Advanced very-high-resolution radiometer

Advanced very-high-resolution radiometer (AVHRR) instruments are a type of space-borne sensor that measure the reflectance of the Earth in five spectral bands that are relatively wide by today's standards. Most AVHRR instruments are or have been carried by the National Oceanic and Atmospheric Administration (NOAA) family of polar orbiting platforms (POES). The first two are centered on the red (0.6 micrometres, 500 THz) and near-infrared (0.9 micrometres, 300 THz) regions, the third one is located around 3.5 micrometres, and the last two sample the thermal radiation emitted by the planet, around 11 and 12 micrometres, respectively. The NOAA satellite has equator crossing times of 0730 and 1930 local solar time.

The first AVHRR instrument was a four-channel radiometer, while the latest version (known as AVHRR/3, first carried on the NOAA-15 platform launched in May 1998) acquires data in a 6th channel located at 1.6 micrometres.

NOAA has at least two polar-orbiting meteorological satellites in orbit at all times, with one satellite crossing the equator in the early morning and early evening and the other crossing the equator in the afternoon and late evening. The primary sensor on board both satellites is the AVHRR instrument. Morning-satellite data are most commonly used for land studies, while data from both satellites are used for atmosphere and ocean studies. Together they provide twice-daily global coverage, and ensure that data for any region of the earth are no more than six hours old. The swath width, the width of the area on the Earth's surface that the satellite can "see", is approximately 2,500 kilometers (~1,540 mi). The satellites orbit between 833 or 870 kilometers (+/− 19 kilometers, 516–541 miles) above the surface of the Earth.

The highest ground resolution that can be obtained from the current AVHRR instruments is 1.1-kilometer (0.68 mi), which means that the satellite records discrete information for areas on the ground that are 1.1 × 1.1 kilometers. This smallest recorded unit is called a pixel. AVHRR data have been collected continuously since 1981.

The primary purpose of these instruments is to monitor clouds and to measure the thermal emission of the Earth. These sensors have proven useful for a number of other applications, however, including the surveillance of land surfaces, ocean state, aerosols, etc. AVHRR data are particularly relevant to study climate change and environmental degradation because of the comparatively long records of data already accumulated (over 20 years). The main difficulty associated with these investigations is to properly deal with the many limitations of these instruments, especially in the early period (sensor calibration, orbital drift, limited spectral and directional sampling, etc.).

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