GLOBAL POSITIONING SYSTEM
Most people have heard the acronym GPS. Whether you are a hiker, biker, camper, hunter or boater, GPS is becoming more and more popular by the day. Handheld GPS units have become readily available and are easier to use than ever. Serious backcountry campers and outdoor enthusiasts can now immerse themselves into the world of satellite-aided navigation, all at the press of a button.
But exactly what is GPS? The following information provides a detailed look into the Global Positioning System. After reading this page, you will have a new appreciation for just how GPS works and its importance in the world of terrestrial, marine and aerial navigation.
WHAT IS GPS?
GPS stands for Global Positioning System. The Global Positioning System is a satellite-based navigation architecture consisting of 24 networked satellites placed into orbit by the U.S. Department of Defense (DOD). GPS permits land, sea and airborne users to determine their three dimensional position, velocity and time, 24 hours a day in all weather, anywhere in the world.
Deployed in 1973, the system was invented for and used exclusively by, the United States Military until the 1980s when the DOD opened up GPS for private and recreational use.
HOW DOES IT WORK?
Three distinct parts make up the Global Positioning System:
1) SPACE: Orbiting Satellites and / or Space Vehicles
2) CONTROL: Command and Control Ground Stations
3) USER: End User GPS Receivers which Display GPS Data
1) SPACE - SATELLITES AND SPACE VEHICLES
The first segment of the system consists of 24 satellites, orbiting 10,900 nautical miles above the Earth in 12-hour circular orbits. This means that it takes each satellite 12 hours to make a complete circle around the Earth and thus successfully orbits the Earth twice per day. The orbits are tilted to the earth's equator by 55 degrees to ensure coverage of polar regions. Powered by solar cells, the satellites continuously orient themselves to point their solar panels toward the sun and their antenna toward the earth. In order to provide holistic, 24 hour coverage everywhere on the Earth's surface, the satellites are divided into six groups of four. Each group is assigned a unique path to follow and the result is six orbital planes which completely surround the Earth.
These satellites send radio signals to Earth that contain information about the satellite. Using GPS ground-based receivers, these signals can be detected and used to determine the receivers' positions (latitude, longitude, height.) The radio signals are sent at two different L-band frequencies. L-band refers to a range of frequencies between 390 and 1550 MHz. Within each signal, a coded sequence is sent. By comparing the received sequence with the original sequence, scientists can determine how long it takes for the signal to reach the Earth from the satellite. The signal delay is useful in learning about the ionosphere and the troposphere, two atmospheric layers that surround Earth's surface. A third signal is also sent to the receivers from the satellite. This signal contains data about the health and position of the satellite.
The satellites are composed 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 such as antennas. The exterior of the GPS satellite has a variety of antennas. The signals generated by the radio transmitter are sent to GPS receivers via the L-band antennas. Another component is the radio transmitter, which generates the signal. Each of the 24 satellites transmits its own unique code in the signal. Internal components such as atomic clocks and radio transmitters.
Each satellite contains four atomic clocks. These clocks are accurate to at least a billionth of a second or a nanosecond. An atomic clock inaccuracy of 1/100th of a second would translate into a measurement (or ranging) error of 1,860 miles to the GPS receiver.
2) CONTROL - COMMAND AND CONTROL GROUND STATIONS
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.
Satellite Monitoring 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 uplinked, 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.
3) GPS RECEIVERS WHICH DISPLAY DATA AND COORDINATES
The user segment includes the equipment of the military personnel and civilians who receive GPS signals. Military GPS user equipment has been integrated into fighters, bombers, tankers, helicopters, ships, submarines, tanks, jeeps, and soldiers' equipment. In addition to basic navigation activities, military applications of GPS include target designation, close air support, "smart" weapons, and rendezvous. With more than 500,000 GPS receivers, the civilian community has its own large and diverse user segment. Surveyors use GPS to save time over standard survey methods. GPS is used by aircraft and ships for enroute navigation and for airport or harbor approaches. GPS tracking systems are used to route and monitor delivery vans and emergency vehicles. In a method called precision farming, GPS is used to monitor and control the application of agricultural fertilizer and pesticides. GPS is available as an in-car navigation aid and is used by hikers and hunters. GPS is also used on the Space Shuttle. Because the GPS user does not need to communicate with the satellite, GPS can serve an unlimited number of users.
WIDE AREA AUGMENTATION SYSTEM (WAAS)
WAAS is an extremely accurate navigation system originally developed for civil aviation. Before WAAS, the U.S. National Airspace System (NAS) did not have the potential to provide horizontal and vertical navigation for approach operations for all users at all locations. With WAAS, this capability is a reality. In addition to providing the FAA and NAS with enhanced aviation navigation and precision flight landings, WAAS is available on some handheld GPS units, providing recreationists with even sharper terrestrial navigation capability.
Unlike traditional ground-based navigation aids, the WAAS covers nearly all of the National Airspace System (NAS). The WAAS provides augmentation information to GPS receivers to enhance the accuracy and reliability of position estimates. The signals from GPS satellites are received across the NAS at many widely-spaced Wide Area Reference Stations (WRS) sites. The WRS locations are precisely surveyed so that any errors in the received GPS signals can be detected. The GPS information collected by the WRS sites is forwarded to the WAAS Master Station (WMS) via a terrestrial communications network. At the WMS, the WAAS augmentation messages are generated. These messages contain information that allows GPS receivers to remove errors in the GPS signal, allowing for a significant increase in location accuracy and reliability.
The augmentation messages are sent from the WMS to uplink stations to be transmitted to navigation payloads on geostationary communications satellites.
The navigation payloads broadcast the augmentation messages on a GPS-like signal. The GPS/WAAS receiver processes the WAAS augmentation message as part of estimating position. The GPS-like signal from the navigation transponder can also be used by the receiver as an additional source for calculation of the user’s position. WAAS also provides indications to GPS/WAAS receivers of where the GPS system is unusable due to system errors or other effects. Further, the WAAS system was designed to the strictest of safety standards – users are notified within six seconds of any issuance of hazardously misleading information that would cause an error in the GPS position estimate.
