ENVISAT ASAR影像地理定位方法

随着ENVISAT ASAR传感器成功通过调试阶段，国内将有越来越多的用户能够获取到ASAR数据。如何进行ASAR数据的地理定位是实际应用中需要解决的第一个关键问题。基于距离一多普勒(RD)模型发展了对ASAR主要影像产品进行地理定位的方法，重点解决了不同影像产品类型距离向方程的构建方法：对于斜距产品，距离向方程由第一斜距和影像距离向像元大小所确定的直线方程描述；精确地距模式影像元数据中仅提供一套SRGR参数，可直接用于建立距离向方程；而中分辨率地距模式影像在元数据中提供了若干套SRGR参数，需根据方位向成像时间与SRGR参数的方位向更新时间的关系，通过插值方法建立距离向方程。利用该方法对APP_1P、APM_1P、APS_1P、IMP_1P和IMM_1P共5景ASAR数据进行了地理定位试验。SAR处理器在生成ASAR数据产品时，已经对影像4个角点的地理位置进行了定位，并将定位结果记录在ASAR数据的元数据中。本文以这些定位结果为基准对所发展的定位方法的相对精度进行了评价，实验结果表明：对于APP_1P、APS_1P和IMP_1P数据产品，经度和纬度相对误差都小于1m；对于中分辨率APM_1P和IMM_1P数据产品，最大经度误差为59．73m，最大纬度误差为83．38m，分别是影像像元大小的0．8倍和1．1倍；总之，本文定位结果和生产这些产品的SAR处理器的定位结果有很高的符合程度，对进一步发展ASAR数据正射校正算法及其他相关雷达数据处理技术有一定的参考意义。

Study on ENVISAT ASAR Image Geolocation Method

As the ASAR sensor has successfully passed the primary validation period, more and more users in China will be able to get ASAR data in the near future. If users apply the data to practical mapping, the first key problem which should be solved is to geolocate the SAR image. Based on Range-Doppler(RD) model, a geolocation method has been developed for the major imaging products of ENVISAT Advanced SAR (ASAR). As one special contribution of this paper, the way to construct range equations for different ASAR products has been solved successfully. The range function of slant range image products is just a linear equation in one order, which can be defined by the first range distance and the range pixel spacing. For precision ground range products, the one and only SRGR record provided by the metadata can be used directly to construct the range equation. But for median resolution ground range products, several SRGR records have been provided by the metadata, the range function should be established through interpolation method according to the relation between the azimuth time for each image line and the azimuth updating time for each SRGR records. Five scenes of ASAR image have been geolocated with this method and the relative geolocation accuracy has been evaluated taking the geographic locations of four image corners derived from SAR metadata as reference. The geographic locations of the four image corners have been calculated and stored in the metadata of SAR image product by the original SAR processor. The product type ID for the five image products is APP -1P, APM -1P, APS -1P, IMP -1P and IMM -1P respectively. It has been shown that the location error in both latitude and longitude direction is below 1.0 meter for precision products such as APP -1P, APS -1P and IMP -1P; For median resolution products such as APM -1P and IMM -1P, the maximum longitude error is 59.73 m, about 0.8 times of the image pixel space; and the maximum latitude error is 83.38m, about 1.1 times of the image pixel space. It shows that the geolocation result achieved by the developed method is quite consistent with that achieved by the SAR processor to produce these ASAR image products. Also, the result will be useful for the development of orth-rectification and the other relevant processing algorithms for ASAR image products.