Differential GPS or DGPS

Posted on 22. Jul, 2011 by admin in Uncategorized

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 accuracy in the calculated position. It was conceived primarily because 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 located close together. The errors are strongly correlated in the next recipients.

A ground-based GPS receiver (reference) who knows exactly its position based on other techniques, get the position given by GPS, and can calculate the errors caused by GPS, comparing with it, known in advance. The receiver transmits error correction receivers next to him, and so these can, in turn, also correct the errors produced by the system within the coverage area of ​​transmission of signals from GPS reference computer.

In short, the structure would DGPS as follows:

• Monitored station (reference) , which knows its position with high accuracy. 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.
  • Transmitter to establish a data link to the receivers one-way end-users.
• User equipment , comprising a DGPS receiver (GPS + receiver link monitored data from the station).

There are several ways to get DGPS corrections. The most commonly used are:

• Received by radio , through a channel prepared for it, as the RDS on a radio FM .
• Downloaded from the Internet or a wireless connection .
• Provided by a satellite system designed for this purpose. In the U.S. there is WAAS in Europe, the EGNOS and Japan theMSAS , all compatible.

The messages that are sent to the recipients next can include two types of corrections:

• A correction applied directly to the position . This has the disadvantage that both the user and the monitoring station will use the same satellites since the correction is based on the same satellites.
• A pseudo-correction applied to each of the visible satellites . In this case the user can make the correction using the 4 satellites better signal to noise ratio (S / N). This correction is more flexible.

The error produced by selective availability (SA) varies even faster than the speed of data transmission. Therefore, with the message that is sent corrections, also sent the validity period of the corrections and trends. Therefore, the receiver must do some interpolation to correct the mistakes made.

If you wish to increase the coverage area DGPS corrections and at the same time minimizing the number of fixed reference receivers will need to model the spatial and temporal errors. In this case we are talking about wide area differential GPS.

With the DGPS can correct errors in part due to:

• Selective Availability (removed from the year 2000 ).
• Spread the ionosphere – troposphere .
• Errors in satellite position ( ephemeris ).
• Errors caused by problems in the satellite clock.

For DGPS corrections are valid, the receiver must be relatively close to a DGPS station, generally, less than 1000 km. The details which are handled centimeter differential receivers, so it can be used in engineering.