The AVHRR radiometer (AVHRR - Advanced Very High Resolution Radiometer) was initially designed for weather observation (determination of cloud covering and surface temperature). Later the data was found to be useful for numerous applications in the field of Earth Observation and AVHRR sensor became one of the most popular sources of remote sensing data.

The AVHRR scanner has a sweep of broad band, capable of measuring radiation in different zones of the electromagnetic spectrum. On having obtained the same image in different lengths of wave it is possible to do a multispectral analysis to define with great precision hydrological, oceanography and meteorological parameters.

Table 1. Explorer module.

Swath	Field of view	IFOV	Spatial resolution
2580 Km.	55.4º a c/lado del nadir	1.3 x 1.3 mrad	1.1 x 1.1 Km. nadir

Three different models of this instrument can be distinguished: AVHRR/1, AVHRR/2 and AVHRR/3. The latter is the one on board the KLM series satellites.

The AVHRR instrument provides with data form different portions of the visible, near infrared and thermal infrared spectrum. Observation from channels corresponding to the visible and near infrared allows the study of vegetation, clouds, lakes, coasts, snow and ice. The three remaining channels operating in the thermal infrared allow to obtain products for land, water, sea surface and clouds temperature. The additional 3A channel present in AVHRR/3 improves the capability for distinguishing among snow, ice and clouds. See table 2.

Table 2. AVHRR sensor.

Instrument	Spectral Resolution	Spatial Resolution	Radiometric Resolution
AVHRR/2 (NOAA-14)	5 bands	1100 m	10 bits
AVHRR/3 (NOAA15,16,17&18)	6 bands	1100 m	10 bits
Instrument	Spectral Resolution - Bands
AVHRR/2 (NOAA-14)	Channel 1 - Visible Channel Channel 2 - Near infrared Channel 3 - Mid infrared Channel 4 - Far infrared Channel 5 - Far infrared
AVHRR/3 (NOAA15,16,17&18)	Channel 1 - Visible Channel Channel 2 - Near infrared Channel 3a - Near infrared 3b - Mid infrared Channel 4 - Far infrared or Thermal Channel Channel 5 - Far infrared or Thermal Channel

AVHRR data transmission modes: AVHRR transmits data in three different ways: High Resolution Picture Transmission (HRPT), Local Area Coverage (LAC) and Global Area Coverage (GAC).

HRPT data is transmitted to a ground station as they are collected. The average instantaneous field-of-view of 1.4 milliradians yields to a ground resolution of approximately 1.1 km around the nadir at a nominal orbit altitude of 833 km.

LAC are high resolution data that is recorded on an on board tape for subsequent transmission during a station overpass. The average instantaneous field-of-view of 1.4 milliradians yields to a ground resolution of approximately 1.1 km around the nadir at a nominal orbit altitude of 833 km.

GAC data is derived from a sample averaging of the full resolution AVHRR data. Four out of every five samples along the scan line are used to compute one average value and the data from only every third scan line is processed, yielding to a 1.1 km by 4 km resolution at the subpoint.

In CREPAD the data is acquired in HRPT format.

1 AVHRR Instrument .... 2 AVHRR Archiving .... 3 CREPAD Products ....