Characteristics of the AVHRR Sensor

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.

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.

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