GPS PPS Sync

20081222

Introduction

This project is a small utility project for synchronizing a personal computer (PS) with a GPS receiver with a pulse per second output.

The 'BSD-style' license allows non-restrictive use for commercial and open source development.

Particulars

NovAtel OEM3, OEM4, and OEMV receivers supported
Accuracy of 5ms or better generally
Some of the source is derived from http://sourceforge.net/projects/gnsstk/

How To

Modify the serial cable connecting the GPS receiver to the PC serial port by:

Connect the PPS output of the receiver to pin 9, ring indicator (RI) pin, of the PC serial port.*
Connect the ground pins of the input/output (I/O) connects to the ground pin, pin 5, on the PC serial port.

*The PPS signal is a TTL signal and not at RS232 voltage levels. This method has worked without a signal converter but if you so desire you can use a TLL to RS232 signal converter (~$50) before connecting to pin 9 of the PC serial port.

An example

In this example, the I/O port cable and the RS232 serial communications cable, RX COM1, are combined using a connector box. The PPS pin, pin 2 of the OEM3 GPS receiver's I/O cable, is connected to pin 9 of the RS232 communcations cable. The ground pins of the I/O cable should be connected appropriately in the connector box as well. A null modem cable is connected to the connector box and the user's PC. For details about OEM3 pin output refer to http://www.novatel.com/Documents/Manuals/om-20000016.pdf.

