[ntp:questions] Re: GPS Time source offset

Brad Knowles brad at stop.mail-abuse.org
Sat Oct 22 12:08:29 UTC 2005

At 7:21 AM +0000 2005-10-22, Bjorn Gabrielsson wrote:

>>  	You may have been unlucky to get a GPS unit that is not
>>  well-suited to your desired task.
>  IF the offset is fairly constant it may very well be a suitable
>  receiver - for some low accuracy timing applications (like ntp).

	On most consumer-grade GPS devices, the serial port 
communications get the lowest priority of anything on the device, and 
the manufacturers tend to cut as many corners as they possibly can. 
The result is that most of them don't support NMEA at all, and those 
few that do have such horribly bad jitter that they are virtually 
worthless as timekeeping devices.

	There are a very few consumer-grade GPS devices (such as the 
Garmin GPS 18LVC) which do not fall into this situation, but they are 
very few and very far between.

Brad Knowles, <brad at stop.mail-abuse.org>

"Those who would give up essential Liberty, to purchase a little
temporary Safety, deserve neither Liberty nor Safety."

     -- Benjamin Franklin (1706-1790), reply of the Pennsylvania
     Assembly to the Governor, November 11, 1755

   SAGE member since 1995.  See <http://www.sage.org/> for more info.

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