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X 2 , Y 2 , Z 2
X 3 , Y 3 , Z 3
P r
P r
X 4 , Y 4 , Z 4
X 1 , Y 1 , Z 1
P r
P r
Unknown location of receiver r
fIGURe 9.1
Determination of the position of user r by triangulation, using range measurements P to mul-
tiple satellites.
The concept of GPS technology was formulated with the following primary objectives:
Suitability for all classes of platforms, such as spaceborne, airborne, marine, land-based,
and individual pedestrian, under a wide variety of dynamics
Availability any time, any weather, anywhere on Earth and its vicinity
Real-time positioning, velocity, and time determination capability
Providing the service to an unlimited number of users worldwide
Positioning on a single global geodetic datum (World Geodetic System, WGS84)
Redundancy provisions to ensure the survivability of the system
Restricting the highest accuracy to a certain class of users (military)
Low cost and low power users' unit
9.3 the MAIn CoMponentS of GpS
The main components of the GPS system are Space , Control, and User segments. The GPS opera-
tional constellation is made up of twenty-four satellites that orbit Earth at the altitude of ~20,000 km
(Figure 9.2). The satellites are placed in six nearly circular orbital planes, inclined at 55 degrees
with respect to the equatorial plane, with nominally four satellites in each plane. This configuration
assures the simultaneous visibility of five to eight satellites at any point on Earth. The constellation
establishes the Space Segment.
GPS satellites are powered by solar energy. During the solar eclipses, they use the backup
batteries carried onboard. Because the satellites tend to drift from their assigned orbital positions,
primarily due to orbit perturbations caused by Earth, the Moon, and planets' gravitational pull,
solar radiation pressure, and so forth, they have to be constantly monitored by the Control Segment
to determine their exact location in space. In order to keep the satellites as close as possible to the
predesigned orbits, each satellite is equipped with small rocket boosters that can be fired when the
orbit correction is needed. The Control Segment consists of 11 monitoring stations, each check-
ing the exact altitude, position, speed, and overall health of the orbiting satellites 24 hours a day.
In 2005 the National Geospatial-Intelligence Agency (NGA) added six more stations to the initial
network of five. Based on these observations, the position coordinates and clock bias, drift, and drift
rate can be predicted for each satellite, and then broadcast to each satellite for retransmission back
to the users. The satellite position is parameterized in terms of predicted ephemeris, expressed in a
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