Example Software Usage

D:\gpsppssync\bin>gpsppssync.exe gpsppssync_options.txt
Performing GPS - PC time synchronization.
Command to COM1:
unlogall COM1
Command to COM1:
log com1 POSB ontime 2.0
88 bytes read.
A valid receiver position was verified.
Command to COM1:
unlogall com1
Command to COM1:
log com1 TM1B ontime 5.0
52 bytes read.
52 bytes read.
Coarse synchronization performed using TM1B message.
Command to COM1:
unlogall com1
Pulse detected: Synchronizing in 4 s.
Pulse detected: Synchronizing in 3 s.
Pulse detected: Synchronizing in 2 s.
Pulse detected: Synchronizing in 1 s.
Sync to PPS successful.
Performing PPS test.
Pulse: 0, dt:0.000, PC(hh:mm:ss.sss): 20:41:52.001 difference:1 ms
Pulse: 1, dt:1.001, PC(hh:mm:ss.sss): 20:41:53.002 difference:2 ms
Pulse: 2, dt:1.993, PC(hh:mm:ss.sss): 20:41:53.994 difference:-6 ms
Pulse: 3, dt:2.994, PC(hh:mm:ss.sss): 20:41:54.995 difference:-5 ms
Pulse: 4, dt:3.996, PC(hh:mm:ss.sss): 20:41:55.997 difference:-3 ms
Pulse: 5, dt:4.997, PC(hh:mm:ss.sss): 20:41:56.998 difference:-2 ms
Pulse: 6, dt:5.999, PC(hh:mm:ss.sss): 20:41:58.000 difference:-0 ms
Pulse: 7, dt:7.000, PC(hh:mm:ss.sss): 20:41:59.001 difference:1 ms
Pulse: 8, dt:8.001, PC(hh:mm:ss.sss): 20:42:00.002 difference:2 ms
Pulse: 9, dt:8.993, PC(hh:mm:ss.sss): 20:42:00.994 difference:-6 ms
Pulse:10, dt:9.994, PC(hh:mm:ss.sss): 20:42:01.995 difference:-5 ms
Pulse:11, dt:10.996, PC(hh:mm:ss.sss): 20:42:02.997 difference:-3 ms
Pulse:12, dt:11.997, PC(hh:mm:ss.sss): 20:42:03.998 difference:-2 ms
Pulse:13, dt:12.999, PC(hh:mm:ss.sss): 20:42:05.000 difference:-0 ms
Pulse:14, dt:14.000, PC(hh:mm:ss.sss): 20:42:06.001 difference:1 ms
Pulse:15, dt:15.002, PC(hh:mm:ss.sss): 20:42:07.003 difference:3 ms
Pulse:16, dt:15.993, PC(hh:mm:ss.sss): 20:42:07.994 difference:-6 ms
Pulse:17, dt:16.994, PC(hh:mm:ss.sss): 20:42:08.995 difference:-5 ms
Pulse:18, dt:17.996, PC(hh:mm:ss.sss): 20:42:09.997 difference:-3 ms
Pulse:19, dt:18.997, PC(hh:mm:ss.sss): 20:42:10.998 difference:-2 ms
Pulse:20, dt:19.999, PC(hh:mm:ss.sss): 20:42:12.000 difference:-0 ms
Pulse:21, dt:21.000, PC(hh:mm:ss.sss): 20:42:13.001 difference:1 ms
Pulse:22, dt:22.002, PC(hh:mm:ss.sss): 20:42:14.003 difference:3 ms
Pulse:23, dt:22.993, PC(hh:mm:ss.sss): 20:42:14.994 difference:-6 ms
Pulse:24, dt:23.994, PC(hh:mm:ss.sss): 20:42:15.995 difference:-5 ms
Pulse:25, dt:24.996, PC(hh:mm:ss.sss): 20:42:16.997 difference:-3 ms
Pulse:26, dt:25.997, PC(hh:mm:ss.sss): 20:42:17.998 difference:-2 ms
Pulse:27, dt:26.999, PC(hh:mm:ss.sss): 20:42:19.000 difference:0 ms
Pulse:28, dt:28.000, PC(hh:mm:ss.sss): 20:42:20.001 difference:1 ms
Pulse:29, dt:29.002, PC(hh:mm:ss.sss): 20:42:21.003 difference:3 ms
Pulse:30, dt:29.993, PC(hh:mm:ss.sss): 20:42:21.994 difference:-6 ms
Pulse:31, dt:30.995, PC(hh:mm:ss.sss): 20:42:22.996 difference:-4 ms
Pulse:32, dt:31.996, PC(hh:mm:ss.sss): 20:42:23.997 difference:-3 ms
Pulse:33, dt:32.997, PC(hh:mm:ss.sss): 20:42:24.998 difference:-2 ms
Pulse:34, dt:33.999, PC(hh:mm:ss.sss): 20:42:26.000 difference:0 ms
Pulse:35, dt:35.000, PC(hh:mm:ss.sss): 20:42:27.001 difference:1 ms
Pulse:36, dt:35.992, PC(hh:mm:ss.sss): 20:42:27.993 difference:-7 ms
Pulse:37, dt:36.993, PC(hh:mm:ss.sss): 20:42:28.994 difference:-6 ms
Pulse:38, dt:37.995, PC(hh:mm:ss.sss): 20:42:29.996 difference:-4 ms
Pulse:39, dt:38.996, PC(hh:mm:ss.sss): 20:42:30.997 difference:-3 ms
Pulse:40, dt:39.997, PC(hh:mm:ss.sss): 20:42:31.998 difference:-2 ms
Pulse:41, dt:40.999, PC(hh:mm:ss.sss): 20:42:33.000 difference:0 ms
Pulse:42, dt:42.000, PC(hh:mm:ss.sss): 20:42:34.001 difference:1 ms
Pulse:43, dt:42.992, PC(hh:mm:ss.sss): 20:42:34.993 difference:-7 ms
Pulse:44, dt:43.993, PC(hh:mm:ss.sss): 20:42:35.994 difference:-6 ms
Pulse:45, dt:44.995, PC(hh:mm:ss.sss): 20:42:36.996 difference:-4 ms
Pulse:46, dt:45.996, PC(hh:mm:ss.sss): 20:42:37.997 difference:-3 ms
Pulse:47, dt:46.998, PC(hh:mm:ss.sss): 20:42:38.999 difference:-1 ms
Pulse:48, dt:47.999, PC(hh:mm:ss.sss): 20:42:40.000 difference:0 ms
Pulse:49, dt:49.000, PC(hh:mm:ss.sss): 20:42:41.001 difference:1 ms
Pulse:50, dt:49.992, PC(hh:mm:ss.sss): 20:42:41.993 difference:-7 ms
Pulse:51, dt:50.993, PC(hh:mm:ss.sss): 20:42:42.994 difference:-6 ms
Pulse:52, dt:51.995, PC(hh:mm:ss.sss): 20:42:43.996 difference:-4 ms
Pulse:53, dt:52.996, PC(hh:mm:ss.sss): 20:42:44.997 difference:-3 ms
Pulse:54, dt:53.998, PC(hh:mm:ss.sss): 20:42:45.999 difference:-1 ms
Pulse:55, dt:54.999, PC(hh:mm:ss.sss): 20:42:47.000 difference:0 ms
Pulse:56, dt:56.000, PC(hh:mm:ss.sss): 20:42:48.001 difference:1 ms
Pulse:57, dt:56.992, PC(hh:mm:ss.sss): 20:42:48.993 difference:-7 ms
Pulse:58, dt:57.993, PC(hh:mm:ss.sss): 20:42:49.994 difference:-6 ms
Pulse:59, dt:58.995, PC(hh:mm:ss.sss): 20:42:50.996 difference:-4 ms
Pulse:60, dt:59.996, PC(hh:mm:ss.sss): 20:42:51.997 difference:-3 ms
Over 60 seconds, the difference in elapsed PC time is -0.004 s
The mean pulse to pc time difference is: -2.314 ms
Command to COM1:
log com1 gpzda ontime 1.0
Command to COM1:
log com1 posa ontime 1.0
The serial port is now closed you may open the port with hyperterminal 
for comparison.
UTC Time (hh:mm:ss.sss)      GPS Week GPS Time of Week (s) (of the PC)
20:42:53.019                     1511 247387.019
20:42:54.010                     1511 247388.010
20:42:55.002                     1511 247389.002
20:42:56.003                     1511 247390.003
20:42:57.004                     1511 247391.004
20:42:58.006                     1511 247392.006
20:42:59.007                     1511 247393.007
20:43:0.009                      1511 247394.009
20:43:1.000                      1511 247395.000
20:43:2.002                      1511 247396.002
20:43:3.003                      1511 247397.003
20:43:4.004                      1511 247398.004
20:43:5.006                      1511 247399.006
20:43:6.007                      1511 247400.007
20:43:7.009                      1511 247401.009
20:43:8.000                      1511 247402.000
20:43:9.002                      1511 247403.002
20:43:10.003                     1511 247404.003
20:43:11.005                     1511 247405.005
20:43:12.006                     1511 247406.006
20:43:13.007                     1511 247407.007
20:43:14.009                     1511 247408.009
20:43:15.000                     1511 247409.000
20:43:16.002                     1511 247410.002
20:43:17.003                     1511 247411.003
20:43:18.005                     1511 247412.005
20:43:19.006                     1511 247413.006
20:43:20.007                     1511 247414.007
20:43:21.009                     1511 247415.009
20:43:22.010                     1511 247416.010
20:43:23.002                     1511 247417.002
20:43:24.003                     1511 247418.003
20:43:25.005                     1511 247419.005
20:43:26.006                     1511 247420.006
20:43:27.008                     1511 247421.008
20:43:28.009                     1511 247422.009
20:43:29.010                     1511 247423.010
20:43:30.002                     1511 247424.002
20:43:31.003                     1511 247425.003
20:43:32.005                     1511 247426.005
Shutting down.

