| Budget Amount *help |
¥3,400,000 (Direct Cost: ¥3,400,000)
Fiscal Year 2004: ¥900,000 (Direct Cost: ¥900,000)
Fiscal Year 2003: ¥1,000,000 (Direct Cost: ¥1,000,000)
Fiscal Year 2002: ¥1,500,000 (Direct Cost: ¥1,500,000)
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| Research Abstract |
Polarimetric radar usually has polarimetric transfer errors in its antennas. It is therefore necessary to remove its influence for correct application of the system. The procedure is called polarimetric calibration. There are several approaches for polarimetric calibration; however, they have disadvantages as well as advantages. We investigated a new approach to use two reference targets, i.e. polarization preserving and polarization selective reflectors. Validation of the approach was made by using the data taken with Spaceborne Imaging Radar C (SIR-C) and showed the its good and performance for spaceborne synthetic aperture radar (SAR) calibration. SAR to be onboard the ALOS (advanced land observing satellite) uses the L-band radar wave and it may suffer from the influence of Faraday rotation within the ionosphere. The amount of Faraday rotation may vary with several parameters, so we have to estimate it and compensate its influence. The countermeasure was also investigated by using
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polarization rotating reflector and the new approach has been propposed.
We need to use the reflectors as described above. We developed new polarization rotating reflector by using the concept of Van Atta array. It has a capability of source tracking, so it does not need precise pointing of the reflector. Polarization rotating function was introduced by using dual-polarization antenna elements. When the reflector is of active design, the influence of direct scattering can be neglected; however, there are serious problems of direct scattering when it is of passive design. We proposed the quarter-wavelength displacement technique to suppress the direct scattering components. The technique was very simple, but effective to suppress the scattered field from the reflectors. We also had to develop polarization selective reflectors having high cross polarization discrimination ratio. When we use polarization grid, it is enough to use thin grid structure. However, it is not so easy to construct a large reflector with very thin grid. We applied the quarter-wavelength displacement technique to the polarization selective reflector using grid, and the result was satisfactory.
Unfortunately, launch of the ALOS satellite is delayed, so we could not conduct the experiment on the PALSAR (Phased array L-band synthetic aperture radar). However, we had an opportunity to test the above algorithm and reflectors in the airborne synthetic aperture radar experiment conducted by JAXA and NICT. We regarded it as a preliminary test of the satellite experiment. We obtained the satisfactory results that the channel imbalance was within 1 dB, channel phase difference was within 6.2 degrees, and cross-polarization noise was less than -25 dB. This proved the validity of our approach and showed the promising role in the ALOS experiment. Less
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