June 26, 2009

Global Positioning System

The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense. GPS was originally intended for military applications, but in the 1980s, the government made the system available for civilian use. GPS works in any weather conditions, anywhere in the world, 24 hours a day. There are no subscription fees or setup charges to use GPS.

How it works

GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. GPS receivers take this information and use triangulation to calculate the user's exact location. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. The time difference tells the GPS receiver how far away the satellite is. Now, with distance measurements from a few more satellites, the receiver can determine the user's position and display it on the unit's electronic map.

A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user's 3D position (latitude, longitude and altitude). Once the user's position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more.

The GPS satellite system

The 24 satellites that make up the GPS space segment are orbiting the earth about 12,000 miles above us. They are constantly moving, making two complete orbits in less than 24 hours. These satellites are traveling at speeds of roughly 7,000 miles an hour.

GPS satellites are powered by solar energy. They have backup batteries onboard to keep them running in the event of a solar eclipse, when there's no solar power. Small rocket boosters on each satellite keep them flying in the correct path.

Here are some other interesting facts about the GPS satellites:

* The first GPS satellite was launched in 1978.
* A full constellation of 24 satellites was achieved in 1994.
* Each satellite is built to last about 10 years. Replacements are constantly being built and launched into orbit.
* A GPS satellite weighs approximately 2,000 pounds and is about 17 feet across with the solar panels extended.
* Transmitter power is only 50 watts or less.

What's the signal?

GPS satellites transmit two low power radio signals, designated L1 and L2. Civilian GPS uses the L1 frequency of 1575.42 MHz in the UHF band. The signals travel by line of sight, meaning they will pass through clouds, glass and plastic but will not go through most solid objects such as buildings and mountains.

Sources of GPS signal errors

Factors that can degrade the GPS signal and thus affect accuracy include the following:

* Ionosphere and troposphere delays — The satellite signal slows as it passes through the atmosphere.
* Signal multipath — This occurs when the GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver.
* Receiver clock errors — A receiver's built-in clock is not as accurate as the atomic clocks onboard the GPS satellites.
* Orbital errors — Also known as ephemeris errors, these are inaccuracies of the satellite's reported location.
* Number of satellites visible — The more satellites a GPS receiver can "see," the better the accuracy. GPS units typically will not work indoors, underwater or underground.
* Satellite geometry/shading — Ideal satellite geometry exists when the satellites are located at wide angles relative to each other. Poor geometry results when the satellites are located in a line or in a tight grouping.


The Global Positioning System, while originally a military project is considered a dual-use technology, meaning it has significant applications for both the military and the civilian industry.


The military applications of GPS span many purposes:

* Navigation: GPS allows soldiers to find objectives in the dark or in unfamiliar territory, and to coordinate the movement of troops and supplies.
* Target tracking: Various military weapons systems use GPS to track potential ground and air targets before they are flagged as hostile Military aircraft, particularly those used in air-to-ground roles use GPS to find targets.
* Missile and projectile guidance: GPS allows accurate targeting of various military weapons including ICBMs, cruise missiles and precision-guided munitions.
* Search and Rescue: Downed pilots can be located faster if they have a GPS receiver.
* Reconnaissance and Map Creation: The military use GPS extensively to aid mapping and reconnaissance.


Many civilian applications benefit from GPS signals, using one or more of three basic components of the GPS: absolute location, relative movement, and time transfer.

The ability to determine the receiver's absolute location allows GPS receivers to perform as a surveying tool or as an aid to navigation. The capacity to determine relative movement enables a receiver to calculate local velocity and orientation, useful in vessels or observations of the Earth. Being able to synchronize clocks to exacting standards enables time transfer, which is critical in large communication and observation systems. An example is CDMA digital cellular. Finally, GPS enables researchers to explore the Earth environment including the atmosphere, ionosphere and gravity field. GPS survey equipment has revolutionized tectonics by directly measuring the motion of faults in earthquakes.

GPS tours are also an example of civilian use. The GPS is used to determine which content to display. For instance, when approaching a monument it would tell you about the monument.

GPS functionality has now started to move into mobile phones en masse. The first handsets with integrated GPS were launched already in the late 1990’s, and were available for broader consumer availability on networks.

Some best GPS-enabled mobile phones

Nokia 6210 Navigator
BlackBerry Bold 9000
Samsung Omnia
Apple iPhone 3G (16GB)
Nokia N96
Samsung i560


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