GNSS is prone to threats, both unintentional and deliberate, due to extremely low signal at the user end. Frequent checks, countermeasures and upgrade in satellite systems could improve positioning accuracy and reliability
Modern society is highly reliant on Global Navigation Satellite Systems (GNSS). In addition to the obvious usage in positioning and navigation, more and more applications are relying on robust timing reference from GNSS. Though it provides accurate and global position, velocity and time, just like any other radio frequency (RF) signal, the technology is highly vulnerable to a range of interference threats. GNSS is particularly prone to unintended and malicious interference due to extremely low signal at the user receiver end after travelling from the satellite transmitter to the user receiver antenna on the earth. The received minimum power of – 157 dBW is expected both for GPS (Global Positioning System) L1C and Galileo E1 open service signals, considering both data and pilot channels.
The Volpe report by the Department of Transportation in the United States from 2001 (Volpe, 2001) and the Royal Academy of Engineering report of 2011 from the United Kingdom (Royal Academy, 2011) describe the wide variety of threats ranging from interruptions in satellite-based services in some regions due to solar storms to the intentional jamming risks from localised to more extensive denials in GNSS availability. The interference sources threatening reliable GNSS operation could be divided into unintentional and deliberate risks.
Unintentional interferences include natural phenomena such as increased levels of ionospheric disturbance and solar flares as well as man-made phenomena such as inherent errors in the satellites or transmitted signals, and unwanted radio frequency transmissions due to television, microwave communication traffic or radar signals. The pressure on the shared use of finite radio spectrum compromises the satellite navigation signals. Moreover, ionospheric scintillation affects GNSS receivers, causing losses of lock, navigation data bit errors, cycle slips, and degradation in the accuracy and availability of the measurements. Intentional GNSS interference sources include jamming, meaconing, and spoofing. Of late, a serious concern has been the increase in the number of illegal jammer devices on the civilian front.
Jammers are also termed personal privacy devices (PPDs) and they are intended to prevent people and vehicles from being tracked within a limited area. Jammers may cause severe damage if their signals are not properly detected and the effects mitigated in user receivers. Meaconing is a form of intentional interference, including interception and re-broadcast of navigation signals. Deliberate deception in the form of spoofing signals on the other hand is a more complex form of interference, deceiving a GNSS receiver to another location by broadcasting a modified and slightly more powerful signal than that received from the GNSS satellites.
The power spectrum (above) and the frequency with respect to time (below) for a low-cost in-car jammer
At present, GNSS is used in a wide range of everyday businesses, such as tracking of valuable cargoes, tagged offenders or vehicles, measuring the distance travelled etc. Unfortunately, these innovations give criminals an incentive to interfere with GNSS services. Intentional interference occurs when a user is deprived of GNSS services deliberately and maliciously. Jamming of GNSS signals can be achieved quite easily using relatively lowcost equipment, for example an inexpensive in-car jammer shown in Fig. 1.
Spread spectrum signals such as those of GPS and the upcoming European Galileo are most vulnerable to a broadband jammer, taking out all the civilian GNSS signals. The Russian frequency division system GLONASS can still give positioning service if the jammer is narrow enough to take out only one or two satellites” signals. Recorded incidents of deliberate jamming have mostly been observed by US and European military authorities, but on the civilian side, the maritime and aviation world remain vastly vulnerable to jamming.
Figure 1: An inexpensive in-car jammer in research use at the Finnish Geodetic Institute
Countermeasures for jamming are available in the form of expensive directional antennas and interference suppression units, but there is a great need to develop low-cost, effective civilian vulnerability alleviation approaches. Spoofing is more difficult to achieve than jamming, as it is necessary to simulate the signals in order to make the receiver lock on to false signals, to cause a functional deception scenario. Furthermore, the consequences of spoofing are far more serious than those of jamming. If the false signals are indistinguishable from the real ones and give a position close enough to be believable, the user may not be aware of the deception and possibly lead to life-threatening danger. An authentication service via encryption/decryption is considered an effective countermeasure to spoofing.
|Countermeasures to GNSS threats|
Consequences of interference to navigation applications may range from a complete loss of signal, false position information or intermittent loss to degradation of accuracy. GNSS signals of current and future generation are affected in different ways depending on the specific type of interference (for example, continuous wave (CW), narrow band (NB), or wide band (WB) interference). Strategies for mitigating the GNSS vulnerabilities were categorised and the following schemes were distinguished: individual interference awareness, global GNSS disruption awareness, legislative countermeasures, backup systems and electronic crosschecks, and authenticable signals in modernised systems.
Knowing you are under interference and reporting it is crucial on an individual receiver basis. Some basic signal checks (such as monitoring the signal”s carrier-to-noisedensity ratio, C/N0) and navigation solution hypothesis testing (such as receiver autonomous integrity monitoring) can detect, diagnose, and characterise an interference situation, including potential jamming and spoofing attacks.
In order to make an effort to mitigate the effects of interference, from intentional or unintentional sources, reliable interference detection must be conducted. In the presence of interference, whether it is unintentional or deliberate, one of the following scenarios will unavoidably occur — loss of accuracy resulting in noisy position solution, severely erroneous position information with significant “jumps” in the position solution, total loss of signal resulting in no-position solution, or appearance of hazardous misleading information.
Current satellite navigation systems are evolving — the modernised US GPS and the Russian GLONASS will make new signals available with further utility coming from the expected deployment of the European Galileo and the Chinese Compass systems. Modernised GNSS signals will generally consist of a pilot channel and a data channel. The pilot channel is a spread spectrum-modulated signal but does not have a low rate data modulated onto it such as the current civilian GPS C/A (coarse/ acquisition) civilian signal. This allows for continued successful operation at lower signal-to-noise ratios, either due to attenuation in urban environments or jamming. Above all, the advent of multiple GNSS constellations and signals from each satellite on multiple frequencies provides more measurements to the GNSS user, and hence brings newer opportunities for receivers to cope better in challenging GNSS interference environments by offering improved positioning accuracy and reliability.