Understanding the Basics of Global Navigation Satellite Systems
The Global Navigation Satellite System (GNSS) has become an essential technology for many industries, from vehicle navigation to timing synchronization for electrical and communication networks, here we explore the basics of GNSS and how it works, as well as the accuracy of GNSS receivers and the factors that can impact their precision.
What is the Difference between GPS and GNSS?
The Global Positioning System (GPS)is a satellite-based radio-navigation system owned by the United States government, it is one of many global navigation satellite systems (GNSS). However, as GPS was the first and then the most widely used GNSS, often people use GPS when they are talking about GNSS – a bit like saying Hoover instead of vacuum cleaner or iPad instead of tablet PC.
What is GNSS?
A global navigation satellite system (GNSS) is a group of synchronized satellites working in concert (collectively called constellations) used for Position Navigation and Time (PNT) solutions on a global basis. It consists of many global constellations of satellites transmitting radio signals used for PNT solutions. The main constellations are Global Positioning System (GPS) (USA) - Glonass (Russia) - Galileo (EU) and Beidou (China).
The PNT solutions provided by these GNSS are used for a wide and growing variety of applications including, but not limited to, vehicle location and navigation (Aviation Maritime, Road and Rail). - Timing synchronisation for electrical and communication networks, Surveying, mining, construction and agriculture Location Based Services (LBS).
How does GNSS work?
The GNSS satellites transmit information about their position and (using atomic clocks) the time when each individual signal was sent.
GNSS receivers can then use signals from multiple satellites to trilaterate their position using their distance from at least four GNSS satellites, this is also why more satellites equal more precision.
If one system is down, some GNSS receivers can pick up signals from all the other systems. And of course, the more satellites your receiver is looking at – the more likely that if your line of sight to one satellite is obstructed by a tree of other obstacle, it can ‘see’ another satellite.
GNSS systems in Australia
Australia is one of few countries in the world with high visibility to six GNSS due to our geographical location. These include not only the main global systems of GPS, GLONASS, Galileo , and BeiDou but also Japan's Quasi Zenith Satellite System (QZSS) and the Indian Regional Satellite Navigation System (IRNSS).
GNSS Receivers are the user interface to any Global Navigation Satellite System (GNSS). The receivers process the Signals In Space (SIS) transmitted by the satellites. GNSS receivers measure the distance to each individual satellite based on the time it takes a satellite SIS signal to reach them.
Even though the information provided by a generic GNSS receiver can be used by a wide range of applications, most of them rely on the receiver's navigation solution - i.e. receiver computed Position, Velocity and Time (PVT).
In open sky conditions, standard accuracy GNSS receivers are accurate to around two meters, however, because GNSS receivers rely on the time it takes a satellite signal to reach them, even the slightest errors (billionths of a second) can negatively impact accuracy.
Errors in satellite orbit position can lead to around 2.5 meters’ loss of accuracy. Satellites clock errors can add another 1.5 meters. And inconsistencies in the troposphere and the ionosphere can add another one and five meters respectively, then throw in the occasional intense burst of solar activity or multi path effects like signals bouncing off building walls and this accuracy can blow out to 10 meters or more.
Luckily, High precision GNSS systems dramatically improve precision using GNSS correction data to cancel out the errors. One way they do this involves monitoring GNSS signals from a base station at a known location. Deviations from the base station’s position are observed and sent to a rover – eg. a vehicle equipped with a GNSS receiver – allowing it to obtain a more accurate position reading. In favourable conditions, this approach can be used to achieve centimetre-level accuracy, provided that the base station and the rover are not too far apart.
What is a Base Station?
Typically, a base station (or Reference Station) is made up of a GNSS Receiver, a GNSS Antenna, Radio Transmitter and a power supply. The station is placed at a known (and fixed) position, the base station’s receiver tracks satellites in the same way (and at the same time) that the rover does.
The errors in the GNSS system (mentioned in the accuracy section above) are monitored at the fixed (and known) location of the base station, and a series of position corrections are computed and can be sent via the base station’s radio to the rover’s receiver. The rover then uses the data to correct its real time position, leading to very high accuracy positioning.