Resilient Navigation in Demanding Maritime Environments

The European GNSS Agency (GSA) joins the international maritime community at TransNav 2015 to discuss the use of GNSS within the maritime sector.

GNSS has revolutionised maritime navigation, including in such restricted maritime waters as port approaches and harbours, where traffic is densest and the risk of collision and grounding is greatest. Indeed, the shift to satellite-based positioning systems has been so far-reaching that, in 2010, the US Coastguard started dismantling its Loran-C ground-based navigation system.

But with the increased reliance on GNSS comes a correspondingly heightened potential risk if anything should go wrong with the signal - one of the reasons the US has since reversed its decision and is now investing in an upgraded eLORAN system as a backup for GPS. It is challenges like this that drove the discussion at the recent TransNav 2015 conference.

A Need for Resilency

The problem is that the low-power GNSS signal is relatively susceptible to interference, whether from natural sources, such as solar storms and ionospheric activity, or reflections (multipath interference) from buildings and local infrastructure. “Large steel structures like cranes can block and reflect the GNSS signals and introduce error,” explains William Roberts, Applications Manager at Nottingham Scientific Ltd (NSL), which specialises in mitigating GNSS interference. “In other words, positioning within a port is very similar to positioning in a downtown city.”

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With the increasing availability of low-cost jamming devices, it is also relatively easy to maliciously jam GNSS receivers. Less easy to do, but an equally serious threat, is ‘spoofing’, or the deliberate distortion of the GNSS signal to give false readings.

It is not surprising that the need for resilient position, navigation and timing (PNT) is at the core of the International Maritime Organisation’s (IMO) recent strategy for e-Navigation, which recommends integrating a range of different sources of PNT information in ship-borne receivers. These systems can then complement each other should any one system fail. More so, they can also provide a means of cross checking position information, for example in the event of a spoofing attempt.

“The most important issue is to measure and analyse the extent of the problem with GNSS in the area concerned and see if there is a real risk to operations,” says Roberts. “GNSS receivers may then need to be made more robust and resilient, with interference mitigation techniques.”

Complementary PNT Systems

There are already several different and complementary sources of PNT information. GNSS provides a three-dimensional position, velocity and time solution when at least four satellites are in view for a given system, such as those belonging to GPS, Glonass, Beidou and Galileo (as from next year). If constellations are combined, such as GPS and Galileo, an extra (fifth) satellite is needed - three for the position, one for the GPS time correction and one for the Galileo time correction. Meanwhile, land-based radio-navigation systems can provide a two-dimensional position using at least three stations. These systems include e-LORAN, which is upgrading and replacing LORAN-C, using solid-state transmitters, precise timing (using atomic clocks) and a data channel to provide correction and integrity messages. 

Various augmentation systems already provide integrity data and corrections to improve PNT accuracy. Some, such as D-GNSS and real-time kinematic carrier-phase enhancement (RTK), use ground based reference transmitters. The Automatic Identification System (AIS), which is mandatory on all passenger ships and on cargo vessels over a certain tonnage, integrates a VHF transceiver with a positioning system like GNSS or LORAN-C.  There are also satellite-based augmentation systems (SBAS) like EGNOS, which incorporates an ionospheric model, is a valuable tool to correct for most natural sources of error.

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Meanwhile, more traditional ship-borne instruments are still valuable, such as Inertia Measurement Units, the gyro-compass, echo-sounder and radar, as well as signals of opportunity (SoOP), which use VHF communications signals in the vicinity to help estimate position by triangulation.

Some resilience may also already be built into the GNSS system. “The Galileo authentication service will provide an added value for general navigation, but especially in restricted waters where the traffic is more intense,” says GSA Head of Market Development Gian Gherardo Calini.

“This service will inform the mariner about the origin of the signals. In other words, if signals come from the Galileo satellites or not, thus representing a protection for the mariner against spoofing.”

An Integrated Solution

The use of multiple-system receivers that can handle navigational signals from two or more GNSS, with or without augmentation, is already one way to achieve more resilient PNT data. But as they all use essentially the same (L1) signal, they are all equally susceptible to jamming.       

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To achieve greater resilience, especially to intentional or unintentional interference, it is necessary to use two or more independent, or frequency-diverse radio-navigation systems. This kind of combined approach provides the redundancy to mitigate the loss of a single system and underlies the eNavigation strategy being implemented by IMO.

The IMO Maritime Safety Committee is finalising a resolution on performance standards for multi-system ship-borne navigation receivers, with support from IALA (International Association of Marine Aids to Navigation and Lighthouse Authorities) and CIRM (the International Maritime Radio Committee). At the same time, new IMO guidelines for resilient PNT are under development and will include both EGNOS and Galileo.

Heads Up

For Roberts, purely technical solutions to a more resilient navigation in demanding maritime environments may still not be enough. Radio-navigation signals like eLORAN, for example, are also susceptible to jamming, though they use different frequencies to GNSS.

“There is a degree of overreliance on what the GNSS receiver tells you,” he says. “It’s becoming treated as a black box navigation tool, with little appreciation of what’s going on behind the screen. You need to understand the navigation system and use your better judgement to identify when there may be errors. This can also mean better awareness of what’s happening around you, so you don’t necessarily believe what’s being plotted on your map.”

More Information

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