Annals of Glaciology 55(67) 2014 doi: 10.3189/2014AoG67A055 39 A low power consumption radar system for measuring ice thickness and snow/firn accumulation in Antarctica Jose A. URIBE,1 Rodrigo ZAMORA,1 Guisella GACITU A,1 AndreÂs RIVERA,1;3 David ULLOA2 1Glaciology and Climate Change, Centro de Estudios CientõÂficos (CECs), Valdivia, Chile E-mail: [email protected] 2Unmanned Industrial Ltda., Valdivia, Chile 3Department of Geography, Universidad de Chile, Santiago, Chile ABSTRACT. In order to measure total ice thickness and surface snow accumulation in Antarctica, we have designed and built a surveying system comprising two types of radar. This system is aimed at having low power consumption, low weight/volume and low construction cost. The system has a pulse- compression radar to measure ice thickness, and a frequency-modulated continuous wave (FM-CW) radar designed to measure hundreds of meters of surface snow/firn layers with high resolution. The pulse- compression radar operates at 155 MHz, 20 MHz of bandwidth; and the FM-CW radar operates from 550 to 900 MHz. The system was tested in December 2010 at Union Glacier (798460 S, 838240 W), West Antarctica, during an oversnow campaign, where Union and other nearby glaciers (Schanz, Schneider and Balish) were covered through 82 km of track. Ice thickness of 1540 m and snow/firn thickness of 120 m were detected in the area. The collected data allowed the subglacial topography, internal ice structure, isochronous and the snow/ice boundary layer to be detected. Here we describe radar electronics, their main features and some of the results obtained during the first test campaign. Further improvements will focus on the adaptation of the system to be implemented on board airplane platforms. KEYWORDS: Antarctic glaciology, glaciological instruments and methods, ground-penetrating radar, polar firn, radio-echo sounding INTRODUCTION rough areas using ground or airborne platforms. Its main Radio-echo sounding (RES) has been a useful method in advantages are its low power consumption and low weight/ glaciology since the discovery of the transparency of cold volume, which were achieved by taking into account, during ice for the propagation of electromagnetic waves (Waite and the design stage, the integration of a pulse-compression radar Schmidt, 1962). Radar applications in polar ice sheets have and a frequency-modulated continuous wave (FM-CW) included the study of ice thickness, subglacial topography, radar, as well as the complete design of each section. We subglacial roughness, subglacial geology and subglacial also reduced the implementation costs in designing and lakes (Peters and others, 2005; Vaughan and others, 2007). building each stage. Low power and compact design are Radar data have also been used to detect internal ice layers, important considerations for long oversnow traverses, as well estimate glacier mass balance and characterize ice dynam- as being a key factor for future implementation of radars on ics (Bingham and Siegert, 2007). In snow and firn, radars unmanned aerial vehicles (UAV), a configuration that can have been used to determine thickness, stratigraphy layers reduce operational costs for glaciological measurements and other properties (Plewes and Hubbard, 2001). Because (Blake and others, 2008; Remy and others, 2012). of the versatility of RES, numerous radar types have been In December 2008, we tested a pulse-compression developed for studying ice masses. Historical reviews of prototype radar designed to measure ice thickness at Union radar systems developed for glaciological purposes are Glacier (798460 S, 838240 W) and the Horseshoe Valley in provided by Gogineni and others (1998) and Plewes and West Antarctica (Fig. 1) (Rivera and others, 2010). Thanks to Hubbard (2001), for example. this experience, the upgraded radar presented in this paper Normally, radars designed to survey ice thickness are was used at Union Glacier, during an oversnow traverse completely independent of high-resolution radars aimed at carried out in December 2010. During this campaign, we measuring snow accumulation (Gogineni and others, 2001; performed ice thickness and snow/firn measurements along Kanagaratnam and others, 2004; Raney and others, 2008; Union, Schanz, Schneider and Balish glaciers (Fig. 8); results Panzer and others, 2013). However, a complete package are presented here to show the performance of the radar multi-frequency radar system, focused on wide-coverage system. These data complement the glaciological baseline of airborne survey, has been developed by the University of Union Glacier and improve knowledge of the basic Kansas, USA. This illustrated the importance of multispectral characteristics of nearby glaciers, where only BEDMAP data radar data to characterize the entire ice column (RodrõÂguez- (Lythe and others, 2001) were available until our first surveys Morales and others, 2013). (Rivera and others, 2010). In this paper, we present a design of a VHF/UHF ground The Union Glacier area was selected