D:\gpsppssync\bin>

Options File Example

; gpsppssync
; NovAtel OEM3, OEM4, OEMV GPS receiver pulse per second PC time synchronization
; Options File
;
; by: Glenn D. MacGougan
; Copyright (c) 2008 - All rights reserved.
; 
; Redistribution and use in source and binary forms, with or without
; modification, are permitted provided the following conditions are met:
; 
; - Redistributions of source code must retain the above copyright
;   notice, this list of conditions and the following disclaimer. 
; - Redistributions in binary form must reproduce the above copyright
;   notice, this list of conditions and the following disclaimer in the
;   documentation and/or other materials provided with the distribution. 
; - The name(s) of the contributor(s) may not be used to endorse or promote 
;   products derived from this software without specific prior written 
;   permission. 
; 
; THIS SOFTWARE IS PROVIDED BY THE CONTRIBUTORS ``AS IS'' AND ANY EXPRESS 
; 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 THE 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.
;______________________________________________________________________________
; FORMAT
; 
; OPTIONFIELD, COMMENT = VALUE ; POST_VALUE_COMMENT
; OPTIONFIELD        - a valid identifier, no spaces
; COMMENT            - a good place to put the units
; VALUE              - the option value, a number, a string, an 
;                      array, a boolean (yes/no, 1/0)
; POST_VALUE_COMMENT - a comment following the value
; 
; Anything after a ';' is a comment.

ReceiverType, (NOVATELOEM3, NOVATELOEM4)                   = NOVATELOEM3
ReceiverSerialPortNumber, (com port number used on the rx) = 1
ComPort, (com port number, e.g. 1 is COM1)                 = 1
PerformPPSTest, (A 60s PPS timing test, yes/no)            = yes ; performed after PPS sync achieved
AllowTimeForHyperTerminalComparison, (yes/no (40s given))  = yes ; performed after PPS sync achieved

; The serial port for the receiver must be set to 115200 baud rate.
; This can be achieved by issueing the following commands over the port using hyperterminal.
; The COM1 must be replaced with the appropriate receiver serial port number if needed.
; For the OEM3: "COM1 115200,N,8,1,N,OFF,ON\r"
; For the OEM4: "COM COM1,115200,N,8,1,N,OFF,ON\r"
; For the OEMV: "COM COM1,115200,N,8,1,N,OFF,ON\r"