To understand the GNSS (Global Navigation Satellite Systems), it's helpful to first understand the relationship between GNSS and GPS. The best way to summarize the concept is that the Global Positioning System (GPS) is only one supporting part of the GNSS network. While GPS is very important to GNSS, there are four other constellations that make up the entire GNSS system. One major function of having multiple systems (constellations) is that it creates better security in the event that one constellation becomes unreliable or cannot work under certain conditions.
The 5 GNSS constellations have been developed by separate governments and the EU respectively:
- US - (GPS)
- QZSS (Japan)
- BEIDOU (China)
- GALILEO (EU)
- GLONASS (Russia)
GPS was the first GNSS system that began in 1978 and became fully available for global use in 1994. GPS was introduced to act as an independent military navigation system after The United States Department of Defense realized the potential value of the technology. The system was intentionally made to provide high accuracy and high security against noise and interference from enemies of war. GPS was first made public by an executive order from Reagan in 1983.
GPS operates in a portion of the radio spectrum between 1 and 2 GHz referred to as the L-Band. This was chosen for the following primary reasons:
- Lower frequencies have greater Ionospheric delay
- Antenna design could be simplified
- The effect of weather on GPS signal propagation would be minimized
Despite being the first GNSS system, GPS is still the most accurate navigation system worldwide today. The newest GPS satellites use rubidium clocks and are synchronized to ground-based cesium clocks for even further accuracy.
QZSS (Quasi-Zenith Satellite System) is a Japanese regional satellite system that is sometimes referred to as the "Japanese GPS".
QZSS uses one geostationary satellite orbit and three in the QZO which feature a figure-eight-shaped orbit with north-south asymmetry.
The first-generation QZSS timekeeping system (TKS) will be based on Rubidium clocks and will carry a basic prototype of an experimental crystal clock synchronization system. TKS (Time Keeping System) technology is a new system that does not require onboard atomic clocks and is already used by navigation satellite systems including GPS, GLONASS, and Galileo. This allows the system to operate optimally when satellites are in direct contact with the ground station, making it a great solution for the QZSS constellation.
QZSS is compatible with GPS which is an important feature to ensure there are sufficient satellites for stability with high-precision positioning.
BEIDOU is a Chinese satellite navigation system that consists of three separate satellite constellations, BeiDou-1 and BeiDou-2, and BeiDou-3.
The BeiDou Satellite Navigation Experimental System (BeiDou-1) was built in 2000 using only three satellites with limited navigation and coverage capabilities. It was mainly used in China and close neighboring regions and was ended in late 2012.
BeiDou-2 (COMPASS) is the second iteration of the system which began operating in December 2011 with partial usage of 10 satellites. The system has been servicing customers in the Asia-Pacific region since 2012.
The 3rd generation system titled BeiDou-3 began in 2015 with the goal of global coverage. The deployment was completed in 2020 which allowed search and rescue transponder capabilities on the system.
GALILEO is the European GNSS system designed to have compatibility with GPS and GLONASS and began providing service in December 2016.
The system's receivers use the "GALILEO Reference System" to track the satellite position by implementing triangulation principles.
In total, the Galileo system is comprised of three main parts which are Space, Ground, and the User.
- The space segment functions to generate and transmit code and carrier phase signals within the Galileo signal structure. It also stores and retransmits the navigation data sent by the Ground Segment.
- The ground segment controls the entire constellation including navigation and dissemination services. Major parts of the ground segment are:
- Two Ground Control Centers (GCC)
- A Telemetry network
- Tracking and Control (TT&C) stations
- Network of Mission Uplink Stations (ULS)
- Network of Galileo Sensor Stations (GSS)
- The user segment is made up of GALILEO receivers whose main purpose tracking the coordinates of satellite constellations and providing timing services. To do so, they receive Galileo signals, measure pseudorange, and complete navigation equations.
GLONASS is Russia's version of GPS which began development in 1976 under the Soviet Union. Since inception, there have been 5 total versions:
- GLONASS (1982)
- GLONASS-M (2003)
- GLONASS-K (2011)
- GLONASS-K2 (2015)
- GLONASS-KM (2025 - Currently in the research phase)
Assisted GLONASS (A-GLONASS) has the same basic functions with more features specifically for smartphones. Turn-by-turn navigation and real-time traffic data are examples of the additional utility available with A-GLONASS and heavily rely on cell towers in their operation to quickly track location with precision.
What makes GLONASS and GPS GNSS different?
The US GPS network utilizes 31 satellites while GLONASS has 24. The US GPS system is slightly more accurate than GLONASS and operates at a lower frequency. This means that GPS provides a stronger network to users and has no major concessions to GLONASS. Because of the greater network strength, GLONASS is generally used as a backup to GPS when signals are lost.