[ntp:questions] lots of GPS modules and info at SparkFun

unruh 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?
>>> Sincerely,
>>> Ron
>> (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.
>> Cheers,
>> David
> 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. 

> Sincerely,
> Ron

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