Differential GPS

Differential GPS
Name	Differential GPS
Abbreviation	DGPS, DGNSS
Type	Satellite navigation augmentation
Status	Operational
Coverage	Regional
Operator	Various (e.g., United States Coast Guard, General Lighthouse Authorities)

Differential GPS. Differential GPS (DGPS) is an enhancement to the standard Global Positioning System that improves location accuracy from the meter-level to the centimeter-level. It works by using a network of fixed, ground-based reference stations to calculate correction data for GPS signals and broadcast it to local receivers. This technique is foundational to many high-precision navigation and geomatics applications, forming the basis for more advanced satellite-based augmentation system networks.

Overview

The fundamental concept behind DGPS involves mitigating errors inherent in standard GPS positioning. These errors, caused by factors like ionospheric delay and satellite clock error, are correlated over a wide geographic area. By placing a receiver at a precisely surveyed location, known as a reference station, the system can compute the difference between the measured pseudorange and the true range. This correction data is then transmitted to rovers operating within the coverage area, allowing them to achieve significantly improved positional fixes. The development of DGPS was driven by the needs of maritime navigation, led by organizations like the United States Coast Guard, and has since become critical for surveying, agriculture, and aviation.

Principles of operation

DGPS operates on the principle of spatial correlation of errors. Errors affecting the GPS signal, such as those from the Earth's atmosphere or imperfections in the GPS satellite clocks, are almost identical for receivers separated by tens to hundreds of kilometers. A reference station, with its coordinates determined through geodetic survey, receives the same signals as a user's receiver. It calculates a correction for each visible satellite by comparing the measured pseudorange to the geometrically computed range. These corrections, which can be for the pseudorange itself (code-phase) or the carrier wave (carrier-phase), are formatted into a standard message, often following protocols from the Radio Technical Commission for Maritime Services, and broadcast to users.

System components

A functional DGPS system requires three primary components. The first is the reference station, a permanent installation at a known location, often managed by entities like the General Lighthouse Authorities in the United Kingdom or the Canadian Coast Guard. The second component is the data link, which transmits correction signals; common links include medium-frequency radio beacons, satellite communication systems like Inmarsat, or terrestrial radio data system networks. The final component is the user segment, comprising mobile receivers, such as those from manufacturers like Trimble Navigation or Leica Geosystems, capable of receiving and applying the transmitted corrections to their GPS or GLONASS measurements.

Applications

DGPS enables a vast array of precision applications across numerous industries. In maritime contexts, it is essential for harbor navigation, dredging operations, and hydrographic survey work conducted by agencies like the National Oceanic and Atmospheric Administration. Within agriculture, it facilitates precision farming techniques such as automated guidance system steering for tractors. The construction industry relies on it for machine control on projects like the Channel Tunnel or Hong Kong International Airport. Furthermore, it is indispensable for geodetic control network establishment, cadastral survey, and monitoring tectonic plate movements in regions like the San Andreas Fault.

Accuracy and limitations

The accuracy of a DGPS solution is typically in the range of 1-3 meters for code-based corrections and can reach centimeter-level with carrier-phase techniques like Real Time Kinematic. Primary limitations include the spatial decorrelation of errors; accuracy degrades as the distance between the user and the reference station increases, a factor described by the User Differential Range Error. System integrity is also a concern, as any error in the reference station's known position or a failure in the data link will directly impact all users. The coverage area is inherently regional, bounded by the range of the correction broadcast signal, unlike global satellite-based augmentation system such as the Wide Area Augmentation System.

Augmentation systems

DGPS is the foundational model for more sophisticated, wide-area augmentation systems. These systems, like the Wide Area Augmentation System in the United States, the European Geostationary Navigation Overlay Service, and the Multi-functional Satellite Augmentation System in Japan, use a network of reference stations to model errors across a continent and broadcast corrections via geostationary satellite. Other related systems include commercial services like OmniSTAR and Veripos, which provide global high-precision corrections via satellite communication. Continuously Operating Reference Station networks, such as those operated by the National Geodetic Survey, provide carrier-phase correction data for Real Time Kinematic surveying and scientific research.

Category:Navigation Category:Global Positioning System Category:Surveying