Acquiring GPS signals with an RTLSDR dongle using gnss-sdr

Once you've installed gnss-sdr and tested your installation with prerecorded data, the next step is to try and get a live GPS solution. The following is based off this node at gnss-sdr.org and this paper. The paper is a really good mix of theory and implementation and I strongly recommend reading it.

The first thing you need to do is calibrate the front end of your RTL-SDR device. The calibration procedure will do the following

1. Download an ephemeris via SUPL. This provides your computer with advance knowledge of the GPS satellite locations, allowing you to get a quick fix.
2. Search for GPS satellites
3. Calculate the doppler shift assuming your RTL-SDR stick is perfect (trust me, it's not!)
4. Using the predicted doppler shift and some fancy math (see the paper, section 5), calculate
1. The actual sampling frequency
2. The actual IF bias

Before running the calibration you need to update the front-end-cal.conf. Make a copy of the version installed (typically to /usr/local/share/gnss-sdr/conf/front-end-cal.conf). On the first couple of lines you will see a number of GPS lat/lon/alt locations commented out using a semicolon. You need to provide your best estimate at lat/lon/alt, bearing in mind altitude is in meters. The easiest way to do this is use a tool like the aptly named mapcoordinates.net. The other change I had to make was to change both SUPL servers to supl.google.com in place of supl.nokia.com - for some reason the nokia servers were not working. Once complete, you can run the front end calibration like so 

sudo front-end-cal --config_file=/path/to/my/front-end-cal.conf

Your output should look something like this

linux; GNU C++ version 4.9.1; Boost_105500; UHD_003.007.003-0-unknown
Initializing... Please wait.
Logging will be done at /tmp
Use front-end-cal --log_dir=/path/to/log to change that.
Trying to read ephemeris from SUPL server...
SUPL: Trying to read GPS ephemeris from SUPL server...
SUPL: Received Ephemeris for GPS SV 1
SUPL: Received Ephemeris for GPS SV 2
SUPL: Received Ephemeris for GPS SV 3
SUPL: Received Ephemeris for GPS SV 5
SUPL: Received Ephemeris for GPS SV 6
SUPL: Received Ephemeris for GPS SV 7
SUPL: Received Ephemeris for GPS SV 8
SUPL: Received Ephemeris for GPS SV 9
SUPL: Received Ephemeris for GPS SV 10
SUPL: Received Ephemeris for GPS SV 11
SUPL: Received Ephemeris for GPS SV 12
SUPL: Received Ephemeris for GPS SV 13
SUPL: Received Ephemeris for GPS SV 14
SUPL: Received Ephemeris for GPS SV 15
SUPL: Received Ephemeris for GPS SV 16
SUPL: Received Ephemeris for GPS SV 17
SUPL: Received Ephemeris for GPS SV 18
SUPL: Received Ephemeris for GPS SV 19
SUPL: Received Ephemeris for GPS SV 20
SUPL: Received Ephemeris for GPS SV 21
SUPL: Received Ephemeris for GPS SV 23
SUPL: Received Ephemeris for GPS SV 24
SUPL: Received Ephemeris for GPS SV 25
SUPL: Received Ephemeris for GPS SV 26
SUPL: Received Ephemeris for GPS SV 27
SUPL: Received Ephemeris for GPS SV 28
SUPL: Received Ephemeris for GPS SV 29
SUPL: Received Ephemeris for GPS SV 30
SUPL: Received Ephemeris for GPS SV 31
SUPL: Received Ephemeris for GPS SV 32
SUPL: Trying to read Acquisition assistance from SUPL server...
Actual RX Rate: 2000000.000000 [SPS]...
Actual RX Freq: 1575420000.000000 [Hz]...
PLL Frequency tune error 0.000000 [Hz]...Actual RX Gain: 40.200000 dB...
Front-end RAW samples captured
Using Volk machine: sse4_2_64_orc
Searching for GPS Satellites in L1 band...
[ . . . . 5 . 7 8 9 . . . . . . . . . 19 . . . 23 . . . 27 28 . 30 . . ]
Total signal acquisition run time 2.73478 [seconds]
Reference Time:
GPS Week: 851
GPS TOW: 276680 22134.400000
~ UTC: Tue Dec 15 20:51:21 2015
Current TOW obtained from SUPL assistance = 276680
Doppler analysis results:
SV ID Measured [Hz] Predicted [Hz]
5 3000.00 871.77
7 2875.00 524.39
8 2500.00 170.24
9 1125.00 -1220.17
19 6000.00 3734.82
23 -750.00 -3032.99
27 687.50 -1703.53
28 4375.00 2306.86
30 3750.00 1621.38
Parameters estimation for Elonics E4000 Front-End:
Sampling frequency =1999997.27 [Hz]
IF bias present in baseband=2151.87 [Hz]
Reference oscillator error =-1.37 [ppm]
Corrected Doppler vs. Predicted
SV ID Corrected [Hz] Predicted [Hz]
5 848.13 871.77
7 723.13 524.39
8 348.13 170.24
9 -1026.87 -1220.17
19 3848.13 3734.82
23 -2901.87 -3032.99
27 -1464.37 -1703.53
28 2223.13 2306.86
30 1598.13 1621.38GNSS-SDR Front-end calibration program ended.

The important outputs are highlighted above, the sampling frequency and the IF bias. The sampling frequency is nominally 2 MHz but deviation of a few Hz are normal. Baseband is defined as 0 Hz, so when the signal is shifted to baseband if the signal is not exactly centered at 0 Hz this bias is calculated as an IF bias. 

To receive GPS, then, we can copy the gnss-sdr_GPS_L1_rtlsdr_realtime.conf file (typically located in /usr/local/share/gnss-sdr/conf) to another directory and make a few edits. Handily enough there are comments in the file like this

NSS-SDR WITH RTLSDR DONGLES USER MUST SET THE CALIBRATED SAMPLE RATE HERE
; i.e. using front-end-cal as reported here:http://www.cttc.es/publication/turning-a-television-into-a-gnss-receiver/

at each place we need to make an edit. The four edits you need to make are

GNSS-SDR.internal_fs_hz=(your sampling frequency)
SignalSource.sampling_frequency=(your sampling frequency)
InputFilter.sampling_frequency=(your sampling frequency)
InputFilter.IF=(your IF bias)

Once complete, you can run the receiver 

gnss-sdr ---config_file=/path/to/your/gnss-sdr_GPS_L1_rtlsdr_realtime.conf

It will likely take a minute or two to get a fix as a certain amount of ephemeris and almanac data must be downloaded from the satellites. You also must have at least four satellites in view. Here is example output once a fix was found:

Using the KML file generated by gnss-sdr I was able to plot the locations generated in Google Earth

The red dots are individual position fixes. The blue arrow points to the skylight where my GPS antenna is mounted. The road is a two-lane road with no shoulder and the boat in the backyard is 20' long. There is a bias which may be due to the fact that the roof occludes satellites to the east (the four satellites I picked up were, at the time, overhead and eastward). I want to move the antenna up to a (plastic) roof vent in the attic to get a better view of the sky.