A navigation satellite is an artificial satellite that forms part of a satellite navigation system and transmits precisely timed radio signals that allow receivers on or near Earth to determine their position, velocity and time. The satellite itself generally does not calculate the user's location; instead, it broadcasts information about its own orbital position and a highly accurate reference time, while a receiver measures when signals arrive from multiple satellites and uses the differences in arrival times to calculate its location.
This principle is known as satellite-based positioning. Global navigation satellite systems, commonly abbreviated GNSS, include the United States Global Positioning System, Russia's GLONASS, Europe's Galileo and China's BeiDou, while regional systems include Japan's QZSS and India's NavIC. The term navigation satellite can therefore refer either to an individual satellite in one of these constellations or, more broadly, to satellites designed to support positioning, navigation and timing services.
The fundamental technical principle is trilateration based on signal travel time. A navigation satellite carries an extremely stable clock and continuously broadcasts a signal containing timing information and orbital data. A receiver compares the transmitted time with the time at which the signal was received and estimates the distance to the satellite by multiplying the measured signal travel time by the speed of light.
Because a receiver's ordinary clock is not as accurate as the satellite clocks, the calculation normally uses signals from at least four satellites to solve simultaneously for three spatial coordinates and the receiver clock error. More satellites provide additional measurements that improve robustness and enable techniques for detecting inconsistent observations. The resulting position is initially expressed in a defined terrestrial reference frame and can then be converted into latitude, longitude and altitude for mapping or navigation applications.
Navigation satellites operate in carefully designed constellations rather than as isolated spacecraft. GPS, for example, uses medium Earth orbit satellites, while Galileo and GLONASS likewise use medium Earth orbit as their principal operational orbit. The satellites continuously move around Earth, so a receiver must account for orbital motion, Earth's rotation and the changing geometry between satellites and the user. The orbital information transmitted by a satellite allows receivers to predict where satellites are and select suitable signals.
Constellation design aims to ensure that users in large portions of the world can simultaneously observe enough satellites at useful elevations. Satellite operators monitor the spacecraft from ground infrastructure, determine orbital and clock parameters, upload navigation information and maintain the constellation by replacing ageing or failed spacecraft.
A navigation satellite normally carries equipment dedicated to generating and transmitting timing and ranging signals rather than taking conventional photographs of Earth's surface. Atomic clocks are particularly important because positioning accuracy depends directly on timing accuracy. Signals are transmitted on several radio-frequency bands, allowing advanced receivers to compare measurements between frequencies and reduce certain propagation errors. The spacecraft also requires antennas, power-generation systems, thermal control, attitude determination and control, computers, communications equipment and propulsion or orbit-maintenance capabilities. Modern navigation payloads can transmit multiple signal types intended for different civilian, commercial, governmental or authorised applications. Ground stations monitor signal performance and the health of the constellation, while sophisticated control systems maintain accurate satellite clocks and orbital information.
Atmospheric conditions are one of the principal sources of positioning error. As a navigation signal travels through the ionosphere and troposphere, its propagation is affected by the physical properties of the atmosphere. The ionosphere can introduce frequency-dependent delays, which can be reduced by using measurements on multiple frequencies and by applying models or correction services. The troposphere produces additional delays that are estimated using atmospheric models and receiver observations.
Other important errors include satellite clock errors, orbital prediction errors, multipath caused by reflected signals, receiver noise, antenna effects and poor satellite geometry. High-precision techniques can use carrier-phase measurements, correction data and continuously operating reference stations to achieve centimetre-level positioning in appropriate circumstances, far beyond the accuracy of an ordinary standalone consumer receiver.
Navigation satellites have applications far beyond turn-by-turn directions. They provide positioning and timing for aviation, maritime navigation, surveying, construction, agriculture, logistics, emergency response, telecommunications, financial systems, electrical grids and scientific research. Precision agriculture can use satellite positioning to guide machinery and support variable-rate operations. Surveyors can use carrier-phase observations and correction services to establish highly accurate coordinates. Telecommunications networks can use GNSS-derived timing to synchronise equipment, while power networks can use precise timing for monitoring and control. Phones and vehicles often combine GNSS information with inertial sensors, Wi-Fi positioning, cellular information, digital maps and other data, meaning the navigation satellite system is frequently one component of a larger positioning architecture rather than the entire navigation solution.
Navigation satellites are often confused with communication or Earth-observation satellites because all are artificial spacecraft in orbit, but their primary functions differ. A communications satellite primarily relays or processes communications traffic, whereas a navigation satellite broadcasts precisely timed ranging and navigation signals. An Earth-observation satellite primarily collects measurements or imagery of Earth's land, ocean or atmosphere.
Navigation systems are also sometimes described as GPS even when the technically correct term is GNSS, because GPS is only one GNSS operated by the United States. Another misconception is that a navigation satellite directly knows where a user is; normally the receiver performs the positioning calculation using signals from the constellation. Satellite navigation also does not require the satellites to remain stationary over a particular location, and most major GNSS satellites are deliberately placed in orbits that allow them to move across the sky.
The importance of navigation satellites lies in their ability to provide a common global reference for position and time without requiring the user to maintain a direct physical connection to a terrestrial navigation infrastructure. Their signals can be received passively, meaning a normal user receiver does not have to transmit a signal back to the satellite to determine its position. This characteristic supports enormous numbers of simultaneous users.
Nevertheless, satellite navigation is not universally available under all conditions: signals can be weakened or blocked indoors, underground, underwater or by physical obstructions, and deliberate or accidental interference can degrade performance. Spoofing can cause a receiver to calculate an incorrect position if it accepts counterfeit signals, while jamming can prevent reliable reception. Consequently, safety-critical systems increasingly use multiple navigation sources, integrity monitoring, augmentation systems and complementary sensors.
