GPS-SIM

GPS-SIM is designed to use the HackRF to simulate GPS. This is a specific application the the GPS-SDR-SIM found at osqzss/gps-sdr-sim

Step 1 - Build Path to Simulate

Export KML file from Google Maps

If trasnmitting a static location, skip to Step 3.

Decide on a starting point and a destination to enter into Google Maps.

Instructions on how to export a KML file from a route built on Google Maps can be found here.

Step 2 - Converting KML to CSV

KML to NMEA .TXT

Labsat has a Free GPS NMEA Simulator Prgram that can be used to convert the KML file produced in Step 1 to a .txt file containing NMEA data.

The process for this is simple. Open the program, load the KML file using the GUI, and enter a few desired options. Set the output rate to 10Hz. You can change how long the path is stationary at the start of the data as well as at the end. You can also change the Ma Speed in km/h.

NMEA .TXT to .CSV

This step may be able to be skipped by using the -g option in Step 3 and using the .txt file generated above. However, this was not thoroughly tested.

Satgen folder contains files needed for creating csv from NMEA txt file

The nmea2um.c file must be compiled, then the usage is simple as described below.

nmea2um <nmea_txt_file> <user_motion_csv_file>

Step 3 - Generate BIN file to Transmit

Building with GCC

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

Generating the GPS signal file

A user-defined trajectory can be specified in either a CSV file, which contains the Earth-centered Earth-fixed (ECEF) user positions, or an NMEA GGA stream. The sampling rate of the user motion has to be 10Hz. The user is also able to assign a static location directly through the command line.

The user specifies the GPS satellite constellation through a GPS broadcast ephemeris file. The daily GPS broadcast ephemeris file (brdc) is a merge of the individual site navigation files into one. The archive for the daily file is:

ftp://cddis.gsfc.nasa.gov/gnss/data/daily/

Example file path for most recent data available on May 7, 2018 @ 16:43 EDT ftp://cddis.gsfc.nasa.gov/gnss/data/daily/2018/brdc/brdc1270.18n.Z

These files are then used to generate the simulated pseudorange and Doppler for the GPS satellites in view. This simulated range data is then used to generate the digitized I/Q samples for the GPS signal.

hackrf_transfer supports I/Q pairs stored as signed bytes. HackRF requires 2.6 MHz sample rate.

The simulation start time can be specified if the corresponding set of ephemerides is available. Otherwise the first time of ephemeris in the RINEX navigation file is selected.

The maximum simulation duration time is defined by USER_MOTION_SIZE to prevent the output file from getting too large.

The output file size can be reduced by using "-b 1" option to store four 1-bit I/Q samples into a single byte.

Usage: gps-sdr-sim [options]
Options:
  -e <gps_nav>     RINEX navigation file for GPS ephemerides (required)
  -u <user_motion> User motion file (dynamic mode)
  -g <nmea_gga>    NMEA GGA stream (dynamic mode)
  -c <location>    ECEF X,Y,Z in meters (static mode) e.g. 3967283.15,1022538.18,4872414.48
  -l <location>    Lat,Lon,Hgt (static mode) e.g. 30.286502,120.032669,100
  -t <date,time>   Scenario start time YYYY/MM/DD,hh:mm:ss
  -T <date,time>   Overwrite TOC and TOE to scenario start time
  -d <duration>    Duration [sec] (dynamic mode max: 300 static mode max: 86400)
  -o <output>      I/Q sampling data file (default: gpssim.bin ; use - for stdout)
  -s <frequency>   Sampling frequency [Hz] (default: 2600000)
  -b <iq_bits>     I/Q data format [1/8/16] (default: 16)
  -i               Disable ionospheric delay for spacecraft scenario
  -v               Show details about simulated channels

The user motion can be specified in either dynamic or static mode, examples:

> gps-sdr-sim -e brdc3540.14n -u circle.csv

> gps-sdr-sim -e brdc3540.14n -g triumphv3.txt

> gps-sdr-sim -e brdc3540.14n -l 30.286502,120.032669,100

Transmitting the samples

The TX port of a particular SDR platform is connected to the GPS receiver under test through a DC block and a fixed 50-60dB attenuator.

HackRF:

> hackrf_transfer -t gpssim.bin -f 1575420000 -s 2600000 -a 1 -x 0

Usage: hackrf_transfer [options]
Options:
  -t <filename>         # Generated binary file with GPS data (required)
  -f <frequency>        # GPS band center frequency (required)
  -s <sample_rate_hz>   # Sampling frequency [Hz]
  -a <set_amp>          # Set Amp 1 = Enable, 0 = Disable
  -x <gain_db>          # Gain start at 0 dB with increments of 1 dB up to 47 dB
Additional unused hackrf_transfer options:
  -r <filename>         # Receive data into file.
  -l <gain_db>          # Set lna gain, 0-40dB, 8dB steps
  -i <gain_db>          # Set vga(if) gain, 0-62dB, 2dB steps
  -n <num_sample>       # Number  of  samples  to  transfer (default  is unlimited).
  -b <baseband_filter_bw_hz> # Set baseband filter bandwidth in MHz.
        Possible values:
        1.75/2.5/3.5/5/5.5/6/7/8/9/10/12/14/15/20/24/28MHz
        default <sample_rate_hz>

Check for external clock (TCXO):

> hackrf_si5351c -n -0 -r 
	Output:
    [  0] -> 0x01	# detected 
    [  0] -> 0x51	# not detected

License

Copyright © 2015-2018 Takuji Ebinuma
Distributed under the MIT License.