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About GPS & GNSS

The Global Positioning System (GPS)

GPS is the US government-owned satellite navigation system that provides a highly accurate, continuous global navigation service independent of other positioning aids. With GPS, a user can receive information about his or her location, velocity, direction and time in any weather conditions, anywhere on the globe, free of charge. Other similar systems include Russia's GLONASS system, Europe's Galileo system, and China's COMPASS system.

GPS, the most widely used system in the world, uses the NAVSTAR (Navigation Satellite Timing and Ranging) satellite system, which consists of 24 active satellites, plus several working backup satellites, totaling, as of 2007, 30 operational satellites. NAVSTAR updates the GPS constellation and individual satellite status every working day.

The GPS System Design

The GPS System consists of three parts:

  • The space segment
  • The control segment
  • The user segment

These three segments operate together to provide users worldwide with accurate three-dimensional data about positioning, velocity, and time.

The Space Segment

The NAVSTAR GPS satellites make up the "space" segment of the GPS system. This constellation consists of 24 satellites in six orbital planes; there are four satellites in each plane (with room for spares). The 12-hour orbit period and altitude of each satellite ensures that GPS receivers have at least six satellites in view from any point on Earth, at any time. Receivers require signals from a minimum of four satellites at a time to compute their current latitude, longitude, altitude and GPS system time.

The GPS satellite signal identifies the satellite and provides users with the positioning, timing, ranging data, satellite status and corrected orbit parameters of the satellite. The satellites can be identified either by the Space Vehicle Number (SVN) or the Pseudorandom Code Number (PRN).

GPS satellites transmit on several L-band frequencies: L1, L2 and L5. L1, which is modulated by the Coarse Acquisition (C/A) code and the Precision (P) code, which is encrypted for military and other authorized users. The L2 carrier is modulated with the P-code and L2C (civilian) code. The L5 signal is proposed as a "safety of life" signal; it falls into a protected range for aeronautical navigation, promising little or no signal interference.

The Control Segment

A master control station, five base stations and three data uploading stations in locations all around the globe make up the "control" segment of the GPS system. The base stations track and monitor the satellites via their broadcast signals, which contain information about the orbit, ranging signals, and clock and almanac data for each satellite. These signals are passed to the master control station, where the orbit parameters (or ephemeredes) are recomputed. The resulting corrections for time and orbit parameters are then transmitted to the satellites via the data uploading stations.

The User Segment

The "user" segment of the system consists of the equipment which tracks and receives the satellite signals. To ensure accurate position, velocity and timing measurements, GPS receivers must be able to process the signals from a minimum of four satellites, simultaneously.

Global Navigation Satellite System (GNSS)

GNSS is a generic term to encompass all global positioning systems. Currently, GNSS includes North America's GPS system and Russia's GLONASS. Eventually, GNSS will also include the EU's Galileo and China's COMPASS.

GNSS systems have a variety of applications for the military, avionic, agricultural and vehicular fields, including:

  • Machine control
  • Surveying
  • Geographic information systems (GIS)
  • Port automation
  • Timing