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An Overview of the GPS System
The Global Positioning System is a space-based
radio-navigation system comprised of a swarm of satellites and a network of ground
stations used for monitoring and control. At least 24 GPS satellites orbit the earth at an altitude
of approximately 11,000 miles providing users with
accurate information on position, velocity, and time
anywhere in the world and in all weather conditions.
GPS is operated and maintained by the Department of
Defense. The National Space-Based Positioning,
Navigation, and Timing Executive Committee (PNT)
manages GPS, while the U.S. Coast Guard acts as the
civil interface to the public for GPS matters. The
Federal Aviation Administration (FAA) is
responsible for the use of GPS in civilian aviation.
History and Development
The
Global
Positioning System was initiated in
1973, primarily for the military, to increase
navigational capability and improve accuracy. By creating a
system that overcame the limitations of many
existing navigation systems, GPS became attractive
to a broad spectrum of users worldwide.
The Global Positioning System has been
successful in virtually all navigation applications,
and because
its capabilities are accessible using small,
inexpensive equipment, GPS is being utilized in a
wide variety of applications across
the globe.
The GPS system contains 24 satellites, plus spares.
These orbit the earth so that at least seven are
visible
at any time from virtually any point on the surface
of
the planet.
How It Works
Satellite Navigation is based on a global network of
satellites that transmit radio signals in medium
earth orbit. Users of Satellite Navigation are most
familiar with the 24 Global Positioning System (GPS)
satellites. The United States, who developed and
operates GPS, and Russia, who developed a similar
system known as GLONASS, have offered free use of
their respective systems to the international
community. The International Civil Aviation
Organization (ICAO), as well as other international
user groups, have accepted GPS and GLONASS as the
core for an international civil satellite navigation
capability known as the Global Navigation Satellite
System (GNSS).
The basic GPS service provides users with
approximately 7-meter accuracy, 95% of the time,
anywhere on or near the surface of the earth. To
accomplish this, each of the 24 satellites emits
signals to receivers that determine their location
by computing the difference between the time that a
signal is sent and the time it is received. GPS
satellites carry atomic clocks that provide
extremely accurate time. The time stamp is included
in the data transmitted by the satellite so that a
receiver can continuously determine the time that
the signal was broadcast.
The signal contains data that a receiver uses to
compute the locations of the satellites and to make
other adjustments needed for accurate positioning.
The receiver uses the time difference between the
time of signal reception and the broadcast time to
compute the distance, or range, from the receiver to
the satellite. The receiver must account for
propagation delays, or decreases in the signal's
speed caused by the ionosphere and the troposphere.
With information about the ranges to three
satellites and the location of the satellite when
the signal was sent, the receiver can compute its
own three-dimensional position. An atomic clock
synchronized to GPS is required in order to compute
ranges from these three signals. However, by taking
a measurement from a fourth satellite, the receiver
avoids the need for an atomic clock.
The Control Segment
The Control Segment of GPS consists of:
Master Control Station:
The master control station, located at Falcon Air
Force Base in Colorado Springs, Colorado, is
responsible for overall management of the remote
monitoring and transmission sites. GPS ephemeris
being a tabulation of computed positions, velocities
and derived right ascension and declination of GPS
satellites at specific times, replace "position"
with "ephemeris" because the Master Control Station
computes not only position but also velocity, right
ascension and declination parameters for eventual
upload to GPS satellites.
Monitor Stations:
Six monitor stations are located at Falcon Air Force
Base in Colorado, Cape Canaveral, Florida, Hawaii,
Ascension Island in the Atlantic Ocean, Diego Garcia
Atoll in the Indian Ocean, and Kwajalein Island in
the South Pacific Ocean. Each of the monitor
stations checks the exact altitude, position, speed,
and overall health of the orbiting satellites. The
control segment uses measurements collected by the
monitor stations to predict the behavior of each
satellite's orbit and clock. The prediction data is
up-linked, or transmitted, to the satellites for
transmission back to the users. The control segment
also ensures that the GPS satellite orbits and
clocks remain within acceptable limits. A station
can track up to 11 satellites at a time. This
"check-up" is performed twice a day, by each
station, as the satellites complete their journeys
around the earth. Noted variations, such as those
caused by the gravity of the moon, sun and the
pressure of solar radiation, are passed along to the
master control station.
Ground Antennas:
Ground antennas monitor and track the satellites
from horizon to horizon. They also transmit
correction information to individual satellites.
The Space Segment
The space segment includes the satellites and
booster rockets that launch them from Cape
Canaveral, Florida. The satellites fly in circular
orbits at an altitude of 10,900 nautical miles with
an orbital period of 12 hours. The orbits are tilted
to the earth's equator by 55 degrees to ensure
coverage of polar regions. The satellites are
powered by solar cells. They continuously orient
themselves so that their solar panels point toward
the sun and their antennae toward the earth. Each of
the 24 satellites, positioned in 6 orbital planes,
circles the earth twice a day.
The satellites consist of:
Solar Panels.
