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positions, and a collection of the latest data on all GPS
satellites
* Update the satellite navigation message regularly.
The control segment has one master control station and five
monitor stations. Three of the monitor stations are also upload
stations.
The master control station is in Colorado Springs, USA. The
master control station is the operational center of the GPS. The
master control station controls all operations in the control
segment. The master control station has an atomic clock, this
clock is the reference for the GPS.
The monitor stations track the satellites 24 hours a day. The
master control station remotely controls the monitor stations
through on-line connections. The monitor stations are in these
locations:
* Ascension island
* Colorado Springs
* Diego Garcia island
* Hawaii
* Kwajalein island.
The monitor stations receive the same information from the
satellites that the GPS receiver unit in the airplane receives.
The monitor stations do these things:
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* Record the accuracy of the satellite clocks
* Collect and relay to the control station meteorologic data
such as barometric pressure, temperature, and dew point.
The master control station uses this data to calculate the
tropospheric signal delay
* Continuously measure the ranges to all visible satellites. The
master control station uses this data to calculate and predict
the satellites orbits.
The master control station uses the upload stations to send this
data:
* Orbit correction commands to the satellites. The satellites
use control rockets to correct their orbits
* The navigation message to the satellites.
The upload stations are on Ascension island, Diego Garcia
island, and Kwajalein island.
GPS Accuracy
Civilian users have access to standard positioning service
(SPS). SPS has an accuracy of 15 - 25 meters for 95% of the
position fixes. For security reasons, the American Department
of Defense intentionally degrades the accuracy for civilian
users to 100 meters for 95% of the fixes.
Military users have access to precision positioning service
(PPS). PPS has an accuracy of 18 meters or less for 95% of the
fixes.
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Ranging
The GPS receivers use the principle of ranging to measure the
distance between the receiver and the satellites. The receiver
always has the location of the satellites in their orbits in
memory.
The receiver measures the time it takes for a radio signal to go
from a satellite to the airplane. Since the receiver knows the
location of the satellite and that the radio signal travels at the
speed of light, it can calculate the distance to the satellite.
The receiver uses one way ranging. The receiver must know
exactly at what time the satellite sent the radio signal. The
receiver compares the satellite signal to a signal that the
receiver makes at the same time as the satellite. The difference
between the two signals (
t) is the time the satellite signal took
to get to the receiver.
Each satellite has an atomic clock to keep accurate time. All the
satellites have precisely the same time. The receiver in the
airplane has an internal clock but it is not atomic. It is not as
accurate. Thus, it is not possible for the receiver to have
precisely the same time as the satellite.
The receiver assumes that its internal clock is off by some clock
bias (
tBIAS). This
tBIAS is an unknown that the receiver
must calculate. The
tBIAS is the difference between the
receiver time and GPS time.
To calculate the airplane position (latitude, longitude, and
altitude) and the
tBIAS, the receiver must know the position of
at least four satellites. The receiver then measures the
distances to all the satellites at the same time. It then solves
for these four unknowns with four range equations:
　
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