January 29, 2013

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GPS Interference: A Threat to NextGen and SESAR?

The US NextGen and European SESAR programs both aim at more efficient use of airspace and better air safety, and have a common vision to integrate and implement new technologies to improve air traffic management (ATM) performance. Though the implementation frameworks for each are radically different, both these programs rely on the Global Positioning System (GPS) for achieving the end objective.

GPS originated as a military positioning aid and was later adapted for public use. Today, GPS has become a ubiquitous aspect in our daily lives. It finds its use in smartphones, cars, ships and aircraft. It is used not only for navigation, but also for precise positioning and timing. For aircraft users, the GPS guided navigation systems are invaluable, as the entire safety of the passengers and the efficiency to navigate are highly dependent on the navigation systems in place.

GPS can be split into three areas -- the ground, space and user segments:

  • The ground, or control segment is used to upload data to the satellites, to synchronize time across the constellation and to track the satellites to enable orbit and clock determination.

  • The space segment consists of the GPS satellites in six orbital planes. 24 satellites make a full constellation, and there are currently (as of January 2011) 30 satellites in service.

  • The user segment consists of the receivers and associated antennae, used to receive and decode the signal to provide position, navigation and timing (PNT) information.

Originally, systems were built in such a way that the GPS satellite signals were extraordinarily difficult to detect, and use, as it was meant for military applications, was achieved by a combination of very low transmitted signal power and signal concealment techniques. But the inherent design of the system has made it vulnerable to interference from more powerful low frequency transmissions. The problem of vulnerability of the GPS network is not new, and was highlighted way back in the year 2001. The vulnerability of the GPS networks has come into question recently following an increase in GPS jamming and spoofing incidents.

Some of the known mechanisms for deliberate interference include:

  • Jamming of GPS signals – the main threat is to lower-altitude aircraft on a GPS, GPS/RNP approach and on the airport surface, where GPS-driven airport maps are being used

  • Rebroadcasting a GPS signal maliciously, accidentally, or to improve reception but causing misreporting of a position

  • Spoofing GPS signals to create a controllable misreporting of position, for example to deceive tracking devices

Mitigation strategies are being put in place and systems are being evaluated for navigation alternatives to overcome interference, jamming and spoofing. Some of the alternatives being considered as are follows:

  • Enhanced GPS receiver jamming protection: Efforts are being made to improve receiver standards, total anti-jam is impractical and retrofits could be cost prohibitive

  • Increase satellite transmitter power: Increasing the transmit power of satellites currently in orbit is impractical, but can be achieved by next generation GPS III constellation – launch is planned for around 2015

  • Anti-spoofing technology: Futuristic, and will have lots of challenges

  • Combining inertial navigation systems (INS) with GPS: One of the conventional methods to avoid spoofing. A major problem in INS systems is that the errors keep accumulating over time in case of “free running” and would eventually increase the path divergence. Perfecting the combination of GPS and INS systems with kalman filter implementation is another topic of research interest

Any transmission that impedes GPS networks is of major concern to the aviation industry. GPS reliability as a key element of NextGen and SESAR will come under pressure as its interference will become further noticeable in the coming years. The future of GPS avionics is tied to the future of the overall Global Navigation Satellite System (GNSS). Are the backup systems being proposed adequate for the emergence of a dependable system? Only time will tell…

References

Report published by “The Royal Academy of Engineering” titled “Global Navigation and Space Systems – reliance and vulnerabilities Report” - last accessed on 22nd Jan 2013.HCL's aviation software team provides a complete suite of solutions from design to support.

http://www.raeng.org.uk/news/releases/shownews.htm?NewsID=633 last accessed on 22nd Jan 2013

http://www.logistics-insight.com/news/terrorist-threat-uk-gps last accessed on 22nd Jan 2013

http://www.npstc.org/gps.jsp last accessed on 22nd Jan 2013