ogrt_preamble.c

The OpenGPSRec receiver software runs on the real time operating system RTAI-Linux. It compiles with

/* ************************************************************************ 
   *                                                                      *
   *                            OpenGPS Receiver                          *
   *                                                                      *
   * -------------------------------------------------------------------- *
   *                                                                      *
   *    Module:   ogrt_preamble.c                                         *
   *                                                                      *
   *   Version:   0.1                                                     *
   *                                                                      *
   *      Date:   13.10.03                                                *
   *                                                                      *
   *    Author:   C. Kelley, G. Beyerle                                   *
   *                                                                      *
   * -------------------------------------------------------------------- *
   *                                                                      *
   * Copyright (C) 2001-2003  C. Kelley, G. Beyerle                       *
   *                                                                      *
   * 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.            *
   *                                                                      *
   * -------------------------------------------------------------------- *
   *                                                                      *
   * OpenGPS is based on Clifford Kelley's OpenSourceGPS distribution.    *
   * The OpenSourceGPS distribution can be obtained from                  *
   *             http://www.home.earthlink.net/~cwkelley                  *
   *                                                                      *
   * -------------------------------------------------------------------- *
   *                                                                      *
   *                         Real-time process                            *
   *                                                                      *
   ************************************************************************ */

/* ******************************* changes ********************************

   26.01.03 - replace 'trackch' by 'RT_Ctrl->leadch'
              use fields RT_Ctrl->accum_missed & ->accum_new
   18.07.03 - Chan[].ms_count wraps at 6000 (instead of 30000)
   18.07.03 - move Ms_Count to struct Chan[]
   31.07.03 - fixed bug in find/read_preamble()
   03.08.03 - copy carrier-aided code tracking from OpenSourceGPS
   ************************************************************************ */

/***********************************************************************
  GPS RECEIVER (GPSRCVR) Ver. 1.02
  12 Channel All-in-View GPS Receiver Program based on 
  Mitel GP2021 chipset
  Clifford Kelley [e-mail]
  Copyright (c) 1996-2002 Clifford Kelley.  All Rights Reserved.
  This LICENSE must be included with the OGR code.
***********************************************************************/

/*
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:

      CONDITIONS

1. Redistributions of GPSRCVR source code must retain the above copyright
notice, this list of conditions, and the following disclaimer.

2. Redistributions in binary form must contain the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.

3. All modifications to the source code must be clearly marked as
such.  Binary redistributions based on modified source code must be
clearly marked as modified versions in the documentation and/or other
materials provided with the distribution.

4. Notice must be given of the location of the availability of the
unmodified current source code, e.g.,
  http://www.Kelley.com/
or
  ftp://ftp.Kelley.com
in the documentation and/or other materials provided with the
distribution.

5. All advertising and published materials mentioning features or use
of this software must display the following acknowledgment:  "This
product includes software developed by Clifford Kelley and other
contributors."

6. The name of Clifford Kelley may not be used to endorse or promote
products derived from this software without specific prior written
permission.

      DISCLAIMER

This software is provided by Clifford Kelley and contributors "as is" and
any expressed or implied warranties, including, but not limited to, the
implied warranties of merchantability and fitness for a particular
purpose are disclaimed.  In no event shall Clifford Kelley or
contributors be liable for any direct, indirect, incidental, special,
exemplary, or consequential damages (including, but not limited to,
procurement of substitute goods or services; loss of use, data, or
profits; or business interruption) however caused and on any theory of
liability, whether in contract, strict liability, or tort (including
negligence or otherwise) arising in any way out of the use of this
software, even if advised of the possibility of such damage.
*/

/* ------------------------------- includes ------------------------------- */

#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/slab.h>

#include <rtai.h>
#include <rtai_sched.h>
#include <rtai_fifos.h>
#include <rtai_shm.h>

#include  "port.h"
#include  "ogr_defines.h"
#include  "ogr_structs.h"

#include  "ogrt_defines.h"
#include  "ogrt_globals.h"

