Global Positioning System Reference
In-Depth Information
a Transit user at the equator could obtain a position fix on the average of once every
110 minutes, whereas at 80° latitude the fix rate would improve to an average of
once every 30 minutes [1]. Limitations applicable to both systems are that each posi-
tion fix requires approximately 10 to 15 minutes of receiver processing and an esti-
mate of the user's position. These attributes were suitable for shipboard navigation
because of the low velocities, but not for aircraft and high-dynamic users [2]. It was
these shortcomings that led to the development of the U.S. Global Positioning
System (GPS).
1.2
Condensed GPS Program History
In the early 1960s, several U.S. government organizations, including the Depart-
ment of Defense (DOD), the National Aeronautics and Space Administration
(NASA), and the Department of Transportation (DOT), were interested in develop-
ing satellite systems for three-dimensional position determination. The optimum
system was viewed as having the following attributes: global coverage, continu-
ous/all weather operation, ability to serve high-dynamic platforms, and high accu-
racy. When Transit became operational in 1964, it was widely accepted for use on
low-dynamic platforms. However, due to its inherent limitations (cited in the pre-
ceding paragraphs), the Navy sought to enhance Transit or develop another satellite
navigation system with the desired capabilities mentioned earlier. Several variants of
the original Transit system were proposed by its developers at the Johns Hopkins
University Applied Physics Laboratory. Concurrently, the Naval Research Labora-
tory (NRL) was conducting experiments with highly stable space-based clocks to
achieve precise time transfer. This program was denoted as Timation. Modifications
were made to Timation satellites to provide a ranging capability for two-dimen-
sional position determination. Timation employed a sidetone modulation for
satellite-to-user ranging [3-5].
At the same time as the Transit enhancements were being considered and the
Timation efforts were underway, the Air Force conceptualized a satellite positioning
system denoted as System 621B. It was envisioned that System 621B satellites would
be in elliptical orbits at inclination angles of 0°, 30°, and 60°. Numerous variations
of the number of satellites (15-20) and their orbital configurations were examined.
The use of pseudorandom noise (PRN) modulation for ranging with digital signals
was proposed. System 621B was to provide three-dimensional coverage and contin-
uous worldwide service. The concept and operational techniques were verified at the
Yuma Proving Grounds using an inverted range in which pseudosatellites or
pseudolites (i.e., ground-based satellites) transmitted satellite signals for aircraft
positioning [3-6]. Furthermore, the Army at Ft. Monmouth, New Jersey, was inves-
tigating many candidate techniques, including ranging, angle determination, and the
use of Doppler measurements. From the results of the Army investigations, it was
recommended that ranging using PRN modulation be implemented [5].
In 1969, the Office of the Secretary of Defense (OSD) established the Defense
Navigation Satellite System (DNSS) program to consolidate the independent devel-
opment efforts of each military service to form a single joint-use system. The OSD
also established the Navigation Satellite Executive Steering Group, which was
 
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