GPS Basic Information
Get to know the basic history and how it works the Global Positioning System. Also known as GPS.
GPS or Global Positioning System is a fully functional Global Navigation Satellite System. This system uses an artificial constellation of 24 medium Earth orbit satellites. These satellites transmit microwave signals, thus enabling a GPS receiver to determine its location, speed, direction and time. This system was developed by United States Department of Defense and was named as NAVSTAR GPS which was given by Mr. John Walsh. NAVSTAR is not an acronym, as is widely believed.
This satellite constellation is managed by United States Air Force 50th Space Wing. The cost is approximately 750 US dollars every year, including the maintenance cost, replacement, research and development. After shoot down of Korean Air Lines Flight 007 in the year 1983, a directive was issued which made the GPS available for civilian use and has been used extensively since then. It has become a very useful tool for making maps, surveying landscapes, commerce and many scientific uses. It also provides time reference which can be used in many applications which include study of earthquake and telecommunication network synchronization.
A GPS receiver simply calculates the distance between itself and three more GPS satellite. Each satellite has an atomic clock in it continually transmits certain data containing its exact time, location of the transmitting satellite and the almanac. The receiver then measures the reception time of the signal. Thus the distance to each satellite is known. Knowing three such distances, a trilateration is formed. By using a fourth satellite, need for a clock at receiver is avoided.
The Global Positioning System is used in a variety of Military and Civilian Applications. It allows soldiers find their objectives in a dark or completely unfamiliar territory and coordinate troop movement and supplies. GPS receivers which military personnel use are called Commanders and Soldier Digital Assistants. A combination of GPS and communication through radio enables real time vehicle tracking.
It is also used in marking targets as hostile and enables the precision guided munitions to allow them engage these targets with high accuracy. Air to Ground roles of military aircrafts use GPS to find targets. GPS also allows targeting for military weapons like ICBMs, Cruise missiles, precision guided missile. Artillery based projectiles are embedded with GPS receivers and can withstand forces up to 12,000G. These are used for 155 mm Howitzers. Any Downed pilot can be easily located if he has GPS receiver. It is widely used by military for reconnaissance and mapping. Some GPS satellites also have nuclear detonation detectors.
GPS helps civilians a lot in surveying and navigation. Its ability to calculate local speed and orientation is extremely useful. Time transfer is possible because of its capability to synchronize clock. A widely used example of use of GPS is CDMA digital cell phone. Each base uses a GPS timing receiver to synchronize the codes with different base stations and thus making it easy inter-cellular hand off and thus support emergency phone calls and other many applications. GPS equipment has also revolutionized tectonics by measuring the fault motion during earthquakes.
The two GPS developers, Ivan Getting and Bradford Parkinson have received national academy of Engineering Charles Stark Draper prize during year 2003. Roger L Easton received National Medal for technology on February 13, 2006. Other similar tracking systems are Beidou, which has been developed by China and is proposed to be expanded into COMPASS; Galilieo, which is been developed by European Union along with many other countries like India and China; GLONASS, which is been developed by Russia is fully available in partnership with India; IRNSS is India`s proposed regional system and QZSS which has been proposed by Japan.
This satellite constellation is managed by United States Air Force 50th Space Wing. The cost is approximately 750 US dollars every year, including the maintenance cost, replacement, research and development. After shoot down of Korean Air Lines Flight 007 in the year 1983, a directive was issued which made the GPS available for civilian use and has been used extensively since then. It has become a very useful tool for making maps, surveying landscapes, commerce and many scientific uses. It also provides time reference which can be used in many applications which include study of earthquake and telecommunication network synchronization.
A GPS receiver simply calculates the distance between itself and three more GPS satellite. Each satellite has an atomic clock in it continually transmits certain data containing its exact time, location of the transmitting satellite and the almanac. The receiver then measures the reception time of the signal. Thus the distance to each satellite is known. Knowing three such distances, a trilateration is formed. By using a fourth satellite, need for a clock at receiver is avoided.
The Global Positioning System is used in a variety of Military and Civilian Applications. It allows soldiers find their objectives in a dark or completely unfamiliar territory and coordinate troop movement and supplies. GPS receivers which military personnel use are called Commanders and Soldier Digital Assistants. A combination of GPS and communication through radio enables real time vehicle tracking.
