GNSS is the umbrella term for any satellite constellation providing global positioning, navigation, and timing. The four operational GNSS systems are GPS (US), Galileo (EU), GLONASS (Russia), and BeiDou (China). Multi-GNSS receivers use signals from all four for improved accuracy and reliability.
Understanding GNSS
The four global constellations
All four global systems occupy medium Earth orbit (MEO) — high enough that around two dozen satellites blanket the planet, yet low enough to keep signals usable and launches affordable. A GPS satellite completes two orbits per sidereal day, so it retraces the same ground track once per sidereal day — appearing about four minutes earlier by the clock each day. The systems differ chiefly in altitude, inclination and signal design. BeiDou is the outlier: alongside its MEO tier it flies geostationary (GEO) and inclined-geosynchronous satellites that concentrate coverage over Asia.
| System | Operator | Altitude | Inclination | Nominal constellation |
|---|---|---|---|---|
| GPS | United States | ~20,200 km | 55° | 24 baseline (~31 in use) |
| GLONASS | Russia | ~19,100 km | 64.8° | 24 |
| Galileo | European Union | ~23,200 km | 56° | 24 + 6 spares |
| BeiDou-3 | China | ~21,500 km (MEO) | 55° | 24 MEO + 3 GEO + 3 IGSO |
How GNSS pins down your position
Each satellite continuously transmits its own position and a time stamp from an on-board atomic clock. The receiver notes the instant each signal arrives and multiplies the delay by the speed of light to obtain a pseudorange — a distance still skewed by the receiver's inexpensive clock. Signals from four or more satellites let it solve four unknowns at once: three coordinates (latitude, longitude, altitude) plus that clock error. Geometry matters as much as count — satellites spread across the sky give a crisp fix, whereas a tight cluster inflates the result, an effect measured as dilution of precision.
Multi-GNSS, accuracy and the spoofing threat
Listening to a single constellation, a receiver in a city street or deep valley may catch only a few satellites. Combining GPS, Galileo, GLONASS and BeiDou can put 30 or more in view, raising availability, shortening the time to first fix and tightening accuracy towards the metre level. A single-constellation fix is roughly 3–8 m; wide-area augmentation such as SBAS sharpens this to about a metre, while carrier-phase methods like RTK and PPP reach a few centimetres. The catch is signal strength: broadcasts crossing ~20,000 km arrive extremely faint and are easily drowned out by jamming or spoofing, so aviation, finance and power grids increasingly rely on multi-constellation, multi-frequency receivers and independent backup timing.