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General tips/data for APT and LRPT decoding

Frequency list

NOAA-15: 137.6200 MHz | NOAA-18: 137.9125 MHz | NOAA-19: 137.1000 MHz | Meteor M-N2: 137.1000 MHz

Obviously for those of you wanting to do LRPT as well you will have to deal with the overlap and shared frequency of NOAA 19. This is always irritating and there isn't much you can do. An APT signal will destroy a digital LRPT signal, and if you try decoding APT with LRPT in the background it will show discernible shapes, but nothing really useful to work with. The best thing to do is to wait a few days for their orbits to diverge from each other.

APT: FM, 34 kHz signal, set filter bandwidth to around 38-50 kHz. If you’re correcting for Doppler you can maybe go towards the lower of the range to block out more of what you don't need, but if not go for 50 to get the whole thing in and account for the +/- Doppler difference, approximately 3 kHz.

LRPT: FM, 120 kHz signal, set filter bandwidth to around 140-150 kHz. Doppler correction isn’t strictly needed as the demodulator tools can pick out the data and lock onto it just fine. An SNR of around 10 dB will decode fine, but you may even get some data coming through around 5-10 if you're lucky. Just don't expect a great, error-free capture. Once you hit around 10-20 dB you're laughing.

For LRPT work remember that it’s the raw I/Q baseband recording that you want if you aren't decoding live, not an audio recording. For APT you want the audio recording.

Another really important thing to mention is the black bars that will frequent your LRPT images. This is due to a bug on the Meteor satellite itself, and the imager will reboot every 6-7 minutes or so. There is nothing you can do about this, other than fix in post if you like. You'll hear an awful screeching noise when this happens.

If you’re switching between receiving the two types of signal don’t forget to check your Doppler/recording settings are suitable for each one! MM3IIG has lost more than one capture because of this kind of silliness…

If you can avoid DC spikes by using offset tuning or a filter function it can help, particularly with LRPT. If it’s baked into your recording and you're using SatDump then there’s an option to block it as the file is read.

You'll need more gain for LRPT signals because of the similar power but lower bandwidth (power density). This can induce intermod/breakthrough depending on your antenna, location, etc. If that happens you may want to look into bandpass filters for 136-138 MHz.

Experiment! Different apps can give different results with the same data. You'll eventually land on a workflow you like with the image channels you like.

Audio Samples

Here's a couple of signal samples of the Atlantic/Europe area to test your software setup with. There's a WAV audio recording for APT and an LRPT sample consisting of a baseband I/Q WAV recording (i16, 500000 kbps sample rate). You'll need to use DC Blocking on the baseband file whether you import it into SatDump or playback in SDR# into the demodulator. The LRPT sample is not great quality (14 deg pass with a fairly low SNR of 9-10 dB, but it still decodes well).

APT Sample

11,025 kHz mono WAV file, (16 MB) Link

LRPT Sample

500 kHz wide baseband I/Q WAV file, (475 MB) Link

vhfsatellite-general.txt · Last modified: 2021/08/14 18:15 by mm3iig