GPS Definitions for Land Surveyors

GPS (Global Positioning System).—The NAVigation Satellite Timing and Ranging (NAVSTAR) GPS is a navigation and positioning system based on a constellation of 24 satellites, operated and maintained by the Department of Defense. Elements of GPS can be categorized as the space segment (i.e., the network of satellites), the user segment (receivers) and the control segment, which consists of a master control station, monitoring stations, and ground antennas. The control segment is responsible for monitoring satellites and data uploading and correction.

GPS, real-time differential (RTD GPS)—Differential GPS carried out with a radio transmitter at the base station. The radio transmissions contain range rate corrections as well as pseudorange corrections for specific epochs, which are sent to the roving receivers in near-real time.

GPS, real-time kinematic (RTK GPS)—A method similar to differential GPS in which errors are eliminated using carrier phase-shift measurements. Pseudorange errors are also computed by a receiver at a base station, and transmitted to roving receivers. When conducted with dual-frequency receivers and at least four satellites, RTKGPS can provide near-centimeter accuracy.

GPS controller unit—A device that is used to receive GPS data from the signals received by the antenna; provide quality indicators to the user; allow user definition of items such as epoch interval, elevation mask angle, point identifier, point descriptor, and height of instrument; and provide interface to items such as radios and modems which are used in conjunction with GPS.

GPS receiver—An electronic device designed to accept signal transmissions from GPS satellites for the determination of position. Single-frequency receivers receive only LI frequency. Dual-frequency receivers receive both LI and L2 frequencies. As a result, the effects of ionospheric errors are diminished, and results over long baselines are more reliable. However, dual-frequency receivers are considerably more expensive than single-frequency receivers.

GPS session—During a GPS control survey, the time during which static receivers are collecting and storing satellite data.

GPS Time—The time system to which all GPS monitoring stations and satellite clocks are referenced. On January 6, 1980, GPS Time was equal to UTC Time, but the two no longer coincide due to GPS Time not being adjusted for leap seconds. They are always an integer number of seconds apart from each other.

GPS Week—The number of weeks elapsed since the week of January 6. 1980. Each new week begins at Saturday/Sunday midnight.

Source: NSPS “Definitions of Surveying and Related Terms“, used with permission.

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