LRPT Decoding and Demodulation Tutorial

This page contains an easy to follow tutorial on the process of Demodulating and Decoding an image from a QPSK signal origniating from the Meteor-M2 Satellite

Prerequisites

This tutorial assumes you already have the required hardware and basic software such as SDR# or HDSDR Installed and working.

To continue you will need to download the following Software which is free of cost and works offline:

QPSK Signal Demodulator

M2_LRPT_Decoder.exe (ver. 2015.7.1.33)

Audacity

Collecting Signal Data

This tutoriual will fous On using SDR# to collect the data, this is possible with an alternative program such as HDSDR or CubicSDR but the process will vary.

Once you have a good elevation pass tune to 137.1/137.9 Mhz and look for the signal. It Looks like this:

Before you start recording make sure you have the frequency centered. Also check the box that says "I/Q Swap". Once you have that done , in the recording section uncheck "Audio" and make sure "Baseband" is checked. Make Sure "Sample Format" says "16 Bit PCM". Then click on record. This will record a very large .wav file. The total pass should give a 2.5Gb file. SDR# automatically stops the recording at 2Gb so make sure to restart it.

Processing Recorded Data

Now open the recorded .wav file in audacity. In the bottom left there is a drop down menu with the label "Project Rate (Hz)". Change it to 13000 Hz. Then at the top click on "Tracks" then "Resample" and make sure to resample to 130000Hz. This is a big file so it will take a while. Once resampled , export the file.

Demodulation

Now that you have the resampled .wav file it's time to demodulate the QPSK signal. You should have downloaded the demodulation program earlier, It will be located in a "software.zip" folder. Look for the file name "LRPTrx.exe" The program should look like this:

Then in the "Input file Name" box , select the file you just resampled in audacity. In the "Output Filename" box select a directory where you want the demodulated file to be located. Also make sure to enable I/Q Swap.

Then hit the start button at the very right of the program, if you followed the above steps correctly you should see 4 groups of constellations as seen in the below image.

LRPTrx does not stop on its own so make sure to stop it yourself whenever done demodulating. Now that you have your demodulated file, open the AMIGOS decoder you donwloaded earlier. It should look like this when you open it:

Now click on the button labeled "72k" and select the demodulated .wav file you got from LRPTrx. AMIGOS should then start decoding automatically.