Global Navigation Satellite System (GNSS) is perhaps one of the biggest scientific achievements in the recent years after internet which is impacting our daily life. While the digital divide is narrowing, dependence on GNSS is being taken for granted by the public, all of whom may not be aware of what it really is. That a constellation of 24 satellites can be used for precision landing by aircrafts to keeping a track of trucks or ships or railway and ensuring its location with time is bringing in economic efficiency without people realizing that its happening.
While our dependence on GNSS is increasing by the day without our realization, we need to prepare ourselves on its vulnerability. We must appreciate that this technology is dual use and is good for precision bombing as well. While defence forces find convenient to use, this technology can be used by non-state actors as well. In a recent news items, it was stated that North Korea tried to jam GPS signals in South Korea. While it did to make any major disturbance but over 30 fishermen reported problem with their navigation systems.
It has been claimed by European Commission that 6-7% of the GDP in western countries is already dependent upon satellite radio navigation or say about Euro 800 million in European Union.
The greatest vulnerability of GNSS be it GPS, GLONASS or in future Galileo is that its signals are weak. These signals are typically less than 100 watts transmitted from a distance of 20,000 kms to 25,000 kms. When received at the surface of Earth they may be very low. It is vulnerable to failure, disruption and interference. These are possible both by natural clauses and by motivated action. The fact that they are coming from a distance of 20,000 kms or more from Ionosphere and Troposphere, leads to some natural disturbances especially around the tropics.
Let us start with what constitutes a GNSS system. The GNSS system consists of three segments:
1) A space segment of at least 24 satellites in 6 orbital planes to make a full constellation (currently GPS has 32 satellite while GLONASS has 24).
2) A Ground Control segment which is used to upload data to satellites, to synchronize time across the constellations and to track satellites to enable orbit and clock determination.
3)A user segment consisting of receivers and associated antennas, used to receive and decode the signal to provide PNT information.
What is GNSS? It is a ranging System with three available carrier frequencies, all multiples of a fundamental frequency. The distance is derived from a coded signal through measuring time distance between transmission between satellite and the reception.
Primary GPS Error Sources
In normal standalone operations of GNSS, it gives a 3D dimensional position of around 5-10 metre accuracy. The accuracy also depends upon user equipment, error sources and configuration for tracking.
A) Space Segment issues
Both Ionospheric and Tropospheric disturbances are responsible for errors. Ionospheric storms pose a threat to GNSS especially over Equator. Augmentation Satellite systems have been developed like WAAS over Americas, Egnos over Europe or Gagan over Asia and MTSAT over Japan and Pacific to take care of these disturbances. These systems are supposed to make the corrections in the Ionospheric disturbances, clock and ephemeris so that a corrected message is transmitted to user receivers capable of receiving such corrections.
International Civil Aviation Organization (ICAO) and GNSS
According to ICAO, GNSS signals from satellites are very weak at the receiver antenna and therefore, are vulnerable to interference. While navigational services provided by conventional aids can also be disrupted by interference, but GNSS typically serves more aircraft simultaneously and the interference may affect wide geographic areas. In addition, GNSS signals are also susceptible Ionospheric and Stratospheric effects. Therefore, GNSS receivers must meet specified performance requirements in the presence of levels of interference defined in Annex 10 to the Chicago Convention of 1944 and used with International Telecommunication Union (ITU) recommendations. Interference above defined levels may cause degradation or loss of service, but avionics standards require that such interference shall not result in hazardously misleading information (HMI). Current GNSS approvals use a single frequency band common to GPS, GLONASS and SBAS (L Band). This makes it easier to intentionally jam GNSS signals as well as unintentional interference. With greater use of Automatic Surveillance Broadcasting (ADS-B) in aviation, the issue of vulnerability needs a greater study for aviation.
