GPS jamming: Keeping ships on the 'strait' and narrow

Big, modern ships are highly automated with networked navigational systems, including differential GPS (DGPS) which offers more accurate positioning (to one metre) than conventional GPS.

However, previous research has found that these highly-sensitive maritime DGPS receivers, are easy to disrupt, using £50 jammer devices, which are widely available on the market.

DGPS signal disruption, particularly when ships are navigating through narrow inshore waters, could result in inaccurate positional information, leading to more maritime accidents.

For the study, academics from the Nottingham Geospatial Institute (NGI) and RNoNA Navigation Centre tested DGPS disruption in the busy shipping lanes of the Norwegian straits where navigational errors account for half of accidents.

Dr Lukasz Bonenberg, senior technical officer at the NGI, said: "Main factors behind maritime accidents in this part of Norway are an influx of foreign vessels, coupled with quickly changing weather conditions and the dangerous nature of the narrow inshore waters.

"In these difficult conditions, with a need for high-accuracy navigation, there tends to be an over-dependence on DGPS technology which can lead to a false feeling of security. These errors have increased significantly since the introduction of DGPS on most ships.

"DGPS jamming from nearby cliffs, for instance, could seriously affect shipping traffic going through the narrow straits and fjord networks. Affected vessels could take a long time to correct their journey or physically stop, which may cause the maritime equivalent of a motorway pile-up," Dr Bonenberg adds.

The trial was conducted with the high-end surveying grade receiver and antenna, which was placed on the shore with the jammer moving towards or away from the receiver on a small boat.

The aim of the trial was to quantify the jamming effect, simulating a vessel's approach to a narrow inshore strait. The researchers found that the DGPS receiver didn't stop functioning altogether. Instead it gave false readings in the on-board navigation system with positional data moving more than 10 metres.

"Observed discrepancies of up to 10 metres are very hazardous, considering the narrow nature of the Norwegian straits, which are frequently affected by poor visibility," explains Lieutenant Commander Oeystein Glomsvoll at RNoNA Navigation Centre.

"GPS jamming is a worldwide growing problem. The technology for jamming is readily available, resulting in many cases of intentional jamming in recent years, and the attention given to this problem has increased."

The research team looked at a solution that would fix and maintain a transporter ship's position more accurately and quickly using additional GPS signal frequencies instead of upgrading navigational systems on board.

Currently, the majority of maritime receivers are DGPS L1 ones. Authors suggest combining this system with the multi-frequency GLONASS receiver -- an alternative navigation system to GPS -- is advantageous.

This is because the frequency band of GPS + GLONASS signals together is much wider than dual L1 and L2 frequency GPS or GLONASS alone, increasing positional precision.

This has been traced back, not only to the increased number of satellites used, but also to the higher elevation and better coverage of the GLONASS satellites operated by the Russian Aerospace Defence Forces.

Dr Bonenberg, from the Faculty of Engineering, said: "The use of multi-constellation receivers and an increase in the frequencies received offers better jamming resilience for close-to-shore navigation."

GPS signalling is currently undergoing modernisation, which includes an additional frequency (L5) and an open L2C code on an L2 frequency. Currently, only a limited number of satellites offer these signals. Data observed during this experiment suggests that use of this modernised signal will offer advantages similar to the multi-frequency GLONASS one.