1.0 Introduction
The mass balance of the Greenland and Antarctic ice sheet plays an important role in the rise of the Earth’s sea level. Rising sea levels over the last century led to the initiation by NASA of a program to measure the surface elevation and thickness of the Greenland ice sheet. Remote sensing instruments, such as airborne altimeters and radio echo sounders, provide a safe, inexpensive and effective means of obtaining these ice parameters.
In 1991, a research effort to measure the surface elevation of the Greenland ice sheet was begun in support of a global climate study. Airborne laser altimetry data were collected and subsequent analysis showed that the surface elevation could be determined with an accuracy of about 20 cm. In 1993, the airborne program was expanded to include a 150-MHz radar echo sounder developed and operated by The University of Kansas to measure the ice thickness. Since the summer of 1993, the radar depth sounder has collected ice-thickness data concurrent with measurements from the laser altimeter and other location measurement instruments.
2.0 Radar System Description
Two coherent radar depth sounders were used to measure ice thickness on this mission. The first system is an improved version of the original system. The second system is the Next Generation Coherent Radar Depth Sounder System (NG-CORDS), which is a compact version of the first system with better noise performance. Both systems were operated from a NASA P-3 aircraft that was also equipped with precision laser altimeter systems, Global Positioning System (GPS) receivers, and a nadir-looking video camera enabling accurate registration of the depth sounder data with precise location information and independent ice surface elevation data.
Both coherent radar systems operate at a center frequency of 150 MHz with a nominal pulse repetition frequency (PRF) of 9200 Hz. The transmitter generates a pulse that is frequency modulated (chirped) over a bandwidth of 17 MHz with a duration of 1.6
ms and a peak power of 200W. The system uses separate transmit and receive antennas that are mounted beneath the port and starboard wings, respectively. Each antenna is a four-element, half-wavelength dipole array. The receiver, protected during transmit events by a blanking switch, amplifies and compresses the received signal in a weighted SAW compressor resulting in a compressed pulse length of about 60 ns and a depth resolution of 5 m in ice (n = 1.78). The end-to-end receiver gain is about 95 dB. The compressed signal is coherently detected, providing in-phase and quadrature (I and Q) analog outputs. Two 8-bit A/D converters digitize these analog signals at a sampling rate of 18.75 mega-samples/s (MSPS). Coherent integration is then performed by summing the complex data vectors from 256 consecutive transmit-receive periods. Power in each record is computed (I2 + Q2) and then integrated further (incoherent integration) by summing four consecutive data vectors. The data are displayed to the user and are also recorded, along with position and time data provided by the on-board GPS receiver, on a removable hard disk. Table 1 lists the principal radar system parameters.|
Parameter |
Value |
Units |
|
Nominal aircraft altitude (AGL) |
500 |
m |
|
Nominal aircraft speed |
110 |
m/s |
|
Radar type |
Coherent – Pulse |
- - |
|
Radar center frequency |
150 |
MHz |
|
Transmitted bandwidth |
17 |
MHz |
|
Transmitted pulse duration |
1.6 |
m s |
|
Compressed pulse duration |
60 |
ns |
|
Peak transmit power |
200 |
W |
|
Pulse repetition frequency (PRF) |
Selectable |
Hz |
|
STC dynamic range |
70 |
dB |
|
Number of coherent integration |
Programmable |
- - |
|
Number of incoherent integration |
Programmable |
- - |
|
Baseband bandwidth |
8.5 |
MHz |
|
A/D dynamic range |
48 (8 bits) |
dB |
|
A/D sampling frequency |
18.75 (53.3 m s) |
MHz |
|
A/D sample length |
950 |
- - |
|
A/D sampling delay |
Programmable; 100-ns precision |
ns |
|
Range (depth) resolution (in ice) |
5 |
m |
|
Range sample spacing (per pixel in ice) |
4.494 & 3.37 |
m |
|
Antenna type |
4 element half l dipoles |
- - |
|
Along-track 3-dB 2-way beamwidth |
66 |
Degrees |
|
Cross-track 3-dB 2-way beamwidth |
18 |
Degrees |
|
Effective 3-dB along-track beamwidth of coherent integration |
7 |
Degrees |
Table 1. Radar System Parameters
We believe the pulse compression, coherent processing and the bistatic antenna arrangement are the features that permit this system to succeed in sounding outlet glaciers as well as the more than 3-km-thick ice sheet. For example, the two-way antenna half-power beamwidths are about 18° and 66° in the planes perpendicular and parallel to the nominal flight path, respectively. The coherent integration serves as a low-pass filter on the data and effectively reduces the along-track antenna beamwidth from about 66° to 7° at the nominal velocity of 110 m/s. Figure 1 illustrates this reduction in sensor beamwidth by projecting the instantaneous field of view on the ice surface for both the raw antenna illumination as well as the coverage after coherent integration.
Figure 1. Instantaneous field of view of the radar depth sounder projected on the ice surface for the raw antenna illumination (before coherent integration) and after coherent integration. Aircraft altitude is 500 m above ground (ice) level and aircraft speed is 110 m/s.
3.0 Experiment Results
Mapping internal ice layers is very important in radio echo sounding (RES) of ice. Internal ice layers are also important to glaciologists. Figure 2 shows an ICARDS radio echogram. This radio echogram was used to aid in deciding the location for the North GReenland Ice Core Project (NGRIP) drill site. The echogram shows there are no internal layer undulations. Also, by following the layers, the Danes identified the deepest visible layer to be 97 kyrs old, just a little above the 120-130 kyrs-old Eemian layer. The Eemian layer is important in the potential solution of the controversial question of whether there was an abrupt warming during that time.
Figure 3 shows an ICARDS radio echogram of the Petermann Glacier. The thickness, location of the grounding line and location of the calving front of the glacier can be identified from the echogram. This echogram, merged with data from the airborne Altimeter Topographic Mapper (ATM), can be used to study the drainage of ice from the vast ice sheet of Greenland.
The raw data were broken down to smaller files and formatted into our standard binary file format for easy processing and future use. We processed all the data with clear visible bottom echos first. Ice surface position and bedrock surface position were derived from these new formatted data using adaptive thresholding and smoothing algorithms before storing them back to the binary files.
4.0 Conclusion
Sea level rise, which affects inhabitants of the Earth, has led to a major research interest in the mass balance of the ice sheets. NASA responded by initiating a program to measure the elevation and thickness of the Greenland ice sheet. Airborne laser altimetry and radio echo sounding provide methods for obtaining the ice parameters. The NASA P-3 flight lines are controlled by specific task-orientated missions from year to year. Every five years the flights are repeated. The depth sounder systems are instrument that provide reliable measurements of the ice thickness.