AVHRR, SPOT Vegetation and MODIS Vegetation Index Anomalies:
The Global Inventory Modeling and Mapping Studies (GIMMS) group at NASA Goddard Space Flight Center processes and produces multi-instrument normalized vegetation index (NDVI) products to support USDA’s Office of Global Analysis (OGA) agricultural monitoring activities.
The principal end-user products consist of three sets of vegetation index anomalies derived from:
The anomalies from each data set are computed as follows:
The following mean baseline periods are used
The NDVIg data set was produced using empirical mode decomposition/reconstruction (EMD) to minimize effects of orbital drift, view geometry, volcanic aerosols and clouds. Data points with missing data are filled through interpolation procedures that take into account long-term mean and nearest-neighbor values. NDVIg are screened for clouds in near real time using thresholds on the channel 5 brightness temperature. These tests are applied to Africa, Australia, and South America only. No snow/ice screening is done in near real time for NDVIg since the older AVHRR instruments did not have a channel 3A (SWIR).
The SPOT Vegetation data are screened for cloud, snow/ice, and band data quality using the QA layer provided by VITO. If a pixel QA indicates cloud, cloud shadow, unknown cloud status, snow/ice, or poor quality in B0/B2/B3 then the pixel is flagged and its value is not used in calculations of means or anomalies.
NOAA-18 data was produced to provide a one-instrument only data series as a cross check on the other two data sets and to provide redundancy in the monitoring system. The dataset uses the same channel 5 thresholds as NDVIg to screen for clouds, but also additional tests based on channels 1,2 and 5 (CLAVR algorithms) are used. Also, the SWIR band (3A) is used in conjuction with channel 5 brightness temperatue to screen out snow and ice.
The MODIS-Terra dataset was produced using the MODAPS MOD09A1 Surface Reflectance 8-day 500 m products. MOD09A1 500m tiles were processed in the standard MODIS Sinusoidal grid tiling system and reprojected/mosaiced to produce this 1km Albers Conical Equal Area dataset. MOD09A1 QC and State QA flags were applied to screen for band quality, clouds, water, and snow/ice. These products are “experimental” until the real-time production is completed and operational.
All anomalies are calculated with reference to cloud-free measurements per pixel over the given reference period. Therefore the respective baseline period for each pixel will vary by the number of cloud-free/good data measurements.
Examples of these anomaly products can be found at http://www.pecad.fas.usda.gov/cropexplorer/fews_briefing/ as part of OGA’s support to the USAID FEWSNET project.