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advances of the digital computer to increase system responsiveness.
Section 16D— NAVSTAR Global Positioning System
16.21. Basics. Deployment of the NAVSTAR GPS constellation of satellites began with the first launch
in 1977. The satellites were launched into precisely controlled orbits, allowing users with GPS
equipment to receive data to determine their position. The phenomenal accuracy of GPS was its major
selling point, but its many different applications were a close second. GPS determines a position
referenced to a common grid known as the World Geodetic System 1984 (WGS 84). The WGS 84 grid
is based upon a mathematical model and compensates for the fact that the earth is not a perfect sphere
(Figure 16.9). Derived using precise satellite measurements, it creates an accurate model of the earth's
surface. As a consequence, WGS 84 provides extremely accurate information when compared to older
traditional datum references. The value of the WGS 84 grid is that positional data can be standardized
worldwide. Many receiver sets are capable of converting WGS 84 data into these other commonly used
references.
Figure 16.9. World Geodetic System 1984.
16.22. Military Applications. In air operations, GPS accuracy can improve en route and terminal
navigation, reducing flight times and fuel consumption. Since it is a three-dimensional system, descent,
approach, and landing operations can be monitored more closely. In combat-related applications, GPS
performance can improve weapons delivery, airdrops, rendezvous, and threat avoidance. Since GPS
requires the use of a common grid, coordination of air, ground, and sea operations can be greatly
improved. These include close air support, search and rescue, rendezvous, and airdrop operations.
16.22.1. For ground forces, GPS can provide similar advantages. Its accuracy will enhance site
surveying, field artillery placement, and target acquisition. First-round artillery effectiveness can be
improved based on precise knowledge of the location of friendly firepower, coupled with forward340
AFPAM11-216 1 MARCH 2001
observer determinations of enemy locations and movement. Special forces can operate with greater
precision closer to enemy forces. GPS can also provide benefits to naval forces. Harbor entry operations
can be improved. Coastal survey operations can be conducted more quickly and effectively. Mine
placement and countermeasure operations can be conducted with greater precision and safety.
16.22.2. In the area of Command, Control, Communications, and Intelligence (C3I), GPS performs a
vital function by providing worldwide nuclear detonation (NUDET) detection capability to the National
Command Authorities (NCA). Each GPS satellite carries a sensor capable of detecting nuclear
detonations. This surveillance capability can be used in verifying future Nuclear Test Ban treaties. More
importantly, it provides the NCA with the means of monitoring and reacting to a nuclear confrontation
should the need arise.
16.23. Civilian Applications. Although originally conceived and developed as a military-only project,
GPS information is available to anyone in the civilian community with a receiver. It provides a broad
spectrum of civilian users with accurate position, velocity, and time information at a reasonable cost. In
addition to military uses and benefits, GPS is being used in such diverse areas as oil and mineral
exploration, geophysical surveys, airborne collision avoidance, and maritime hazard systems. Anyone
can buy hand-held GPS receivers with 100-meter accuracy for less then $300. The potential applications
for GPS are boundless. The civilian community has a great deal of interest in the applications of GPS,
and more sophisticated uses are developed every day.
16.24. General System Description. The GPS system is made up of three segments—space, user, and
control.
16.24.1. Space Segment. The space segment is composed of 24 satellites plus 3 operational spares in 6
orbital planes (Figure 16.10). The orbits are arranged precisely such that a minimum of four satellites are
in view at all times worldwide. The design life of each satellite is 7.5 years with an average mission
duration of 6 years.
Figure 16.10. GPS Constellation.
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16.24.2. User Segment. The user segment consists of user equipment (UE) sets (Figure 16.11), test
equipment, and associated support equipment. The UE set, using data transmitted by the satellites,
determines the user's position, altitude, and velocity. Transmissions from the satellites also allow the UE
set to evaluate the accuracy of the navigational information being received. This is based on built-in
checks of its own performance, the configuration of the satellite constellation in view, and the jammingto-
　
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