Applications of AVHRR data to
Global Change Research

The goal of this document is to describe the proceedure to process AVHRR data, and to provide examples of processed data.

Characteristics of the AVHRR Sensor

Average Orbital Height: 830 km (515 Miles)
Spatial Resolution:
Global Area Coverage (GAC): 4.4 kilometers
Local Area Coverage (LAC): 1.1 kilometers (Available over the U.S. and in some parts of the world)
Swath Width: 2800 kilometeres
Coverage: 2 times per day per satellite
Records Data in 5 Wavelength Intervals (bands)
  1. Visible Green and Red (0.58 to 0.68 microns)
  2. Near Infrared (0.72 to 1.10 microns)
  3. Mid Infrared (3.53 to 3.93 microns)
  4. Thermal Infrared (10.3 to 11.3 microns)
  5. Thermal Infrared (11.5 to 12.5 microns)
Bands 1 and 2 record reflected energy
Band 3 records reflected energy during the day and emitted energy at night.
Bands 4 and 5 records emitted thermal (heat) energy

Geographic Coverage

The image above shows AVHRR coverage on January 4, 1995 at 00:30 GMT (Eastern time is GMT -5 hours). The satellite was moving from southeast to northwest (ie. ascending). The horizontal lines are at one minute increments.

Geographic Rectification and Registration

As the satellite passes over the earth, it scans the surface beneath it at an angle of +/- 63 degrees. As a result, the spatial resolution varies from 1.1 km directly beneath the satellite, to 5.5 km at the edge of the swath. This causes geometric distortions to occur. The image on the left shows an example of the unrectified data. In our processing, we use an automated routine that utilizes satellite tracking information to determine where the satellite was when it acquired the data. Once the position of the satellite is known, the data can be registered to and earth location. This registered data, can then be rectified to any map projection. The image on the right is the rectified version of the image shown on the left. Note that this is the same data shown in the example of geographic coverage shown above.

AVHRR Sensor Calibration

Before the data can be used in a quantitative manner, it needs to be calibrated. The calibration algorithm is based on information contained in the Polar Orbitor User's Guide. Briefly, the visible and near infrared data (channels 1 and 2) are converted from raw counts to percent albedo using a linear relationship determined prior to launch. The thermal infrared data (channels 3, 4, and 5) are converted from raw counts to radiances with a linear relationship that is based on the raw count value associated with cold space (roughly 3 degrees Kelvin) and the raw count value associated with the temperature of an onboard target (approximately 300 degrees Kelvin). A slight nonlinearity in channels 4 and 5 is corrected using a quadratic function of radiance. Lastly, the infrared radiances are converted to temperature using the inverse Planck function.

Examples of the Use of AVHRR Data

Volcanic Eruption Clouds
Lake Superior
Global Vegitation Composite
Sea Surface Temperature Archive-URI

Ordering Data

The Satellite Active Archive (SAA) is a system designed to provide easy access to data from NOAA's satellites. SAA contains descriptive information about those data sets, and permits users to search inventories of data holdings for availability based upon geographic and date requirements. In addition to providing advanced online data query and product request capabilities, SAA also provides an online graphical browse tool which can assist the user in determining geographic coverage of individual data sets and display online digital representations of those data sets. Once the data requirements have been determined an order may be placed electronically, and data may be delivered either electronically or through physical medium.

NOAA Satellite Act ive Archive

The Pathfinder AVHRR Land data sets are global, multichannel, land surface data derived from the Advanced Very High Resolution Radiometers (AVHRR) on the NOAA/TIROS operational meteorological satellites (NOAA-7, -9, and -11) which provide a continuous daily and composite data set from July 1981 through the present. These data, when complete, will enable studies of global vegetation and surface characteristics over more than a decade. Every effort has been made to provide a consistently processed data record by cross-calibrating instruments and minimizing changes to the processing algorithm during the processing of the 1981 to present data.

NOAA/NASA AVHRR Pathfinder Home Page

Technical Information

An online version of the Polar Orbitor User's Guide. This document gives you many of the details about the sensors, data structure, processed products, and placing orders.

Polar Orbitor User's Guide