GLOBAL POSITIONING SYSTEM

GLOBAL POSIITIIONIING SYSTEM
The Global Positioning System (GPS) is a network of 24
Navstar1 satellites orbiting Earth at 11,000 miles up.
Established by the U.S. Defense Department for military
applications, access to GPS is now free to all users, including
those in other countries. The system’s positioning and timing
data are used for a variety of applications, including air, land,  and sea navigation; vehicle and vessel tracking; surveying and
mapping; and asset and natural resource management. With
military accuracy restrictions lifted in May 2000, the GPS can
now pinpoint the exact location of people as they move about
with their receivers powered on. This development has ushered
in a wave of new commercial applications for GPS.
GPS Components
The first GPS satellite was launched in 1978. The first 10
satellites were developmental satellites. From 1989 to 1993,
23 production satellites were launched. The launch of the
twenty-fourth satellite in 1994 completed the $13 billion
constellation. The satellites are positioned so that signals
from 6 of them can be received nearly 100 percent of the time
at any point on earth.
The GPS consists of satellites, receivers, and ground control
systems. The satellites transmit signals (1575.42 MHz)
that can be detected by GPS receivers on the ground. These
receivers can be portable or mounted in ships, planes, or cars
to provide exact position information, regardless of weather
conditions. They detect, decode, and process GPS satellite
signals to give the precise position of the user.
The GPS control or ground segment consists of five
unmanned monitor stations located in Hawaii, Kwajalein
in the Pacific Ocean, Diego Garcia in the Indian Ocean,
Ascension Island in the Atlantic Ocean, and Colorado Springs,
Colorado. There is also a master ground station at Falcon Air
Force Base in Colorado Springs, Colorado, and four large
ground antenna stations that broadcast signals to the satellites.
The stations also track and monitor the GPS satellites.
System Operation
With GPS, signals from several satellites are triangulated to
identify the exact position of the user. To triangulate, GPS
measures distance using the travel time of a radio message
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from the satellite to a ground receiver. To measure travel time,
GPS uses very accurate clocks in the satellites. Once the distance
to a satellite is known, knowledge of the satellite’s location
in space is used to complete the calculation. GPS receivers
on the ground have an “almanac” stored in their computer
memory that indicates where each satellite will be in the sky
at any given time. GPS receivers calculate for ionosphere and
atmosphere delays to further tune the position measurement.
To make sure both satellite and receiver are synchronized,
each satellite has four atomic clocks that keep time to within 3
nanoseconds, or 3 billionths of a second. For cost savings, the
clocks in the ground receivers are not that accurate. To compensate,
an extra satellite range measurement is taken.
Trigonometry says that if three perfect measurements locate a
point in three-dimensional space, then a fourth measurement
can eliminate any timing offset. This fourth measurement
compensates for the receiver’s imperfect synchronization.
The ground unit receives the satellite signals, which
travel at the speed of light. Even at this speed, the signals
take a measurable amount of time to reach the receiver. The
difference between when the signals are sent and the time
they are received, multiplied by the speed of light, enables
the receiver to calculate the distance to the satellite. To measure
precise latitude, longitude, and altitude, the receiver
measures the time it took for the signals from several satellites
to get to the receiver (Figure G-2).
GPS uses a system of coordinates called the Worldwide
Geodetic System 1984 (WGS-84). This is similar to the latitude
and longitude lines that are commonly seen on large wall maps
used in schools. The WGS-84 system provides a built-in, standardized
frame of reference, enabling receivers from any vendor
to provide exactly the same positioning information.
GPS Applications
The GPS system has amply proven itself in military applications,
most notably in Operation Desert Storm where U.S. reliable navigation system, sophisticated troop maneuvers
could not have been performed. This could have prolonged
the operation well beyond the 100 hours it actually took.
With GPS, troops were able to go places and maneuver in
sandstorms or at night when even the troops who were native
to the area could not. Initially, more than 1000 portable commercial
receivers were purchased for their use. The demand
was so great that before the end of the conflict, more than
9000 commercial receivers were in use in the Gulf region.
They were carried by ground troops and attached to vehicles,
helicopters, and aircraft instrument panels. GPS receivers
were used in several aircraft, including F-16 fighters, KC-135
tankers, and B-52s. Navy ships used GPS receivers for rendezvous,
minesweeping, and aircraft operations.
Navstar Satellites
GPS-equipped Vehicles
Figure G-2 Signals from four satellites, captured by a vehicle’s onboard
GPS receiver, are used to determine precise location information.
