WAYPOINT Critical positioning
Dr Andy Norris, an active Fellow of The Nautical Institute and the Royal Institute of Navigation, examines the significance of relative position-based sensors for improving situational awareness
Information based on absolute position – knowing where you are on the surface of the earth – is highly important to safe navigation. So, too, is relative position – where you are in relation to everything else in the area, including other vessels, navigational aids and any hazards to navigation. Good situational awareness should make use of both.
The concept of absolute position was developed some centuries ago, with the realisation that the Earth was near-spherical, and that astronomical objects, such as the Sun, stars and planets, could be used (when visible) to determine the unique position of the observer on the Earth’s surface.
GNSS is based on a very similar basic concept to astronavigation but, of course, gives massively higher accuracy and availability. Not least, AIS allows vessels and navigational marks to transmit their own determined absolute position to all vessels in the vicinity, greatly aiding safe navigation for everyone.
However, we should never rely too much on the integrity of any information that is solely referenced to absolute position. Even when using multiple satellite services, there are many possibilities for significant inaccuracies when estimating our own absolute position. These include failure to receive signals from satellites in certain areas and the ever-growing menace of GNSS jamming and spoofing.
Proceed with caution
Positional information received by AIS should always be treated with caution. We have no knowledge of whether the equipment on the vessel sending the signal has been properly set up and maintained.
Some transmissions could even be spoofs. Even when this information is accurate, AIS does not tell the whole story. Numerous navigational hazards do not transmit AIS information, including many small craft and navigational marks – and, not least, floating or submerged wreckage. Never put 100% reliance on the positional accuracy of charted features, either. The situation might have changed since the last survey.
NEVER PUT 100% RELIANCE ON THE POSITIONAL ACCURACY OF CHARTED FEATURES. THE SITUATION MIGHT HAVE CHANGED SINCE THE LAST SURVEY
Real-time hazard avoidance is fundamentally based on relative position: how far away are any potential hazards in the area, and what is their bearing and relative speed? Key navigational aids, such as the human eye, radar and sonar, all give independent estimates of the presence and relative motion of potential hazards, with no fundamental dependency on GNSS and/or knowing their absolute position. Neither do they need any additional information from the target, although detecting the target can be greatly aided by lights and radar reflectors, etc.
Navigation aids based on relative position provide a totally independent check on the integrity of displayed absolute position data, including most charted features. However, you should also bear in mind the particular weaknesses of such aids when assessing the complete scene. Not least, the further away the target is, the more their performance degrades, declining steadily to zero. It is possible to jam or spoof most relative position sensors available today, but in reality this is rarely encountered outside of war zones.
Into the future
In the future, inertial sensors will increasingly contribute to the safety of maritime navigation, including enhancing the accuracy of transmitted AIS data. They are relative position based aids that determine the actual movement of the vessel from any known position and are achieving ever greater accuracy. This enables an estimate of the vessel’s current absolute position to be independently and automatically determined, potentially over hours or even days. Importantly, they are totally immune to jamming and spoofing.
jamming and spoofing. Unfortunately, they will not prevent spoof data being transmitted from other vessels. Therefore, the optical, radar and sonar scenes will remain just as important as they are today.
GNSS, when sensibly used, makes a valuable contribution to the safety of navigation, but we must always bear in mind its weaknesses and the fact that collision avoidance is fundamentally based on the concept of relative navigation. The importance of always using information from relative position based sensors when making real-time navigational decisions must never be overlooked.
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