[ntp:questions] custom NMEA messages

juergen perlinger juergen.perlinger at t-online.de
Sat Dec 26 22:28:56 UTC 2015


On 12/24/2015 09:43 PM, Leon McCalla wrote:
> Hi,
> 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.
> 

According to the data sheet at

 http://usglobalsat.com/store/download/688/bu353s4_ds.pdf

the GPS puck can emit GPRMC and GPZDA -- both are supported by the NMEA
driver. (Funny enough, only the data sheet mentions GPZDA... the manuals
don't. But GPRMC is a must have, and it's listed in the manual and in
the data sheet.)

If neither of those can be seen, IMHO the most likely reasons are:

 1. You do not see enough sky for the receiver to make a good fix.
 2. Somehow the device got misconfigured.

In the second case,

  http://usglobalsat.com/store/downloads/NMEA_commands.pdf

might give you basic information for some experiments, including a
possible reset to factory settings.

But unless the receiver emits GPRMC periodically, something is
definitely wrong with the puck, its setup or its location.

My immediate experience with SiRF chipsets is limited, though, and
someone else might have better ideas.

Cheers,
	Pearly
	


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