KTEC), and the University of Kansas.
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