1.3 Global Systems
Since GPS became available for civilian use, there have been any number of navigation systems that have leveraged the same technology and approaches. From full global systems, to local ground based systems, there are over 20 systems currently in operation.
Global Primary Systems or GNSS are those that are full satellite constellations; that have global coverage. Regional systems are systems with only a few satellites that are designed to cover a specific area.
Satellite Based Augmentation Systems (SBAS) use the positioning information from GNSS combined with corrections sent via communication satellites to provide a commercial service to users. Global SBAS have global coverage, while Regional SBAS are restricted to specific areas.
Ground Based Augmentation Systems (GBAS) work in a similar way to SBAS, however, rather than using the communications satellites for corrections, they utilise a ground based method of relay, such as radio waves or mobile phones.
These are best explained by the hierarchy of satellite systems, as shown in Figure 1.3(a). We will discuss most of these systems in further detail throughout the book.
Global Primary Systems
There are currently four main global primary systems:
GPS
As previously discussed, GPS is run by the US Department of Defense, and was primarily developed for military purposes, however, since becoming available for civilian use has radically changed the navigation landscape.
GLONASS
The GLObal NAvigation Satellite System (GLONASS), deployed by the Russian Federation, is becoming an increasingly important system. The first GLONASS launch was in October 1982. Most satellites are launched three at a time on a Proton launch vehicle from Baikonur.
Overall, GLONASS is similar to GPS in terms of the satellite orbits and signal structure but there are some important differences. The GLONASS constellation uses three orbital planes rather than the six used by GPS.
While GPS satellites all broadcast the same radio frequency but different codes, GLONASS satellites all broadcast the same code but each satellite uses a slightly different frequency.
GLONASS also has two levels of service. Civil users have access to the Channel of Standard Accuracy which provides horizontal position accuracy of 60m (99.7% confidence) and vertical position accuracy of 75m (99.7 % confidence). Like the GPS, the GLONASS Channel of High Accuracy is only available to authorised users.
Galileo
Galileo is the European Union’s GNSS. The European Union identified that they had several issues in relying on GPS or GLONASS for positioning systems, including:
- sovereignty and security of Europe
- present system not fully meeting civil users’ performance requirements
- needing to ensure that users are not exposed to risks linked to a virtual monopoly
- capacity for European industry to compete on a fair basis in a fast growing market.
Identification of these issues led to the formal recommendation to the European Union to involve Eur