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.