HUTSCAT – Airborne Dual-Frequency Ranging Scatterometer
The HUTSCAT (Helsinki University of Technology Scatterometer) is a helicopter-borne non-imaging FW-CW scatterometer operating simultaneously at frequencies 5.4 GHz (C-band) and 9.8 GHz (X-band) and all four linear polarizations. HUTSCAT was designed and constructed during 1987-1990 by the Laboratory of Space Technology (now the Space Technology group in Aalto University Department of Radio Science and Engineering).
Figure 1. HUTSCAT mounted under Bell Jet Ranger helicopter.
The main parameters of HUTSCAT are presented in Table I. HUTSCAT measures the backscattered power as a function of measurement range at a resolution of 0.68 m. At each channel 20 backscattered power spectra per second are measured. Simultaneously with the backscattering measurements, the target is recorded by a video camera, and DGPS-coordinates of the flight track are saved in a laptop computer. A typical flight altitude is 100 m and the flight speed is 25 m/s. The backscattered power spectra data are converted to absolute backscattering coefficients using internal, external and range calibration coefficients. Internal and external calibration is used to eliminate short-term and long-term variations in the backscattered power level. Internal calibration is conducted by a delay line which connects transmitted power to the receiver. External (i.e. absolute) calibration is achieved with active radar calibrators.
Table 1. Technical characteristics of HUTSCAT.
|
Center frequencies |
5.4 and 9.8 GHz |
|
Modulation |
FM-CW |
|
Sweep bandwidth |
230 MHz |
|
Polarization |
HH, HV, VV, VH |
|
Transmitted power |
10 mW |
|
Measurement range |
20 to 167 m |
|
Range resolution |
0.68 m |
|
Incidence angle |
0° to 45° of nadir |
|
Antenna effective two-way 3 dB beamwidth |
4.7° (5.4 GHz) 4.4° (9.8 GHz) |
|
Antenna sidelobe level |
< -16.5 dB (5.4 GHz) < -16.5 dB (9.8 GHz) |
|
Antenna polarization isolation |
26.0 dB (5.4 GHz) 28.5 dB (9.8 GHz) |
|
Relative accuracy of σ°, 90% confidence interval |
±0.3 dB (5.4 GHz) ±0.3 dB (9.8 GHz) |
|
Absolute accuracy of σ°, 90% confidence interval |
±0.8 dB (5.4 GHz HH) ±0.5 dB (5.4 GHz VV, VH) ±0.5 dB (9.8 GHz) |
|
Noise equivalent σ° |
< -25.0 dB (5.4 GHz HH) < -28.5 dB (5.4 GHz VV) < -46.5 dB (5.4 GHz VH) < -21.0 dB (9.8 GHz HH) < -48.0 dB (9.8 GHz VH) |
HUTSCAT has a fixing device for Bell 206 JetRanger helicopter. The fixing device holds boxes containing the 5.4 GHz and 9.8 GHz RF-parts and antennas and a video camera. The signal processing and control unit is accommodated inside the helicopter. The fixing device is approved by Finland National Board of Aviation. The total weight of HUTSCAT hardware when accommodated onboard the helicopter is around 160 kg.
Reference
- M. Hallikainen, J. Hyyppä, J. Haapanen, T. Tares, P. Ahola, J. Pulliainen, M. and Toikka, "A helicopter-borne eight-channel ranging scatterometer for remote sensing - Part I: system description," IEEE Trans. Geoscience and Remote Sensing, vol. 31, no. 1, pp. 161-169, 1993.