[time-nuts] OT - GPS and North

Brooke Clarke brooke at pacific.net
Mon Nov 23 00:35:34 UTC 2009


Hi Antonio:

----------------
That's an interesting question.

It turns out that one of the key military applications of GPS, in addition to 
position and time, is to find North to high accuracy.  This is needed to be 
known to better that one grad (1/6400 of a circle).  In that past it was done 
using a theodolite integrated with a gyroscope (AG8 (Kern DKM1) North Finding 
System).  This system was heavy, cost the government about $250k and was very 
easy to break.
http://www.prc68.com/I/Alidade.shtml#AG8

I think starting with the PLGR96 and now the DAGR there are two or more methods 
of accurately finding North.
http://www.prc68.com/I/PLGR.shtml
http://www.prc68.com/I/DAGR.shtml#GLS
http://www.prc68.com/I/DAGR.shtml#AzD

In one method you place the receiver on stake #1, go to stake #2 (while not 
obstructing the antenna to allow carrier phase tracking sats) then at stake #2 
press enter and the GPS gives you the bearing and distance between the stakes.

In another method two GPS receivers are used.  One at stake #1 and the other 
first is synchronized to the one at stake #1 then moved to stake #2, press 
enter, and go back to stake #1 and reconnect the two units.  Again you get 
bearing and distance between the stakes based on carrier phase processing 
inside the GPS unit.

---------------------

Many GPS receivers have a built in World Magnetic Model.  It's just a 12th 
order polynomial fitted to the magnetic vector by Lon, Lat, Ele, date & time. 
They're good for 5 years, see:
http://www.prc68.com/I/Sensors.shtml#EMF

For example the military DAGR has a built in magnetic compass that has the 
deviation corrected based on the location and date so you can select either 
true or magnetic bearings.  Using the magnetic compass drains the battery so 
there's an option to turn it off when not in use.  There's no tilt correction 
so the GPS receiver needs to be held level when using the mag compass.

Have Fun,

Brooke Clarke
http://www.prc68.com

iovane at inwind.it wrote:
 > Does a stationary (not in motion) GPS receiver know where the North is?
 >
 > As far
 > as I can understand, it doesn't, isn't it?
 >
 > Antonio I8IOV
 >
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