ogr_msgdump.c

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

/* ************************************************************************ 
   *                                                                      *
   *                            OpenGPS Receiver                          *
   *                                                                      *
   * -------------------------------------------------------------------- *
   *                                                                      *
   *    Module:   ogr_msgdump.c                                           *
   *                                                                      *
   *   Version:   0.1                                                     *
   *                                                                      *
   *      Date:   14.07.03                                                *
   *                                                                      *
   *    Author:   G. Beyerle                                              *
   *                                                                      *
   * -------------------------------------------------------------------- *
   *                                                                      *
   * Copyright (C) 2003  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.            *
   *                                                                      *
   * -------------------------------------------------------------------- *
   *                                                                      *
   *              Dump nav data and pseudoranges to file                  *
   *                                                                      *
   ************************************************************************ */

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

   02.04.03 - 

   ************************************************************************ */


/*

-----------------------------------------------------------------

message id 0x1a

    ofs   type   quantity
    --------------------------
     0    char   PRN-1
     1    char   elevation [deg]
     2    word   signal quality
     4    char   tracked?  1 = yes / 0 = no
     5    char   status bits
     6    char   ??

message id 0x16

    ofs   type   quantity
    --------------------------
     0    float  pseudorange increment
     4    float  f1
     8    double pseudorange
    16    float  f2
    20    char   PRN-1

message id 0x36

 - ephemeris and almanac data -> message id 0x36
 - 10 LONGs x number of sats in view per 6 seconds

    ofs   type   quantity
    --------------------------
     0    long   50 Hz counter
     4    long   GPS word
     9    char   PRN-1   

message id 0x38

    ofs   type   quantity
    --------------------------
     0    long   phase counter
     8    char   track byte
     9    char   unknown
    10    char   unknown
    11    char   unknown
    12    word   doppler
    14    double pseudorange
    22    long   511500 Hz counter
    26    word   signal strength
    28    double time of week
    36    char   PRN-1

-----------------------------------------------------------------
1.5 Message ID: 0x16
-----------------------------------------------------------------

General description:  
several hints indicate that this record could be related to 
velocity??/Doppler???  information:

* The delta_pseudorange_rate field is very similar to the difference 
  in pseudoranges divided by the time increment.

* Also, using the phase field of the 0x38 record as a reference,
  and studying the behaviour of this field compared to the 
  rate of increase of that phase we found the following:
 
                   Sat Id   Signal_Q   Mean(cm)   std (cm)
                     10      12000      2.0         19 
                     02       8500     -1.3         16
                     05       6000      0.3         18
                     15      12500      0.8         17
                     18       6500     -0.2         18

  We find that this delta_pseudorange rate follows the phase rate 
  a bit more closely than the differences
  of pseudoranges shown in the other table. The improvement is 
  more clearly seen in those sats with a lower signal. 
  That would be consistent with its being a more precise 
  measurement (integrated Doppler??) of the same quantity. Again, 
  beware that this has been obtained by examining a couple 
  minutes of data, and this interpretation could be wrong.

* These 0x16 records are not sent until enough sats are acquired.

* F1 and F2 seems to be some sort of correction (Kalman filter 
  parameters/variances??) that is used when aditional info is 
  gathered.  Some observations:

  - F1 has similar values (usually 0 point something) for each group 
    of messages that are sent together for the different sats.
  - F2 can be real big (in the order of thousands) in a cold start for 
    a different position for the FIRST satellites. After a while, 
    F2 drops to its normal values of about 5.
  - In a warm start (same position,a bit later) F2 values are very 
    low from the beginning.
  - In its stable state, F2 used to be larger (about 20-30 as opposed 
    to 3-5) with SA on.

 
Name         delta_pseudorange_rate (m/s)
Position     bytes 1-4
Type         float
Description  A float that closely (usually within a meter) resembles
             the increment of the pseudorange for that sat in a second.

Name         f1
Position     bytes 5-8
Type         float
Description  Similar values for a group of messages that are sent 
             together for the different sats. 
             See general description above.

Name         pseudorange
Position     bytes 9-16
Type         double
Description  Pseudorange (ms). Same as in the preceding 0x38 record.

Name         f2
Position     bytes 17-20
Type         float 
Description  See general description above.

Name         svid
Position     byte 21
Type         BYTE
Description  SVID (PRN-1)

--------------------------------------------------------------
1.4 Message ID: 0x36
---------------------------------------------------------------

General description: 
timing info + something else. These records are only sent once we 
have computed a valid pseudorange for a satellite (see power-on 
description below), and disappear if there is troubles (when
tracked_byte of record 0x38 becomes 0).

