Overview

GPS users employ differential techniques to achieve the highest possible positional accuracy. A straightforward manner of doing this is to use two GPS receivers, a base and a rover, as discussed in Chapter 5. However, this may not be cost effective in many instances. An alternative, which could significantly reduce the survey cost without degrading the positional accuracy, is to use one of the available GPS data and correction services. If such a service is available within the project area, a GPS user would only need one receiver to be used as a rover; no base receiver is required. A number of various GPS services are currently available with various levels of accuracy and cost. Some services are even provided with no user fees required.

For high-accuracy static positioning, primarily for establishing new control points, GPS users may use one of the highly precise permanent GPS reference station networks established by several organizations around the world. These services are currently available free of charge at the global level [e.g., the International GPS Service for Geodynamics (IGS) network] as well as at the regional level [e.g., the Continuously Operating Reference Station (CORS) network in the United States] [1 3]. The Canadian Active Control System (CACS) is another regional GPS service, which is available to users at nominal fees [4]. The reference stations within these systems are operating on a continuous basis, and provide access to the modern reference frames, such as the ITRF and the improved NAD 83.

Some countries around...