Last reviewed 16 Sept 2026 · Facts as of 16 Sept 2026 · 8 min read
Global navigation satellite systems (GNSS)
GNSS is the general term for satellite systems that provide positioning, navigation and timing worldwide.
| System | Operator | Key features |
|---|---|---|
| GPS (NAVSTAR) | United States | Satellites in 6 orbital planes inclined at about 55°, altitude about 20 200 km, orbital period about 12 hours (11 h 58 min); a minimum of 24 satellites (more are operational) |
| GLONASS | Russia | 3 orbital planes inclined at about 64.8°, altitude about 19 100 km |
| Galileo | European Union | 3 orbital planes inclined at about 56°, altitude about 23 200 km; civilian-controlled |
| BeiDou (BDS) | China | Mixed constellation of medium Earth orbit, geosynchronous and geostationary satellites |
| NavIC (IRNSS) | India (ISRO) | Regional system with satellites in geostationary and geosynchronous orbits, covering India and a region of about 1500 km around it |
| QZSS | Japan | Regional augmentation/complementary system |
Satellite-based augmentation systems (SBAS) improve accuracy and integrity using geostationary satellites and ground reference stations — e.g. GAGAN (GPS Aided GEO Augmented Navigation, developed by ISRO and AAI for India), WAAS (USA), EGNOS (Europe).
Segments of GPS
- Space segment — the constellation of satellites broadcasting signals, with precise atomic clocks.
- Control segment — a master control station (in Colorado, USA), monitor stations and ground antennas that track satellites, compute orbits (ephemerides) and clock corrections, and upload navigation messages.
- User segment — receivers (navigation, mapping and geodetic/survey-grade) and antennas that compute position, velocity and time.
Signals
- Carrier frequencies (GPS): L1 = 1575.42 MHz, L2 = 1227.60 MHz, L5 = 1176.45 MHz.
- Codes: C/A (coarse/acquisition) code — civilian code on L1; P(Y) code — precise (encrypted military) code on L1 and L2; modernised civil codes L2C and L5.
- Navigation message — satellite ephemeris (precise orbit), almanac (approximate orbits of all satellites), clock corrections, ionospheric model parameters, health status.
Principle of positioning
Pseudorange (code) positioning
The receiver measures the travel time of the signal from satellite to receiver:
( ≈ 299 792 458 m/s.) This measured range is called a pseudorange because the receiver clock is not synchronised with satellite clocks.
- Each pseudorange defines a sphere centred on the satellite; the position lies at the intersection (trilateration).
- Four unknowns — X, Y, Z coordinates and receiver clock bias — so at least four satellites must be observed simultaneously.
- A receiver clock error of just 1 microsecond corresponds to a range error of about 300 m — hence the clock bias must be solved.
Carrier-phase positioning
The receiver measures the phase of the carrier wave (wavelength about 19 cm for L1). The integer number of cycles between satellite and receiver (integer ambiguity) is unknown and must be resolved; once fixed, carrier phase gives millimetre to centimetre relative accuracy — the basis of survey-grade GNSS. Cycle slips (loss of lock) must be detected and repaired.
Sources of error
| Error | Description / mitigation |
|---|---|
| Satellite clock error | Corrected by broadcast clock parameters; eliminated in differential methods |
| Orbit (ephemeris) error | Differences between predicted and actual orbits; precise ephemerides for post-processing |
| Ionospheric delay | Signal delay by free electrons in the ionosphere — largest error source for single-frequency receivers; dual-frequency observations largely eliminate it |
| Tropospheric delay | Delay in the lower atmosphere (dry and wet components) — modelled; larger at low elevation angles |
| Multipath | Signals reflected from buildings, water, vehicles reach the antenna — choose open sites, choke-ring antennas, elevation mask |
| Receiver noise | Instrumental noise |
| Antenna phase centre variation | Calibrated antennas; consistent antenna orientation |
| Selective availability | Deliberate degradation of civilian accuracy by the US — discontinued in May 2000 |
| Satellite geometry | Expressed by dilution of precision (DOP) |
Dilution of precision
DOP measures how satellite geometry amplifies measurement errors:
- GDOP (geometric), PDOP (3D position), HDOP (horizontal), VDOP (vertical), TDOP (time).
- Low DOP = good geometry (satellites well spread across the sky); high DOP (satellites clustered) = poor accuracy.
- Position error ≈ DOP × range measurement error.
- Vertical accuracy is usually poorer than horizontal accuracy because satellites are above the horizon only.
Positioning methods
| Method | Description | Typical accuracy |
|---|---|---|
| Absolute (single point) positioning | One receiver, code pseudoranges | Several metres |
| Differential GPS (DGPS) | A base station at a known point computes pseudorange corrections and transmits them to rovers | Sub-metre to a few metres |
| Static relative positioning | Two or more receivers observe simultaneously for long sessions (e.g. an hour or more); carrier phase post-processed | Millimetres to a centimetre — control networks, deformation monitoring |
| Rapid (fast) static | Shorter sessions with good geometry and dual frequency | Centimetre level over short baselines |
| Kinematic (stop-and-go) | Rover moves after initialisation; carrier phase maintained | Centimetre level |
| Real-time kinematic (RTK) | Base station transmits carrier-phase corrections by radio or internet (NTRIP) to the rover; ambiguities fixed in real time | About 1–2 cm horizontal (short baselines) — stake-out, detail surveys, machine control |
| Network RTK / CORS | A network of continuously operating reference stations generates corrections over a region — no own base needed | Centimetre level |
| Precise point positioning (PPP) | Single receiver with precise orbit and clock products, dual frequency | Decimetre to centimetre after convergence |
The Survey of India operates a CORS network providing real-time corrections for high-accuracy positioning across the country.