[time-nuts] FTS4060 Saga

Brooke Clarke brooke at pacific.net
Wed May 17 14:32:48 EDT 2006


I've been comparing s/n 1227 aginst GPS for a couple of weeks and am 
within a tick or two of getting the c field optimized. I think the 
current actual offset is less than +2.2E-13, but R^2 is only at 0.2 so 
am not really sure yet.

A problem that shows up every now and then is that the standard 
deviation on a 500 second average is above 10 ns (typical is 9 ns).  For 
example here is some data all this morning (17 May 06):
Time Interval (ps)   time       sigma
975803               04:49:20     9
969817               04:57:40    98
919359               05:06:00   240
838596               05:14:20   349
957634               05:22:40   150
979542               05:31:00     9
917433               05:39:20   244
903613               05:47:40   267
913995               05:56:00   251
936876               06:04:20   206
976696               06:12:40     9
973410               06:21:00    62
959098               06:29:20   144
976590               06:37:40     9

I have a couple of ideas of what might be causing this.  One is that 
there were no satellites to track and the crystal in the M12+T drifted, 
but think this is almost impossible since the elevation mask now is at 
20 degrees.  The other is multipath.  Is there any information on how 
multipath effects the M12+T?  I would think that in 0-D mode and 
tracking only one satellite then multipath would be a factor, but if 
tracking a bunch of satellites then multipath on one of them would not 
be a factor. 

Since I'm logging the sigma when I see this I just toss the data.  
Notice that for the 3 points where sigma is 9 ns the TI looks like:
Where the spread is under 5 ns which is good.

I would like to know what's causing this.  Any thoughts?

Have Fun,

Brooke Clarke

w/Java http://www.PRC68.com
w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml

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