Differential GPS

October 1998

ON THE ROAD TO PRECISION FARMING – Differential Global Positioning

The Global Positional System (GPS) is a U.S. space-based technology which provides free, worldwide, full-time electronic positioning and navigation services.

There are a number of errors inherent in the GPS system. The position error for any calculated location is generally quoted as +/-40 meters (+/- 130 feet). To complicate the accuracy picture, the Department of Defense introduces intentional random errors in the satellite signal (called Selective Availability) which increase the general error to +/- 100 meters (330 feet).

To compensate for these errors, the industry has developed a concept called Differential Global Positioning that is intended to help correct these errors.

Differential GPS uses one or more base stations to calculate and provide corrections to the inherent errors of the satellite system. Each base station is at a precisely known location, and has a high-accuracy GPS receiver. After each position solution, the base station calculates a correction for the three numbers defining location (latitude, longitude and elevation).

Real-Time DGPS

For real-time GPS, these correction factors are transmitted in a way that can be received by the user’s GPS receiver and are applied to the position solution. The correction procedure is as follows:

	You take a position fix in the field with the GPS receiver (50-100 meters accuracy)
	At the precise same instant, a GPS base station also calculates it’s position.
	The base station calculates a correction factor (3) by comparing the known base station position (3A) with the base station GPS calculated position (3B).
	The computer looks at the uncorrected receiver position (50-100 meter accuracy) and then...
	Applies the correction factor that was determined in Step 3 which gives ...
	A final, differentially corrected position for the field data (1-5 meter accuracy).

Post Processing DGPS

The corrections from the base station can (and often are) stored. This data is labeled with the time and can be used to correct a set of GPS data at a later time. This process is called post-processing and is useful for data from a GPS received that does not have a DGPS receiver.

Real-Time DPGS in Michigan

The US Coast Guard provides marine radio beacons on many waterways that serve as base stations. These beacons provide an accuracy of +/- 3.0 m if the user is within 150 km.

Fee-for-Service DGPS comes in two flavors:

	broadcast on a subcarrier of commercial FM stations; limited coverage area (20 - 30 mi) with accuracies of 10m, 3m, or 1m; providers include Accqpoint and Differential Corrections, Inc.
	satellite-based wide-area DGPS (used by GreenStar); accuracy is sub-meter in latitude and longitude, and 2.5 m in altitude; providers include Omnistar and SatLoc.

This page was last updated on 10/08/01.