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signal ratios being experienced by the set.
Figure 16.11. Handheld User Equipment Set.
16.24.3. Control Segment. The control segment (Figure 16.12) includes a network of monitor stations
and ground antennas placed throughout the world. The monitor stations track all satellites in view and
342 AFPAM11-216 1 MARCH 2001
monitor general health of the system. Data from the monitor stations is sent to and processed at the
Master Control Station (MCS). This data is then used to refine and update the satellites' navigational
signals. These corrections are transmitted to the individual satellites via ground antennas. The
operational master control station is collocated with the Consolidated Space Operations Center at
Peterson Field CO. Three ground antenna stations are located at Diego Garcia, Ascension Island, and
Kwajalein. Five monitor stations will be positioned in Hawaii, Colorado, and at the three ground antenna
locations.
Figure 16.12. GPS Control Segment.
16.25. Theory of Operation. A UE set is capable of determining position, velocity, and time
information by receiving ranging signals from a number of satellites. By measuring the difference
between signal transmission and reception times and multiplying that time interval (Dt) by the speed of
light, range to the satellite can be determined. In a general sense, this is very similar to the way TACAN
DME functions, with one important difference.
16.25.1. TACAN DME is an active system in that a signal must be sent from the aircraft to the selected
TACAN ground station. The ground station in turn sends a reply signal back to the aircraft. The
TACAN set then measures the Dt and then computes and displays the range to the station. GPS is a
passive system; no signal is transmitted by the UE set to the satellite. How does the user or receiver
determine when the signal was transmitted by the satellite? The solution is to encode the satellite signal
so the receiver knows when it was transmitted.
AFPAM11-216 1 MARCH 2001 343
16.25.2. In order to encode the signal with its transmission time, the satellite generates what is known as
a pseudorandom noise (PRN) sequence or code. This code is broadcast continuously from each satellite.
At the same time, the UE set simultaneously generates an identical code. When the set receives the
satellite's signal, it compares it with the code that it has been generating. If a signal arrives at the
receiver with the same code generated two seconds ago, we know that the satellite's signal took 2
seconds to reach us (Figure 16.13).
Figure 16.13. PRN Code Comparison.
16.25.3. If we know the satellite's location in space and our distance from it, we know we are
somewhere on the sphere having the satellite as its center. With two satellites in view, the user's position
is somewhere on the circle representing the intersection of two spheres. A third satellite provides an
additional sphere of position whose intersection with the other two will define a three-dimensional
navigation fix, with timing errors. Figure 16.14 illustrates this concept in two dimensions for clarity. A
fourth allows us to eliminate most of the timing errors. The accuracy of the navigation fix would be
dependent on (1) the accuracy of the measurement process (how accurately is the digital signal
processed), (2) the accuracy of the satellite positions, and (3) the accuracy and stability of the satellites'
clocks and the receiver clock. The user equipment should be able to track the satellite's signal to within 3
nanoseconds (3 x 10-9 seconds). This is equivalent to a 1-meter error in position. If navigational
accuracy on the order of 10 meters is desired, we must be able to establish satellite position at a
particular time to within at least 10 meters. This is not a trivial problem. Since the satellite is moving
and is subject to complex gravitational attractions and solar winds, measuring and predicting its position
within 10 meters as a function of time is quite difficult. Fortunately, this ephemeris (orbital) data is
transmitted to the receiver in the form of almanac data.
16.25.4. Ground stations continuously monitor each satellite so its position can be corrected and passed
to the other satellites in the network. Each satellite in reception range transmits its coordinates, a time
factor correction and other data. The receiver solves simultaneous equations for the unknown receiver
coordinates and the correct time. Four channel GPS receivers can use one channel per satellite to
maintain continuous lock on and update. Receivers with less than four channels must continuously
switch frequencies and hunt for new satellites that can limit the system responsiveness.
344 AFPAM11-216 1 MARCH 2001
Figure 16.14. Resolution of Position.
　
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