Thenewlydevelopedairborneradio-echosoundingsystem oftheAWIasaglaciologicaltool U. Nixdorf, 1 D. Steinhage, 1 U. Meyer, 1 L. Hempel, 1* M. Jenett, 2 P. Wachs, 3 H. Miller 1 1 AlfredWegener Institute for Polar and Marine Research, P.O. Box 120161, D-27515 Bremerhaven, Germany 2 Technische Universita« t Hamburg^Harburg, Arbeitsbereich Hochfrequenztechnik,Wallgraben 55, D-27071Hamburg, Germany 3 Aerodata FlugmeÞtechnik GmbH, Hermann-Blenk-StraÞe 36, D-38108 Braunschweig, Germany ABSTRACT . Since 1994 the AlfredWegener Institute AWI) has operated an air- borne radio-echo sounding system for remote-sensing studies of the polar ice caps inAnt- arctica and in Greenland. It is used to map ice thicknesses and internal layerings of glaciers, ice sheets and ice shelves, and is capable of penetrating ice thicknesses of up to 4km.ThesystemwasdesignedandbuiltbyAWIincooperationwithAerodataFlugmeÞ- technik GmbH,Technische Universita«t Hamburg^Harburgandthe Deutsches Zentrum fu« r Luft- und Raumfahrt e.V.The system uses state-of-the-art techniques, and results in high vertical 5m) as well as along-track 3.25m) resolution. The radar signal is a 150MHz burst with a duration of 60 or 600ns.The peak power is1.6kW, and the system sensitivity is190dB.The short backfire principle has been adopted and optimized for an- tennae used on Polar2 , a Dornier 228-100 aircraft, resulting in an antenna gain of 14dB each. Digital data recording allows further processing.The quality of the recorded data canbe monitored on screen andas online analogue plots during the flight. MOTIVATION Inthecontextofglaciologicalwork,radarstudiesarehighly valuable in several respects. Mass-balance parameters such as ice thickness and subglacial topography can be surveyed over wide areas Drewry, 1983; Hodge and others, 1990). These parameters are input to numerical models of ice-flow dynamics to derive responses of the polar ice caps to global changes Huybrechts and Oerlemans, 1990; Huybrechts, 1994).Moreover,theextentoficestreams,whichplayanim- portant role in the understanding of the stability of ice sheets Bentley,1998), canbe determined Bell and others, 1998).The mappingof internal radio-echo sounding RES) horizons enables glaciologists to identify optimal sites for deep ice-core drilling Dahl-Jensen and others,1997).With radar systems, the information deduced from ice-core studies GRIP Project Members,1993; Grootes and others, 1993) can be extrapolated from the drill sites to the whole areacoveredby radar profiles Hempel andThyssen,1993). For these reasons, the AlfredWegener Institute AWI) has operated an airborne RES system for glaciological studies since1994 in Greenland and Antarctica. SYSTEMDESIGN Radarsystemshavebeenusedsincethe1950sinremote-sen- singoperations in Greenlandand Antarctica Waite,1959). The first measurement of ice thickness with a standard US AirForceradioaltimeterSCR-718isdescribedbyWaiteand Schmidt 1962). A historical review of radar systems in gla- ciology is givenby Gogineni andothers 1998). Anoverview of the technical details of different frequently used airborne RES systems canbe found in Bogorodskyandothers 1985). TECHNICALDATA:HIGH-FREQUENCY COMPONENTS AWI operates two Dornier Do 228-100 aircraft that canbe equippedwith different scientific instruments dependingon the various scientific programmes. The system presented here was built byAWI in cooperation with Aerodata Flug- meÞtechnik GmbH, Technische Universita«t TU) Ham- burg^Harburg and the Deutsches Zentrum fu« r Luft- und Raumfahrte.V. DLR). Figure1showsthe differentcompo- nents of the system, from the signal generator to the data re- cording andanalysis. The transmitted high-frequency HF) signal is produced byasynthesizer,andtheoutputfrequency f 0 canbechosenin 1MHzstepsbetween100and200MHz.Innormaloperation it is set to150MHz with an output power setting of P synth = 10dBm.The output frequency f 0 can also be easily changed during the flight.The limiting factor is the bandwidth of the antennae which is about 20MHz around the centre fre- quency of 150MHz and is far smaller than the bandwidth of the HF components. A timing board based on an oscillator withareferencefrequencyof 10MHzcontrolsthesignalgen- eration, with pulse lengths  p of60nsor600nsburst)anda pulse repetition frequency f p of 20kHz. In``toggle mode''the pulse length ofconsecutivetraces is switchedbetween 60 and 600ns.Thusused,thesystemallowsbothhighresolutionand maximum depth-sounding during each flight. The signal is Annals of Glaciology 29 1999 # International Glaciological Society * Present address: danet GmbH, GutenbergstraÞe 6^12, D-64331Weiterstadt,Germany. 231 Downloaded from https://www.cambridge.org/core. 12 Nov 2021 at 13:26:23, subject to the Cambridge Core terms of use.