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OpenGPSSim - A simple software GPS signal correlator
Version: 0.1.1
OpenGPSSim is a _simple_ GPS receiver simulation tool. OpenGPSSim
is based on Clifford Kelley's (cwkelley@earthlink.net)
OpenSourceGPS distribution (see www.home.earthlink.net/~cwkelley).
Several routines are taken from Antonio Tabernero Galan's gar2rnx
tool (see artico.lma.fi.upm.es/numerico/miembros/antonio/async/)
* This program is free software; you can redistribute it and/or modify *
* it under the terms of the GNU General Public License as published by *
* the Free Software Foundation; either version 2 of the License, or *
* (at your option) any later version. *
* *
* This program is distributed in the hope that it will be useful, *
* but WITHOUT ANY WARRANTY; without even the implied warranty of *
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the *
* GNU General Public License for more details. *
* *
* You should have received a copy of the GNU General Public License *
* along with this program; if not, write to the Free Software *
* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. *
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The files 'gpsfuncs.cpp', 'gpsrcvr.cpp' and 'gp2021.cpp' are
modified versions of the files with the same name from Clifford
Kelley's OpenSourceGPS distribution. The unmodified files can
be obtained from http://www.home.earthlink.net/~cwkelley
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OpenGPSSim is composed of three programs:
'ogsxmit' calculates a downconverted, 2-bit quantized GPS signal
and writes the data to './data/ogsraw.dat'. Navigation data is read
from input files, one file for each satellite. Sampling frequency is
5.7 MHZ, 4 samples are stored in one byte (8 bit). Therefore,
'ogsxmit' produces about 1.4 MByte per second simulation time.
Simulation time and other parameters are set in parameter file
'./bin/xmit_par.dat'.
'ogsrcvr' reads the downconverted GPS signal from './data/ogsraw.dat',
tracks it and - if successful - writes the navigation message (1 frame,
1500 bits) to files './data/nav-out-PRN' (PRN=1,...,32)., in-phase and
quad-phase values are written to './data/iq-out-PRN', carrier frequency
doppler is stored in './data/carfrq-out-PRN'. Parameters are set in
parameter file './bin/rcvr_par.dat'.
'ogsencdec' translates navigation messages between binary and ASCII
formats.
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Installation:
1) unzip the zip-archive
2) cd to subdirectories 'encdec', 'xmit' and 'rcvr' and compile,
i.e. (OGSROOT : OpenGPSSim root directory)
cd ${OGSROOT}/encdec; make cd ${OGSROOT}/xmit; make cd ${OGSROOT}/rcvr; make
executables 'ogsencdec', 'ogsxmit' & 'ogsxmit' are placed in
subdirectory '${OGSROOT}/bin'.
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Example simulation procedure:
1) generate navigation messages:
First the 1500 bit navigation message has to be generated. (1500 bit
at 50 Hz corresponds to 30 seconds simulation time. For longer time
periods the same message is re-used. Subcommutation of subframe 4 & 5
is not yet implemented).
Two template files are found in './data/':
nav-01-template-01-06.txt
nav-11-template-01-06.txt
Files names can be chosen freely. However, the satellite PRN must
be present and the extensions has to be '.txt' (indicating ASCII format).
cd ${OGSROOT}
./bin/ogsencdec nav-01-template-01-06.txt
./bin/ogsencdec nav-11-template-01-06.txt
translates both files into 1500 byte binary format; the result is written
to:
nav-01-template-01-06
nav-11-template-01-06
[Remark: If you own a Garmin GPS receiver you might be able to use
Antonio Tabernero Galan's async/gar2rnx tool to extract the navigation
data from real observations
(see artico.lma.fi.upm.es/numerico/miembros/antonio/async/).
An example file is '${OGSROOT}/data/393540x36.g12', the navigation data
can be extracted using 'ogsencdec' as well. The command
cd ${OGSROOT}
./bin/ogsencdec -l -a 393540x36.g12
lists all PRN present in this data file. Then, e.g.
cd ${OGSROOT}
./bin/ogsencdec -s 1 -a 393540x36.g12
extracts all navigation frames transmitted by satellite PRN=1.
A pair of files (binary and ASCII) is created for each complete
navigation frame found:
nav-PP-WWWW-SSSSSS-AA-BB
nav-PP-WWWW-SSSSSS-AA-BB.txt
where
PP : PRN
WWWW : GPS week number
SSSSSS : time of week [sec]
AA : page number in subframe 4
BB : page number in subframe 5
Files with extension 'txt' are ASCII files, file without extension
are binary files.]
2) once the navigation messages are generated, calculation of simulated
GPS signal can be started with:
cd ${OGSROOT}
./bin/ogsxmit -n nav-PRN-template-01-06
Control parameters are read from './bin/xmit_par.dat'. 'ogsxmit' writes
output to './data/ogsraw.dat'. Currently, a simulation time period
is set to 45 seconds in './bin/xmit_par.dat'. Therefore,
'./data/ogsraw.dat' grows to more than 60 MByte.
3) the signal tracking process is started by
cd ${OGSROOT}
./bin/ogsrcvr -i -r
produces for each PRN found in the signal output files (in our example
PRN 1 and 11 are present):
nav-out-01 and nav-out-11
iq-out-01 and iq-out-11
carfrq-out-01 and carfrq-out-11
4) analyse navigation data files
cd ${OGSROOT}
./bin/ogsencdec -r nav-out-01
./bin/ogsencdec -r nav-out-11
translates the contents of nav-out-01 and nav-out-11 to ASCII format
and writes it to
nav-out-01.txt
nav-out-11.txt
The values found in these files should correspond to those of the
input files
nav-01-template-01-06.txt
nav-11-template-01-06.txt
It is instructive to plot the in-phase and quad-phase values stored
in data files:
iq-out-01 and iq-out-11
(these are binary files - data formats are described in
'./doc/dataformats.txt')
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Notes:
'ogsxmit' writes about 1.4 MByte of data per second simulation
time. Thus, a 30 seconds simulation (1 data frame) produces
about 42 MByte of data.
Currently, 'ogsxmit' does _not_ take into account GPS satellites
positions and receiver location. Thus, calculation of receiver
position is not performed.
The simulation is _much_ slower than real time: it takes about
20 sec to construct and about 10 sec to track 1 sec of data on
a 450 MHZ PC. Tracking speed can be increased by reducing the
number of active channels (parameter 'number active channels'
in parameter file 'bin/rcvr_par.dat', maximum value is 12).
The software receiver is a 12 channel, single wavelength receiver,
C/A code only, carrier and code 2nd order Costas PLL (code aiding
not yet implemented). PLL parameters set by parameter file
'./bin/rcvr_par.dat'
File formats are described in './doc/dataformats.txt'
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Send comments and suggestions to gbeyerle@gfz-potsdam.de
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