as a testing site radar system aimed at measuring cold ice thickness and high- because of its complex surface and subglacial topography, resolution surface snow/firn. This system is intended to survey the presence of a blue-ice area and the availability of in situ 40 Uribe and others: Radar for measuring ice and snow in Antarctica Table 1. Pulse-compression radar parameters Description Value Unit Frequency range 145±165 MHz Resolution in ice* 4:2 m Chirp length 1±10 ms Pulse repetition frequency (PRF) 10 (max) kHz Peak power 200 (max) W Coherent integration 64±4096 traces Trace length 2048 samples Sampling frequency 40:5 MHz Receiver gain 83:8 (max) dB Receiver noise figure 3:3 dB *See Discussion section for more details. Table 2. FM-CW radar parameters Description Value Unit Frequency range 550±900 MHz Resolution in air 0:5 m PRF 10 kHz Transmitter power 21 dBm Coherent integration 64±256 traces Trace length 4096 samples Range 480 m Sampling frequency 45 MHz Receiver gain 80:3 dB Receiver noise figure 4:1 dB waveform. This method uses long chirp-modulated pulses to increase the average transmitted power (Mahafza, 2000). Given the low attenuation of high frequencies in cold ice, it is possible to operate in VHF band, employing common RF amplifiers, electronics components and small antennas. FM- CW radars are often used as high-resolution systems because, despite their high bandwidth, they do not require Fig. 1. Location map of the testing site at Union Glacier (box in high-speed data acquisition. Figure 2 shows a reduced block lower image). diagram of the overall radar system. The pulse-compression system is based on an Advanced COherent Radar Depth Sounder (ACORDS) system by Namburi (2003), and the FM- mass-balance and ice-dynamic data collection. This area is CW system is based on the radar presented by Kanagar- located only 40 km upstream of the local grounding line at atnam and others (2004). Constellation Inlet, in the Ronne Ice Shelf. The proximity of The digital system generates the chirp waveform at our study area to the local grounding line zone is of interest 155 MHz to the pulse-compression radar transmitter, and due to possible upward grounding line migrations already the baseband frequency sweep (50±400 MHz) to the FM- forecast in the region (Hellmer and others, 2012). Hence, CW system, through two separate direct digital synthesizer contributions to build up a glaciological baseline of Union (DDS) systems. It also performs the analog-to-digital con- Glacier are important for understanding possible future versions of three received analog signals: two signals glacier dynamic responses to ongoing atmospheric and (channels 1 and 2) from the pulse-compression radar oceanic changes. receiver, and the intermediate frequency (IF) signal from Here we present the technical details of our system the FM-CW radar front-end. The two channel schemes have including the signal generation, RF systems, the RF power different amplification gains (low and high gain) of the amplifier and the digital data capture systems, as well as received signal to increase the dynamic range of the pulse- some of the results obtained. compression radar. This method was proposed by Namburi (2003) to avoid sensitivity time control (STC) systems. A geodetic Javad Lexon GPS receiver was used, which SYSTEM OVERVIEW provides NMEA183 messages and pulse per second (PPS) The pulse-compression method permits one to improve the signals for precise time marking and basic georeferencing of range resolution and the signal-to-noise ratio (SNR) that each radar trace captured and precise georeferencing in would have been obtained from the uncompressed pulse post-processing. All digitized data are coherently averaged Uribe and others: Radar for measuring ice and snow in Antarctica 41 Fig. 2. General block diagram of the radar system. inside the digital system, then sent to the computer through Furthermore, trimmer capacitors to the input and output an Ethernet link, where the raw data are stored. The matching networks were added for manual tuning to obtain computer runs a custom program that controls all radar a flat frequency response from 145 to 165 MHz. Finally, after parameters and preprocesses the raw data in real time for several laboratory tests, we reduced the current polarization viewing purposes. Tables 1 and 2 show the most important of the transistor drain (Idq) from 900 mA (data-sheet parameters of pulse compression and the FM-CW radars. recommendation) to 100 mA, reducing the total power consumption. With this configuration, we obtained 200 W of peak power (10 kHz of pulse repetition frequency (PRF), THE PULSE-COMPRESSION SYSTEM 10 ms chirp long) from the transmitter output in the complete Transmitter 145±165 MHz band. The pulse-compression radar transmitter (Fig. 3) consists of three stages: a pre-amplifier, a power driver amplifier and Receiver the final power stage. The power driver amplifier produces a The receiver (Fig. 4) is designed to produce two outputs from maximum output of 5 W.