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[TowerTalk] RE: Compass question

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Subject: [TowerTalk] RE: Compass question
From: (Chuck Counselman)
Date: Thu Feb 13 12:07:44 2003
At 6:48 AM -0800 2/13/03, Jim Lux wrote:
>The GPS receiver doesn't measure any bearings... it measures 
>distance to the satellites, and does "tri-lateration" not 

What Jim says is quite correct for consumer-grade (basically, 
inexpensive) GPS receivers, including both handhelds and those 
intended for use on boats.  AFAIK, an LCD-displayed arrow that points 
north and continues pointing north if you rotate the receiver about 
the local vertical axis, is simply a high-tech version of a Boy-Scout 
compass, using a solid-state "flux gate" rather than a pivoted needle 
to sense the direction of the ambient magnetic field.  The receiver's 
firmware may also compensate for the difference between magnetic and 
astronomic norths.

At least three exceptional cases are worth knowing about:

1. Even the cheapest handheld or boat receiver can correctly 
determine and display the direction of its recent _motion_, including 
its latest, or near-current, velocity with respect to the ground 
immediately below, and its position track, or bread-crumb trail.  I 
refer to its transverse motion, *not* the orientation of its body.

2. The original-equipment GPS vehicle navigation system in my BMW 
automobile, and probably many other automobiles, correctly determines 
and displays the "compass" direction or azimuth in which automobile 
is pointed, i.e., headed, as opposed to merely the direction that it 
it is traveling or has traveled.  The system in my car does this not 
by reference to magnetic field, but by including measurement data 
from wheel-rotation sensors, a steering-angle sensor, and a rate 
"gyro" together with GPS-satellite-signal measurement data in its 
navigation calculations.  (It is very cool to watch the dashboard 
display continuing to show the correct vehicle heading as you make 
all sorts of maneuvers inside an underground parking garage, far 
beyond the reach of any GPS satellite signal.

3. Some GPS receivers made for ships and aircraft use multiple 
antennas, e.g., on the tips of the wings, the top of the fuselage, 
and the top of the vertical stabilizer ("tail") of an airplane, to 
determine all aspects of the vehicle's body orientation of "attitude" 
(roll, pitch, yaw/heading...) directly from observations of signals 
received from the GPS satellites, independently of magnetic or 
inertial sensor (gyro, accelerometer, artificial-horizon...) inputs, 
although vehicle navigation/guidance systems may also (and preferably 
do) integrate all available relevant inputs.

73 de Chuck, W1HIS
Inventor/designer/developer of the world's first civilian GPS 
receiver, and inventor of 30 patents relating to GPS positioning and 
navigation, and a couple relating to cellphone location and other 
terrestrial radiolocation technologies
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