[time-nuts] GPS Satellite Elevation Observation

Brooke Clarke brooke at pacific.net
Sat Apr 15 17:15:04 EDT 2006


I'm seeing distortion in the time interval between GPS and the FTS4060 
standard.  The time interval jumps about 60 ns for about 4 hours and I 
think it's because the satellite signal(s) have multipath.  I would 
expect multipath if the satellite was being tracked at or very near the 
horizon.  In TAC32 you can plot the horizon by setting the mask angle to 
zero and letting the program run for a number of days.

But, when you increase the elevation mask angle I think the horizon plot 
is just moved up.  I think this may be a bug in version 2.7.2.

Because of the orbit inclination there's a region to the north where 
there never will be a satellite.  So for a given latitude there's some 
elevation mask which would never let any satellite be near the horizon.  
But that is wasting the good fixes that can be obtained when satellites 
are to the West, South or East at lower elevation angles.  It would be 
better if TAC32 would allow the elevation mask to be different for 
different azimuth angles.  I think this would improve the quality of 
time interval measurements or to say it another way it would be useful 
in getting rid of bad time intervals.

I'm sensitive to this because I'm in a canyon and need a mask angle of 
about 60 degrees.  The above problem may have been caused by setting the 
mask angle to 50 degrees.  I'm now replotting the horizon in TAC32 with 
the mask set for 60 degrees and expect to see a solid circle, not the 
noisy horizon plot that was there (which was moved up from the zero 
degree plot).

After about 4 weeks my FTS4060 plot shows -7 ns/day but with an R^2 
value of only 0.54.  Part of the reason for such a low correlation may 
be the above mask angle allowing skimming passes?

Have Fun,

Brooke Clarke

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

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