GNSS (Global Navigation Satellite System)

Overview

A Global Navigation Satellite System (GNSS) is a space-based infrastructure that provides global positioning, navigation, and timing (PNT) services. GNSS consists of satellite constellations in orbit, ground control segments, and user receivers. By transmitting precisely timed radio signals, GNSS enables any equipped receiver—on land, at sea, or in the air—to determine its geographic location (latitude, longitude, altitude) and time anywhere on Earth, as long as it has an unobstructed view of multiple satellites.

GNSS technology has revolutionized navigation, geospatial science, and critical infrastructure worldwide. It is indispensable in aviation, maritime operations, land transportation, telecommunications, energy, banking, scientific research, emergency response, and daily consumer applications.

Major GNSS Constellations

Four global GNSS constellations provide worldwide coverage, with several regional systems enhancing performance for specific areas:

• GPS (Global Positioning System): Operated by the United States, GPS was the first operational GNSS and remains the most widely used. Its constellation consists of at least 24 satellites in Medium Earth Orbit (MEO), broadcasting on multiple frequencies for civilian and military use.

• GLONASS: Russia’s GNSS, similar in structure to GPS but with different frequencies and orbital inclinations, provides robust coverage, especially at high latitudes.

• Galileo: The European Union’s system, featuring advanced signal structures, interoperability, and high accuracy with integrity features (such as signal authentication).

• BeiDou (BDS): China’s GNSS, utilizing a hybrid constellation of MEO, geostationary (GEO), and inclined geosynchronous (IGSO) satellites, offers global and enhanced regional services.

Regional Systems

• QZSS (Quasi-Zenith Satellite System): Japan’s system focuses on improved coverage and accuracy in Asia-Oceania, especially in urban and mountainous environments.

• NavIC (Navigation with Indian Constellation): India’s regional system provides high-accuracy services over the Indian subcontinent and surrounding regions.

Most modern receivers are multi-constellation and multi-frequency, combining signals from several systems to improve accuracy, reliability, and resilience in challenging conditions.

How GNSS Works

Trilateration

Trilateration is the core method used by GNSS receivers to calculate position. By measuring the time delay of radio signals from at least four satellites, a receiver calculates the distance to each satellite. These distances define intersecting spheres; their intersection pinpoints the receiver’s position and corrects its internal clock.

• Minimum of four satellites: Needed to solve for latitude, longitude, altitude, and time bias.
• Timing accuracy: Essential, as even nanosecond errors can result in meter-level position errors.

Satellite Orbits

GNSS satellites typically orbit in Medium Earth Orbit (MEO) (about 19,000–23,000 km altitude), arranged in multiple orbital planes for continuous, overlapping coverage. This ensures that users can always see enough satellites for positioning.

Some systems use additional geostationary or inclined orbits (GEO/IGSO) for regional augmentation and communication.

Signal Structure

GNSS satellites broadcast on L-band frequencies (1–2 GHz), chosen for reliable atmospheric penetration and compact antennas. Signals include:

• Ephemeris: Precise satellite orbital and clock data.
• Almanac: Coarse orbital data for all satellites, aiding acquisition.
• Ranging codes: Unique codes per satellite for signal separation.
• Navigation data: Integrity, corrections, and (for some services) authentication.

Most systems use code-division multiple access (CDMA) so signals from different satellites can be distinguished.

GNSS Performance Criteria

Four key metrics define GNSS performance:

• Accuracy: Closeness of measured position/time to true value. Typical civilian accuracy: 3–5 meters, with sub-meter or centimeter-level accuracy achievable using augmentation.
• Integrity: Trustworthiness of information and timely alerting of errors—critical for safety applications.
• Continuity: Likelihood that service continues without interruption for the required duration.
• Availability: The percentage of time the system meets the above requirements.

These criteria are regulated and standardized for critical applications, such as aviation, by organizations like ICAO.

Sources of Error

GNSS accuracy can be degraded by:

• Atmospheric delays:
  • Ionospheric delay: Variable, frequency-dependent delay corrected by dual-frequency receivers or models.
  • Tropospheric delay: Caused by weather; mitigated by models and corrections.
• Multipath: Reflections from surfaces cause signal timing errors, especially in cities.
• Satellite/receiver errors: Clock drift and orbital uncertainties, minimized by ground monitoring and corrections.
• Obstructions: Buildings, foliage, and tunnels can block or weaken signals.
• Interference: Jamming and spoofing are significant concerns for security and critical infrastructure.
• Satellite geometry (DOP): Poor satellite distribution increases uncertainty.

