About

gpsdLogger is a gpsd client which publishes Position/Location Information (P/LI) to a shared memory segment, compatible with gpsLogger and MGEN. It does not set time, as it is expected that systems using gpsdLogger will also use ntpd (or similar software) for time management. (Otherwise, gpsLogger, without the d, is a more suitable program, as it manages time and logs P/LI in a single program.)

It was created to address the situation where sole access to a GPS receiver could not be ensured, and therefore gpsd was used to multiplex GPS data access. Although gpsd provides timing information suitable for use with ntpd, it does not publish P/LI in a format that MGEN can understand. Rather than patch MGEN, this utility was designed to replicate gpsLogger's shared memory publishing, allowing MGEN to acquire P/LI from gpsd.

Prerequisites

You will need the following packages to build gpsdlogger:

Slackware et al.

• gcc
• gpsd (from SlackBuilds)
• make
• scons (only to build gpsd)

Debian et al. (Ubuntu, etc.)

• build-essential
• libgps-dev

For troubleshooting, you will probably also want:

• gpsd-clients
• gpsd-tools

Red Hat et al. (CentOS, Fedora, etc.)

• gcc
• gpsd
• libgpsd-devel
• make

Additional Packages

You probably also want ntp (or ntpd) to set the time on the system, and mgen or similar to consume the data published by gpsdlogger. (Neither of these are requirements for building nor running gpsdlogger.)

Building

make

Running

You need gpsd configured for your GPS (which is outside the scope of this document) and listening for network connections. (This means gpsd and gpsdlogger both need not to be blocked by SELinux or iptables.)

After that, just run gpsdlogger and it will automatically attempt to connect to the local gpsd instance.

gpsdlogger will close all logfiles and exit upon receipt of SIGTERM.

Runtime Options

Debug Mode

By default, gpsdlogger forks to the background and doesn't print anything to stdout and stderr. If you enable "debug mode" with -d, it won't fork and will print additional information to stdout/stderr, in addition to normal logging.

Logging Control

By default, logs are written to /tmp/gpsdlogger in filenames named after the GMT timestamp (YYYYMMDD.HHMMSS) when gpsdlogger first gets fix. This path can be controlled with -l (lowercase "ell" or "Lima") and does not need to exist (though the parent directory does). You should change it from the default to keep the logs safe from automatic cleaners and system reboots.

An example filename is 20210213.131214, which would be created if gpsdlogger first got fix at 13:12:14 GMT on 13 Feb 2021.

If "truetime" logging is enabled (see below), an additional log with more information will be created, with the same filename format, but using the system clock timestamp, in GMT, at time of first fix, and with the extension .tt (so the truetime logfile for the above timestamp would be 20210213.131214.tt).

Syslog Facility

gpsdlogger logs to syslog with identifier gpsdlogger and tries to include the PID with each log message. By default, it uses the "daemon" logging facility, but you can change it with -f to any valid facility name. Recommendations are one of "cron", "daemon", or "local0" through "local7". (See syslog(3) for more details.)

Logging

gpsdlogger only logs to the logfile(s) when it has fix. It will log to syslog whenever it doesn't have fix, and whenever it acquires fix, at log level "debug". (Note that most systems don't actually record "debug" syslog messages, so you may need to enable them when troubleshooting.)

It waits up to two seconds for gpsd to return fresh information, but may log more often if gpsd indicates that it has received an update.

Normally, gpsdlogger only logs the following information:

• GPS timestamp
  • Hour
  • Minute
  • Second
  • Microsecond
• GPS position
  • Latitude
  • Longitude
  • Altitude

Important Note

If no logfile exists in logpath, gpsdlogger hasn't gotten fix yet.

If a logfile exists but it's not getting updated, and gpsdlogger is still running, it doesn't have fix.

Truetime Logging

If "truetime" logging is enabled (with -t), gpsdlogger will also log the following information to the "truetime" logfile:

• GPS timestamp (seconds)
• Fix mode (not status)
• Position (lat/long/alt)
• Track
• Speed over ground
• Climb
• Estimated error for all values (in order, ept, epy, epx, epv, epd, eps, and epc)

See the documentation for the TPV object for explanations of the above fields and their possible values.

Licensing

This software is licensed only under Version 2 of the GNU Public License (GPL). It is provided for Government use with Restricted Rights, as defined in the FAR, and remains the property of Fortian.

This software incorporates:

• previously unpublished property of Fortian, which is not public domain and remains the property of Fortian

• software derived from gpsLogger, which, under 17 USC §105, is public domain software

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Fortian is a registered trademark of Fortian Inc.