What Is DGPS?
Differential GPS is a way of improving the accuracy of GPS. Data from a receiver at a known stationary location (base or reference) is used to correct the data received from a receiver at an unknown location (rover). DGPS lets users with GPS and DGPS receivers know their location within 10 meters of accuracy and is usually within 3 meters or less, depending on the quality of the user's equipment and their distance from the DGPS stationary location. These corrections can be applied in real time or by post processing.
Why Have DGPS
Differential GPS makes the difference between finding a street and finding a particular house on that street.
Typical Sources of GPS Errors
- Ionosphere and Troposphere - As the satellite signal passes through the atmosphere it slows down.
- Signal Multi-Path - The satellite signal can be reflected off of objects such as tall buildings, mountains and other large rock surfaces. This causes the signal to increase its travel time.
- Receiver Clock Errors - The clock in a receiver is not an atomic clock as it is in the satellite and the built-in clock can generate small errors in timing.
- Ephemeris Errors - These are inaccuracies of the reported position of a satellite.
- Low Number of Visible Satellites - The fewer satellites signals the receiver receives will result in a less accurate location reading. Buildings, high terrain and trees are just some things that can block satellite signals.
- Bad Satellite Geometry - Bad satellite geometry exists when the satellites are located either in a line or are closely grouped together.
How DGPS Works
Differential GPS basically involves two GPS receivers. One of these receivers is stationary (base or reference station) and the other is roving and making position measurements. As the base station knows its location exactly, it can determine satellite signal errors. This is done by measuring the ranges to each satellite using the received signals which are compared to the actual ranges calculated from its known location. These differential corrections for each tracked satellite are transmitted to the roving GPS receiver and applied to its calculations. Transmissions for real-time use can be over FM radio frequencies, by satellite or by beacon transmitters that are maintained by the U.S. Coast Guard. Calculations can also be recorded for post-processing.
Real-Time Corrections - base station and rover communication
The base station transmits corrected data instantly to the rover.
Real-Time Corrections - W.A.A.S., satellite subscription service, radio beacon
The base station uploads the corrected data to privately-owned satellites and these satellites transmit the corrections to subscribing rovers. The Federal Aviation Administration Wide-Area Augmentation System (F.A.A. W.A.A.S.) provides service to the aviation community and OmniStar is an example of a satellite based commercial GPS correction service.
The U.S. Coast Guard maintains a network of differential monitors and transmits corrections over radio beacons.
Base stations record corrections and then upload the files for storage onto large capacity servers. Public and private agencies distribute this information electronically.