27-01-2012, 12:35 PM
Global Positioning System
By Moloud Heidary
What I want to talk about?
Definition of GPS.
Determining position.
Applications in military, navigation, surveying,… .
Definition:
The Global Positioning System, usually called GPS, is the only fully-functional satellite navigation system(allow small electronic devices to determine their location (Longitude, Latitude, and Altitude) in within a few meters using time signals transmitted along a line of sight by radio from satellites. Receivers on the ground with a fixed position can also be used to calculate the precise time as a reference for scientific experiments.)
Definition (cont…):
GPS has become a vital global utility, indispensable for modern navigation on land, sea, and air around the world, as well as an important tool for map-making and land surveying. GPS also provides an extremely precise time reference, required for telecommunications and some scientific research, including the study of earthquakes. GPS receivers can also gauge altitude and speed with a very high degree of accuracy.
NAVSTAR:
The United States Department of Defense developed the system, officially named NAVSTAR GPS (Navigation Signal Timing and Ranging Global Positioning System), and launched the first experimental satellite in 1978.
GPS as public good:
Although the cost of maintaining the system is approximately 400 million$ per year, including the replacement of aging satellites, GPS is available for free use in civilian applications as a public good. In late 2005, the first in a series of next-generation GPS satellites was added to the constellation, offering several new capabilities, including a second civilian GPS signal called L2C for enhanced accuracy and reliability.
This taxi in Kyoto equipped with GPS navigation, is an example of how GPS technology can be applied in routine activities.
Accuracy:
The GPS (Global Positioning System) is a "constellation" of at least 24 well-spaced satellites that orbit the Earth and make it possible for people with ground receivers to pinpointtheir geographic location. The location accuracy is anywhere from 1 to 100 meters depending on the type of equipment used . In coming years it reduce into 1cm with DGPS(. The DGPS eliminates selective availability (SA) and provides a much more accurate determination of time and position). The GPS is owned and operated by the U.S. Department of Defense, but is available for general use around the world.
How does it work:
GPS works like this:
A minimum of 24 GPS satellites are in orbit at 20,200 kilometers (12,600 miles) above the Earth. The satellites are spaced so that from any point on Earth, at least four satellites will be above the horizon.
Each satellite contains a simple computer, atomic clocks, and various radios. With an understanding of its own orbit and the clock, the satellite continually broadcasts its changing position and time. The satellites use their on-board atomic clocks to keep precise time, but are otherwise very simple and unsophisticated when compared to other modern spacecraft.
Determining Position
A GPS receiver "knows" the location of the satellites. By estimating how far away a satellite is, the receiver also "knows" it is located somewhere on the surface of an imaginary sphere centered at the satellite. It then determines the sizes of several spheres, one for each satellite. The receiver is located where these spheres intersect.
1- Satellite's position is determined relative to the Earth.
2- Location on Earth is located relative to the satellite.
3- THEN the Location's position on the Earth can be determined from the VECTOR sum of the other two measurements. All measurements must are done to such a precision that the location on the Earth is known to within 15 m.
METHODS:
The distance from the satellite is determined by the time it takes for a radio wave to reach the site from the satellite.
distance = (speed of light) x (time of flight)
This is very simple but there are a few difficulties:
The receiver clock is not exactly synchronized with the satellite clock so the time of flight will be imprecise.
The satellite and receiver are in different velocity reference frames and gravitational regimes .
The speed of light is 300,000 km/s in a vacuum. However, while traveling through the Earth Ionosphere and Troposphere, the radio waves travel at slightly slower speeds.
Method(cont…)
The location is a vector and must also include direction. In order to do this, distances from several satellites are required. This is called triangulation. We wish to find our latitude, longitude and height above the center of the Earth. These are three different numbers and would require distances to three different satellites
Receivers:
If the receiver is also equipped with a display screen that shows a map, the position can be shown on the map.
If a fourth satellite can be received, the receiver/computer can figure out the altitude as well as the geographic position.
If you are moving, your receiver may also be able to calculate your speed and direction of travel and give you estimated times of arrival to specified destinations.
Some specialized GPS receivers can also store data for use in Geographic Information Systems (GIS) and map making.
Receivers (cont…):
Many different types of receivers have become less and less expensive in recent years. They are now portable enough to put in your pocket. Internal active antenna are sensitive enough to pick up 8 satellites even under tree cover. An external antenna is still better and more flexible.
GPS 315
Affordable and easy to use, the GPS 315 is the ultimate outdoor guide. Nine easy-to-follow graphic navigation screens show bearing, heading, distance, speed and more on a high-resolution.
Applications:
GPS has different uses such as:
Military
Navigation
Mobile satellite communication
Location based services
Surveying
agriculture
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. Civilian GPS receivers are required to have limits on the velocities and altitudes at which they will report coordinates.
Navigation
GPS is used by people around the world as a navigation aid in cars, airplanes, and ships. Personal Navigation Devices(PND) such as hand-held GPS are 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 with PDAs, cell phones, car computers, or vehicle tracking systems.. The system can be used to automate harvesters, mine trucks, and other vehicles.
Mobile Satellite Communications :
Satellite communications systems permit "remotes” to communicate with "hubs" via satellites. A typical system uses satellites in geosynchronous orbit: this requires a directional antenna (usually a "dish") that is pointed at the satellite. When the "remote" is portable, as on a ship or a train, the antenna must be pointed based on its current location. Essentially all modern antenna controllers incorporat a GPS receiver to provide this location information.
Location-based services :
GPS functionality can be used by emergency services and location-based services to locate mobile phones. Assisted GPS is a GPS technology often used by the mobile phone because it reduces the power requirements of the mobile phone and increases the accuracy of the location obtained.
Location-based services :
GPS receivers come in a variety of formats, from devices integrated into cars, phones, and watches, to dedicated devices .The availability of hand-held GPS receivers for a cost of about US$90 and up has led to recreational applications including location-based games like the popular game Geocaching. Geocaching involves using a hand-held GPS unit to travel to a specific longitude and latitude to search for objects hidden by other geocachers.
Surveying:
More costly and precise receivers are used by land surveyors to locate boundaries, structures, and survey markers, and for road construction. There is also a growing demand for Machine Guidance such as Automatic Grade Control systems that use GPS positions plans to automatically control the blades and buckets of construction equipment.
Agriculture :
GPS Machine Guidance is used for tractors and other large agricultural machines via auto steer or a visual aid displayed on a screen, which is extremely useful for controlled traffic and row crop operations and when spraying. As well as guidance, GPS used in harvesters with yield monitors can provide a yield map of the paddock being harvested.
