Polarimetric calibration of synthetic aperture radar onboard the ALOS satellite

极化雷达的天线通常存在极化传输误差。因此，有必要消除其对系统正确应用的影响。这个过程被称为极化校准。有几种极化校准方法；然而，它们有优点也有缺点。我们研究了一种使用偏振保持和偏振选择两种参考目标的新方法。利用星载C型成像雷达（SIR-C）的数据对该方法进行了验证，验证了该方法在星载合成孔径雷达（SAR）标定中的良好性能。ALOS（先进陆地观测卫星）上的SAR使用l波段雷达波，它可能受到电离层内法拉第旋转的影响。法拉第旋转的量可能随几个参数的变化而变化，因此我们必须估计它并补偿它的影响。并对采用多偏振旋转反射器的对策进行了研究，提出了新的对策。我们需要使用如上所述的反射器。我们利用Van Atta阵列的概念研制了一种新型偏振旋转反射器。它具有源跟踪能力，因此不需要精确指向反射器。利用双极化天线单元引入极化旋转功能。当反射器为主动设计时，直接散射的影响可以忽略；然而，在被动设计时，直接散射存在严重的问题。我们提出了四分之一波长位移技术来抑制直接散射分量。该技术非常简单，但能有效地抑制反射器的散射场。我们还必须开发具有高交叉偏振辨别比的偏振选择性反射器。当我们使用极化网格时，使用薄网格结构就足够了。然而，用非常薄的网格构造一个大的反射器并不是那么容易的。将四分之一波长位移技术应用于栅格偏振选择反射器，取得了满意的效果。不幸的是，由于ALOS卫星发射延迟，我们无法在PALSAR（相控阵l波段合成孔径雷达）上进行实验。然而，我们有机会在JAXA和NICT进行的机载合成孔径雷达实验中测试上述算法和反射器。我们认为这是卫星实验的初步试验。我们得到了令人满意的结果，信道不平衡在1 dB以内，信道相位差在6.2度以内，交叉极化噪声小于-25 dB。这证明了我们的方法的有效性，并在ALOS实验中显示了有希望的作用。少

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 … More 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