StellarStation stores satellite messages in the cloud, a selection of which users can download directly from StellarStation Amateur. The data stored online is a binary file with raw 32bit IQ samples and a sample rate of 112.5 kHz. There are a number of ways to decode this binary data. Two potential options for reading raw samples are open sources tools like SDR# or GNU Radio.
Decoding IQ Data With SDR# on Windows
- Convert the binary file into a SDR# compatible .wav file using the "SoX" tool. SoX is a command line utility for sound files, which can be downloaded from Sourceforge. Once the software is extracted, open command prompt to run the SoX executable included in the extracted directory with these commands.
- For data stored before 2018/2/26 8:30 (UTC)
sox -e float -t raw -r 112500 -b 32 -c 2 -v 0.0001 [path to downloaded IQ file.bin] -t wav -e float -b 32 -c 2 -r 112500 [path to output file.wav] gain -h -30
For data stored after 2018/2/26 8:30 (UTC)
sox -e float -t raw -r 112500 -b 32 -c 2 [path to downloaded IQ file.bin] -t wav -e float -b 32 -c 2 -r 112500 [path to output file.wav]
-r: Sampling rate: Default sampling rate for IQ files downloaded from StellarStation is 112.5kHz.
-b: Bit rate: Original 32bit data is required to be converted to SDR# compatible 16bit data
-e: Data representation: Floating point
-c: Channel: 2ch for representing I-phase and Q-phase data
- Download SDR# (Windows SDR Software Package) from Airspy. Installation is not required for this software. Instead, you can just run the SDRSharp.exe file included in the package. In order to run our data, please choose the converted wav file by choosing "File player" in the "Source" option in SDR#.
- Our recommendation is to use the "File Player" plugin in SDR#, which can describe waterfall thumbnails of an entire file and also expand capability. You can find instructions for adding this tool on this RTL-SDR.com blog post. Please note that in the process of the installation, you will need to add the "magic line" into FrontEnds.xml file, since this is the file which is called from <frontendPlugins> section in the SDRSharp.exe.Config text file.
Screenshot of SDR# with File Player plugin
Decoding IQ Data With GNU Radio
- GNU Radio is an open source SDR development toolkit available on almost any OS. You can find the GNU Radio installation guide here.
- You can play an IQ data file with file sink directly. Here is an example flowchart:
Screenshots from GNU Radio Companion
- Select a "File Source" block, drag it into your work area, and choose a IQ file.
- If you play it as real speed, you also need to create a "Throttle" block using the same method and set the sample rate to 112.5kHz.
- The "Multiply Const" amplifier block acts like a volume control. If you feel that the signal level is too high, please add this block and set a value less than 1.
- "QT GUI Sink" is a utility sink block. When you select this block, you can view spectrum, time-domain waveform, waterfall, and constellation graphs.