福爾摩沙衛星二號(福衛二號)為我國第一顆遙測與科學用途之高解析度光學衛星，其搭載之光學酬載為遙測照相儀(Remote Sensing Instrument，RSI)，具有4米的高空間解析度和8位元之輻射解析度。為維持光學酬載RSI之輻射品質，定期的在軌絕對輻射校正是不可或缺，亦即為本文之研究目標，包括特定目標及相互輻射校正。特定目標以國際上常用之美國內華達州為校正場址，透過地表反射率之實地量測，利用輻射傳送模式(6S)，配合實際福衛二號影像之灰階值檢驗並校正其間之輻射轉換係數。在相互輻射校正方面，則以近同步觀測之 Landsat-8 OLI (Operational Land Imager)為參考影像進行校正。結果顯示，利用特定目標及相互輻射校正所獲之轉換係數與最新儀器校正數值之差異約±5%以內，符合國際上對於絕對輻射校正誤差的要求，代表本研究所建構之絕對輻射校正方法具相當可行性。 此外，RSI與一般高輻射解析度之光學感測器不同，RSI具有可根據地表反射特性調整動態量程(Dynamic Range)的功能，以有效地獲取高、低反射地區之整合觀測。因此，本研究根據MODIS (Moderate-Resolution Imaging Spectroradiometer)之全球地表反射率產品(MOD09)、太陽照射之幾何與福衛二號光學特性，透過輻射傳送模式(6S)模擬福衛二號RSI各頻段於大氣層頂所觀測之顯輻射率，並配合RSI儀器之輻射轉換模式，建構福衛二號各頻段之全球全球動態量程，除可避免RSI影像之輻射飽合或訊雜比過低外，亦可提升影像之輻射解析度。 ;FORMOSAT-2 satellite (FS-2) is the first Earth observation satellite equipped with delicate optical sensor operated by the National Space Organization (NSPO) in Taiwan. The main payload on FS-2 is Remote Sensing Instrument (RSI) with high spatial resolution (8 m) and 8-bits radiometric resolution. For any optical sensors, in-orbit radiometric calibration is essential for ensuring the accurate radiance observation. Therefore, the primary goal of this study is to construct the absolute radiometric calibration procedure including the vicarious-calibration by using field-measurement reflectance and the cross-calibration by applying Landsat-8 Operational Land Imager (OLI) image as a reference. Then, the desert areas in Nevada are also applied to be a calibration site in this study. In addition, the radiative transfer code, Second Simulation of the Satellite Signal in the Solar Spectrum (6S), is employed to drive the radiance RSI observing on the top of atmosphere. Furthermore, the radiometric conversion coefficients of RSI could be estimated based on the relationship between the retrieved radiance and the digital number. The results of this study show that the radiometric conversion coefficients are similar to the dim file in terms of the differences between them within ±5% for most part of spectral bands. Moreover, an option of selecting gain factor is designed for RSI to subtly adjust the dynamic range of radiometric signal. This function make RSI can successfully acquire images in both dark and bright regions. For this objective, the related 6S model input data such as solar geometry, RSI spectral response function and Moderate-Resolution Imaging Spectroradiometer (MODIS) global land surface reflectance product (MOD09) are employed to derive the potential ranges of reflected radiance on the top of atmosphere for each spectral band of FS-2 RSI. Eventually, the global map of dynamic range map for FS-2 RSI could be constructed with the calibrated radiometric coefficients to eliminate the saturation of output signal and enhance the target signal to noise ratio over bright and dark surfaces, respectively, as well as efficiently extent the radiometric resolution.
