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Yes, George, US Navy ships are made of steel.  Very, very special steel.  Just 
kidding - I don't do the "look at me I'm so smart" game.  My posting (10801) 
was in response to your "It's always possible that I've misunderstood the 
technology, and the makers have some clever trick up their sleeve that I've 
missed. If so, no doubt some list member will put me right" in post 10794.

The advantage of these "electronic" compasses is that you can put the sensor 
assembly anywhere you want - no need for "Navigator's Balls."  Find the sweet 
spot and put it there.  Then, if needed, punch in an offset (this for the 
high-end, i.e. expensive equipment).  Doesn't fix the three major drawbacks: 
1) eliminating the vertical component of the magnetic field when the vehicle 
is heeled, 2) moving in a circle to calibrate, and 3) effects of an 
alteration to the ship's configuration, i.e. additions or deletions of 
equipment, unless you redo the offset.

1) is reduced through gimbaling (two axis systems) or by using a third axis 
sensor and then "calculating out" the vertical component.  A third sensor 
leads directly to "why bother?"  Three magnetic sensors and a computer or 
three accelerometers and a computer (inertial).

2) has been partially solved by an engineer at Delphi, an automotive industry 
parts supplier - they do the automotive "electronic compasses."  With his 
system you move in a partial circle - the electronics assumes an ellipse and 
calculates the ellipse locii.  The locii location, in relation to the curve 
location, determines the quadrant.  It's patented, so the rest of the 
suppliers will have to wait it out or devise their own system.

3) is not a factor in an inertial system.  "Why bother" won for US Navy ships.

Joe


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