The Soyuz-Fregat launch vehicle carrying GIOVE-B on launch pad, 2008 Photo: ESA - S. Corvaja 2008
That is how New Scientist described the results of a GPS jammer during tests off the UK coast. Although illegal in many jurisdictions, the devices are available online for as little as $30. While these results are dramatic, as a report from the Royal Academy of Engineering published this week shows, some GPS vulnerabilities may be more subtle and growing dependence on GPS needs to be moderated by greater awareness of threats to the system’s integrity.
The academy’s study has identified an increasing number of applications where position-navigation-timing, PNT, signals from global navigation satellite systems, GNSS, are used with little, or no, non-GNSS based back-ups available. The trend is for GNSS to be used in a growing number of safety of life critical systems. Unfortunately, the integrity of GNSS is insufficient for these applications without augmentation. Non-GNSS based back-ups are often absent, inadequately exercised or inadequately maintained.
The original implementation of GNSS, the US operated GPS comprises ground based, space based and receiver segments, all of which are susceptible to failures of various types. There are also some common mode failure mechanisms which can affect whole classes of receiver or even the entire satellite constellation.
A failure, or loss of signal due to some outside influence, can result in a range of consequences depending on the application; in a telecommunications network, a small loss in the efficiency of data handling may occur while the system ‘freewheels’ until a signal is restored: in a surveying application where timing is not critical, some delays may occur before the survey can be properly completed. In such applications, a temporary loss of GNSS signals might be considered an
inconvenience. However, where systems are used in safety of life critical
applications, the consequences can be more severe – in some situations, even if operators are well versed in procedures for a loss of GNSS signals, the number of interlinked systems simultaneously activating alarms can lead to eroded situational awareness of operators in what could well be an emergency situation.
GNSS have system-level vulnerabilities: GPS satellites have on rare occasion broadcast dangerously incorrect signals, a reduced number of satellites visible could prevent availability of a position fix, and GNSS receivers can incorrectly process valid signals to give unpredictable results.
GNSS signals are very weak: typically less than100 watts transmitted from a
distance of 20,000 km to 25,000 km. When received at the surface of the earth, the signal strength may be as low as –160 dBW (1 x 10–16 ) watts, with a spectrum spread out effectively below the noise floor in the receivers. Deliberate or unintentional interference with this signal can easily defeat the signal recovery or overload the receiver circuitry.
Furthermore, signals are vulnerable to disruptions in the atmospheric medium they pass through, and receivers can also unintentionally lock onto reflections of the signals, known as multipath, giving unexpectedly large errors.
These causes can have quite different effects on users, such as partial or complete loss of the positioning and timing service, poorer accuracy, very large jumps in position, velocity or time, and ‘hazardously misleading information’ (HMI) that is to say, believable data that is dangerously wrong in safety critical applications.
Report from the Royal Academy of Engineering.
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