[time-nuts] FTS4060 C Field Time Constant

Brooke Clarke brooke at pacific.net
Fri Mar 4 13:38:35 EST 2005


I've been experimenting with the elevation mask setting and also paying 
attention to which SVs are being tracked and just started paying 
attention to the actual elevation angles of the satellites.  Also 
looking at the standard deviation that the SR620 counter generates along 
with the mean value of 1,000 observations.  I think SDEV is a measure of 
the multipath.

I'm convinced that I have a serious multipath problem and the solution 
is to only track satellites that are very high in the sky.  But as far 
as I can tell there's no easy way to do it.

Currently I have the elevation mask at 50 degrees and sometimes am 
tracking only one satellite, which is all that's required in the 
position fixed timing mode.

For example (have not tried this yet) if I set the elevation mask higher 
than 50 (my first guess is somewhere in the 60 to 70 degree range) then 
I'm concerned that the GPS receiver will output 1 PPS when there is no 
satellite present.

I'm now checking the TAC32 screen after each average of 1,000 seconds.  
About once per day there is a very high angle pass and typically it's 
just one satellite, and the SDEV is in the 30 to 40 ns range.  Most of 
the day (with a 50 degree El angle mask) the SDEV is more like 150 ns.

Maybe I need a custom computer data acquisition program that only plots 
when the SDEV value is small.  Or maybe there's some other simpler answer?

Have Fun,


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

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