Key Global Satellite Navigation Systems and Constellations

Global Navigation Satellite Systems (GNSS) are satellite constellations that transmit signal data from space to provide precise positioning, navigation, and timing (PNT) services across the globe. Satellite positioning operates primarily through the principle of trilateration, requiring signals from a minimum of four operational satellites to calculate latitude, longitude, altitude, and atomic clock time bias. Beyond global constellations, countries also deploy Regional Navigation Satellite Systems (RNSS) and Satellite-Based Augmentation Systems (SBAS) to satisfy defense sovereignty, enhance regional positioning accuracy, and power critical civilian infrastructure like aviation, telecommunication, and disaster management.

Global Navigation Satellite Systems (GNSS)

Global systems maintain complete planetary coverage, typically utilizing Medium Earth Orbits (MEO) arranged across multiple orbital planes.

Global Positioning System (GPS)

• Country / Operator: United States (US Space Force)
• Operational Status: Fully operational since 1995 (development began in 1978 as NAVSTAR).
• Constellation Architecture: Operates with a minimum core of 24 active operational satellites (currently maintaining around 31) distributed across 6 MEO orbital planes at an altitude of approximately 20,200 km inclined at 55 degrees.
• Frequencies and Atomic Clocks: Transmits signals on L1 (1575.42 MHz), L2, and L5 frequencies using onboard Rubidium and Cesium atomic clocks.
• Civilian vs. Military Services: Offers Standard Positioning Service (SPS) for public use and high-precision, encrypted Precise Positioning Service (PPS) for military applications. Selective Availability (SA) was permanently turned off in May 2000 to enhance civilian accuracy.

GLONASS (Global Navigation Satellite System)

• Country / Operator: Russia (Roscosmos)
• Operational Status: Operational since 1993; fully restored to complete global coverage by 2011.
• Constellation Architecture: Consists of 24 operational satellites arranged in 3 MEO orbital planes at an altitude of 19,100 km with a high inclination angle of 64.8 degrees.
• Key Features: High orbital inclination offers superior positioning accuracy and coverage in high northern latitudes and polar regions compared to other systems.
• Frequency Mechanism: Traditionally used Frequency Division Multiple Access (FDMA), transitioning toward Code Division Multiple Access (CDMA) in modern GLONASS-M/K series.

Galileo

• Country / Operator: European Union (European Union Agency for the Space Programme – EUSPA)
• Operational Status: Declared operational for initial services in 2016; civilian-controlled independent system.
• Constellation Architecture: Designed for 30 satellites (24 operational plus spares) across 3 MEO orbital planes at an altitude of 23,222 km.
• Key Features: Uses Passive Hydrogen Maser (PHM) clocks providing sub-meter horizontal positioning accuracy for public applications.
• Services Provided: Public Regulated Service (PRS) for government and emergency services, Open Service (OS), and dedicated Search and Rescue (SAR) integrated with the Cospas-Sarsat ecosystem.

BeiDou Navigation Satellite System (BDS)

• Country / Operator: China (China National Space Administration – CNSA)
• Operational Status: BDS-3 reached full global operational capability in June 2020.
• Constellation Architecture: Employs a hybrid orbit structure featuring 35 active satellites placed in Medium Earth Orbit (MEO), Geostationary Earth Orbit (GEO), and Inclined Geosynchronous Orbit (IGSO).
• Key Features: Offers global positioning alongside indigenous short-message communication capabilities.

Regional Navigation Satellite Systems (RNSS)

Regional systems focus coverage over specific geographical zones using Geosynchronous (GSO) and Geostationary (GEO) orbits.

NavIC (Navigation with Indian Constellation / IRNSS)

• Country / Operator: India (Indian Space Research Organisation – ISRO)
• Coverage Area: Covers the primary landmass of India and extends up to 1,500 km beyond national boundaries.
• Constellation Architecture: Designed with a 7-satellite core operational constellation—3 satellites in Geostationary Orbit (GEO) and 4 satellites in Inclined Geosynchronous Orbit (IGSO) at an orbital altitude of 35,786 km.
• Frequencies and Signals: Uses dual-frequency L5 (1176.45 MHz) and S-band (2492.028 MHz). Second-generation NVS series satellites (beginning with NVS-01) incorporate the civil L1 band and indigenously developed Rubidium atomic clocks.
• Service Classes: Standard Position Service (SPS) for open civilian usage and Restricted Service (RS) for encrypted defense operations.

QZSS (Quasi-Zenith Satellite System)

• Country / Operator: Japan (Cabinet Office of Japan)
• Coverage Area: Covers Japan and the broader Asia-Oceania region.
• Constellation Architecture: Uses a 4-satellite regional setup operating in highly inclined, elliptical Quasi-Zenith Orbits (QZO) and GEO orbits, designed to ensure at least one satellite remains directly overhead (near zenith) over Tokyo at all times to bypass urban canyon signal blockages. Fully interoperable with US GPS.

Satellite-Based Augmentation Systems (SBAS)

SBAS technology relies on additional GEO satellites and ground reference networks to send differential corrections and integrity monitoring signals to enhance existing GNSS positioning accuracy, primarily for civil aviation navigation.

GAGAN (GPS Aided GEO Augmented Navigation)

• Jointly developed by ISRO and the Airports Authority of India (AAI).
• Uses GSAT series satellites (such as GSAT-8, GSAT-10) to augment US GPS signals over the Indian Airspace and Flight Information Regions (FIR).
• Enables Approach with Vertical Guidance (APV-1) landings, allowing aircraft to land safely at equipped regional airports without relying on ground-based Instrument Landing Systems (ILS).

Major Global SBAS Frameworks

• WAAS (Wide Area Augmentation System): Operated by the Federal Aviation Administration (FAA) in North America.
• EGNOS (European Geostationary Navigation Overlay Service): Operated by the European Space Agency and EU for European airspace.
• MSAS (Multi-functional Satellite Augmentation System): Operated by Japan for East Asian airspace.

Structural Comparison of Satellite Navigation Systems

Essential Facts and Key Data

• Trilateration Principle: A minimum of 4 satellites is required to compute 3D spatial coordinates (X, Y, Z) and solve for receiver time offset.
• Atomic Clock Technology: NavIC uses Rubidium atomic clocks. Galileo uses Passive Hydrogen Maser clocks for precision timing.
• NavIC L1 Band Addition: The NVS-01 satellite introduced the civilian L1 band (1575.42 MHz) to allow direct compatibility with low-power consumer wearables and smartphones without requiring dedicated S-band chips.
• 3GPP Standardization: NavIC is standard-certified by the 3rd Generation Partnership Project (3GPP), driving its integration into commercial mobile chipsets.
• IMO Recognition: NavIC is recognized by the International Maritime Organization (IMO) as a component of the World-Wide Radio Navigation System (WWRNS) for ocean navigation within 1,500 km of Indian borders.
• Disaster Messaging: NavIC features a dedicated short-message broadcast service used to transmit cyclone alerts, tsunami warnings, and high-seas advisories directly to deep-sea fishermen.