[ntp:questions] lots of GPS modules and info at SparkFun
Ron Frazier (NTP)
timekeepingntplist at c3energy.com
Thu Feb 9 16:59:16 UTC 2012
On 2/9/2012 4:18 AM, David J Taylor wrote:
>> Hi David,
>> I'm certainly considering that board. It looks very attractive. I'm
>> going to have to read over the documentation page you've put
>> together. It looks very comprehensive.
>> Chris Albertson said to get a unit which had:
>> 1) - a stationary mode, or possibly only a stationary mode
>> 2) - self survey
>> 3) - PPS
>> 4) - serial output
>> 5) - NTPD driver support
>> Now I know the Sure board has 3) and 4). And I presume you can do 5)
>> through the generic NMEA driver. However, do you know if it can do
>> 1) and 2)? Actually, does it have native PPS, or are you adding it
>> by using the LED output?
> (1) and (2) are not required for the microsecond level accuracy you
> are going to see in an NTP application, Ron. In common with other GPS
> timing devices, the Sure board doesn't have those features, or if it
> does, I've not used them. It's my understanding that they only become
> significant below the microsecond level. Other uncertainties will
> contribute rather more than (1) or (2) will reduce. (1) might help if
> you don't have a reliable view of the sky - in practice with today's
> sensitive receivers being on the top floor of a building with just the
> roof in the way may well be enough, particularly in a domestic
> environment. See how well other GPS devices perform in your location.
> Any further questions which aren't answered on my Web page, please ask.
I just finished reading your Sure board web page. That's a really good
write up. I now understand how you're getting the PPS signal. Could
you possibly share the rev level of the board, since, if they change the
design, you patch instructions may not work any more? Some boards don't
have a rev level.
As suggested by Brent Gordon's post, and confirmed in the manual for the
Trimble Copernicus II, I have switched my GPS to output only the GPZDA
sentence and changed the NMEA driver to respond to that. I don't think
the GPZDA sentence has any position information, so it's length should
be very consistent. Whatever the reason, the results are amazing.
Although I've only been running this way for 8 hours, my peak offsets
are now in the +/- 6 ms range. Of course, we'll have to see if the
system continues this way, or if my GPS goes phycho again. This is
twice the level of performance and accuracy than I had before.
Now, I will probably try some PPS stuff, just for intellectual reasons
and just because it would be really cool to see that chart drop under 2
or 1 ms. However, for my simple purpose of just keeping all my PC
clocks right to the second, if I can keep this thing under 10 ms of
error, I'm pretty happy with it. And this is still USB only.
Assuming it's possible to program the Sure board for GPZDA, it would be
interesting to see if doing that affects the performance you're seeing.
Of course, if you're using PPS, the specific content of the NMEA
sentence may not matter as much.
(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
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timekeepingdude AT c3energy.com
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