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
>"tri-angulation"
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
<http://www-eaps.mit.edu/faculty/counselman.htm>
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