| The Global Positioning System (GPS) is a U.S. | | | | contain information about the satellite. Using GPS |
| space-based radio navigation system that provides | | | | ground-based receivers, these signals can be |
| reliable positioning, navigation, and timing services to | | | | detected and used to determine the receivers' |
| civilian users on a continuous worldwide basis -- freely | | | | positions (latitude, longitude, height.) The radio signals |
| available to all. | | | | are sent at two different L-band frequencies. L-band |
| For anyone with a GPS receiver, the system will | | | | refers to a range of frequencies between 390 and |
| provide location and time. GPS provides accurate | | | | 1550 MHz. Within each signal, a coded sequence is |
| location and time information for an unlimited number | | | | sent. By comparing the received sequence with the |
| of people in all weather, day and night, anywhere in | | | | original sequence, scientists can determine how long it |
| the world. | | | | takes for the signal to reach the Earth from the |
| The GPS is made up of three parts: satellites orbiting | | | | satellite. |
| the Earth; control and monitoring stations on Earth; | | | | The signal delay is useful in learning about the |
| and the GPS receivers owned by users. GPS satellites | | | | Ionosphere and the Troposphere, two atmospheric |
| broadcast signals from space that are picked up and | | | | layers that surround Earth's surface. A third signal is |
| identified by GPS receivers. Each GPS receiver then | | | | also sent to the receivers from the satellite. This |
| provides three-dimensional location (latitude, longitude, | | | | signal contains data about the health and position of |
| and altitude) plus the time. | | | | the satellite. |
| Individuals may purchase GPS handsets that are | | | | The second part of the GPS system is the ground |
| readily available through commercial retailers. Equipped | | | | station, comprised of a receiver and antenna, as well |
| with these GPS receivers, users can accurately locate | | | | as communication tools to transmit data to the data |
| where they are and easily navigate to where they | | | | center. The omni-directional antenna at each site, |
| want to go, whether walking, driving, flying, or | | | | acting much like a car radio antenna, picks up the |
| boating. GPS has become a mainstay of | | | | satellite signals and transmits them to the site |
| transportation systems worldwide, providing | | | | receiver as electric currents. The receiver then |
| navigation for aviation, ground, and maritime | | | | separates the signals into different channels |
| operations. | | | | designated for a particular satellite and frequency at |
| Disaster relief and emergency services depend upon | | | | a particular time. Once the signals have been isolated, |
| GPS for location and timing capabilities in their | | | | the receiver can decode them and split them into |
| life-saving missions. Everyday activities such as | | | | individual frequencies. |
| banking, mobile phone operations, and even the | | | | With this information the receiver produces a general |
| control of power grids, are facilitated by the accurate | | | | position (latitude, longitude, and height) for the |
| timing provided by GPS. Farmers, surveyors, | | | | antenna. Later, the data collected by the receiver can |
| geologists and countless others perform their work | | | | be processed again by scientists to determine |
| more efficiently, safely, economically, and accurately | | | | different things, including another set of position |
| using the free and open GPS signals. | | | | coordinates for the same antenna,this time with |
| Three distinct parts make up the Global Positioning | | | | millimeter accuracy. |
| System. The first segment of the system consists | | | | The third part of the system is the data center. The |
| of 24 satellites, orbiting 20,000 km above the Earth in | | | | role of the data center is two fold. It both monitors |
| 12-hour circular orbits. This means that it takes each | | | | and controls the global GPS stations, and it uses |
| satellite 12 hours to make a complete circle around | | | | automated computer systems to retrieve and |
| the Earth. In order to make sure that they can be | | | | analyze data from the receivers at those stations. |
| detected from anywhere on the Earth's surface, the | | | | Once processed, the data, along with the original raw |
| satellites are divided into six groups of four. Each | | | | data, is made available to scientists around the world |
| group is assigned a different path to follow. This | | | | for use in a variety of applications. Since global GPS |
| creates six orbital planes which completely surround | | | | sites are constructed and monitored by different |
| the Earth. | | | | institutions all over the world, there are many |
| These satellites send radio signals to Earth that | | | | different data center locations. |