ST2276
Scientech Technologies Pvt. Ltd.
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In December of 1993, GPS reached Initial Operational Capability, with a minimum of
24 satellites in orbit. On July 17, 1995 the Air Force announced that GPS had met all
requirements for Full Operational Capability with 24 Block II SVs in orbit. With over
50 companies supplying a selection of over 275 GPS receivers to a global market, the
well established user community of navigators, surveyors, geologists, geodesists, time
and frequency users, and many thousands of recreational user has come to accept GPS
as a viable military and civilian system.
Civil and Military GPS :
While controlled and maintained by the DoD, the GPS user community has a large
civil component. In the 1977 National Plan for Navigation, published by the U. S.
Department of Transportation (DoT), the NAVSTAR GPS user community was
planned to include 27,000 military receivers. While the potential for a civil-sector user
base was recognized, the document did not include plans for a civil GPS service
(U.S. DoT 19773-14; 3-15). A decade later the Federal Radio navigation Plan (FRP)
(U.S. DoD and DoT 1986) stated that GPS would be available to civil users,
worldwide, on a continuous basis but with accuracy limited to 100 meters (95
percent).
In these radio navigation documents position accuracy is usually specified as a two
standard deviation (95 percent) radial error or 2drms (2 distance root mean squared)
uncertainty estimate. For GPS the 95 percent probability and 2drms accuracy are
equivalent (DoD and DoT 1995, A-2). The 1985
Comprehensive Global Positioning
System User Policy
defined both a military, encrypted, Precise Positioning Service
and a "lower level of accuracy" Standard Positioning Service (U.S. DoD and DoT
1986, B-32).
Standard Positioning Service :
The Standard Positioning Service (SPS) is defined in the most recent FRP as: the
standard specified level of positioning and timing accuracy that is available, without
restrictions, to any user on a continuous worldwide basis. The accuracy of this service
will be established by the DOD and DOT based on U. S. security interests. SPS
provides a predictable positioning accuracy of 100 meters (95 percent) horizontally
and 156 meters (95 percent) vertically and time transfer accuracy to UTC within 340
nanoseconds (95 percent).
Precise Positioning Service :
The FRP defines the Precise Positioning Service (PPS) as: the most accurate direct
positioning, velocity, and timing information continuously available, worldwide, from
the basic GPS. This service is limited to users specifically authorized by the U.S.
P(Y)-code capable military user equipment provides a predictable positioning
accuracy of at least 22 meters (95 percent) horizontally and 27.7 meters (95 percent)
vertically and time transfer accuracy to UTC within 200 nanoseconds (95 percent)
(DoD and DoT 1995, A-36). By the time the 1992 FRP was published, the projected
1995 estimate of 53,000 civil users of GPS exceeded the projected number of military
users estimated at 19,000 (U.S. DoD and DoT 1993, 3-41). Civil users now constitute
the majority of GPS users. The 1994 FRP estimates the current total number of GPS
users at over 500,000 in the United States alone (U. S. DoD and DoT 1995, 3-7).
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