A-GPS (Assisted GPS)
A technique that downloads satellite orbital data over the internet or cellular network before a fix attempt, dramatically reducing the time it takes a device to lock onto satellites compared to a cold start.
Reference
GPS & GNSS Glossary
Plain-English definitions for the terms you'll encounter when reading about GPS, satellite navigation, and location tracking. Search below or jump to a letter.
A
Almanac
A coarse dataset broadcast by every GPS satellite that describes the approximate orbits of all satellites in the constellation. Receivers use it to know which satellites to look for before downloading the more precise ephemeris data.
B
BeiDou
China's national satellite navigation system, also called BDS. It became globally operational in 2020 and now provides a full GNSS constellation offering accuracy comparable to GPS, with some devices using both for better coverage.
BLE (Bluetooth Low Energy)
A short-range wireless standard optimized for low power consumption. BLE trackers such as Apple AirTags and Tile use nearby devices' Bluetooth radios as a crowdsourced finding network rather than dedicated satellites.
C
Cold Start
The process of acquiring a GPS fix when the receiver has no stored almanac, ephemeris, or last-known position. Cold starts are slowest because the device must download all satellite data from scratch, typically taking 30-90 seconds or more.
Constellation
The complete set of satellites in a given navigation system orbiting Earth. GPS has 24-32 satellites in its constellation; a receiver works by selecting the best-positioned satellites in view at any moment.
D
DGPS (Differential GPS)
A technique that uses known fixed reference stations to broadcast correction signals, canceling out errors caused by atmospheric delays and satellite clock drift. DGPS improves consumer GPS accuracy from several meters down to roughly 1-3 meters.
Dead Reckoning
A navigation method that estimates current position based on a previously known location plus speed, heading, and elapsed time. Many vehicle navigation systems combine GPS with dead reckoning to maintain a position estimate when satellite signals are blocked, such as in tunnels.
Doppler Shift
The change in frequency of a satellite signal caused by the relative motion between the satellite and the receiver. Receivers measure this shift to calculate velocity and to help track which satellites are in view.
E
Ephemeris
A precise dataset broadcast by each satellite describing its exact orbital position for the next few hours. Receivers use ephemeris data to calculate accurate satellite positions when computing a location fix.
F
Fix
A confirmed position calculated by a GPS or GNSS receiver. A 2D fix (latitude and longitude) requires at least three satellites; a 3D fix (latitude, longitude, and altitude) requires at least four.
G
Galileo
The European Union's civil GNSS constellation, operated by the EU Agency for the Space Programme (EUSPA). Galileo offers comparable accuracy to GPS and is fully interoperable with it, meaning modern receivers can use both systems simultaneously.
Geocoding
The process of converting a text address into geographic coordinates (latitude and longitude). Reverse geocoding does the opposite: it converts coordinates into a human-readable address. Both are handled by software APIs, not the GPS satellite system itself.
Geofencing
A software-defined virtual boundary around a real-world geographic area. When a tracked device crosses the boundary, the system can trigger an alert, a notification, or an automated action. Geofencing is one of the most common features of fleet and personal GPS trackers.
GLONASS
Russia's Global Navigation Satellite System, operated by the Russian Aerospace Defence Forces. Like GPS and Galileo, GLONASS provides global coverage, and many smartphone chipsets support it alongside GPS for faster and more reliable fixes.
GNSS (Global Navigation Satellite System)
The umbrella term for all satellite-based positioning systems, including GPS (USA), GLONASS (Russia), Galileo (EU), and BeiDou (China). A GNSS receiver can use signals from multiple constellations simultaneously to improve accuracy and availability.
GPS (Global Positioning System)
The US government's satellite navigation system, operated by the US Space Force. It consists of at least 24 satellites in medium Earth orbit and provides free positioning, navigation, and timing services to anyone with a compatible receiver worldwide.
H
Hot Start
A GPS fix attempt where the receiver already has a valid almanac, recent ephemeris, and a last-known position stored in memory. Hot starts are the fastest acquisition mode, often achieving a fix in under 5 seconds.
