NOAA KLM User's Guide

Appendix J.1

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APPENDIX J.1: AVHRR/3

The AVHRR/3 is an imaging system in which a small field of view (1.3 milliradians by 1.3 milliradians) is scanned across the earth from one horizon to the other by continuous 360 degree rotation of a flat scanning mirror. The orientation of the scan lines are perpendicular to the spacecraft orbit track and the speed of rotation of the scan mirror is selected so that adjacent scan lines are contiguous at the subsatellite (nadir) position. The analog data output from the sensors is digitized on board the satellite at a rate of 39,936 samples per second per channel. Each sample step corresponds to an angle of scanner rotation of 0.95 milliradians. At this sampling rate, there are 1.362 samples per IFOV. A total of 2048 samples will be obtained per channel per Earth scan, which will span an angle of ± 55.4 degrees from the nadir (subpoint view). All six spectral channels of the AVHRR/3 are registered so that they all measure energy from the same spot on the earth at the same time.

As with earlier versions of the AVHRR instrument, the GAC processing of the AVHRR/3 data makes the frame rates directly compatible by only using the data from every third AVHRR/3 scan. The data are further reduced by averaging the value of four adjacent samples and skipping one sample of each channel of AVHRR data across each scan line used. Figure J.1-1 shows the relationship of GAC and LAC data, while Figure J.1-2 shows the signal position as a function of scan angle for the AVHRR/3 instrument.

Figure showing simulated Earth surface footprints of GAC and LAC data

Figure showing AVHRR/3 signal position as a function of scan angle

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