GPS using an RTL-SDR stick

Using a $20 RTL-SDR stick with 1ppm TXCO and a simple mod to power an active GPS antenna, it is possible to download and decode GPS signals in real time.

Software

GNSS-SDRLIB:  https://github.com/taroz/GNSS-SDRLIB

RTKLIB: http://www.rtklib.com/

Both packages are Open Source, with a default build target is Windows.
Both packages are Open Source (GPL2 and BSD 2-clause, respectively). The default build target for both packages is Windows, although RTKLIB has been compiled under Linux.

Please note that GNSS-SDRLIB is not to be confused with GNSS-SDR!

Resources

Useful presentation by the author of GNSS-SDRGUI for a summer school course. Also check the manuals included with GNSS-SDRLIB and RTKLIB.

Step-By-Step Implementation

1. Ensure RTL-SDR stick is working in Windows. If your driver is not working, try using the Zadig driver installation method outlined here.
2. Install GNSS-SDRLIB and RTKLIB to any convenient directory
3. Open GNSS-SDRGUI and select the following options
1. Input Type: RTL-SDR
2. [x] RTCM MSM, Port 9999
3. Change "output interval" dropdown to 10 hz
4. [x] Plot Tracking
5. [x] All GPS, GLOSNASS, Galileo satellites 
6. (optional) enter approximate lat/lon into MISC and click the "..." button to get  current satellite locations in relation to your location.
4. Click "Start", a number of command consoles will open then close for each satellite being tracked.
5. Click "M" for log
6. Now, open RTKNAVI
1. Click on the "I" button
1. check "rover", type TCP Client, format RTCM3
2. click OPT button and set address to "localhost" and port to "9999"
3. click OK
4. Click OK
2. Click on the "start" button
3. Within a few seconds you should see satellites in the Rover:Base SNR pane
4. Once a solution exists it will update lat/lon in the left pane
5. Click "Plot" to generate a plot of the random walk of lat/lon over time

This is what your GNSS-SDRGUI should look like:

This is what your RTKNAVI input should look like:

If everything is working you should get a GPS solution, as shown in this video

I was also able to decode a sample recorded by a RTL-SDR stick connected to an Intel Compute Stick which could serve as a portable GPS receiver.

Next Actions

Try using GNSS-SDR and/or GNURadio to decode the GNSS signal; this would provide native, Linux compatible headless execution and a cursory Google suggests this has been done successfully with the RTL-SDR stick.