Each satellite is equipped with solar array panels.
These panels capture energy from the sun, which
provides power for the satellite throughout its
life.
External components
include the antennae ("antennas"). GPS satellites
sport a variety of antennas. The signals, generated
by onboard radio transmitters, are sent to GPS
receivers via the L-band antennas. Each of the 24
satellites includes its own unique code in the
signal.
Internal components
include atomic clocks and radio transmitters. Each
satellite contains four atomic clocks. These clocks
are accurate to at least one billionth of a second
(nanosecond). Because distances between the
satellites and the aircraft's receivers are based on
time, an atomic clock inaccuracy of 1/100th of a
second would translate into a measurement error of
1,860 miles.
The User Segment
The user segment includes the equipment of the users
who receive the GPS signals.
The aviation community uses GPS extensively.
Pilots, equipped with GPS receivers,
use satellites as precise reference points to trilaterate the aircraft's position anywhere on or
near the earth. GPS is already providing benefits to
aviation users, but relative to its potential, these
benefits are just the beginning. The foreseen
contributions of GPS to aviation promise to be
revolutionary. With air travel nearly doubled in the
21st Century, GPS can provide a cornerstone of the
future air traffic management (ATM) system that will
maintain high levels of safety, while reducing
delays and increasing airway capacity. To promote
this future ATM system, the FAA's objective is to
establish and maintain a satellite-based navigation
capability for all phases of flight. Garmin is the
leader in aviation GPS.
The User
Segment - Aviation
Satellite navigation is being widely used by
aviators throughout the world to overcome many of
the deficiencies in today's air traffic
infrastructure. With its accurate, continuous,
all-weather, three (GPS only) and four (GPS with
augmentations) dimensional coverage, satellite
navigation offers an initial navigation service that
satisfies many of the requirements of users
worldwide. Unlike current ground-based equipment
such as LORAN, satellite navigation permits accurate
aircraft position determination anywhere on or near
the surface of the earth.
More specifically, an aggressive exploitation of
satellite navigation technologies provides
substantial benefits to both the providers of such
services in the region, as well as the individual
and combined user communities. The implementation of
this technology in a country or region provides the
following benefits to aviation transportation:
·
Enhanced safety of flight throughout the region
·
Seamless navigation service based on a standardized
navigation service and common avionics
·
More efficient, optimized, flexible, and
user-preferred route structures
·
Increased system capacity
·
Reduced separation minimums resulting in increased
capacity and capabilities
·
Significant savings from shortened flight times and
reduced fuel consumption
·
Reduced costs to each individual State while
increasing overall benefits to individual States and
the entire region
·
Further economies from reduced maintenance and
operation of unnecessary ground-based systems
·
Improved ground and cockpit situational awareness
·
Increased landing capacity for aircraft and
helicopters
Additionally, the implementation of this technology
adds a margin of safety to operations within the
expected coverage area by providing four-dimensional
positioning, as opposed to the two-dimensional
positioning of traditional systems. This reduces
accidents by providing a consistent navigation
capability that does not change, regardless of
location, replacing major portions of current
ground-based navigation infrastructures, and
simplifying avionics suites. It also offers a
precision approach capability at any airport within
that region. All aircraft equipped with certified
GPS/WAAS receivers have the needed accuracy,
integrity, and availability for them to use GPS as a
primary navigation aid, and thus experience the
benefits of seamless travel.
Arrival
Aircraft arriving at the terminal area use set
instructions to lead them into the local area to
begin their landing approach. The current Standard
Terminal Arrival Routes (STARS) are based upon the
placement of navigation aids, aircraft performance,
and obstructions to flight. Through more accurate
and continuous position information, GPS will offer
more flexible routes, easing congestion, saving time
and fuel, especially at high-density airports.
Departure
Aircraft departing from the terminal area must
comply with set instructions that will lead them
safely to their enroute departure point. The current
standard instrument departures (SIDs) are based on
factors such as navigational aids available,
aircraft performance, and obstructions to flight.
Because of its accurate and continuous location
information, GPS will offer direct and flexible
departure routed, ease congestion, and save time and
fuel while maintaining high levels of safety.
Enroute
Control and navigation of aircraft over land must
rely on the use of ground hardware. Aircraft must
normally fly from point to point to navigate to
their destination. Flight paths are rarely direct.
With the advent of GPS, exact positional information
is available to pilots. This enables direct routes,
reduced flight times and reduced fuel consumption.
Landing
Landings based on GPS will eliminate many of the
time and fuel-consuming maneuvers currently in use.
Additionally, GPS can enable the addition of
vertical guidance to landing scenarios where this
capability did not formally exist. Vertical guidance
is a key component to increasing safety.
Surface
Surface traffic at airports is frequently busy.
Controlling and monitoring that traffic becomes
increasingly difficult as visibility decreases. The
FAA is examining ways to use GPS with other
technologies to help identify and locate surface
vehicles during all kinds of weather conditions.
That information could be used to help aviators and
controllers safely navigate in the surface
environment.
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