Global Navigation Satellite System (GNSS) refers to any satellite constellation that provides global positioning, navigation, and timing services.

Several GNSS are currently available:

• Galileo (EU)
• GPS (USA)
• GLONASS (Russia)
• BeiDou (China)

There are also two regional navigation satellite systems (RNSS) in the form of Japan’s Quasi-Zenith Satellite System (QZSS) and the Indian Regional Navigation Satellite System (IRNSS, also known as NavIC).

Using signals from space, each of these systems transmits ranging and timing data to GNSS-enabled receivers, which then use this data to determine location.

GNSS performance is assessed using four criteria:

• Accuracy: the difference between a receiver’s measured and real position, speed or time.
• Integrity: a system’s capacity to provide a threshold of confidence and, in the event of an anomaly in the positioning data, an alarm.
• Continuity: a system’s ability to function without interruption.
• Availability: the percentage of time a signal fulfils the above accuracy, integrity and continuity criteria.

This performance can be improved using regional satellite-based augmentation systems (SBAS), such as the European Geostationary Navigation Overlay Service (EGNOS), Japan’s QZSS and India’s GAGAN. All of these are based on GPS.