Thursday, December 24, 2015

Acquire GPS signals from rtl_sdr recording

This one took a few evenings of head-scratching to finally resolve.

I want to be able to record I/Q off an RTL-SDR stick and then post-process it with gnss-sdr. The rtl_sdr software by default dumps interleaved unsigned shorts - that is an (uchar)i_sample, then a (uchar)q_sample, then an (uchar)i_sample, so on and so forth.

gnss-sdr has a couple of different SignalSources and a couple of different item_types and I initially tried varying combinations of those hoping something would work out of the box. Part of my confusion was the existence of an Osmosdr_Signal_Source which uses the RTL-SDR stick and outputs gr_complex without any intermediate conversion. I figured there would be a way to use this SignalSource with raw RTL-SDR dumps but this is not the case. In any case, with the current gnss-sdr infrastructure one must convert the rtl_sdr dump into a file format that gnss-sdr will read.

There are two components to getting an rtl_sdr dump into gnss-sdr. The first is converting the input file from interleaved uchars to gr_complex. The second is properly configuring your input file.

Converting rtl_sdr interleaved uchar into gr_complex

This is fairly well documented in this post at the Amateur Radio stackexchange. There are two methods proposed, one is writing up a flowgraph in GNURadio to convert the file. The second is a short C program posted by Dr. Paul Brewer which performs the exact same operation in 45 lines of code (with comments!) I opted for the C code. Once you compile the program, the process looks like this:
  1. Acquire dump using "rtl_sdr /path/to/mydump.bin -s 2.048e6 -f 1575.420e6 &"
  2. Convert file using "rtlsdr-to-gqrx /path/to/mydump.bin /path/to/complex.bin"
A third option is to write a radio receiver in GNURadio which uses the RTL-SDR stick to directly write a file in the appropriate format. I will go through this in a future post, as it is a good introduction into gnuradio using only two blocks.

Creating an input file

You can use the "gnss-sdr_GPS_L1_rtlsdr_realtime.conf" as a template and make the following changes:

  1. SignalSource.implementation=File_Signal_Source
  2. SignalSource.filename=/path/to/complex.bin
  3. Change every sampling_frequency to the sampling frequency used in the initial acquisition
  4. Change the InputFilter.IF to your IF
Once you make these changes you should be able to decode the file.

Bonus: Have your cake and eat it too!

You can skip the whole mess by modifying two line of code in gnss-sdr_GPS_L1_rtlsdr_realtime.conf which will let you dump the raw I/Q in complex format to file while in parallel running the tracking loop and getting a GPS solution.

To enable this,
  1. Set SignalSource.dump=true
  2. Set SignalSource.dump_filename=/path/to/dump.bin
Now you can track in realtime and generate a data file to post-process simultaneously!

As always any questions, comments or suggestions please leave a message.

2 comments:

  1. Hi. I am looking for captured raw data for simulation. I have already installed GNSS SDR and tried their simulation file. I have tracked in realtime (as well as non real time) and did not successfully obtain a position. Your raw data would be of great help. Thanks!

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  2. Hi Jan, I saw your email on the gnss-sdr list and hadn't had a chance to reply yet. I can procure some data in the near future for you. The only data I have right now is from my house and I'm reluctant to put that online.

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