DGPS - Differential GPS

An improvement over the Global Positioning System (GPS), Differential GPS (DGPS) indicates the difference between the actual values of distance and those provided by the satellites. The infrastructure of the Differential GPS is set up in the form of reference stations based on earth. A 'pseudorange' is the approximate measurement provided by the satellites. The difference between the 'pseudoranges' is broadcast from the reference stations to the GPS receiver stations.

GPS in Brief
A Global Positioning System (GPS) works with the help of three components; the satellites, control system and the users who navigate with its help. The satellites installed with high-precision atomic clocks revolve around the earth at a height of 20,200 km every 12 hours. The control system, which includes a 'Master Control Station' and 'ground antennas', work in tandem with the satellites. The antennas which are a part of the control system, track the satellites and forwards the information to the 'Master Control Station; which in turn checks the changes in the time and positioning of the satellites. This information is transmitted back to the satellites via antennas. The cycle of transfer of information, explained above, ensures that the satellite is delivering accurate information.

Working of DGPS
The reference station established on ground receives signals from the satellites, after which it calculates the difference in the positioning of its own location. The reference station provides the users with the necessary corrections in the distance measured by the GPS system. These corrections are transmitted by means of ultra high frequency waves (UHF). Only those users, within the range of 370 km of the reference stations can benefit from the service. However, even the DGPS can generate errors resulting from the distortions produced in the troposphere and ionosphere. The ephemeris errors may also lead to the users receiving incorrect information. Thus, the information provided by the DGPS loses accuracy as we move away from the reference station. The errors in the DGPS may range from 0.22 to 0.67 km per 100 km.

Post Processing
The DGPS system finds out the exact locations of unknown points by using reference points, known as 'survey markers'. This technique is referred to as Post-Processing. Let us find more about the DGPS used in different nations.

DGPS in Different Countries
The 'United States Coast Guard' provides a DGPS system in the USA and Canada. The radio frequencies known as 'longwave' are used in the system. The radio frequencies used for the system range between 285 kHz and 325 kHz. Australia will also be using the DGPS for air navigation in the near future. It will replace the 'Instrument Landing System' used earlier.

• United States: The coast guard agency which looks after the DGPS in the USA, was appointed after a joint decision taken by the Federal Highway administration, National Geodetic Survey, United States Department of Transportation and the Federal Railroad Administration. The Nationwide DGPS also known as NDGPS, covers the whole area of the USA.
• Europe: The DGPS system used in Europe is known as the European DGPS Network; the maritime administrations of Sweden and Finland created the system. It was started in order to keep a check on the security in the islands located between these countries. The European DGPS Network was introduced in Ireland and the United Kingdom in 1998, and was established by the General Lighthouse Authorities of the respective nations. The system became fully operational in 2002 and was operated on the 300 kHz band.

The DGPS system has helped us overcome the errors in the GPS system. It proves beneficial in various operations of the defense forces as well as for civilians. The concept of GPS seems to be analogous with the Internet technology, where locating information and data is made easier for the users. With the improvements in technology, limitations and problems in the DGPS are being removed. Let's hope, the DGPS of tomorrow is more accurate and error free.