Teletrac Prism TM, Информация по установке

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Teletrac, Inc. - Prism TM Information and Installation Guide
42 1622-0300 B1 3/18/04
So by ranging from three satellites we can narrow our position to just two points in space.
To decide which one is our true location we could make a fourth measurement. But usually one of
the two points is a ridiculous answer (either too far from Earth or an impossible velocity) and can
be rejected without a measurement.
A fourth measurement does come in very handy for another reason however, but we'll tell you
about that later.
Next we'll see how the system measures distances to satellites.
Step 2: Measuring Distance from a Satellite
We saw in the last section that a position is calculated from distance measurements to at least
three satellites. But how can you measure the distance to something that's floating around in
space? We do it by timing how long it takes for a signal sent from the satellite to arrive at our
receiver.
T
HE
B
IG
I
DEA
M
ATHEMATICALLY
In a sense, the whole thing boils down to those "velocity times travel time" math problems we did
in high school. Remember the old: "If a car goes 60 miles per hour for two hours, how far does it
travel.?"
Velocity (60 mph) x Time (2 hours) = Distance (120 miles)
In the case of GPS we're measuring a radio signal so the velocity is going to be the speed of light
or roughly 186,000 miles per second. The problem is measuring the travel time.
S
YNCHRONIZING OUR
W
ATCHES
The timing problem is tricky. First, the times are going to be awfully short. If a satellite were right
overhead, the travel time would be something like 0.06 seconds. So we're going to need some
really precise clocks. We'll talk about those soon.