# L1B -- NOAA Polar Orbiter Level 1b Data Set (AVHRR)

GDAL supports NOAA Polar Orbiter Level 1b Data Set format for reading. Now it can read NOAA-9(F) --- NOAA-17(M) datasets. NOTE: only AVHRR instrument supported now, if you want read data from other instruments, write to me (Andrey Kiselev, dron@at1895.spb.edu). AVHRR LAC/HRPT (1 km resolution) and GAC (4 km resolution) should be processed correctly.

## Georeference

Note, that GDAL simple affine georeference model completely unsuitable for the NOAA data. So you should not rely on it. It is recommended to use the thin spline warper (tps). Automatic image rectification can be done with ground control points (GCPs) from the input file. NOAA stores 51 GCPs per scanline both in the LAC and GAC datasets. In fact you may get less than 51 GCPs, especially at end of scanlines. Another approach to rectification is manual selection of the GCPs using external source of georeference information.
Precision of the GCPs determination depends from the satellite type. In the NOAA-9 -- NOAA-14 datasets geographic coordinates of the GCPs stored in integer values as a 128th of a degree. So we can't determine positions more precise than 1/128=0.0078125 of degree (~28"). In NOAA-15 -- NOAA-17 datasets we have much more precise positions, they are stored as 10000th of degree.
Image will be always returned with most northern scanline located at the top of image. If you want determine actual direction of the satellite moving you should look at LOCATION metadata record.

## Data

In case of NOAA-10 in channel 5 you will get repeated channel 4 data.
AVHRR/3 instrument (NOAA-15 -- NOAA-17) is a six channel radiometer, but only five channels are transmitted to the ground at any given time. Channels 3A and 3B cannot operate simultaneously. Look at channel description field reported by `gdalinfo` to determine what kind of channel contained in processed file.

## Metadata

Several parameters, obtained from the dataset stored as metadata records.

Metadata records:

- SATELLITE: Satellite name

- DATA_TYPE: Type of the data, stored in the Level 1b dataset (AVHRR HRPT/LAC/GAC).

- REVOLUTION: Orbit number. Note that it can be 1 to 2 off the correct orbit number (according to documentation).

- SOURCE: Receiving station name.

- PROCESSING_CENTER: Name of data processing center.

- START: Time of first scanline acquisition (year, day of year, millisecond of day).

- STOP: Time of last scanline acquisition (year, day of year, millisecond of day).

- LOCATION: AVHRR Earth location indication. Will be Ascending when satellite moves from low latitudes to high latitudes and Descending in other case.

## See Also:

- Implemented as `gdal/frmts/l1b/l1bdataset.cpp`.

- NOAA Polar Orbiter Level 1b Data Set documented in the ``POD User's Guide'' (TIROS-N -- NOAA-14 satellites) and in the ``NOAA KLM User's Guide'' (NOAA-15 -- NOAA-16 satellites). You can find this manuals at NOAA Technical Documentation Introduction Page

- Excellent and complete review contained in the printed book ``The Advanced Very High Resolution Radiometer (AVHRR)'' by Arthur P. Cracknell, Taylor&Francis Ltd., 1997, ISBN 0-7484-0209-8.

- NOAA data can be downloaded from the Comprehensive Large Array-data Stewardship System (CLASS) (former SAA). Actually it is only source of Level 1b datasets for me, so my implementation tested with that files only.