#include  "ogrt_prototypes.h"

MODULE_LICENSE( "GPL");

/* ------------------------------- defines -------------------------------- */

/* ------------------------------ local variables ------------------------- */

//  'TLMFifo' holds first word (TLM), 'HOWFifo' holds second word (HOW).
//  'TLMFifo' is fed from 'HOWFifo', 'HOWFifo' from variable 'bit'.
//  Shift bits to TLMFifo/1 until TLMFifo contains TLM word and 
//  HOWFifo HOW word.
unsigned long TLMFifo[NOFCHN], 
              HOWFifo[NOFCHN];

/* ------------------------------ prototypes ------------------------------ */

/* ------------------------------ procedures ------------------------------ */

/* ------------------------------------------------------------------------- *
FUNCTION find_preamble()
RETURNS  None.

PARAMETERS
  ch   channel number (0-11)
  bit  the latest bit from the satellite

PURPOSE
  This function finds the preamble in the navigation message.
  The preamble 0x8b is found in the first 8 bits of the TLM
  word (first word of each subframe).

WRITTEN BY
  Clifford Kelley
* ------------------------------------------------------------------------- */

void find_preamble( INT16 ch, INT16 bit)
{
  int d29, d30;
  INT16  parity0, parity1;
  unsigned long TLM, HOW, pattern;

/*
 *  'TLMFifo' holds first word (TLM), 'HOWFifo' holds second word (HOW).
 *  'TLMFifo' is fed from 'HOWFifo', 'HOWFifo' from variable 'bit'.
 *  Shift bits to TLMFifo/1 until TLMFifo contains TLM word and 
 *  HOWFifo HOW word.
 */

  TLMFifo[ch] = (TLMFifo[ch]<<1) + (( HOWFifo[ch]>>29) & 0x1);

  HOWFifo[ch] = (HOWFifo[ch]<<1) + bit;

  pattern = (( TLMFifo[ch] ^ PREAMBLE)  & 0x3fc00000L);

  if ( pattern == 0)
    Preamble_Found[ch] = 1;
  else if ( pattern == 0x3fc00000L)
    Preamble_Found[ch] = -1;
  else  
    Preamble_Found[ch] = 0;

  if ( Preamble_Found[ch])
  {

    TLM = TLMFifo[ch];
    HOW = HOWFifo[ch];

//  found inverted preamble pattern?
    if ( Preamble_Found[ch] < 0)
    {
      TLM = ~TLM;
      HOW = ~HOW;
    }

    parity0 = (xors( TLM & PARBIT1)<<5)+(xors( TLM & PARBIT2)<<4)+
              (xors( TLM & PARBIT3)<<3)+(xors( TLM & PARBIT4)<<2)+
              (xors( TLM & PARBIT5)<<1)+(xors( TLM & PARBIT6));

    if ( parity0 == (TLM & 0x3f))  // is parity of TLM ok?
    {

      d30 = (HOW >> 30) & 0x1;
      d29 = (HOW >> 31) & 0x1;

      parity1 = ((d29^xors( HOW & PARBIT1)) << 5) + 
                ((d30^xors( HOW & PARBIT2)) << 4) +
                ((d29^xors( HOW & PARBIT3)) << 3) + 
                ((d30^xors( HOW & PARBIT4)) << 2) +
                ((0  ^xors( HOW & PARBIT5)) << 1) +
                ((d29^d30^xors( HOW & PARBIT6)));

      if ( parity1 == (HOW & 0x3f))  // is parity of HOW ok?
      {

//  we at start of 3rd word, i.e. 1.2 sec after start of subframe 
//  (therefore, Chx_20MS_EPOCH = 10, Chx_1MS_EPOCH = 0)
        ch_epoch_count_load( ch, 0xa00);
//        ch_epoch_count_load( ch, (0x1f & ch_epoch_check(ch)) | 0xa00); 

//  D1-D24 = D30 ^ d1-d24. (Note: for TLM d30 is 0 by construction!)
        if ( d30)
          HOW = HOW ^ 0x3fffffc0L;

