AVHRR
AVHRR
Advanced Very High Resolution Radiometer

Introduction

The Advanced very high resolution radiometer (AVHRR) consists of a four to five channel scanner (depending on the model). These instruments are carried on various satellites such as the TIROS-N and the NOAA-11 satellite. It can sense in the visible, near-infrared, and thermal infrared bands of the electromagnetic spectrum. The AVHRR orbits the Earth at an altitude of 833 km and scans a width of 2399 km. It has a resolution of 1.1 square kilometers and circles the Earth 14 times a day (approximately 100 minutes per orbit).

Spectral Channels

The AVHRR on board the NOAA-11 polar orbiting satellite has 5 spectral channels:

  • Channel 1 (0.58 to 0.68 micrometers): used for mapping clouds and surface during the day
  • Channel 2 (0.75 to 1.1 micrometers): used to delineate surface waters and when combined with channel 1 can be used to monitor vegetation
  • Channel 3 (3.55 to 3.93 micrometers)
  • Channel 4 (10.5 to 12.5 micrometers)
  • Channel 5 (10.5 to 12.5 micrometers): channels 3,4 and 5 are combined and used for measuring sea surface temperature (SST), temperature of other surfaces, mapping clouds, and measuring the extent of snow and ice.

Data Acquisition

Data from the AVHRR is acquired in three formats:

  • High Resolution Picture Transmission (HRPT): Full resolution image data that is downloaded as the satellite flies over the ground station.
  • Local Area Coverage (LAC): Areas of particular interest are recorded at full resolution using an on-board tape recorder and played back as the satellite flies over the ground station.
  • Global Area Coverage (GAC): Scans at full resolution, but some of the data is either discarded or averaged. The data is then recorded on a tape recorder and later transmitted to a ground station.

Data Processing

One of the major "contaminants" to the AVHRR are clouds. Since clouds contain ice crystals, clouds cause the AVHRR to register frozen temperatures. There are several methods for dealing with clouds. One method is to use the channel 4 difference method. In this method, a 3x3 pixel grid is constructed around the pixel of interest. If the difference between the maximum and minimum pixel value exceeds some value, then the pixel is rejected.

A second method is to use channel 2. Channel 2 is used to measure albedo. If the albedo exceeds some value, it is assumed that the pixel contains clouds. If the difference in albedo changes dramatically from one pixel to the next (using the 3x3 method again), it is assumed that clouds are responsible. A third method is to use differences between channel 3 and channel 4. Channel 3 and 4 absorb very different proportions of water vapor. If the measured difference between these two channels is significant, then it is assumed there is an abundance of water vapor and therefore cloud contamination.

Another major source of error are edges. Pixels towards the edge of a scan field are usually distorted and therefore ignored.

Applications

There are many applications for the data from the AVHRR. The visible data can be used to map large areas, measure the extent of snow and/or ice, or monitoring vegetation. The infrared datasets can be used to measure sea surface temperature. By monitoring these variables, scientists will be better able to study the effects of global climate change.

Northeastern United States. Wind direction is parallel to lines formed by clouds (cloud streets)(from http://asd-www.larc.nasa.gov/~baum/AVHRR/streets.html)

An interesting use of AVHRR is to study eddies formed by the Gulf Stream. The Navy wants to be able to predict where these eddies will move because if a submarine can hide in these eddies, they would be virtually undetectable. Below is a picture of these eddies formed by the Gulf Stream.

From http://dcz.gso.uri.edu/amy/avhrr.html
Sea surface temperature data from the Chesapeake Bay.

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