The Advanced Very High Resolution Radiometer (AVHRR) is a sensor embarked on the National Oceanic and Atmospheric Administration (NOAA) family of polar orbiting platforms. AVHRR instruments measure the reflectance of the Earth in 5 relatively wide spectral bands. The first two are centred around the red (0.6 micrometer) and near-infrared (0.9 micrometer) regions, the third one is located around 3.5 micrometer, and the last two sample the thermal radiation emitted by the planet, around 11 and 12 micrometers, respectively.
The first AVHRR instrument actually was a 4-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 micrometer.
The primary purpose of these instruments is to monitor clouds and to measure the thermal emission (cooling) 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).
The AVHRR instrument also flies on the METOP series of satellites. The three planned METOP satellites are part of the Eumetsat Polar System (EPS) run by Eumetsat.
More information about AVHRR can be found here.