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.
Crops of the US West Coast and Cyclone Batsirai from a single pass of NOAA-19 at 1702.5 MHz. NOAA satellites use a HRPT-like "GAC" broadcast to play back stored image data, support coming to LeanHRPT soon! pic.twitter.com/cTdRBzDbnd
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.
Thanks to @ZSztanga's work & recording on GAC, here it is decoded from a dump to Svalbard at 15:42 UTC the 03/02/2022.
The signal is LHCP on L-Band (1702.5Mhz), BPSK at 2.66M with all onboard instruments (AVHRR at 4km/px).
We've also seen a tweet by @OK9UWU that shows a much longer image of a full orbit.
Getting my fair bit on this. Had two setups recieving ,both had LHCP feeds with high pol isolation so its surprising i even managed to get anything.
230cm dish and second is my 70cm dish on rotator. https://t.co/t55n9he3p5pic.twitter.com/w0hsTKBlvs