[alberg30] GPS

[Robert Kirk]

From: Robert Kirk <kirk at neptune.gsfc.nasa.gov>


>Maybe I am seeing things, but when I leave my GPS sitting here by the 
>window on solid ground the lat/lon seems to circle around a bit, I assume 
>because of the SA. It looks as though there is a random pattern put on the 
>signal all the time that lets it wobble in  a circle - CEP (circular error 
>probable, as we missileers used to say) of a hundred meters or so? If the 
>SA is ever actually "off" you couldn't prove it by my readings.
>                                                 - Gordon White A-275

Gordon... No, you're not seeing things. The spec for SA dithering is <100 
meters 95% of the time, so that the bad guy will have that big a CEP when 
trying to hit a hardened missile silo which needs a *direct* hit.

An interesting real-time display of SA in action is at;

   http://www.op.dlr.de/~igex98op/monitor/monitor.htm

and then select "horizontal position error".  If they were plotting a 
differentially corrected position, the circle would be around 10 meters or 
less.  A colleague of mine used to broadcast  excellent differential 
corrections from some precise equipment here at Goddard, over the Bay on a 
ham radio frequency  so  the tracking was rock solid.  You can get (almost) 
as good yourself by buying a differential receiver and tuning in the Coast 
Guard 's differential signal.  Or more fun, build one of your own. See:

http://www.tapr.org/tapr/html/kitsf.html

if you are handy with a soldering iron.

Cheers,
Bob Kirk
Isobar #181


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