It is also used in marking targets as hostile and enables the precision guided munitions to allow them engage these targets with high accuracy. Air to Ground roles of military aircrafts use GPS to find targets. GPS also allows targeting for military weapons like ICBMs, Cruise missiles, precision guided missile. Artillery based projectiles are embedded with GPS receivers and can withstand forces up to 12,000G. These are used for 155 mm Howitzers. Any Downed pilot can be easily located if he has GPS receiver. It is widely used by military for reconnaissance and mapping. Some GPS satellites also have nuclear detonation detectors.
GPS helps civilians a lot in surveying and navigation. Its ability to calculate local speed and orientation is extremely useful. Time transfer is possible because of its capability to synchronize clock. A widely used example of use of GPS is CDMA digital cell phone. Each base uses a GPS timing receiver to synchronize the codes with different base stations and thus making it easy inter-cellular hand off and thus support emergency phone calls and other many applications. GPS equipment has also revolutionized tectonics by measuring the fault motion during earthquakes.
The two GPS developers, Ivan Getting and Bradford Parkinson have received national academy of Engineering Charles Stark Draper prize during year 2003. Roger L Easton received National Medal for technology on February 13, 2006. Other similar tracking systems are Beidou, which has been developed by China and is proposed to be expanded into COMPASS; Galilieo, which is been developed by European Union along with many other countries like India and China; GLONASS, which is been developed by Russia is fully available in partnership with India; IRNSS is India`s proposed regional system and QZSS which has been proposed by Japan.
AN INTRODUCTION TO DIFFERENTIAL GPS
CONTENTS
- Introduction to GPS
- Introduction to differential GPS (DGPS)
- DGPS applications
- Miscellaneous
- Various links scattered throughout this document
GPS
GPS stands for Global Positioning System and it allows users to determine their location on land, sea, and in the air around the Earth. It does this using satellites and receivers. There are currently 24 satellites in orbit operated by the US Department of Defense that provide worldwide coverage 24 hours a day, 7 days a week, in all weather.How the system works is by the satellites sending information to receivers. This information includes time, position, and satellite strength among other things. The receivers pick up this information and use it to determine the users location. Using the signals from at least 4 satellites, a receiver can determine latitude, longitude, and elevation. Some receivers can then convert the latitude and longitude into other coordinate system values.
The accuracy of GPS depends on several factors such as which receiver is being used, the surroundings it's being used in, and Selective Availability. Selective Availability is the Department of Defense deliberately interfering with the satellite signals to reduce positional accuracy to around 30m - 100m. With Selective Availability receivers are divided into two types: precise positioning systems (PPS) and standard positioning systems (SPS). PPS receivers are used by the military and are not affected by Selective Availability. Currently there are efforts under way to end the use of Selective Availability.
DGPS
Differential GPS uses position corrections to attain greater accuracy. It does this by the use of a reference station. The reference station (or base station) may be a ground based facility or a geosynchronous satellite, in either case it is a station whose position is a known point. When a station knows what it's precise location is it can compare that position with the signals from the GPS satellites and thus find the SA error. These corrections can then be immediately transmitted to mobile GPS receivers (real time DGPS), or the receiver positions can be corrected at a later time (post processing).The use of DGPS can greatly increase positional accuracy (in general, the better it is the more expensive it is). Some surveying systems can give subcentimeter readings. There are a lot of different differential providers that supply real time and post processing corrections, many are private companies. The availability of these services varies greatly depending on what part of the country you are in, but the US Coast Guard covers the US coastline and the number of private and governmental providers is increasing, so I imagine that someday the entire US will be covered.
DGPS APPLICATIONS
Differential GPS is currently being used for many things, and it is a growing technology. One of it's more popular applications is in air navigation. By using it a pilot can receive constant information about where the plane is in 3 dimensions. It is also becoming a hot topic in precision farming. (MORE) (MORE) Farmers can use DGPS to map out their crops, map crop yields, and control chemical applications and seeding. It is also proving to be useful in ground and hydrographic surveying. Another application is in weather forecasting, where atmospheric information can be gained from it's effects on the satellite signals. There has also been at least one experiment where it was used for beach morphology and monitoring.
DGPS can also be used for train control for such things as avoiding collisions and routing. There is even been research into using it to help the visually impaired in getting around in cities. There is also at least one project that is working on using DGPS for car navigation. In the sports world it is finding a place in balloon and boat racing. I think that the future of DGPS applications will only be limited by imagination and money, and I predict that it will eventually become an integral part of much of our technology.
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