GNSS (Global Navigation Satellite System) is the umbrella term for systems that compute the three-dimensional position of a receiver from time signals broadcast by satellites in Earth orbit. The operating principle is trilateration: by measuring the distance to at least four satellites, the receiver's latitude, longitude, altitude and time components are solved.
Global navigation satellite systems
- GPS (USA, 1995): 31 active satellites at an average orbital altitude of 20,200 km. The L1 (1575.42 MHz) and L2 (1227.60 MHz) frequencies are used for civilian and military purposes.
- GLONASS (Russia, 1995/2011): 24 satellites with FDMA-based signalling; complements GPS at high latitudes.
- Galileo (EU, 2016): 30-satellite capacity, civilian-focused; targets sub-metre accuracy. The High Accuracy Service (HAS) launched in 2023.
- BeiDou (China, 2020): 35 satellites in a global, fully operational state. Uses a mix of MEO, GEO and IGSO orbits.
- Regional: NavIC (India), QZSS (Japan).
Multi-GNSS receivers typically double or triple the number of satellites in view, supporting position validation and continuity in difficult environments such as urban canyons or under tree cover.
Accuracy levels
- Standalone GNSS: 2–10 m horizontal accuracy; carries an inherent error margin from multipath reflections and atmospheric delay.
- SBAS (WAAS, EGNOS, MSAS): 1–3 m accuracy from differential corrections broadcast by geosynchronous satellites.
- DGPS: 0.5–2 m using corrections from fixed reference stations.
- RTK (Real-Time Kinematic): 1–2 cm via carrier-phase corrections; used in geodesy, precision agriculture and machine control.
- PPP (Precise Point Positioning): 5–20 cm after convergence using precise orbit and clock data.
HDOP (horizontal dilution of precision) expresses the effect of satellite-view geometry on accuracy; HDOP < 2 is considered "good".
Assisted GNSS (A-GNSS)
On mobile devices A-GNSS downloads ephemeris and almanac data over the cellular network, reducing the cold-start time-to-first-fix (TTFF) to the order of seconds. Standalone GNSS can take 30–60 seconds for TTFF, while A-GNSS brings it down to 1–5 seconds.
Geo-fencing
Geo-fencing defines a virtual geographic area and detects entry/exit events into that area. Typical applications include time-stamping for field personnel, asset security and automated task triggering. Two common geometries are used — polygonal (vertex-based) and circular; the inside/outside test is performed with ray casting or the Haversine distance formula.
Wireless asset tracking
For long-life, low-cost asset tags, dead-battery and energy-budget optimisation are critical.
- NB-IoT (3GPP Release 13, 2016): 200 kHz narrow band, ~20 km range, deep in-building penetration. For stationary or rarely moving assets.
- LTE-M (Cat-M1): 1.4 MHz, higher bandwidth and cell handover support; for moving assets.
- LoRaWAN: unlicensed ISM bands (868/915 MHz), 2–15 km, very low power. The typical choice for tags with multi-year battery life.
- BLE (Bluetooth Low Energy): short-range (1–50 m) localisation; RSSI-based proximity, or sub-metre with AoA/AoD.
Standards and data formats
The NMEA 0183 protocol is the industry standard at the GNSS receiver output; the GGA, RMC, GSA and GSV sentences carry position, velocity, satellite count and health information. Geographic coordinates are usually expressed in the WGS 84 (1984) datum; this system, encoded as EPSG:4326, is widespread for interoperability between mapping applications. In Türkiye the TUREF projections (TM30/TM33/TM36) are used for large-scale mapping.