In message <[EMAIL PROTECTED]>, Nate Williams writes:
>>
>> "What is a PPS signal ?"
>>
>> Typically handheld/boat naviation stuff. The NMEA or other
>> serial timecodes are at best in the 1msec class.
>
>Again, for me this is acceptable. It would be nice to have it better
>than this, but the kernel's of all the OS's I'm using have at best 1ms
>precision for all of the applications being used (FS timestamps,
>application program timestamps, etc...).
Well, when I say "at best" I mean it. One NMEA boat-navigation unit
I had access to over last winter had +/- 400msec performance.
>As I mentioned to Warner, is there any way to know how good a particular
>model of a GPS receiver is?
measure it. It's not that hard actually, because you can trust the
FreeBSD clock to be allright over short time intervals, so you
timestamp the events (NEMA / PPS / Whatever) and analyse the pairwise
difference between them:
for instance:
xxxxx2.100000
xxxxx3.140000
xxxxx4.120000
gives you *two* datapoints: +.040000 second and -.020000 second.
Find the stddev of a couple of thousand samples and you have a good
number which is correct to within a factor sqrt(two) or so of the
real jitter.
--
Poul-Henning Kamp FreeBSD coreteam member
[EMAIL PROTECTED] "Real hackers run -current on their laptop."
FreeBSD -- It will take a long time before progress goes too far!
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