[ntp:questions] custom NMEA messages

Brian Inglis Brian.Inglis at SystematicSw.ab.ca
Sat Dec 26 21:39:09 UTC 2015

On 2015-12-24 13:43, Leon McCalla wrote:
> I have a globalsat BU-353-S4 GPS receiver that I would lie to use as a source for NTPd.  When looking at the messages that it produces, unlike the periodic messages I would expect every second, I get messages in bursts of threes followed by a pause. If the face of an analog clock represented a 3 second window I receive messages at 12,1,4,8.
> One option is to use the UTC messages as an estimate to set the clock initially then NEVER use the gps again but this defeats the purpose of using a GPS for NTPd.  My alternative is to try to find some stability in the madness before subjecting NTPd to these seemingly unstable messages.
> looking at the messages in detail, I can see that a GPGBS message is produced in the middle of the burst and based on wireshark messages, this single message appears every 3.000 seconds. While this is not every second as a traditional PPS message is expected, I would like to see if NTPd can work with something less frequent but highly stable.
> how do I compile a custom driver to read NMEA messages that are not part of the default NMEA driver? The GPGBS message looks like this... http://www.trimble.com/OEM_ReceiverHelp/V4.44/en/NMEA-0183messages_GBS.html
> PS if im going crazy for no reason and NTPd is capable of working with this garbage please let me know.

Uses a Sirf Star IV chipset which can send NMEA or Motorola Oncore-like binary messages.
Details or URLs for docs should be on the included CD.
Can be reconfigured using (probably Windows) utility, proprietary manual commands via a terminal comm program e.g. PuTTY, or a program which can talk to Oncore-like binary protocol devices.

Increasing baud rate from NMEA default 4800 to 9600 improves jitter, but USB devices are intended for positioning applications, not timing, as USB comms adds microseconds of jitter, and provides no PPS signal.

Normal timing setup is to have the device send only one NMEA GPRMC message at 1s interval, and tweak driver fudge time1 until offset measured against other sources averages around zero.

If you can get it set up to behave like an Oncore-like device, you may be able to use the Oncore driver.

Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada

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