AVHRR Composites = Advanced Very High Resolution Radiometer U.S. Alaska: 1989 - Present

Normalized Difference Vegetation Index (NDVI) Composites are produced from multiple Advanced Very High Resolution Radiometer (AVHRR) daily observations that have been composited together to create a nearly cloud-free image showing maximum greenness. An NDVI ratio is produced from bands one and two of the AVHRR composite to produce a derived NDVI band composite. Normalized Difference Vegetation Index (NDVI) Composites are produced from multiple Advanced Very High Resolution Radiometer (AVHRR) daily observations that have been composited together to create a nearly cloud-free image showing maximum greenness. An NDVI ratio is produced from bands one and two of the AVHRR composite to produce a derived NDVI band composite. The following NDVI products are available: Conterminous U.S. and Alaska Weekly and biweekly NDVI composites date from January 1989 to present. U.S. composites are cast to the Lambert Azimuthal Equal-Area projection. Alaska composites are cast to the Albers Equal-Area Conic projection. Distribution of the NDVI composites depends on the age of the image: 7- and 14-day (current) NDVI composites are weekly and biweekly updates produced within the last four weeks. The data includes a composite for each of the acquired AVHRR bands, as well as a derived NDVI band. 7- and 14-day (recent) NDVI composites are weekly and biweekly updates produced from the end of the previous year until four weeks prior to present. The data includes a composite for each of the acquired AVHRR bands, as well as a derived NDVI band. 14-day (historical) NDVI composites consist of biweekly composites produced from January 1989 to the end of the previous year. The data includes a composite for each of the acquired AVHRR bands, as well as a derived NDVI band. Satellite zenith, solar zenith, relative azimuth, and acquisition dates are also provided.归一化差分植被指数（Normalized Difference Vegetation Index, NDVI）合成产品由多组先进甚高分辨率辐射计（Advanced Very High Resolution Radiometer, AVHRR）逐日观测数据合成而来，用于生成近乎无云的最大绿度影像。利用AVHRR合成数据的1、2波段计算NDVI比值，可得到衍生的NDVI波段合成产品。 归一化差分植被指数（Normalized Difference Vegetation Index, NDVI）合成产品由多组先进甚高分辨率辐射计（Advanced Very High Resolution Radiometer, AVHRR）逐日观测数据合成而来，用于生成近乎无云的最大绿度影像。利用AVHRR合成数据的1、2波段计算NDVI比值，可得到衍生的NDVI波段合成产品。 当前可获取以下NDVI产品： 覆盖美国本土及阿拉斯加地区的周度、双周度NDVI合成数据，时间跨度为1989年1月至今。美国本土合成数据采用兰伯特方位等面积投影（Lambert Azimuthal Equal-Area projection），阿拉斯加合成数据采用阿尔伯斯等面积圆锥投影（Albers Equal-Area Conic projection）。NDVI合成产品的分发依据影像时效分为以下三类： 7天与14天（当前）NDVI合成产品：为近四周内产出的周度、双周度更新数据，包含所有获取到的AVHRR波段合成数据以及衍生NDVI波段。 7天与14天（近期）NDVI合成产品：为上一年年末至四周前产出的周度、双周度更新数据，包含所有获取到的AVHRR波段合成数据以及衍生NDVI波段。 14天（历史）NDVI合成产品：为1989年1月至上一年年末产出的双周度合成数据，包含所有获取到的AVHRR波段合成数据以及衍生NDVI波段。此外还会提供卫星天顶角、太阳天顶角、相对方位角以及观测日期数据。