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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.