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Written by Jeffrey Gillan
The Polar Orbiting Environmental Satellites (POES) are a cooperative effort between NASA and NOAA.
The Advanced Very High Resolution Radiometer (AVHRR) is a multi-spectral sensor flying on NOAA’s Polar Orbiting Environmental Satellites (POES). The high resolution refers to its fast temporal ability. At any given time AVHRR is active on two satellites orbiting the Earth in opposite directions, allowing for total global coverage twice daily. A sensor on many satellites over the years, AVHRR data has been continually collected since 1981 and is still currently operational. AVHRR applications encompass meterological, climatological, and land use. Its broad instantaneous field of view (i.e., resolution) of 1.1 km lends itself to observing surface conditions on a regional or continental scale and has been used to monitor food crops, wildfires, desert encroachment, and vegetation changes.
Its temporal ability is a major advantage for tracking fast changing surface conditions on a broad scale. Another advantage of AVHRR data is the availability of pre-processed products. Weekly and bi-weekly NDVI products are archived and available at no charge. Its broad spatial resolution, however, can also be a disadvantage if your area of interest is relatively small.
MODIS - Moderate-resolution Imaging Spectroradiometer, OrbView-2
Image source: NOAA
AVHRR Level 1b image
Image source: NOAA
An AVHRR image showing heat signatures and smoke.
Image source: NOAA
A composite NDVI product derived from AVHRR bands 1 and 2.
AVHRR sensors can acquire data in two different modes. Local Area Coverage (LAC) data has a spatial resolution of 1.1 km. Global Area Coverage (GAC) data has a spatial resolution of 4 km. The data usually has 10-bit radiometric precision. The exact band specifications have varied over time for the different ACHRR sensors that are placed aboard the different satellites.
The AVHRR swath width is 2,900km
A Level 1b (radiance) scene is 2,400×6,400km
POES satellites have polar orbits at a relatively low orbit of 830 to 870 km above the Earth. They circle the earth 14 times per day allowing for complete global coverage twice per day. The AVHRR sensor is active on two satellites in opposite orbits; one ascending, one descending. This double coverage ensures that every place on Earth is observed every six hours.
Much of AVHRR data is free of cost.
From the USGS Earth Explorer http://earthexplorer.usgs.gov/, weekly and biweekly NDVI composites of the entire U.S. are available from January 1989 to present. Also available are specific orbit segments from 1992-1999. These are Level 1b products (radiance values).
NOAA Comprehensive Large Array-Data Stewardship (CLASS) http://www.class.ngdc.noaa.gov/saa/products/welcome;jsessionid=0B932BE066C8A7EC6B3948A290405A77 has Level 1B products from 1981 to present. The images can be downloaded free through an FTP site. The files are usually between 45-60 MB.
Imagery downloaded from the USGS Earth Explorer comes in .IMG format. This format is recognized in many visualization packages such as Erdas Imagine, ENVI and ArcGIS
The CLASS system receives and archives the AVHRR data in its native binary format and requires a bit of software writing to read the data. Fortunately, there are a number of commercial vendors who support this format and provide programs that would read, display, geo-reference, calibrate, and much more. An example of one such program is Erdas Imagine from Leica Geosystems. Please see the HELP FAQ under satellite data viewing software to find a list of other vendors. http://www.class.ngdc.noaa.gov/notification/faq.htm#11
There are no special hardware or software requirements for using AVHRR data.
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