The next generation GNSS will be based on multiple frequencies as it has already happened in the case of Indian Regional Navigation Satellite System (IRNSS) which is also using S-Band. This will reduce the likelihood of unintentional interference and will make intentional interference more difficult. Enhanced services depending upon the availability of multiple frequencies would, however, be degraded by interference with one frequency. GNSS provides precise time information to support the applications. The majority of these applications use GNSS in a non-critical manner; timing receivers are used with other time distribution systems and do not have demanding absolute accuracy requirements. Systems can coast for a considerable amount of time on internal quartz clocks before needing another GNSS time update. The most notable exception is multilateration, which can have a critical dependence on GNSS time. State regulators and ANS providers can take the measures to reduce the likelihood that GNSS service will be lost. They can assess the residual risk and develop strategies to reduce the impact on aircraft operations in the event of a service disruption.
In all cases reduced precision and even outages may occur, but the integrity of subsystem should ensure, for example, safety of life.
We also note that Ionospheric scintillation impacts are significantly worsened by intentional or accidental jamming. Navigation loss can be mitigated by deeply integrated GNSS-inertial navigation systems. Resilience to ionospheric effects will be somewhat improved through the use of signals from multiple constellations. Therefore, receivers that are interoperable between GPS, GLONASS and GALILEO (yet to become operational) have already become available in the market.
Some limited measures against interference can be taken within the GNSS receiver itself. Many receivers could be configured to detect interference through monitoring the received signal strength indicator, flagging up a warning if this is suspected. Some checks in software could be implemented to detect basic signal spoofing. Some military GNSS receivers use more radical measures for jamming in their design, such as a very high dynamic range in the signal input capability, and the use of smart antenna arrays that can detect and attenuate jammer signals, but these are unlikely to be adopted in civilian receivers owing to the power requirements and additional cost.
Another aspect of vulnerability of GNSS, especially the Space segment is the Atomic Clock. GNSS is highly dependent upon the predictable performance of the atomic clock carried on board the satellites. Occasionally these clocks have produced errors and behaved unpredictably leading to a huge error. While atomic clocks can be repaired and replaced on the ground, this may not be the case for satellite based atomic clock.
Radiation impact on satellites can be yet another cause for defect. When passing through a radiation belt around the earth especially during a solar storm unpredictable behaviour can take place leading to errors.
B) Ground Segment
Attack on ground segment– The GPS ground segment is designed to withstand military attacks, but could easily still have some vulnerabilities to terrorist or cyberattacks. As more GNSS systems become available, there are more targets available to attack (remote monitoring stations could be especially vulnerable) although relative independence of the systems provides some protection.
C) User Segment
Unlike the ground and space segment, the GNSS user segment is extremely diverse and uncoordinated, comprising of GPS receivers. These include decryption-capable military receivers, certified safety-of life aviation receivers, scientific survey receivers and receivers embedded in mass-market mobile phones, and numerous others. Different manufacturers design GNSS receivers to different level of receivers but even a simplest GNSS is a high complex mixture of hardware and software. The common threat is that all receivers are designed to interface with broadband GNSS signals, as documents in the “signal-in-space interface controlled document” (ICD) definition, for example, GPS service 10. This diversity means system vulnerabilities in GNSS will not affect all users, but still could affect one particular user badly, possibly globally, where receivers from one manufacture with a latent software bug are used widely.
The wide range of applications dependent on GNSS signals and seemingly unrelated services could fail simultaneously as a result of disruption to the GNSS signals resulting from the vulnerabilities. Dependence on GNSS connects many otherwise independent services to form an accidental system with a single point of failure. Erroneous GPS signals in an urban area could cause road accidents whilst disrupting the dispatch and navigation of emergency vehicles and causing their communications systems to fail. At sea in fog or at night, jamming could cause collisions between ships or with obstructions whilst causing emergency beacons to broadcast false positions, delaying search and rescue.
