利用星载微波辐射计SSM／I多通道、多时相亮温数据开展了中国陆地覆盖特征的季节变化研究。通过对国内外星载微波辐射应用研究分析，提出了归一化极化指数(NDPI)的概念。处理了1997年4月、7月、10月和1998年1月的(每月的20、24日各两天)多通道SSM／I数据，在此基础上计算形成了第一幅中国陆地区域的归一化微波极化指数图，开展了中国陆地区域覆盖特征随季节变化的研究。研究结果表明，不同的陆地覆盖特征有特征的NDPI值，NDPI随季节而变化，植被、水分是引起NDPI变化的主要因子。

By processing and analyzing space borne multi frequency microwave radiometry data acquired by SSM/I in different time, we aim at carrying out research on seasonal change of land cover characteristics in China. Through a short review on passive microwave remote sensing applications,we choose Normalized polarization index( NDPI ) to describe land cover characteristics. Because NDPI is independent of surface temperature, it diminishes difference caused by acquiring time in data in various regions. It is also shown that NDPI is actually a normalized emissivity of different polarization. Its main influence factors are vegetation biomass and soil surface moisture. With biomass increasing, NDPI becomes smaller;with soil moisture increasing, NDPI increases also. Multi frequency SSM/I data acquired in April,July,October of 1997 and January of 1998(20 th and 24 th of every month )were processed and analyzed. Then the SSM/I NDPI maps over land in China were put forward for the first time. These maps express seasonal change of land cover characteristics in China. By analyzing all of these data and maps, we found that specific land cover has specific NDPI value and NDPI value changes with season.It is concluded that the NDPI maps has great potential to monitor land cover characteristics. The general trend of NDPI is (from high to low):desert, semi desert, plateau(grassland), agricultural region and forest .The research illustrates that 19GHz NDPI is a good indicator of seasonal change of land cover characteristics. This paper also provides a new method to research on global change (such as vegetation and soil moisture change) because we have demonstrated in the paper that a NDPI map covering all of China can be formed by using only two day's SSM/I data,and furthermore, this map is affected little by cloud and is independent of surface temperatrue.