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While the GPS system was developed originally to meet
the needs of the military community, new ways to use its
capabilities are continually being found, from the exotic to
the mundane. Among the former is the use of GPS for
wildlife management. Endangered species such as Montana
elk and Mojave Desert tortoises have been fitted with tiny
GPS receivers to help determine population distribution patterns
and possible sources of disease. In Africa, GPS
receivers are used to monitor the migration patterns of large
herds for a variety of research purposes.
Handheld GPS receivers are now used routinely in field
applications that require precise information gathering,
including field surveying by utility companies, mapping by
oil and gas explorers, and resource planning by timber
companies.
GPS-equipped balloons are used to monitor holes in the
ozone layer over the polar ice caps. Air quality is being monitored
using GPS receivers. Buoys tracking major oil spills
transmit data using GPS. Archaeologists and explorers are
using the system to mark remote land and ocean sites until
they can return with proper equipment and funding.
Vehicle tracking is one of the fastest-growing GPS applications.
GPS-equipped fleet vehicles, public transportation
systems, delivery trucks, and courier services use receivers
to monitor their locations at all times.
GPS data are especially useful to consumers when they
are linked with digital mapping. Accordingly, some automobile
manufacturers are offering moving-map displays guided
by GPS receivers as an option on new vehicles. The displays
can even be removed and taken into a home to plan a trip.
Some GPS-equipped vehicles give directions to drivers on
display screens and through synthesized voice instructions.
These features enable drivers to get where they want to go
more rapidly and safely than has ever been possible before.
GPS receivers are also included in newer mobile phones, and
add-on receivers are available for hand-held computers,
such as the Palm III (Figure G-3). GPS is also helping save lives. Many police, fire, and emergency
medical service units are using GPS receivers to determine
the police car, fire truck, or ambulance nearest to an
emergency, enabling the quickest possible response in life-ordeath
situations.
When GPS data are used in conjunction with geographic
data collection systems, it is possible to instantaneously
Figure G-3 Palm III users can clip on Rand
McNally’s StreetFinder GPS receiver, turning
the unit into a portable navigational tool.
Customized maps from the StreetFinder software
can be downloaded to the Palm III, along
with address-to-address directions accessible via
the Internet. With trip information stored on the
Palm III, the GPS receiver enables the user to
track travel progress and manage itinerary
changes en route.
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arrive at submeter positions together with feature descriptions
to compile highly accurate geographic information systems
(GIS). When used by cities and towns, for example,
GPS can help in the management of the geographic assets
summarized in Table G-1.
Some government agencies, academic institutions, and
private companies are using GPS to determine the location
of a multitude of features, including point features such as
pollutant discharges and water supply wells, line features
such as roads and streams, and area features such as waste
lagoons and property boundaries. Before GPS, such features
had to be located with surveying equipment, aerial photographs,
or satellite imagery. With GPS, the precise location
of these and other features can be determined with a
hand-held GPS receiver.
GPS and Cellular
GPS technology is even being used in conjunction with cellular
technology to provide value-added services. With the
push of a button on a cellular telephone, automobile drivers
and operators of commercial vehicles in some areas
can talk to a service provider and simultaneously signal
their position, emergency status, or equipment failure
information to auto clubs, security services, or central dispatch
services. This is possible with Motorola’s Cellular Positioning and
Emergency Messaging Unit, for example, which offers
mobile security and tracking to those who drive automobiles
and/or operate fleets. The system is designed for sale to systems
integrators that configure consumer and commercial
systems that operate via cellular telephony. The Cellular
Positioning and Emergency Messaging Unit communicates
GPS-determined vehicle position and status, making it
suited for use in systems that support roadside assistance
providers, home security monitoring firms, cellular carriers,
rental car companies, commercial fleet operators, and auto
manufacturers seeking a competitive advantage.
As an option, the OnStar system is available for select
vehicles manufactured by General Motors, which uses a
GPS receiver in conjunction with analog cellular phone technology
to provide a variety of travel assistance services,
including emergency response. At the push of a button, a cellular
call is placed to an OnStar operator. Although digital
technology is more advanced, OnStar uses analog cellular
because it has the broadest geographic coverage in the
United States. Over 90 percent of the country is covered by
the analog system, whereas digital coverage is less than 30
percent. OnStar has worked to “clear” the OnStar emergency
button call through all analog cellular phone companies
so that it will go through no matter which carrier is used
locally. GPS comes into play by providing the OnStar operator
with the precise location of the vehicle.
Summary
Because of its accuracy, GPS is rapidly becoming the location
data-collection method of choice for a variety of commercial,
government, and military applications. GPS certainly has
become an important and cost-effective method for locating
terrestrial features too numerous or too dynamic to be
mapped by traditional methods. Although originally funded
by the U.S. Defense Department, access to the GPS network
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is free to all users in any country. This has encouraged applications
development and created an entirely new consumer
market, particularly in the area of vehicular location and
highway navigation.