Global+Positioning+System

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What is Global Positioning System (GPS)
The term //GPS// (//Global Positioning System//), is commonly misused in order to identify a "//[|receiver]//" (//Image on right//). The device which relays, receives and displays the information of your position from approximately four satellites.

The Global Positioning System (GPS) is actually a worldwide radio-navigation system formed from a constellation of 27 [|satellites]. 24 of which are currently in use, while the other 3 are present incase of an emergency, or any others break down.

These satellites weigh between 3000 and 4000 pounds, and are [|solar powered]. They glide through space at an altitude of approximately 11,000 nautical miles (above the Earth's surface), and circulates around the globe at about 12,000 miles. Which allows the satellites to make a complete orbit around the earth twice a day. The orbit of each satellite, is arranged so the user has at least 4 satellites over-head, any where in the world, allowing the user to receive a relatively good signal. Five ground stations that make sure the satellites are working properly. GPS uses these satellites as reference points to calculate positions accurate to just a few meters. In fact, with advanced forms of GPS you can make measurements to better than a centimeter! Each satellite is equipped with an accurate clock to let it broadcast signals coupled with a precise time message. The ground unit receives the satellite signal, which travels at the speed of light. Even at this speed, the signal takes a measurable amount of time to reach the receiver. The difference between the time the signal is sent and the time it is received, multiplied by the speed of light, enables the receiver to calculate the distance to the satellite. Here's a graphic to illustrate the complete functionality.

**How GPS works?**

 * //Here's how GPS works in five steps://**
 * 1) The basis of GPS is "triangulation" from satellites.
 * 2) To "triangulate," a GPS receiver measures distance using the travel time of radio signals.
 * 3) To measure travel time, GPS needs very accurate timing.
 * 4) Along with distance, you need to know exactly where the satellites are in space.
 * 5) Finally you must correct for any delays the signal experiences as it travels through the atmosphere. You must also correct for the clock differences between the GPS receiver and the satellites.

A GPS receiver operates by measuring its distance from a group of satellites in space which are acting as precise reference points. The GPS system consists of 24 satellites, so there will always be more than four visible from anywhere on earth. The 24 satellites are at a height of 22,200 km located in six orbital planes. Each satellite earth in 12 hours. The satellites are constantly monitored by the department of Defense, which knows their exact location and speeds at every moment. This information is relayed back to the satellites. All the satellites have atomic clocks of unbelievable precision on board and are synchronized so that they are generating the same PN code at the same time. These satellites are constantly transmitting the PN code and the information about their location and time. A GPS receiver which is at the ground is also generating the same PN code, although not in synchronism with that of the satellites. This is because of the necessity to make GPS receivers inexpensive.



__Photo courtesy U.S. Department of Defense__ __Artist's concept of the GPS satellite constellation

The use of spread spectrum in GPS__ The use of spread spectrum in the GPS system accomplishes two things. Firstly, the signals from the satellites can be kept from unauthorized use. Secondly, and more importantly in a practical sense, the inherent processing gain of spread spectrum allows reasonable power levels to be used.

We now know that if we have the locations of the satellites and distances from us, we can accurately determine our position. But we need to know how the distance is computed. In fact there are a couple of ways. Here are some methods to compute distance.
 * How exactly is the distance computed?**
 * Direct measurement with a "ruler."
 * Inferred distances by measuring angles in triangles.
 * Distance measurement using the speed of light (light propagation time).

As mentioned above GPS methods are related to measuring light propagation time but not directly. How to measure distance by light? There are a couple of methods. First, distance can be measured directly by sending a pulse and measuring how it takes to travel between two points. This most common method is to reflect the signal and the time between when the pulse was transmitted and when the reflected signal returns. Such systems, called bi-directional, are used in radar and satellite laser ranging that require single millimeter accuracy. They require a clock capable of timing accuracy of seconds (3 picoseconds). The clock stability need is. A clock with this longtime stability would gain or lose 0.03 seconds in a year. Such equipment is expensive, costing for satellites about $1m. More on this later.

Navigation
GPS is used by people around the world as a navigation aid in cars, airplanes, and ships. The system can also be used by computer controlled harvesters, mine trucks and other vehicles. Hand-held GPS receivers can be used by mountain climbers and hikers. Glider pilots use the logged signal to verify their arrival at turn points in competitions. Low cost GPS receivers are often combined in a bundle with a PDA, car computer, or vehicle tracking system.

Precise Time Reference
Many synchronization systems use GPS as a source of accurate time, hence one of the most common applications of this use is that of GPS as a reference clock for time code generators or [|NTP clocks] (Network Time Protocol). For instance, when deploying sensors (for seismology or other monitoring application), GPS may be used to provide each recording apparatus with some precise time source, so that the time of events may be recorded accurately.

The atomic clocks on the satellites are set to "GPS time", which is the number of seconds since 00:00:00 UTC, January 6, 1980. Today, GPS time is 14 seconds ahead of [|UTC] (Coordinated Universal Time), because it does not follow leap seconds. Receivers thus apply a clock-correction offset (which is periodically transmitted along with the other data) in order to display UTC correctly, and optionally adjust for a local time zone. New GPS units will initially show the incorrect time after achieving a GPS lock for the first time. However, this is usually corrected on the display within 15 minutes once the UTC offset message is received for the first time.

