[ntp:questions] lots of GPS modules and info at SparkFun
unruh at invalid.ca
Sat Feb 11 00:54:52 UTC 2012
On 2012-02-09, Ron Frazier (NTP) <timekeepingntplist at c3energy.com> wrote:
> 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.
> Hi David,
> 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.
PPS, at least on Linux, can give over 1000 times better accuracy.
> 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.
GEt the manual from Mediatex MTK NMEA Packet User Manual, which gives a
far far more extensive set of nmea programming instructions for the
chipset that Sure uses.
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