How the components of Seattle Public Utilities' systems interact.

GPS (Global Positioning System) refers to the navigation and positioning satellites operated by the United States. The advent of analogous systems from Russia (known as GLONASS), China (COMPASS), Japan (QZSS) and the GALILEO system proposed by the European Union, has given rise to the relatively new acronym “GNSS” (Global Navigation Satellite System).

GNSS includes all existing systems and augmentations and leaves room for more to be added in the future. Receivers that can track GPS as well as GLONASS satellites and eventually GALILEO satellites are designated as GNSS units. Receivers that track only the U.S. satellites are called GPS receivers. While GPS and GNSS receivers are quite different internally, the operation and results from the user's point of view are nearly identical.

When a receiver is permanently mounted for fiducial purposes (i.e., for the network) it is called a “Continuously Operating Reference Station” or CORS. A station comprises a receiver, antenna, permanent (or semi-permanent mount) together with power and communications facilities. A receiver/antenna that is mobile (i.e., on a folding tripod or pole) or on a semi-permanent mount in a temporary or campaign style is referred to as a “rover.” Tolt Dam's monitoring receiver and antenna combinations are permanent installations used for monitoring rather than fiducial reference purposes. By virtue of their continuous outputs they are known as “Continuously Operating Monitoring Stations” or COMS.

The term VRS (Virtual Reference Stations), or RTN (Real-Time Networks), is widely used to describe real-time correction networks; Trimble is the original provider of a VRS network solution and has a trademark on the term. VRS networks are comprised of a number of GPS or GNSS receivers installed in a region and linked to a central control center to create a real-time network for precise positioning.

Without a VRS network, surveyors, engineers, and contractors must use additional GNSS receivers as base stations and single-baseline differential techniques to obtain accurate positions. The VRS eliminates the cost and inconvenience of individual base stations while providing robust, reliable results.