[time-nuts] Lady Heather for homebrew GPSDO

Bob Camp kb8tq at n1k.org
Sat Dec 17 15:42:04 EST 2016


There are a pretty small number of things that a GPSDO worries about that are
not in the standard NMEA data structures:

1) DAC
2) Temperature
3) Lock state
4) Time error 
5) Maybe a “quality of lock” metric

A lot of GPSDO’s put out more than that in their status messages. There is a 
lot of repetition between NMEA messages and that carries over to the custom

More or less anything that starts with $P is considered a specialized / custom
message. You could easily have a $PTNT (for TimeNuts of course not for blowing
things up) message or set of messages. If you added a “version” field to the list above, and
a “number of fields to follow”, you probably would have a useful string to use. 

$PTNT,1,5,32768,27.232,1.1.3,+22.868, -13.45,100 would be version 1, 5 fields, DAC 32768 out
of who knows how many (hmm…), State 1.1.3 (out of how many), Temp 22.868 C, time error -13.45 ns, lock 
quality 100%.

You could take care of the dac issue by going to a float with a defied range of 0 to 1. State
is a bit more difficult. It depends a lot on how things are implemented. We probably would need 
it a bit better defined or put it in another message. 


> On Dec 17, 2016, at 2:22 PM, Jim Harman <j99harman at gmail.com> wrote:
> Hi all,
> I have experimented with LH 5 just monitoring a GPS receiver and am very
> impressed with the results.
> As a next step, I would like to use LH to monitor a homebrew GPSDO, and I
> think it would be easier to modify the GPSDO firmware to emulate an
> existing device rather than customize LH to work with the logging data that
> my system currently produces.
> In addition to NMEA data from the GPS, my system can output the DAC and TIC
> (phase error) values as well as the temperature, Since I control the
> firmware, I can produce pretty much any data format as long as it is
> clearly documented, but I would prefer a text-based rather than binary
> protocol and not to have to reformat all the NMEA data.
> Does this approach make sense, and if so which of the several standard
> GPSDOs would it be best to emulate?
> Thanks in advance for your insights
> -- 
> --Jim Harman
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