DIFFERENTIAL GPS
The DGPS (Differential GPS) or Differential GPS is a system that provides GPS receivers
corrections of the data received from GPS satellites, in order to provide greater precision in the
calculated position. It was designed mainly due 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 close together. The errors are strongly correlated
receptors in coming.
A GPS receiver fixed on the ground (reference) who knows exactly where it is based on other
technical, get the position given by GPS, and can calculate the errors caused by GPS,
compared with theirs, known beforehand. This receptor transmits the error correction
receptors close to it, so these can, in turn, also correct errors produced by the system
within the coverage area transmitting signals of GPS equipment in question.
In short, the DGPS structure would be as follows:
Monitored station (reference), who knows his position with very high precision. This station
consists of:
or a GPS receiver.
or a microprocessor to calculate the GPS system errors and to generate the structure of
message sent to recipients.
or transmitter, for establishing a unidirectional data binding to the receptors of users
end.
User equipment, comprising a DGPS receiver (GPS receiver + data binding from the
Monitored station).
There are several ways to get DGPS corrections. The most commonly used are:
1. Received by radio, through a channel prepared for it, as the RDS on the FM band.
2. Downloaded from the Internet, or wireless connection.
3. Provided by a satellite system designed for that purpose. In the U.S. there is
WAAS, Europe EGNOS and MSAS in Japan, all compatible.
In 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 as the instructor station shall use the same satellites, for corrections based on
those same satellites.
A correction applied to the pseudo distances from each of the satellites visible. In this case the
user can

With DGPS can correct errors in part due to:
Selective Availability (eliminated from the year 2000).
Propagation by the ionosphere - troposphere.
Errors in satellite position (ephemeris).
Errors caused by problems in the satellite clock.