Many thanks to Ryzerth from SDR++ for his invaluable help
A simple GNU Radio based flowgraph to decode ITOS HRPT (e.g from NOAA-2).
Documentation on my website
Additional info on the SigID Wiki
A baseband for testing purposes is available here NOAA2.wav, but you only need the ITOS_Decoder.grc file if you have your own baseband.
Open the file with GNU Radio. Adjust the path of the file in the Wav File Source block and the wav_samp_rate_mhz variable with the sample rate of the file, in MHz (e.g. 4 for 4 MHz).
Start the decode, it should produce some calibration ramp in the raster.
If the image is slanted, adjust samples_per_line slowly until the image is perfectly straight.