[ntp:questions] lots of GPS modules and info at SparkFun
Ron Frazier (NTP)
timekeepingntplist at c3energy.com
Sun Feb 19 18:27:43 UTC 2012
On 02/19/2012 11:51 AM, Dave Hart wrote:
> On Tue, Feb 14, 2012 at 18:52, Ron Frazier (NTP)
> <timekeepingntplist at c3energy.com> wrote:
>> I don't think that page mentions it, but I now have my unit outputting only
>> the GPZDA NMEA sentence, which seems to give the most accurate timing
>> information and the least jitter. I don't think this sentence has any
>> position information.
> GPGGA is also a decent choice for fixed position GPS -- it's
> effectively constant length, and does provide position.
> Dave Hart
Thanks for the info. Actually, I was using GPGGA before I went to
GPZDA, and I know several others are using it. I read about GPZDA in a
forum post somewhere, and the manual for one of the new Trimble GPS's
recommends it for timekeeping. I don't need position info for this
application. If I recall correctly, I was getting +/- 15 ms accuracy
out of this unit when I was using GPGGA and now I'm getting +/- 6 ms
accuracy with GPZDA. Even in a fixed position application, the position
reported by the GPS can vary by 100 feet or so depending on the quality
of the satellite signals, as I've seen on a couple of my navigation
units. So, even if you're sitting still, you can appear to be moving,
so much so that my TomTom sometimes decides to replan the route while
I'm sitting in a parking lot. I think having the lat and long numbers
in the NMEA sentence, which are varying, not only makes the sentence
longer, but perhaps, creates more jitter in the output. I have a theory
that, every additional element the GPS has to compute and place in the
NMEA sentence introduces more variable time delays into the production
and output of the sentence, and hence more jitter and less accuracy.
I'm definitely getting superior performance from this unit using GPZDA.
I also upped the baud rate from 4800 to 57600. That also appears to
have reduced jitter and improved accuracy.
(PS - If you email me and don't get a quick response, don't be concerned.
I get about 300 emails per day from alternate energy mailing lists and
such. I don't always see new messages very quickly. If you need a
reply and have not heard from me in 1 - 2 weeks, send your message again.)
timekeepingdude AT c3energy.com
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