The DGPS or Differential GPS is a system that provides to GPS receivers to make corrections of the data received from GPS satellites, in order to provide greater accuracy in the calculated position. It was conceived mainly because of the introduction of selective availability (SA).
The rationale lies in the fact that the errors produced by the GPS system apply equally (or very similar) to the receptors that are close together.
A GPS receiver fixed on the ground (reference) knows exactly where it is based on other techniques and gets the position given by GPS. It can calculate the errors produced by the GPS system, comparing it with its errors. This receptor transmits error correction to receivers next to him, and so these can, in turn, also correct errors produced by the system within the coverage area transmitting signals of GPS equipment.
In short, the DGPS structure could be presented as follows:
* Station Monitored (Reference), who knows his position with very high precision. This station consists of:
- a GPS receiver
- a microprocessor which calculates the GPS system errors and generates the structure of the message sent to recipients
- transmitter-for establishing a unidirectional data link.
* User equipment, comprising a DGPS receiver (GPS receiver + data binding monitored from the station).
There are several ways to get DGPS corrections. The most commonly used are:
1. Received by radio, through a channel prepared for it, like the RDS on the FM band.
2. Downloaded from the Internet, or wireless connection.
3. Provided by a satellite system designed for that purpose. It exists in the U.S. as WAAS, in Europe as EGNOS and as MSAS in Japan.
Messages sent to recipients may include next two types of corrections:
* A correction applied directly to the position. This has the disadvantage that both the user and the instructor station shall use the same satellites, for corrections based on the same satellites.
* A pseudo-correction applied to each of the satellites visible. In this case the user can make the correction with the 4 satellites. This correction is more flexible.
The error produced by selective availability (SA) varies even faster than the speed of data transmission. Therefore, together with the message that is sent to correction, it also sends the time of validity of the corrections and trends. Therefore, the receiver must do some kind of interpolation to correct the mistakes.
If you wish to increase the coverage area of DGPS and to minimize the number of fixed reference receivers, you will need to get the spatial and temporal variations of the errors. In that case we would be talking about the wide area of differential GPS.
With DGPS you can correct errors in part due to:
* Selective Availability (eliminated from the year 2000).
* Spread the ionosphere - troposphere.
* Errors in satellite position (ephemeris).
* Errors caused by problems in the satellite clock.
For valid DGPS corrections, the receiver has to be relatively close to a DGPS station, generally, less than 1,000 km. The accuracy achieved may be about two meters in latitude and longitude, and about 3 m in altitude.
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