Innovative Service Improves GPS Accuracy

The Global Positioning System (GPS), a worldwide radio-navigation system, is the tool of choice to collect spatial data for Geographic Information Systems (GIS). GIS data collected with GPS can support a variety of activities, including mapping and spatial analysis. A stand-alone GPS receiver provides accuracies of approximately 5-15 meters in relation to the global reference frame; however, GPS positioning is subject to several error sources, such as atmospheric effects, satellite orbits and satellite clocks. These factors cause errors in the range-to-satellite, which amount to errors in position. Differential GPS (DGPS) corrections can be used to reduce these errors. The MTO, in partnership with the Ministry of Natural Resources (MNR), other provinces, territories and the federal government, have developed a new differential correction service to produce DGPS corrections, thereby increasing GPS standalone accuracy to about 1-2 meters (for high quality GPS receivers).

The Canadian Differential Global Positioning System, or CDGPS, is the result of four years of development. The CDGPS radio receives a DGPS correction model via the MSAT-2 geostationary satellite, calculates corrections localized to the user, and outputs these corrections to a GPS receiver in standard format. This free service is available for the entire Canadian landmass, including the far north. Other free DGPS services, such as the marine beacon and Wide Area Augmentation System (WAAS), fall short in this respect.

Other commercial services involve yearly subscription plans in order to receive corrections. The radio has a one-time cost of about $1,500. This does not include the cost of GPS equipment. After purchase, the service is free of charge.

CDGPS is the only service that provides corrections yielding positions directly relating to the Canadian Spatial Reference System - North American Datum 1983 (NAD83). WAAS, for example, outputs positions on the World Geodetic System 1984, which is different from NAD83 by as much as a meter. CDGPS was declared operational in October 2003.

Shortly after, the Geometrics Office conducted several tests to determine the increase in accuracy possible using CDGPS corrections with various grades of GPS receivers under different conditions. The tests confirmed the expected accuracy of about 1.5 meters for high quality GPS receivers using CDGPS.

What this means for the MTO is that anyone can purchase a standard piece of equipment to augment a high quality GPS receiver, in order to achieve meter-level positioning for their GIS data or mapping products. The development of CDGPS technology represents a progressive movement to enhance existing GPS positioning.

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