ST2276
Scientech Technologies Pvt. Ltd.
5
Experiment 1
Objective :
Understanding the principle of GPS Technology
Theory :
The Global Positioning System (GPS) is and earth orbiting-satellite based navigation
system. GPS is an operational system, providing users worldwide with twenty-four
hour a day precise position in three dimensions and precise time traceable to global
time standards. GPS is operated by the United States Air Force under the direction of
the Department of Defense (DoD) and was designed for, and remains under the
control of, the United States military. While there are now many thousands of
commercial and recreational civil user’s worldwide, DoD control still impacts many
aspects of GPS planning, operation, and use. Primarily designed as a land, marine,
and aviation navigation system, GPS applications have expanded to include
surveying, space navigation, automatic vehicle monitoring, emergency services
dispatching, mapping, and geographic information system georeferencing. Because
the dissemination of precise time is an integral part of GPS, a large community of
precise time, time interval, and frequency standard users has come to depend on GPS
as a primary source of control traceable through the United States Naval Observatory
to global time and frequency standards.
History of GPS :
Developed in the 1960s, the Navy Transit satellite navigation system still provides
some service as a two-dimensional (horizontal) positioning system. Good (200 meter)
Transit positioning requires knowledge of the user altitude as well as a model of user
dynamics during the fix, a process of integrating satellite signal Doppler shifts (the
change in received signal frequency caused by the changing range) during the fly-over
of the satellite. Another Navy system, based on the Timation satellites carried stable
clocks (quartz, rubidium, and cesium) over the course of the program in the 1960s and
70s and was the precursor to the precise time capabilities of GPS (Easton 1978). GPS
began in 1973 as a test program using ground-based transmitters at the U. S. Army
Proving Ground at Yuma, Arizona, later augmented with early versions of GPS
satellites first launched in 1978. During the 1980s, GPS, although not yet fully
operational and requiring careful planning for missions during times of satellite
availability, was increasingly used by both military and civilian agencies. Land, air,
and sea navigation, precise positioning, carrier phase survey techniques, and precise
time and frequency dissemination were all accomplished to a limited extent during the
initial phases of GPS deployment (Klepczynski 1983). By 1989 ten development
satellites, termed Block I satellites, had been successfully launched. By 1990, 43
laboratories requiring precise time were using GPS to synchronize their atomic clocks
(Clements 1990). By 1994, 24 Block II and IIA operational GPS space vehicles
(SVs) had been launched. The Block IIA SVs can store up to 14 days of uploaded
data in case contact is lost with ground stations and can operate for 180 days with
degraded navigation receiver performance. The next generation of space vehicles, the
Block IIR SV s will incorporate changes to include the capability of maintaining
precise time keeping without Control Segment uploads for periods of up to 210 days
by exchanging data between GPS SV s (Rawicz, Epstein, and Rajan 1992).
HiK-Consulting