[time-nuts] FTS4060 Saga
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?
w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml
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