福衛五號遙測儀之全球增益數研究與開發

Global Gain Map Generation For Formosat-5 RSI Observations

許國賢(Kuo-Hsien Hsu) ； 林孟岳(Meng-Yue Lin) ； 曾聖凱(Sheng-Kai Zeng) ； 林唐煌(Tang-Huang Lin) ； 劉振榮(Gin-Rong Liu)

《航測及遙測學刊》 22卷1期 (2017/03) Pp. 13-21

https://doi.org/10.6574/JPRS.2017.22(1).2

摘要

福爾摩沙衛星二號(以下簡稱福衛二號)為我國第一枚自主作業的遙測與科學用途之高空間解析度衛星，其搭載之遙測光學酬載為遙測照相儀(Remote Sensing Instrument, RSI)，具有8米的高空間解析和8位元之輻射解析度。福爾摩沙衛星五號(以下簡稱福衛五號)擁有與福衛二號相似之多頻譜波段的遙測照相儀且具較高之12位元輻射解析度，為接續福衛二號之觀測任務。與一般高解析光學感測器不同，遙測照像儀擁有調整輻射動態範圍之功能，以提升高反射及低反射地區整合觀測之能力，即根據地表的反射特性調整動態量程(Dynamic Range)之增益數(Gain Factor)，以避免高反射率地區出現灰階值飽和情形，並可加強低射率地區的訊噪比。因此，本文根據MODIS(Moderate-Resolution Imaging Spectroradiometer)所提供之全球地表反射率產品(MOD09)，結合太陽之幾何參數與福衛二號及福衛五號之光學特性，應用輻射傳送模式6S(Second Simulation of a Satellite Signal in the Solar Spectrum)分別計算福衛二號及福衛五號在大氣層頂所觀測的輻射率，配合RSI儀器之輻射轉換模式，建構福衛二號及福衛五號之全球增益數分布圖，以提供動態量程選用之參考依據。以福衛二號RSI的驗證結果顯示，本文所建構之全球增益數分布與現行之增益數分布，於陸地部分及部分頻段之海洋區域具有相近趨勢，顯示此方法具可行性。而在福衛五號之全球增益數反演結果方面，海洋、陸地和極區分別為四、二、一，與實際地表反射狀態相當穩合，顯示此法將可有效地提供未來福衛五號全球增益數之設置與選用參考，進而提升取像之成功率。

關鍵字

福衛二號 ； 福衛五號 ； 遙測照相儀 ； 動態量程 ； 增益數 ； 全球地表反射率

並列摘要

FORMOSAT-2 (FS-2) is the first operational remote sensing satellite of the National Space Organization (NSPO) for science in Taiwan. The Remote Sensing Instrument (RSI), with high spatial resolution (8m) and 8-bits radiometric resolution, is the primary payload of FS-2. FORMOSAT-5 (FS-5) RSI has the similar spectral bands and bandwidths to FS-2 RSI but better radiometric resolution (12-bits) and spatial resolution in multispectral bands (4m). To fit the large fluctuation of surface reflectivity, an option of selecting gain factor is designed for the proper dynamic range of radiometric signal. When observing a high reflectivity region, the lower gain number should be selected to avoid the possible saturation of output signal, while a larger gain number would be favorable for a low reflectance region to enhance the signal to noise ratio (S/N Ratio). Therefore, the objective of present research is to generate global gain number of each spectral band for RSI FS-2/FS-5. For this topic, the Moderate-Resolution Imaging Spectroradiometer (MODIS) global surface reflectance product (MOD09) associated with solar geometry and Second Simulation of a Satellite Signal in the Solar Spectrum, (6S, a radiative transfer code) are employed to derive the reflected radiance in each spectral band at the top of atmosphere. With the radiometric coefficients of RSI, the maps of global gain number can be constructed accordingly. The generated gain number maps of FS-2 RSI agree with the operational maps of National Space Organization (NSPO) in most land areas and some marine areas. The results of FS-5 RSI also conform well to the criteria of gain setting in the areas of land, ocean and polar region in terms of surface reflectivity. The overall result suggests that the global gain maps generated in this study can competently offer the setting of FS-5 RSI gain number for the global observation after launched.

並列關鍵字

Formosat-5 ； Formosat-2 ； Dynamic range ； Gain factor ； RSI ； Global surface reflectance