AHVRR (Advanced Very High Resolution Radiometer) Composites

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）合成产品基于多幅先进甚高分辨率辐射计（Advanced Very High Resolution Radiometer, AVHRR）的每日观测数据经合成处理后生成，旨在得到近乎无云的最大绿度影像。通过AVHRR合成产品的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合成产品的生成流程与前述一致，即基于多幅先进甚高分辨率辐射计每日观测数据合成得到近乎无云的最大绿度影像，并通过AVHRR合成产品的1、2波段计算NDVI比值，生成衍生NDVI波段合成产品。
以下为可用的NDVI产品：覆盖美国本土与阿拉斯加的周度、双周度NDVI合成产品，时间跨度为1989年1月至今。其中美国本土合成产品采用兰伯特方位等面积投影（Lambert Azimuthal Equal-Area projection），阿拉斯加合成产品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三类：7天与14天（当前）NDVI合成产品为近四周内生成的周度、双周度更新数据，该数据集包含所有采集到的AVHRR波段的合成结果，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生成的周度、双周度更新数据，数据集包含所有采集到的AVHRR波段的合成结果及衍生NDVI波段。
14天（历史）NDVI合成产品为1989年1月至上一年年末生成的双周度合成产品，数据集包含所有采集到的AVHRR波段的合成结果及衍生NDVI波段。
数据集还提供卫星天顶角、太阳天顶角、相对方位角及数据采集日期信息。