Differential GPS (DGPS): How it Works and Its Benefits

Differential GPS (DGPS)

The DGPS (Differential GPS) is a system that provides GPS receivers with corrections to the data received from GPS satellites in order to provide greater precision in the calculated position. It was designed mainly due to the introduction of selective availability (SA). The rationale lies in the fact that the errors produced by the GPS system apply equally (or very similarly) to the receivers close together. The errors are strongly correlated in coming receivers.

A GPS receiver fixed on the ground (reference) that knows exactly where it is based on other techniques, gets the position given by GPS, and can calculate the errors caused by GPS compared with its known position. This receiver transmits the error correction to receivers 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.

DGPS Structure

In short, the DGPS structure is as follows:

• Monitored station (reference): Knows its position with very high precision. This station consists of:
  • A GPS receiver.
  • A microprocessor to calculate the GPS system errors and to generate the structure of the message sent to recipients.
  • A transmitter for establishing a unidirectional data binding to the receivers of end users.
• User equipment: Comprising a DGPS receiver (GPS receiver + data binding from the Monitored station).

Ways to Get DGPS Corrections

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 and 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.

Sources of Error Correction

With DGPS, errors can be corrected 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.