Tagged: GAC

Receiving NOAA Global Area Cover (GAC) Images with LeanHRPT

A few weeks ago we posted about how @ZSztanga and @aang254 were able to record and decode Global Area Cover (GAC) images from polar orbiting NOAA weather satellites. GAC images are low resolution, but they provide an image of the entire orbit. The GAC signal is only transmitted over the USA.

A week earlier than @ZSztanga and @aang254 above decoded GAC, another software called LeanHRPT by @Xerbo also implemented a GAC decoder. LeanHRPT is available on Windows, Linux and MacOS, and ready to download binaries are available on the releases page. You'll need the LeanHRPT demodulator too, in order to initially demodulate the signal.

@Xerbo also notes that @dereksgc has also released a useful Python script for predicting NOAA GAC transmissions. It shows when a particular NOAA satellite will begin and end their GAC transmission, as well as the frequency, polarization and elevation of the satellite. 

GAC Transmission Prediction Tool

Global Area Coverage (GAC) Images Decoded from NOAA Satellites

Thank you to @ZSztanga and @aang254 for submitting news about their recent success at decoding the L-Band Global Area Coverage (GAC) signal from polar orbiting NOAA satellites. GAC images are low resolution, and described by NOAA as follows:

Global Area Coverage (GAC) data set is reduced resolution image data that is processed onboard the satellite taking only one line out of every three and averaging every four of five adjacent samples along the scan line.

While it's low resolution, the interesting thing about this data is that you get an image of the entire orbit, not just the data from your current location as you'd receive with the standard 137 MHz APT or L-Band HRPT signal. The catch is that the signal is usually only transmitted over the USA, and you'll need a motorized or hand tracked L-Band satellite dish setup to receive it.

We note that GAC data is not to be confused with the Direct Sounding Broadcast (DSB) signal decoding software we posted about in 2020. 

@ZSztanga has provided some more information about what images are available and who can receive it, and @aang254's tweet below provides some images and additional information:

With @aang254 we decoded GAC from NOAA satellites. It's basically a dump of reduced resolution data from the whole orbit. It includes all the instruments and is transmitted on L-band along with HRPT (mostly over USA, rarely above Europe and only NOAA-19 dumps outside the US). All the decoders are in SatDump.

There is also a schedule available (https://noaasis.noaa.gov/cemscs/polrschd.txt) that includes all the dumps in the upcoming week. It might be a bit hard to interpret, but basically there is a date and the ground station name (SVL stands for Svalbard and it is the only one receivable in Europe). Entries with "GAC" or "PBK" are referring to the GAC transmission.

We've also seen a tweet by @OK9UWU that shows a much longer image of a full orbit.