Korean Journal of Remote Sensing, Vol.26, No.2, 2010, pp.123~131 Detection of Artificial Displacement of a Reflector by using GB-SAR Interferometry and Atmospheric Humidity Correction Jae-Hee Lee*, **, Hoonyol Lee*, Seong-Jun Cho**, Nak-Hoon Sung**, and Jung-Ho Kim** *Department of Geophysics, Kangwon National University **Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources Abstract : In this paper we applied Ground-Based Synthetic Aperture Radar(GB-SAR) interferometry to detect artificial displacement of a reflector and performed an atmospheric humidity correction to improve the accuracy. A series of GB-SAR images were obtained using a center frequency of 5.3 GHz with a range resolution of 25 cm and a azimuth resolution of 0.324, all in fullpolarization (HH, VV, VH, HV) modes. A triangular trihedral corner reflector was located 160 m away from the system, and the artificial displacements of 0-40 mm was implemented during the GB-SAR image acquisition. The result showed that the RMS error between the actual and measured displacements, averaged in all polarization data, was 1.22 mm, while the maximum error in case of the 40 mm displacement was 2.72 mm at HH-polarization. After the atmospheric correction with respect to the humidity, the RMS error was reduced to 0.52 mm. We conclude that a GB-SAR system can be used to monitor the possible displacement of artificial/natural scatterers and the stability assessment with sub-millimeter accuracy. Key Words : GB-SAR, interferometry, displacement, atmospheric humidity correction. hoonyol@kangwon.ac.kr 123
Korean Journal of Remote Sensing, Vol.26, No.2, 2010 Table 1. Parameters for the GB-SAR experiment. Measurement parameters Center frequency 5.3 GHz Range bandwidth 600 MHz IF Bandwidth 1 khz Number of sampling 1601 Power 33 dbm Azimuth Step 0.05 m Azimuth length 5 m 124
Detection of Artificial Displacement of a Reflector by using GB-SAR Interferometry and Atmospheric Humidity Correction (a) (b) (c) Fig. 1. Photos of the experimental setup. (a) GB-SAR system. (b) Aerial view of experiment location. The 4 check symbols are the stable targets while the triangle represent the location of the triangular trihedral corner reflector. (c) Side view of the reflector on top of a acrylic plate. (d) Top view of the reflector with the scale-bar on the plate. (d) (a) HH (b) VV (c) VH (d) HV Fig. 2. GB-SAR amplitude images of (a) HH, (b) VV, (c) VH, and (d) HV polarization. The reflector signal is much stronger in copolarization than cross-polarization images due to the nature of a triangular trihedral corner reflector used in this experiment. 125
Korean Journal of Remote Sensing, Vol.26, No.2, 2010 Table 2. Coefficients of atmospheric correction function. Df = 4pR(ah b), [47% h 58%] a b HH 3.61 10-5 1.73 10-3 VV 3.74 10-5 1.78 10-3 VH 3.13 10-5 1.50 10-3 HV 3.34 10-5 1.59 10-3 l j = 4p R (1) l j n j 0 = 4p (R n R 0 ) = 4p r n (2) l l d j = 4p (3) dr l Table 3. RMS errors between the measured and actual displacements before and after atmospheric humidity corrections RMS error Before Atmospheric After Atmospheric humidity Polarization Correction(mm) Correction(mm) HH 1.56 0.22 VV 1.12 0.62 VH 0.76 0.78 HV 1.45 0.47 Average 1.22 0.52 126
Detection of Artificial Displacement of a Reflector by using GB-SAR Interferometry and Atmospheric Humidity Correction (a) (b) (c) Fig. 3. Comparisons between the actual and measured displacements in (a) HH, (b) VV, (c) VH, and (d) HV polarization. (d) Fig. 4. Atmospheric relative humidity during the two days including the measurement time. 127
Korean Journal of Remote Sensing, Vol.26, No.2, 2010 (a) (b) (c) Fig. 5. Relationship between relative humidity and phase/range values extracted from the stable targets (4 targets 6 measurement = 24 data points) at each polarization. (d) Dj = 4p(ah b) (4) R a b 128
Detection of Artificial Displacement of a Reflector by using GB-SAR Interferometry and Atmospheric Humidity Correction (a) (b) (c) Fig. 6. Comparisons between the actual and the atmospherically corrected displacement in (a) HH, (b) VV, (c) VH, and (d) HV polarization. (d) 129
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