Military
GPS allows accurate targeting of various military weapons including cruise missiles and precision-guided munitions, as well as improved command and control of forces through improved locational awareness. The satellites also carry nuclear detonation detectors, which form a major portion of the United States Nuclear Detonation Detection System. Due to the potential for GPS receiver technology to be used in improvised weaponry, the US Government has classified civilian receivers for controlled export. This means that, in general, a US-based manufacturer can not export a receiver unless it has limits on the velocities and altitudes at which it will report position and speed information.

Ships
Boats and ships are important users of GPS, which allows them to find their way anywhere on the world's seas and oceans, even in the absence of any other reference points. Maritime GPS units include functions specifically useful for use on water, such as “man overboard” (MOB) functions that allow instant recording and retrieval of the location at which a person has fallen overboard, which simplifies rescue efforts. GPS capabilities may be integrated into larger systems for navigational or other use aboard the boat or ship (e.g. Chartplotters). GPS also allows to increase the security of shipping traffic by enabling AIS. Further applications of GPS is tracking vessels on the globe. Usually, GPS will be integrated using the NMEA 0183 interface.

Bycicle
GPS is used by Cyclists for racing and touring. Cyclists often prefer to use quieter narrower streets. GPS navigation allows cyclists to plot their course in advance and follow this course, without having to stop frequently to refer to separate maps. Some GPS receivers are specifically adapted for cycling with special mounts and housings.

On Foot
Hikers, climbers, and even ordinary pedestrians in urban or rural environments can use GPS to determine their position, with or without reference to separate maps. In isolated areas, the ability of GPS to provide a precise position can greatly enhance the chances of rescue when climbers or hikers are disabled or lost (if they have a means of communication with rescue workers). Many models of inexpensive handheld GPS receivers are available for these uses, and they are one type of personal navigation device used for these applications.

Surveying
More costly and precise receivers are used by land surveyors to locate boundaries, structures, and survey markers, and for road construction.

GPS for the visually impared
For information about navigation systems for the visually impaired, including MoBIC, Drishti, Brunel Navigation System for the Blind, NOPPA, BrailleNote GPS, and Trekker, refer to the main article GPS for the visually impaired.

GPS on airplanes
Most airlines allow private use of ordinary GPS units on their flights, except during landing and take-off, like all other electronic devices. Additionally, some airline companies disallow use of hand-held receivers for security reasons, such as unwillingness to let ordinary passengers track the flight route. On the other extreme, some airlines integrate GPS tracking of the aircraft into their aircraft's seat-back television entertainment systems, available even during takeoff and landing to all passengers.

**How GPS is used along with problems in the real world!**
Global Positioning system (GPS) technology, satellites, and computer chips make it possible to track our movements and determine a person's current location. If, when you lock your keys in your car, the car company can remotely unlock it for you with a radio signal, the car company can determine your location. The cell phones and other wireless appliances many people now carry allow our location to be determined. Devices installed in rental cars, to locate them if they are stolen, can also be used to monitor or track drivers. (A rental car agency fined a man for speeding, based on information from a tracking device in the car.) A company sells wireless watchband transmitters for Children, so parents can monitor them. The federal government ordered that all wireless and cell phones have tracking capabilities to locate a phone making a 911 call. This is obviously useful for emergencies, but once the tracking technology is there, what else can it be used for? Some worry that we will be pestered with advertising calls as we walk or drive past a store having a sale. Some worry about abuse by government, saying that the government's ability to track and locate everyone by accessing the wireless telephone provider's system, the rental car system, and so forth, is a threat to our freedom. Will there be options for owners to to turn off tracking features? It is essential that such questions be considered early in the development of new technologies and applications so that privacy conerns can influence both the technical design and the laws mandating particular features.

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 * What are the main components of Global Positioning System and how does it operate?
 * Investigate the use of GPS's for military, civilian, and non military government use.
 * What are the issues to do with countries having control of this tool?

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Geosynchronous Satellites
The three spare satellites are commonly identified as the [|Geosynchronous Satellites]. These three orbit the earth over a period of 24 hours, indicating that they are at "parking orbit". The transponders send signals and are picked up by the object, and calculations made by the internal system define the position of the object on the scale used to measure the position with respect to the earth as frame of reference.

- There is various usage; locating hidden objects like nuclear facilitates or hidden targets to be used by military. To locate and communicate with earth based satellite systems. - It helps in navigation of ships and planes and also space crafts. - In order to connect, and receive a proper connection, a minimum of six satellites are above any location on the planet. - In order to receive an accurate signal, three electronic fields, in the shape of spheres are generated in order to give an accurate pinpoint of one's location.

List of Sources on GPS (Bibliography)

 * [|www.howstuffwork.com/gps]- A general overview of how a GPS and reciever work, along with several interective, step by step explanations on the mathematical procedures which take place in space between the satellites


 * [|www.math.tamu.edu/~dallen/physics/gps/gps.htm]


 * [|www.mitrecaasd.org/proj/satnav] - Brief information of what GPS is, how it is used in Aviation, and it's future!


 * [|www.speedace.ifno/gps_global_positioning_system.htm] - An in-depth look into the history of GPS, and how it's being used today!


 * http://en.wikipedia.org/wiki/GPS - A wide variety of information regarding the history, the use, and the technological development of the GPS