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.