Several jamming incidents in 2016 highlight the increasing reliance on GNSS by commercial aviation and vulnerabilities of PNT-dependent devices and systems to real-world GNSS threats.
Notices to Airmen (NOTAMs) and other warnings to pilots and crews reported GPS signal jamming near major international airports. Aircraft approaching or flying over these airports were advised to avoid using RNAV technology to plan their approach or landing, due to the presence of GPS signal jamming.
The NASA Aviation Safety Reporting System (ASRS) database contains records of pilot-reported incidents, which rose from 11 in 2013 to 28 in 2015, and they continued to grow in 2016.
There is also increasing pressure on the GPS spectrum, from Ligado plans to deploy wireless systems in the U.S. that transmit on adjacent frequencies, to Europe’s new Radio Equipment Directive with a GNSS Adjacent Band Compatibility test. The U.S. Department of Transportation recently found that high-precision GPS receivers were more likely to be affected by adjacent-band noise.
Large international airports lie near busy roads and parking lots and are more likely to encounter small in-vehicle personal privacy device jammers used by employees to disrupt tracking systems. These devices are capable of disrupting Area Navigation (RNAV) approaches that rely on GPS.
It is obviously important to test the effects of such interferers on safety-critical devices and systems so that their performance can be evaluated. There are strong guidelines and standards that apply to GNSS receivers designed for use by commercial aviation. This means that it is very rare for onboard GNSS receivers to output hazardously misleading information to other flight-deck control and management systems. But jamming does obviously affect aviation equipment by rendering it inoperative.
Monitoring the RF signal environment around an airport to understand where interference is occurring, and potentially what’s causing it, has become vital. This can be done using one or more interference detectors, and can produce extremely enlightening results. Information on the frequency of events and jammer types can also be collected and used to better protect systems installed at the airport.
The aviation industry has been deliberately cautious in adopting GPS technology, and is well educated about the specific vulnerabilities of GPS. But GPS interference is now widespread, and as the industry is becoming more dependent on GNSS, safety measures should be featured in every airport’s and every airline’s risk assessment framework.
The collection and analysis of statistics quantifying frequency and type of interference in GNSS bands should be an essential part of any mitigation strategy where GNSS is used to provide position or precise timing data critical for business to operate.
A number of systems and products are available to help businesses mitigate against threats to satellite positioning, navigation and timing (PNT) systems. Test instruments including GPS interference detection systems, real-world threat scenarios, and a range of professional services can all help to evaluate the risks and impacts of relevant threats.
For example, in Spirent’s Robust PNT Framework, once risks and threats have been evaluated, algorithms and/or hardware modifications to receivers and/or antennas can be devised. Once implemented, the effectiveness of these mitigation measures can be evaluated using the same Framework, which can generate simulation scenarios using captured interference waveforms.
This use of real-world interference waveforms in a realistic simulator-generated scenario can provide a great deal of information on likely effects of interference on GNSS systems and devices. Further, airports can implement operational measures to mitigate interference.
While the onus seems to fall on receiver manufacturers to mitigate against interference, they can only offer part of the solution. The remainder is institutional. A robust approach to identifying and dealing with threats must be mandated.
Even with robust receivers and a concerted effort to reduce threats, some will persist. Hence, a backup strategy must continue to be explored and funded at an appropriate level.
GUY BUESNEL is market segment manager, GNSS vulnerabilities, and PAUL CRAMPTON is senior systems engineer, both at Spirent Federal Systems.