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OpenGPSSim是一个gps模拟程序

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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 ([e-mail]) 
  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.            *

  *****************************************************************
    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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