Limitations of GPS
Extract from a report published in March 2011 by the Royal Academy of Engineering, London (www.raeng.org.uk/gnss). Reproduced with permission. (c) Copyright 2011, RAENG.
Reliance on GNSS for PNT is high and increasing
The use of GNSS for a variety of purposes has become so convenient and ubiquitous that there is a strong tendency among users to treat it as a given. At every level, examples of reliance on GPS for positional, navigational and timing uses without fully tested and exercised non-GPS back-ups have been observed. In the great majority of cases, the loss of these services in an individual application will cause only local or isolated inconvenience, but the possibility exists for wider, single mode or common mode failures with more serious consequences.
Although it is currently rare for safety critical systems to be wholly reliant on GNSS, related services that are otherwise independent may have GNSS as a common point of failure, with consequences for the performance of safety critical tasks. Such conditions occur where, for example in emergency services, navigation and positional data is required to help perform safety critical tasks efficiently, even though its absence would not interfere with the actual emergency response task once the location of the emergency had been reached. There are also a number of primary safety critical systems being developed in fields such as transport.
GPS, Galileo, Compass and GLONASS – common vulnerabilities
The risk of a common mode failure affecting an entire GNSS constellation or even multiple constellations cannot be ruled out. The Earth is subject to extreme solar events from time to time and these have the potential to disrupt the GNSS signals, and the satellites themselves. The disruption may be temporary or may cause complete satellite failure. Such super-storm events are not predictable, but studies estimate that these so-called ‘Carrington events’ will occur with a probability in the order of 1-in-100 per year.
Space Weather events of lesser magnitude will occur more frequently. More than once per decade, at UK latitudes, there may be an interruption to high accuracy GNSS services. There should be no direct safety of life issues if the integrity subsystem informs the users that the navigation solution is degraded, but the absence of the service will have varying levels of impact, which could be mitigated if an alternative navigation system is available.
Risk from jamming is growing. As GNSS becomes more widely used for revenue generation or protection, the rewards from criminal activity aimed at disrupting the system grow. Already it is known that criminals have used GPS jamming in connection with theft of high value vehicles and the avoidance of road user charges. The cost of jamming equipment is low and while users of such equipment are concerned only with the jamming of devices on a single vehicle, the area affected by that jamming signal can be large. It is expected that the introduction of Galileo, with its additional frequency bands and compatibility with GPS will make jamming more difficult, but not significantly so for the determined criminal.
The risk from spoofing is emerging and may become serious.
(made by the Royal Academy of Engineering, London, March 2011)
The provision of a widely available PNT service as an alternative to GNSS is an essential part of the national infrastructure. It should be cost effective to incorporate in civil GNSS receivers and free to use. Ideally it should provide additional benefits, such as availability inside buildings and in GNSS blindspots.