An improvement over Global Positioning Systems (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 'pseudoranges' is broadcast from the reference stations to the GPS receiver stations.
A Global Positioning System (GPS) works with the help of three components; satellites, control system and users who navigate with its help. 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 track the satellites and forward 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. This cycle of transfer of information ensures that the satellite delivers accurate information.
Working of DGPS
The reference station established on ground receives signals from the satellites, after which it calculates the difference in positioning of its own location. The reference station provides the users with 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 DGPS can have errors resulting from the distortions produced in the troposphere and ionosphere. Ephemeris errors may also lead to users receiving incorrect information. Thus, the information provided by DGPS loses accuracy as we move away from the reference station. Errors in DGPS may range from 0.22 to 0.67 km per 100 km.
DGPS finds out the exact locations of unknown points by using reference points, known as 'survey markers'. This technique is referred to as post-processing.
DGPS in Different Countries
The 'United States Coast Guard' provides DGPS in the USA and Canada. Radio frequencies that range between 285 kHz and 325 kHz, known as 'longwave', are used in this system. Australia will also be using DGPS for air navigation in the near future. It will replace the 'Instrument Landing System' which was used earlier.
- United States: The Coast Guard Agency, which looks after the DGPS in the US was appointed after a joint decision taken by 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 of USA.
- Europe: The DGPS used in Europe is known as the European DGPS Network; the maritime administrations of Sweden and Finland created this system. It was started in order to keep a check on security in islands located between these countries. The European DGPS Network was introduced in Ireland and United Kingdom in 1998; it was established by the General Lighthouse Authorities of the respective nations. The system became fully operational in 2002 and operated on the 300 kHz band.