Augmentation Systems

To improve GNSS accuracy, integrity, and availability, various augmentation systems are used:

• Satellite-Based Augmentation Systems (SBAS): Networks like WAAS (US), EGNOS (EU), MSAS (Japan), and GAGAN (India) provide real-time corrections and integrity information via geostationary satellites, enabling meter-level accuracy and high integrity for aviation and other users.

• Ground-Based Augmentation Systems (GBAS): Localized corrections for airports and harbors, supporting precise landings and port operations.

• Precise Point Positioning (PPP): Uses globally distributed reference stations to deliver real-time orbit and clock corrections for centimeter-level accuracy anywhere.

Key Applications

Science and Geodesy

• Tectonic monitoring: Plate motion, crustal deformation, and sea-level studies.
• Atmospheric science: GNSS radio occultation profiles the atmosphere for weather and climate research.
• Global time transfer: Synchronizes atomic clocks worldwide.

Transportation

• Aviation: En-route navigation, RNAV, RNP, precision approaches, and landings.
• Maritime: Vessel navigation, port operations, and search-and-rescue.
• Land transport: Vehicle routing, fleet management, rail safety, and intelligent transport systems.

Industry and Infrastructure

• Telecommunications: Network synchronization and handoff.
• Energy: Power grid phase synchronization and fault detection.
• Banking: Transaction timestamping for security and audit.

Surveying and Mapping

• Land surveying: Rapid, accurate mapping for engineering and cadastral purposes.
• Construction: Site layout, monitoring, and machinery guidance.

Agriculture

• Precision farming: Automated guidance, variable-rate application, and yield monitoring.

Public Safety

• Emergency response: Accurate caller location (E112/E911), asset tracking, and disaster management.

Emerging Technologies

• Autonomous vehicles and drones: Real-time navigation and safety.
• IoT: Asset tracking, smart cities, and environmental monitoring.

Standards and International Cooperation

GNSS is governed and harmonized by international standards and working groups:

• International GNSS Service (IGS): Provides precise GNSS data and corrections.
• International Committee on GNSS (ICG): UN forum for provider coordination and interoperability.
• ICAO: Sets aviation GNSS standards for safety, performance, and procedures.
• RTCM: Develops data formats for differential corrections and marine navigation.

Glossary of GNSS Terms

• GNSS: Global Navigation Satellite System; umbrella term for all satellite-based PNT services.
• GPS: US GNSS; most widely used.
• GLONASS: Russia’s GNSS.
• Galileo: EU’s GNSS.
• BeiDou: China’s GNSS.
• QZSS: Regional GNSS for Japan.
• NavIC: Regional GNSS for India.
• Trilateration: Position calculation by measuring distances to multiple satellites.
• Ephemeris: Precise, time-tagged satellite orbit and clock data.
• Almanac: Coarse, long-term orbital data for all satellites.
• SBAS: Satellite-Based Augmentation System.
• GBAS: Ground-Based Augmentation System.
• PPP: Precise Point Positioning.
• DOP (Dilution of Precision): Measure of satellite geometry’s effect on accuracy.
• Multipath: Errors from reflected signals.
• Integrity: Trustworthiness and error notification of GNSS data.
• Continuity: Service reliability over a specified period.
• Availability: System’s operational readiness to meet performance criteria.
• Ionospheric/Tropospheric delay: Atmospheric effects that slow GNSS signals.
• Spoofing: Malicious transmission of false GNSS signals.
• Jamming: Intentional or unintentional GNSS signal interference.

Conclusion

GNSS is a cornerstone of modern life, enabling precise navigation, reliable timing, and global connectivity across all sectors. As technology advances, the integration of multiple constellations and augmentation systems continues to enhance performance, resilience, and the range of applications—from scientific discovery to everyday convenience.

For organizations and individuals alike, understanding GNSS is essential for leveraging its full potential in innovation, safety, and operational excellence.