No-one has oversight over the increasing range of services that are dependent on GNSS or the complex ways in which they interact, and therefore no-one can reliably fully predict the consequences of a significant disruption of GNSS signals. This is an unsatisfactory situation for important services that are dependent on GNSS. This can be addressed through analysis of the effect of different types of disruption to the service, then making the decision between investing in detection, adding extra robustness or supplementing with an independent source of PNT data that has been shown to be unaffected by the types of GNSS failure that cause concern. In carrying out such an analysis, it will be important to consider whether the service that is being analysed forms part of a larger service and whether GNSS failures might simultaneously disrupt other services that are relied on to provide resilience.
Types of Interference to GNSS
Multipath vulnerabilities: Multipath is a situation when a receiver picks up reflected signals as well the normal signals direct from the satellite. GNSS signals can reflect off relatively distant objects, e.g. buildings, and cause gross errors in position accuracy if the receiver falsely locks onto this reflected signal instead of the direct signal.
Spoofing (false signals) & Meacoing (delay and rebroadcasting) both of which re-load erroneous satellites ephemeras are currently less common then jamming, although accidental meacoing could be caused by proximity of a GPS antenna with poor impedance matching.
Under the category deliberate interference there are three distinct types namely Jamming, Spoofing and Meacoing.
Automatic Identification System (AIS) used on ships for identify and locate vessels out at sea electronically, is now required for all international sea going ships for over 300 tonnes. However, poachers and smugglers want to by-pass it and so do terrorists. As a result, there is a growth of false IMO signals detected in 2014. Therefore, the fraud or avoidance of GNSS signal is easy, cheap and growing.
GNSS signals are no doubt vulnerable. They can reflect of building causing gross errors (multi-paths), high power transmitters, ultra-wide band radar, television, VHF, mobile satellite services, personal electronic devices can interfere with GNSS signals – even causing complete loss of lock. In 2002, a poorly installed CCTV camera in isle of man caused GPS within a kilometre be blocked. In March 2014, an 80 metre ship ran ground in North Umber land, UK as it was depending upon an unapproved GPS chart plotter and ran a ground. In 2015, during DEFCON 23 conference on Hacking, two hackers demonstrated how they have created a low cost emulator using of the shelf components, an open source code from the internet. A Wind Word report claimed 59% increase in GPS manipulation in 2013-14.
State Responsibility & GNSS
The US government accountability office, which is a non-partisan investigated arm of US Congress, had tasked the Department of Transport and Home and Security in 2014 to jointly develop backup capability in response to potential and natural and man-made threats to GPS systems.
2) Russian Federation
The Russian Ministry of Defence has launched a project to secure GLONASS against enemies’ destructive signals and is funding 300 million rubles to two Russian research institutions.
3) United Kingdom
UK government has carried out exercises to look into the vulnerabilities of GNSS at strategic locations.
On 18th/22nd May, 2015, the UN Office of the Outer Space Affairs (UNOOSA) and Russian Federal Space Agency (ROSCOSMOS) co-organized a workshop on Application of GNSS in Siberia where more than hundred delegates representing twenty-one countries participated to discuss issues of possible networking for joint efforts for risk mitigation and correction of GNSS signals from interference. They also discussed increased use of GNSS for disaster monitoring and support. This five-day workshop resulted in developing a proposal to set up an international education and training centre at Morocco, Nigeria, India, Brazil and China.
ICAO has repeatedly emphasised the state responsibility to prohibit all actions leading to destruction of GNSS signals and have asked it for setting up regulatory framework governing GNSS repeaters, pseudolites, spoofers and jammers. Aviation is another great potential user of GNSS especially ADS-B. However, the threat of destruction of GNSS signals remains. The shift from primary use of Radars to ADS-B for Aircraft Traffic Management is one of the biggest technology changes that aviation community will experience in the next decade. While there are many benefits associated with this change, there is a growing discomfort among operators how to comply with the regulatory mandate of FAA to be equipped with ADS-B by 1st Jan, 2020 in the US airspace.
By Sanat Kaul, Chairman, International Foundation for Aviation, Aerospace & Development (IFFAAD)-India Chapter. The article is based on a presentation made at the 4th Manfred Lach Conference on Conflicts in Space and Rule of Law held in Montreal on 27th – 28th May, 2016