[ale] Re: WWVB to computer

[Michael H. Warfield]

On Tue, 2007-12-18 at 13:29 -0500, Christopher Fowler wrote:
> On Tue, 2007-12-18 at 13:09 -0500, Michael H. Warfield wrote:
> > In this day and age of cheap GPS modules, I wouldn't even consider
> > going the WWVB route. 

> GPS propagation inside Quality Services in Suwanee?  Feasible?

	Probably.  Depends.  I've stuck a 12 channel GPS next to a window and
received the minimum of 4 satellites very reliably.  If it's a
time-optimized unit, once it knows it's location, it can go into time
tracking mode and maintain quality time reference with even fewer
satellites.  Just find any fixed location where it can get a half way
decent signal and nail it there.

	Here's an example (I make no claims for this unit, I have no experience
with it - just an example and I have no idea about its price)...

	http://www.furunogps.com/GT8031.html

	* "Accuracy is not sacrificed 15 nanoseconds with regard to UTC (1
sigma)"

	* "Autonomous transition from ?Survey? to ?Fixed Observation Point?
lets the receiver calculate its position and then use the averaged
position as the fixed observation point"

	One full cycle of WWVB RF at 60KHz is 16.7 usec and this is speced to
15 nsec off UTC.

	That unit is NMEA with WAAS and DGPS compensation (not really needed
for time sourcing, I suspect) with a 1 per second reporting cycle.  I
saw a few units with a 5 per second reporting cycling giving you tighter
precision on the reports from the receiver to the server.  I believe
there is even some communication latency logic built into ntpd and gpsd
to compensate for the delays in the serial / USB cables but that's
getting beyond the ability of the system to resolve and maintain.

	Mike
-- 
Michael H. Warfield (AI4NB) | (770) 985-6132 |  mhw at WittsEnd.com
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