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The Advanced Very High Resolution Radiometer (AVHRR) is carried aboard the NOAA polar orbiting satellites. These satellites operate in a near-polar sun synchronous orbit at a height of about 850 km and provide global coverage roughly twice daily. The AVHRR has:
For the IR channels, the noise equivalent differential temperature (NEDT) is approximately 0.12 C when viewing the ocean surface.
First the AVHRR data is extracted from the raw High Resolution Picture Transmission (HRPT) telemetry data using hrptin, lac, or gac. (hrptin replaces the function avin). hrptin is designed to extract AVHRR data from the HRPT data collected with the TeraScan data acquisition system. lac extracts AVHRR data from Local Area Coverage (LAC) data tapes provided by the U.S. National Oceanic and Atmospheric Administration (NOAA). gac extracts AVHRR data from Global Area Coverage (GAC) data tapes provided by the NOAA. GAC data are a lower resolution version of the LAC data that are archived globally.
LAC and GAC data are described in detail in the Polar Orbiters Data Users Guide Kidwell (1988). In all three cases, counts in the range 0 to 1023 along with the data necessary to convert counts to radiometric temperatures are extracted form the HRPT data stream.
The hrptin function converts the output (whether or not it has been navigated) to radiometric temperatures. For visible channels the results are percent albedo; for the infrared channels the results are temperature in degrees Celsius/Kelvin. Occasionally lines are missing from the HRPT telemetry stream; hrptin can also fix missing lines by duplicating nearby lines.
Small errors in the spacecraft clock and horizon tracker produce small errors (1 to 10 km) in the satellite location. For many applications these errors are not important and correcting this error can be ignored. For many studies, however, precise locations in small areas are required. To correct these errors, navigate the data with the navbox function.
The ingested AVHRR dataset is navigated by means of the functions navbox2 and nav2. navbox2 selects boxes that contain iregular land/water boundaries, and nav2 correlates image data with those boundaries.
The 3.7u channel (Channel 3) of the AVHRR sensor historically has been plagued with sensor noise. If left uncorrected, this noise will produce errors in the MultiChannel Sea Surface Temperatures (MCSST) that are produced by nitpix. The function smear has been developed to reduce channel noise variance.
As noted, producing estimates of Sea Surface Temperature is done with the function nitpix. The last step in the chain is to orient the data to a master using the function fastreg. You can view the dataset any time after ingest (hrptin) using the TeraVision program.
Last Update: $Date: 2001/10/05 00:00:38 $
