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Satellite Positioning
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DGPS & GPS Satellite Positioning systems
The anachronisms DGPS and GPS stand for Differential Global Positioning System and Global Positioning System. These system are used in order to indicate a geophysical location of Latitude and Longitude in order to be able to find out with varying degrees of accuracy where you are on the face of the earth. These systems are extensively used for navigation purposes on Land, Sea for giving accurate location information. In recent years the technology has also been used in Industry and Agriculture for gathering information as to the location at any given times of such things as Loaders and Cranes in order to see where things are picked up from and where they are put down as well as giving locations for well drilling and exploration. In the agricultural field they are used for gathering information as to whereabouts of Headers (Combine Harvesters) whilst harvesting. This information is stored along with information from Yield Monitors so that a map can be made of the varying yield across a paddock. This information can the be used to construct a treatment plan for fertiliser or herbicides so that the application rate can automatically be adjusted whist the sprayer or spreader is on the move. The system can also be used for guidance of various types of machinery. Both systems are identical in their basic operation. Both use the same type of radio receivers and both receive the same and signals from the same orbiting satellites to triangulate the exact position operation. The GPS system is less accurate because many factors such as atmospherics and local terrain can affect the signal which can effectively mean that the receiver unit can give a position which is several tens of meters out. The American military who own the satellites also have a thing call Selective Availability which means that from time to time they purposely make the signal give the wrong locations (although this feature is now switched off the capability still remains). This is ok for most purposes but not if you require repeatable accuracy down to the sub metre level and in order to do this we require a Differential signal which is transmitted from a ground station up to a commercial satellite and then re transmitted to anyone who has an enabled DGPS receiver. This signal effectively tells the DGPS receiver to correct the other signals being received by a certain amount which improves the result to a repeatable sub metre location. The other noticeable difference is that the GPS unit uses free to air signals whilst the DGPS unit requires a Differential signal which is charged for (there are exceptions but we will not cover them here) |
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