Polarimetric calibration of synthetic aperture radar onboard the ALOS satellite

极化雷达天线中存在极化传递误差。因此，有必要消除其影响，以正确应用该系统。该过程称为偏振校准。存在用于极化校准的若干方法;然而，它们具有缺点以及优点。提出了一种利用偏振保持和偏振选择反射器作为参考目标的新方法。利用星载成像雷达C（SIR-C）的实测数据对该方法进行了验证，结果表明该方法具有良好的星载合成孔径雷达（SAR）定标性能。ALOS（Advanced Land Observing Satellite）上搭载的SAR使用L波段雷达波，可能会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而变化，因此我们必须估计它并补偿它的影响。同时，还利用 ...更多信息 偏振旋转反射器和新的方法已经提出，我们需要使用上述反射器。利用货车阿塔阵列的概念，研制了一种新型的偏振旋转反射镜。它具有源跟踪的能力，因此不需要反射器的精确指向。利用双极化天线单元引入极化旋转函数。当反射器为有源设计时，直接散射的影响可以忽略，而当其为无源设计时，直接散射的问题严重。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，但有效地抑制了来自反射器的散射场。我们还必须开发具有高交叉偏振鉴别比的偏振选择性反射器。当我们使用极化栅极时，使用薄栅极结构就足够了。然而，用很薄的网格来构造大的反射器并不是那么容易。我们将四分之一波长位移技术应用于栅格偏振选择反射器，取得了令人满意的效果，但由于ALOS卫星发射延迟，无法在相控阵L波段合成孔径雷达（PALSAR）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。我们把它看作是卫星试验的初步试验。实验结果表明，信道不平衡度小于1dB，信道相位差小于6.2 °，交叉极化噪声小于-25dB。这证明了我们的方法的有效性，并显示了在ALOS实验的前景。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