WAYPOINT - Integrity and the Aid to Navigation
Dr Andy Norris, an active Fellow of the Nautical Institute and the Royal Institute of Navigation, examines the strengths and weaknesses of common aids to navigation
Aids to navigation have a long history. Lighthouses go back to Roman times at least. Undoubtedly, well before then, enterprising sea users would have erected primitive marks, both onshore and in shallow waters. Today, they continue to evolve, underlining their immense significance to safe navigation.
Perhaps the best definition of the term ‘aid to navigation’ is given by IALA:
‘Any device or system, external to a vessel, which is provided to help a mariner determine position and course, to warn of dangers or of obstructions, or to give advice about the location of a best or preferred route.’
Understanding the strengths and weaknesses of aids to navigation is key to being a successful navigator. Traditional aids, such as buoys, beacons and lighthouses enhance awareness of the boundaries between safe and dangerous areas.
Together with a compass and appropriate chart, they allow the navigator to determine the vessel’s absolute position. They provide a valuable independent method of checking the accuracy of information received from systems like GNSS and Radar – provided they are visible.
Radar reflectors and racons
Radar reflectors on traditional aids to navigation enhance a Radar’s ability to indicate them on the display, making it easy to assess their ranges and bearings from the vessel. Visually comparing the Radar image of an area containing several aids to navigation with charted data helps ensure they are identified correctly.
If an aid to navigation has an installed Radar beacon (racon), this further assists the correct identification of its position on the Radar display. With care, it allows the position of your vessel to be estimated with reasonable integrity from just one aid to navigation. Any appreciable difference from the position indicated by the GNSS shows that there is a problem, potentially with GNSS or the Radar.
AIS
AIS aids to navigation have different attributes to racons. On a GNSS-stabilised Radar display, with all related systems correctly set up and functioning, calculated aid to navigation positions will be clearly indicated and should show a good tie-up with Radar responses from physical aids to navigation, assisting situational awareness. A lack of Radar response from an indicated AIS position may mean it is currently out of Radar range – or that there is a significant positional or Radar error.
Importantly, an AIS aid to navigation can either transmit its GNSS continuously-derived position (typically from GNSS) showing where it actually is, or its surveyed position showing where it ought to be. In this case, a GNSS error over the local area should be visible. To be sure, you must trawl through the data within the transmitted AIS Message 21, which can get very complicated!
Virtual AIS aids to navigation need careful attention. They are invaluable in using Radar or ECDIS to show critical data that is not clearly visible to the eye, nor plotted on a chart, such as the position of a recent wreck. However, if there is no other supporting information available, be ultra-cautious about the accuracy of this information. For instance, positional data contained within the transmissions may be inaccurate, due to authorities needing to react quickly in an emergency situation. AIS data is also easy to spoof and this is virtually impossible to detect without supporting visual or documented information.
TRADITIONAL AIDS, SUCH AS BUOYS, BEACONS, LIGHTHOUSES AND MARKERS, ENHANCE AWARENESS OF THE BOUNDARIES BETWEEN SAFE AND DANGEROUS AREAS
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