//  compute the subframe id number
        Chan[ch].sfid = (HOW & 0x700) >> 8;

//  was:  Chan[ch].TLM  = (TLM >> 2) & 0x3fff;
        Chan[ch].TLM  = (TLM >> 6) & 0xffff;

        if ( Chan[ch].sfid >= 1 && Chan[ch].sfid <= 5)  
        {                         

/*
 *  tow in HOW word is given in units of 6 seconds (4*1.5 seconds), 
 *  tow refers to the start of _next_ subframe;
 *  field 'tow_HOW' uses units of seconds!
 */
          Chan[ch].tow_HOW  = ((HOW & 0x3fffffffL) >> 13) * 6;

//          rt_printk( "Chan[%d].tow_HOW = %ld\n", ch, ((HOW & 0x3fffffffL) >> 13));

          Chan[ch].tow_sync = 1;

// ms_count = 0,...,5999; start of 3rd word (30 bits per word, 20 ms per bit)
          Chan[ch].ms_count = 1200;
        }
        else
          Chan[ch].tow_sync = 0;

      }   // --- if ( parity1 == (HOW & 0x3f)) ---
    }
  }       // --- if (((pream^TLM) & 0x3fc00000L) == 0) ---

  return;
}

/* ------------------------------------------------------------------------- *
FUNCTION read_preamble()
RETURNS  None.

PARAMETERS
  ch   channel number (0-11)
  bit  the latest bit from the satellite

PURPOSE
  Read preamble of the navigation message.
  It is assumed that we already know the preample's start

WRITTEN BY
  Clifford Kelley
* ------------------------------------------------------------------------- */

void read_preamble( INT16 ch, INT16 bit)
{
  int d29, d30;
  INT16  parity0, parity1;
  unsigned long TLM, HOW;


  TLMFifo[ch] = (TLMFifo[ch]<<1) + (( HOWFifo[ch]>>29) & 0x1);

  HOWFifo[ch] = (HOWFifo[ch]<<1) + bit;

//  we expect preamble for ms_count == 1200
  if ( Chan[ch].ms_count == 1200)
  {

    TLM = TLMFifo[ch];
    HOW = HOWFifo[ch];

//  found inverted preamble pattern?
    if ( Preamble_Found[ch] < 0)
    {
      TLM = ~TLM;
      HOW = ~HOW;
    }

    parity0 = (xors( TLM & PARBIT1)<<5)+(xors( TLM & PARBIT2)<<4)+
              (xors( TLM & PARBIT3)<<3)+(xors( TLM & PARBIT4)<<2)+
              (xors( TLM & PARBIT5)<<1)+(xors( TLM & PARBIT6));

    d30 = (HOW >> 30) & 0x1;
    d29 = (HOW >> 31) & 0x1;

    parity1 = ((d29^xors( HOW & PARBIT1)) << 5) + 
              ((d30^xors( HOW & PARBIT2)) << 4) +
              ((d29^xors( HOW & PARBIT3)) << 3) + 
              ((d30^xors( HOW & PARBIT4)) << 2) +
              ((0  ^xors( HOW & PARBIT5)) << 1) +
              ((d29^d30^xors( HOW & PARBIT6)));

    if ( parity0 == (TLM & 0x3f) && parity1 == (HOW & 0x3f)) 
    {

      if ( d30)
        HOW = HOW ^ 0x3fffffc0L;

      Chan[ch].TLM  = (TLM >> 6) & 0xffff;

//  extract subframe id number
      Chan[ch].sfid = (HOW & 0x700) >> 8;

//  extract HOW
      Chan[ch].tow_HOW = ((HOW & 0x3fffffffL) >> 13) * 6;

//      rt_printk( " Chan[%d].tow_HOW = %ld\n", ch, ((HOW & 0x3fffffffL) >> 13));
    }
    else
      Chan[ch].tow_sync = 0;
  }

  return;
}

/* ------------------------------- end of file ---------------------------- */