Cheap serial GPS



  • Just a quick heads-up that a UK Ebay seller has listed some very cheap 2nd-hand serial GPS units using the SIRFstarIII chipset. The model is Navibe GM725 from Compass Systems.

    I acquired a couple of these (best offers are accepted) and have them working on my pfSense HA pair.

    They come terminated with an RJ11 plug but can easily be adapted to work with a serial port, and take 5v power from a USB port.

    The SIRF module itself does have a PPS output, but this would need a little hardware hacking to use, as it'd need level conversion from TTL to RS232. I managed to find the docs if anyone needs them.



  • Excuse my ignorance but what would you use these with pfsense for?



  • Hi,

    I would be interested in how to find these on eBay please?



  • @Nnyan:

    Excuse my ignorance but what would you use these with pfsense for?

    pfSense can use the time signal provided by the GPS as a reference s/cock/clock/ for NTP.


  • Banned

    pfsense can use the time signal provided by the GPS as a reference cock for NTP.

    This is significant, reference cocks are not easy to find. There are so many variations that it is really difficult to find a high quality, standardized cock to reference all of the other cocks to. Most cocks advertised as "reference cocks" actually jitter far too much to be usable.



  • @pfBasic:

    pfsense can use the time signal provided by the GPS as a reference cock for NTP.

    This is significant, reference cocks are not easy to find. There are so many variations that it is really difficult to find a high quality, standardized cock to reference all of the other cocks to. Most cocks advertised as "reference cocks" actually jitter far too much to be usable.

    ;D



  • @Nnyan:

    Excuse my ignorance but what would you use these with pfsense for?

    To run a Stratum 0 NTP service for all local IT systems. Benefits:

    Zero external NTP traffic leakage, resulting in reduced opportunity for fingerprinting etc.
    Reduced dependence on external services outside my direct control.

    I run a local recursive nameserver for similar reasons.

    HTH


  • Banned

    @pfBasic:

    reference cocks are not easy to find.


  • Netgate Administrator

    So no PPS output at the plug?

    Steve



  • @stephenw10:

    So no PPS output at the plug?

    Steve

    Only by opening the device and soldering a patch wire into the correct pin (#21) on the SIRF module. There's spare wires in the cable to route it to the host. As I said it would also need level conversion to interface to the hosts RS232 port. I believe the DCD input is commonly used for the PPS signal.


  • Netgate Administrator

    Without PPS you are really losing much of the benefit of GPS ntp sync. Those are cheap but the USB connected GPS devices (which cannot do PPS) are more convenient.

    Steve



  • @stephenw10:

    Without PPS you are really losing much of the benefit of GPS ntp sync. Those are cheap but the USB connected GPS devices (which cannot do PPS) are more convenient.

    Indeed. The OP was aimed at those who aren't averse to some tinkering, and maybe gain some satisfaction from building their own solutions cheaper than commercial options. Some of the reasons I use pfSense actually. More here:

    http://www.catb.org/gpsd/gpsd-time-service-howto.html


  • Netgate Administrator

    No argument from me. If I had something with a spare serial port I'd probably buy one.  ;D

    Steve


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