L
L1 / L2 / L5
The radio frequency bands used by GPS satellites: L1 (1575.42 MHz) is the primary civilian signal; L2 (1227.60 MHz) is used for ionospheric correction in professional receivers; L5 (1176.45 MHz) is a newer, higher-power signal with better urban performance, now supported by modern smartphones.
Latitude and Longitude
The coordinate system used to specify any location on Earth. Latitude measures how far north or south of the equator a point is (0 to 90 degrees); longitude measures how far east or west of the Prime Meridian (0 to 180 degrees). Together, they uniquely identify every point on the globe.
M
MEO (Medium Earth Orbit)
The orbital band at roughly 20,000 km altitude where GPS, GLONASS, Galileo, and BeiDou satellites operate. This altitude provides a good balance between orbital speed, signal strength, and global coverage.
Multipath
Signal error caused when GPS signals bounce off buildings, terrain, or other surfaces before reaching the receiver, creating multiple signal paths. Multipath is a major source of positioning error in urban environments and is one reason GPS is less accurate in cities than in open fields.
N
NMEA (National Marine Electronics Association)
A standard data format used by GPS receivers to output position, velocity, and time data as readable text sentences. NMEA 0183 sentences such as GPGGA and GPRMC are widely used by navigation software to parse raw GPS output.
O
OBD-II
The on-board diagnostics port found in most vehicles sold after 1996. Many GPS trackers plug directly into the OBD-II port for power and vehicle data, allowing them to report speed, fuel level, engine codes, and location without any wiring.
P
PNT (Positioning, Navigation, and Timing)
The three core services provided by GNSS constellations. Positioning gives location; navigation enables route planning; timing provides a precise reference clock used by everything from financial markets to power grids.
Pseudorange
The approximate distance between a GPS satellite and a receiver, calculated by measuring signal travel time. It is called a pseudo-range because it contains timing errors that must be corrected before an accurate position fix can be computed.
R
RFID (Radio-Frequency Identification)
A technology that uses radio waves to identify and track tags attached to objects. Unlike GPS, RFID tags are passive (no battery), very short-range, and do not provide continuous position data. RFID is common in inventory management; GPS is used for real-time outdoor tracking.
RTK (Real-Time Kinematic)
A GPS correction technique that uses a fixed reference station to broadcast carrier-phase correction data to a roving receiver in real time, achieving centimeter-level accuracy. RTK is used in precision agriculture, land surveying, and autonomous vehicle guidance.
S
SBAS (Satellite-Based Augmentation System)
A family of systems that broadcast GPS correction signals from geostationary satellites to improve accuracy and integrity. Examples include WAAS (North America), EGNOS (Europe), and MSAS (Japan). SBAS typically improves accuracy to about 1-3 meters.
Selective Availability
A deliberate accuracy-degrading feature the US government applied to civilian GPS signals from 1990 until May 2000, limiting accuracy to about 100 meters. Since it was switched off, civilian GPS accuracy has improved to a few meters under normal conditions.
T
Telematics
The combined use of telecommunications and informatics to send, receive, and store information relating to remote objects, most commonly vehicles. Fleet telematics systems combine GPS location data with engine diagnostics, driver behavior scores, and fuel usage into a single management platform.
Trilateration
The mathematical method GPS uses to determine position by measuring the distance from at least three known points (satellites). It differs from triangulation, which uses angles. GPS measures signal travel time to compute distances, then finds the point where those distance spheres intersect.
TTFF (Time to First Fix)
The amount of time a GPS receiver takes to calculate its first reliable position after being powered on or woken from sleep. TTFF depends on whether the device has a hot, warm, or cold start, ranging from under a second to several minutes.
W
WAAS (Wide Area Augmentation System)
The FAA-operated SBAS for North America. A network of precisely-surveyed ground reference stations detects GPS errors and uplinks corrections to geostationary satellites, which rebroadcast them to WAAS-enabled receivers, improving accuracy to roughly 1-3 meters.
Warm Start
A GPS fix attempt where the receiver has stored almanac and an approximate last position, but its ephemeris data has expired (typically after 4-6 hours offline). Warm starts are faster than cold starts but slower than hot starts, usually achieving a fix in 5-30 seconds.