Vulnerabilities of satellite positioning systems
La vulnerabilidad más notable de los GNSS es la posibilidad de ser interferida la señal (la interferencia existe en todas las bandas de radionavegación). Existen varias fuentes de posible interferencia a los GNSS, tanto dentro de la banda como fuera de esta, particularmente por enlaces de microondas terrestres punto a punto permitidos por varios estados (1559 – 1610 MHz). Estos enlaces se irán eliminando gradualmente entre los años 2020 y 2025.
Las señales de los sistemas GNSS son vulnerables debido a la potencia relativamente baja de la señal recibida, pues provienen de satélites y cada señal cubre una fracción significativamente grande de la superficie terrestre.
En aviación, las normas y métodos recomendados (SARPS")) de la OACI para los GNSS exigen un nivel de rendimiento específico en presencia de niveles de interferencia definidos por la máscara de interferencia del receptor. Estos niveles de interferencia son generalmente acordes al reglamento de la Unión Internacional de Telecomunicaciones (UIT). La interferencia de niveles superiores a la máscara puede causar pérdida de servicio pero no se permite que tal interferencia resulte en información peligrosa o que induzca a error.
Types of interference
Interference can be voluntary or involuntary.
The probability and operational consequences of this interference vary with the environment. It is not considered a significant threat as long as states exercise adequate control and protection of the electromagnetic spectrum, both for existing and new frequency allocations. Furthermore, the introduction of new GNSS signals on new frequencies will ensure that unintentional interference does not result in complete loss of service (output), although it will experience some deterioration in its performance.
The majority of reported GNSS interference has been determined to come from onboard systems, and experience with GNSS installation has identified several sources of unintentional interference.[5] Portable electronic devices can also cause interference to GNSS and other navigation systems.
Terrestrial sources of interference currently include mobile and fixed VHF communications,[6] point-to-point radio links in the GNSS frequency band, television station harmonics, certain radar systems, mobile satellite communications systems and military systems. Large cities with considerable sources of radio frequency (RF) interference, industrial sites, etc., are more prone to inadvertent interference than remote regions, where such interference is highly unlikely. The likelihood of such interference depends on state spectrum regulation, frequency management, and enforcement in each state or region.
Due to their low power, GNSS signals can be blocked by low-power transmitters. Although there have been no recorded cases of intentional jamming targeting civil aircraft, for example, the possibility of intentional signal obstruction should be considered and assessed as a threat. If the impact is minimal, the potential threat is low as there is no motivation to interfere. The magnitude of the potential impact may increase as GNSS becomes more widely used and relied upon.
Spoofing is the intentional corruption of navigation signals to cause aircraft to deviate and follow a false flight path. Simulation of satellite GNSS signals is technologically much more complex than simulation of conventional ground-based radio navigation aids. Simulation of GBAS data broadcasting is as difficult as simulation of conventional landing aids.
Although signal simulation jamming can theoretically induce navigation errors in a given aircraft, it is very likely to be detected by normal procedures.[7] Ground proximity warning systems (GPWS) and airborne collision avoidance systems (ACAS) provide additional protection against collisions with terrain and with other aircraft. In view of the difficulty of interfering with GNSS through simulation, no unique operational measures are considered necessary to mitigate it.
Ionospheric and other atmospheric effects
Heavy precipitation only attenuates GNSS satellite signals by a small fraction of a dB and does not affect operations.
Tropospheric effects are addressed by system design and do not represent an aspect of vulnerability. But there are two ionospheric phenomena that must be considered:
Other vulnerabilities
It is also necessary to consider the vulnerabilities of the ground and space segments of GNSS. There is a risk of insufficient number of satellites in a given constellation due to lack of resources to maintain it, launch or satellite failures. A failure of the constellation control segment or human error can cause the failure of multiple satellites in a constellation.
Another risk is the interruption of service or its degradation during a state of national emergency situation. Countries that provide signals for satellite navigation can deny their availability, this is called selective availability. The owner of a satellite navigation system has the ability to downgrade or eliminate satellite-based navigation services over any territory they wish. Thus, if satellite navigation becomes an essential service, countries without their own satellite navigation systems will become customers of the states that provide these services.
In the case of air traffic, if the signal denial is regional, all civil GNSS signals would be blocked and the affected airspace would be closed to civil air traffic.
Another less likely situation would be the degradation or denial of signals from the main satellites or the booster satellites throughout the coverage area.
In the assessment of operational risks related to GNSS vulnerabilities, two main aspects must be considered:
By considering these issues on an airspace basis, air navigation service providers can determine whether mitigation is needed and, if so, to what level. Mitigation is required for disruptions that have significant effects and moderate to high probabilities of occurring.
New signals and major satellite constellations will significantly reduce the vulnerability of GNSS. The use of stronger signals and the diverse frequencies planned for GPS, GLONASS and Galileo will effectively eliminate the risk of inadvertent interference, since it is very unlikely that a source of such interference will affect more than one frequency simultaneously.
More satellites (even multiple constellations) will eliminate the risk of complete GNSS outages due to scintillation, and frequency multiplicity will mitigate the effect of ionospheric changes. Future geostationary satellites will mitigate the effect of the ionosphere on SBAS by using satellites whose lines of sight are at least 45° apart.
More robust signals and new GNSS frequencies make it more difficult to intentionally interfere with all GNSS services. More major satellite constellations reduce the risk of system failure, operational errors or service interruptions. They can also continue to provide global service in the unlikely event that the provider of a GNSS element modifies or denies service due to state emergency situations.
Strong system management and funding are essential for the continued operation of GNSS services and to mitigate the aforementioned system vulnerabilities, barring potential global service disruption due to a national emergency. An effective means of mitigating global disruption vulnerability is for service providers to adopt a regional denial policy in the event of a national emergency.