Import AVHRR/3

Import AVHRR/3

The NOAA AVHRR/3 reader enables SNAP to import NOAA AVHRR/3 HRPT and AVHRR/3 LAC Level-1b data products.

Note: The AVHRR/3 reader is implemented in line with the "NOAA KLM User's Guide" ( ) and is capable of reading data products in version v2 to v4.

Note: A good source for AVHRR/3 data products is

Data Calibration

While AVHRR pixel data is read into memory it is calibrated on the fly. Scan lines with calibration errors are detected and corresponding sample values are set to a special no-data-value. SNAP will exclude those no-data-values from pixels statistics and displays them in a separate image overlay.

Features

The AVHRR/3 reader supports data products from the NOAA -15, -16, -17, and -18, as well as from the NOAA-N' satellites. The following features characterize the reader:

• autodetection of the existence of an Archive Retrieval System (ARS) Header Record.
• autodetection of products in 8 bit, 10 bit and 16 bit format, as long as they contain all 5 AVHRR bands.
• autodetection whether band 3a or band 3b or both are present in a single scene.
• reading of products which contain AVHRR/3 Local Area Coverage (LAC) and High Resolution Picture Transmission (HRPT) data sets.
• reading extensive Metadata from the header record.
• calibrating the raw values by applying calibration coefficients contained in the metadata.
• reading different quality flags.
• setting the no data value, if either the data or the calibration coefficients are invalid.
• geolocating the data products according to the given navigation data (solar zenith angle, satellite zenith angle, relative azimuth angle, latitude, longitude).
• setting the start and end date of the data acquisition.

The products contain the CLAVR-x  (Clouds from AVHRR Extended cloud mask) cloud band. A draft of the CLAVR-x cloud mask theoretical basis document can be found at .

The reader has been tested with products obtained from the NOAA CLASS server which conforms to the format version 2 used for the NOAA -15, -16 and -17 satellites. Recently, NOAA has changed to the new version 3 format. The AVHRR/3 reader already supports both existing 10-bit packed format versions. AVHRR/3 data products can optionally have an Archive Retrieval System (ARS) Header Record. If this header is present, it is automatically detected by the reader. In particular, the AVHRR/3 reader implementation has the following features:

1. Read geometry data at tie-points.
2. Provide geometry data at pixels using interpolation.
3. Read raw counts and calibrate them to geo-physical values for all spectral channels.
4. Channel 3a/3b usage detection.