[ntp:questions] Questions about using GPS receiver to synchronize PowerBook G4

Brad Knowles brad at stop.mail-abuse.org
Sun Mar 27 17:37:35 UTC 2005

At 2:56 AM -0500 2005-03-27, Ningyu Liu wrote:

>  1) Is the PPS signal necessary for the time synchronization? Can I get
>  1 ms accuracy from the NMEA message alone?

	I'm pretty sure you're going to need the PPS, and even then there 
is no guarantee you'll be able to get 1ms accuracy, because a lot 
depends on the GPS device, your machine and how loaded it is doing 
other things, etc....

>  2) Since the PowerBook G4 doesn't have a RS-232 port, can I use a
>  USB GPS receiver or a RS-232 GPS receiver with a USB adaptor?, or
>  maybe a bluetooth GPS receiver?

	USB won't work.  It will introduce *way* too much latency and 
jitter.  Moreover, most general-purpose GPS devices won't work, since 
they're designed to provide high accuracy position information, and 
not accurate time information.

	You need a GPS device that is specifically designed to provide 
high accuracy time information, and then you need the cables 
necessary to directly connect that to your machine.

	Since you don't have serial ports and USB is right out, I don't 
know what other options you will have available to you.

>  3) Is there any other way to synchronize this laptop?

	I'm not sure I've got a good answer to that question.  I do 
pretty well on my PowerBook G3 running MacOS X 10.3.8 and ntpd 
4.2.0a at 1.1320 (from a recent ntp-dev tarball), and a few selected 
upstream time sources that are not too far away on the Internet 
(about 20-45ms latency, on average), but then I'm not running a 
timing-critical application.

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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