[ntp:questions] bluetooth gps receiver time synchronization accuracy

Brad Knowles brad at stop.mail-abuse.org
Tue Oct 11 10:39:20 UTC 2005

At 10:51 AM +0200 2005-10-11, Ger wrote:

>  I'm looking for a way to time-synchronize a pda with a fixed host, and I'm
>  thinking about using a bluetooth gps receiver (ntp is not accurate enough).
>  I was wondering what the accuracy is when you synchronize the gps receiver's
>  clock to the pda clock, any ideas? I know the most bluetooth gps receivers
>  have an accuracy around the 1 microsecond range (to GPS time), but it's not
>  the same on the pda i guess.

	There are two problems here:

		1.  GPS receivers designed for positioning information rarely handle
			NMEA serial port communications with adequate accuracy to be useful
			in a timing-sensitive situation.  Internally they may have all the
			information you want/need, but their communications capabilities
			are very limited (in terms of performance and latency) and are
			at the bottom of the priority stack in terms of doing all 
the things
			that a GPS device needs to do.

		2.  Bluetooth adds a great deal of unknown latency and jitter, all by
			itself.  Excessive latency and jitter will turn a useful time
			reference into a doorstop or boat anchor, at least as it pertains
			to being useful in a timing sensitive situation.

	There are GPS devices designed to handle NMEA serial 
communications with minimal latency, and when paired with a decent 
hardware/OS/application configuration, can result in providing 
excellent quality time to you and your clients.  But these devices 
tend to be a lot more expensive, and it takes some work to get them 
set up correctly.

	The Garmin GPS 18LVC apparently is one of the consumer-oriented 
GPS devices that happens to handle serial port communications 
reasonably well.  But you'll need to do some work to get the serial 
connection made between your server and the device.  Of course, all 
GPS devices are sensitive to being able to receive the necessary 
signals from the satellites, so you may have to locate the Garmin 
device somewhere outside your house (maybe on the roof, or high on a 
pipe), with a clear view of the sky.

	I think you'd also have a challenge finding the code that would 
successfully compile on a PDA -- the NTP clients I know of for PDAs 
tend to be the simplest possible SNTP clients written in a language 
like Java, and therefore reasonably portable.  The full-blown code to 
support NTPv3 or NPTv4, including the ability to use GPS refclocks, 
that's a totally different matter.

	I'm sorry to say that this process is *not* as simple as grabbing 
a Bluetooth GPS receiver, pairing that with your PDA, and then 
everything magically works.

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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