Teletrac Prism TM, Installation Information

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Teletrac, Inc. - Prism TM Information and Installation Guide
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E
XTRA
M
EASUREMENT
C
URES
T
IMING
O
FFSET
If everything were perfect (i.e. if our receiver's clocks were perfect) then all of our satellite ranges
would intersect at a single point (which is our position). But with imperfect clocks, a fourth
measurement, done as a cross-check, will NOT intersect with the first three.
So the receiver's computer says "Uh-oh! There is a discrepancy in my measurements. I must not
be perfectly synced with universal time."
Since any offset from universal time will affect all of our measurements, the receiver looks for a
single correction factor that it can subtract from all its timing measurements that would cause
them all to intersect at a single point.
That correction brings the receiver's clock back into sync with universal time, and bingo! - you've
got atomic accuracy time right in the palm of your hand.
Once it has that correction it applies to all the rest of its measurements, and now we've got
precise positioning.
Step 4: Knowing Where a Satellite is in Space
On the ground all GPS receivers have an almanac programmed into their computers that tells
them where in the sky each satellite is, moment by moment.
C
ONSTANT
M
ONITORING
A
DDS
P
RECISION
The basic orbits are quite exact but just to make things perfect, the GPS satellites are constantly
monitored by the Department of Defense.
They use very precise radar to check each satellite's exact altitude, position and speed.