GPS Simulation

Introduction

This page details experiences using LimeSDR to simulate GPS.

These experiments were inspired by the excellent procedure written up here [1]. We want to use a similar process to target real devices, and have had luck with a qstarz 818XT bluetooth gps device a couple of inches from an antenna. Here I am visiting Trinity College Cambridge.

Setup

Software to git clone - https://github.com/osqzss/gps-sdr-sim
Follow the instructions on the github page for how to compile, it is a very easy procedure on Ubuntu with build-essential package installed.

$ gcc gpssim.c -lm -O3 -o gps-sdr-sim

Note there is a setting in gpssim.h for USER_MOTION_SIZE default 3000 max duration at 10MHz (300 seconds). You can increase that to 6000 or more to get longer default running times.
The default sample rate for gps-sdr-sim is 2.6e6, 16 bit I/Q data format. LimeSDR will need 10e6, and 8 bit interleaved I/Q data format, so the strategy is to create an rf data file non-realtime and then transmit that with a simple gnuradio python script created in gnuradio-companion. The gps-fake-out project [2] links to a grc file, or it's easy to create your own. That example project simultaneous transmits the rf data file and also collects rf data for later analysis with Matlab and SoftGNSS. I found it useful to replace the file sink with an fft display slightly offset, and 20e6 input rate.

The last puzzle piece needed are ephemeris data to feed gps-sdr-sim (required), RINEX v2 format ( read all about it here [3] - especially the file name format). There is a global network of Internation GNSS Service installations [4] providing up to date data, which may be accessed with anonymous ftp from the Goddard Space Flight Center

ftp -p cddis.gsfc.nasa.gov

Login anonymous 'ftp' and email for password. Use the merged GPS broadcast ephemeris file found in /pub/gps/data/daily/2017/brdc/. The filename convention is

'brdc' + <3 digit day of year> + '0.' +  <2 digit year> + 'n.Z' 

'n' for gps (don't get the 'g' files, that is glonass), and 'Z' for compressed. Day of year can be found with

$ date +%j

Get yesterdays - for example, today, Feb 28, 2017, I would get 'brdc0580.17n.Z', uncompress

$ uncompress brdc0580.17n.Z

Pick a place - All you need now is a location to go, Google maps is good for entering latitude,longitude and seeing where it goes, or pick a spot, right click and pick "Directions to here" and a little url hacking to get the coordinates, like 1.8605853,73.5213033 for a spot in the Maldives. To do: use the gpssim with a user motion file instead of a static location, there is even support for Google Earth and SatGen software.

Execution

Get ready to host some large files, ranging from 5 to 20GB in size, if going with a larger USER_MOTION_SIZE full duration and/or trying 16 bit. Create the rf data file, using 10e6 samples per second in interleaved 8bit I/Q sample format, using the day of year 059 merged broadcast ephemeris file:

$ ./gps-sdr-sim -e brdc0590.17n -l 1.8605853,73.5213033,5 -t 2017/02/28,22:00:00 -o gpssim_10M.s8 -s 10e6 -b 8 -v
Using static location mode.
     9.313e-09    0.000e+00   -5.960e-08    0.000e+00
     9.011e+04    0.000e+00   -1.966e+05    0.000e+00
     1.86264514923e-09   1.77635683940e-15     319488      1938
    18
Start time = 2017/02/28,22:00:00 (1938:252000)
Duration = 600.0 [sec]
02   78.1   5.0  25142702.4   4.5
04  305.9  10.6  24630434.2   4.0
10  244.0  20.9  23656748.6   3.2
12  174.6  31.9  22801339.9   2.6
13   59.8  27.2  23001942.1   2.8
15   80.1  60.3  20615340.0   1.7
18  273.8  42.7  21969027.9   2.1
20    3.4  36.7  22141445.5   2.3
21  322.3  14.4  24860118.2   3.7
24  152.1  21.2  23574508.7   3.2
25  227.1  49.6  21537006.8   1.9
26  310.2   0.2  25799081.3   5.1
29    2.7  52.0  21259731.6   1.8
32  211.7   0.4  25733242.7   5.0
Time into run =  1.6

then get some coffee - it's a slow single threaded process which is why we have to create a data file and then transmit it instead of realtime radio broadcast. When done make sure your gnuradio-companion graph is setup with the right source filename, data types, sink driver, antenna, etc. Anything miss-matches can cause it to frustratingly run but not work.