Name         c_50     
Position     Bytes 1-4
Type         unsigned long
Description  50 Hz counter, STARTING from the beginning of the week, 
             that is, c_50/50 corresponds to TOW. It gets incremented 
             in 30 count intervals, so that the resolution is 0.6 sec.
             
Name         unknown
Position     bytes 5-8 
Type         BYTE[4]
Description  seem to vary randomly.

Name         svid
Position     byte 9
Type         BYTE
Description  SVID (PRN-1)

------------------------------------------------------------------
1.3 Message ID: 0x38 (37 byte long)
-------------------------------------------------------------------
Data related to a particular sat (indicated in the last byte). 
It is in this record where the most interesting info seems to be sent.


Name         phase_counter   
Position     bytes 1-4
Type         unsigned long 
Description  A counter that gets increased by about 60-70 millions
             per seconnd. The lower 11 bits give us the fractional 
             phase (it mirrors the corresponding field in the 0x1a
             record), while the upper bits (12-32) represent the integer 
             number of cycles. Just after starting the unit those 
             bits will be equal to the Integrated_phase field (see 
             below), but after a while the won't, because the phase_counter
             will roll over. The best way to compute the carrier phase 
             is thus: 
                     
                carrier_phase = integrated_phase (whole cycles) +
                                (phase_counter AND 2047)/2048.

             that is, the integrated_phase field keeps track of the 
             integer number of cycles, so we don't have to worry
             about the rollover of phase_counter. From phase_counter
             we use only its lower 11 bits to get the fractional phase.

Name         track_byte
Position     byte 5
Type         Byte  
Description  0x00 indicates some sort or problem with the sat. 
             This is the case for sats with low elevation and weak 
             signal. In that case, the other data may be unreliable. 

Name         unknown   
Position     bytes 6-8
Type         byte[3]
Description  Seem to be grouped into an integer (bytes 6-7) and a 
             flag byte (byte 8) that is always? 00 or ff.
             The integer?? moves while the tracked_byte is 00 
             (the sat is not properly tracked). 
             When the sat is locked, the number gets frozen.

Name         delta_f
Position     bytes  9-10 
Type         unsigned int (interpreted as signed by substracting 32768)
Description  If we consider this field a signed integer (not as usually 
             interpreted by the machine, but considering 32768 as the 
             zero so that the value we use is actually delta_f-32768),
             it is positive for those sats with larger increments of 
             pseudoranges (going away from us) and negative for those 
             with smaller increments of pseudoranges (approaching us):

             Sat Id    delta_pseudorange   delta_f-32768
             -------------------------------------------
               10           6329               1323
               02           5638              -2402
               05           6534               2428
               15           5863              -1186
               
             The relation between those two quantities is linear and the 
             constant relating them (in a least squares fit) is very 
             close to the L1 wavelenght. That would mean that this 
             field could be a measurement/estimation? of the Doppler 
             shift (in Hertz) for each sat.

             For the above data an aproximate fit is:

               delta_f = (delta_pr - 6080) / lambda

             delta_pr is obviouly a measure of the relative speed of the
             satellite, and the origin 6080 m/s would be the false speed 
             caused by the clock drift. Once that bias is removed, the 
             relation would be:
             
               delta_f = relative_speed / lambda 
                       = L1 * (relative_speed/c)  = Doppler shift (Hz)

Name         Integrated Phase
Position     Bytes 11-14
Type         unsigned long
Description  integrated phase (whole cycles). The ratio of its increment 
             with the increment of the pseudorange field corresponds 
             with the inverse of the L1 wavelenght. Once locked, 
             the difference between both fields rarely exceed 100 cm. 

          
Name         pseudorange
Position     Bytes 15-22
Type         double
Description  Pseudorange in meters. Due to the particular idiosincrasy 
             of the GPS12, these numbers can get real large, because 
             the GPS12 allows the clock error to accumulate in them, 
             at a rate (in my unit) of about 6000 m/s.

Name         c_511500
Position     Bytes 23-26
Type         Unsigned long
Description  511500 Hz timer. 511500 ticks corresponds to a receiver's 
             second.  At the start it is incremented by exactly 511500 
             units, but once enough sats have been acquired, it can 
             vary one or two units (probably as the GPS tries to sync 
             its measurements to an exact GPS system time second.     

Name         signal_Q
Position     Bytes 27-28 
Type         Unsigned int.
Description  Measure of the signal strength or quality for that 
             satellite.  It has the same value as in the corresponding 
             field of the next 0x1a record.

Name         tow  
Position     Bytes 29-36 
Type         Double
Description  Time of week in seconds. At the start it is incremented