WAYPOINT - Thinking in three dimensions
Dr Andy Norris, an active Fellow of The Nautical Institute and the Royal Institute of Navigation, looks at calculating and negotiating the distance from land – at sea and in the air
Many of the aids that we use for safe navigation give information to help establish our position over the Earth’s surface in two-dimensional form. These include charts, radar, GNSS, AIS, navigation lights and the optical and acoustic scenes. However, safe maritime navigation is heavily influenced by the three-dimensional reality of the situation, just as it is for aircraft and submarines. Unfortunately, surface vessels cannot readily use movement in the vertical dimension to avoid obstacles – although they can be carefully handled in a way that minimises their draught.
Ships may well spend a good deal of time manoeuvring close to land – beneath them. In direct contrast, the time that aircraft spend manoeuvring close to the land is extremely short. Aircraft have good control of their vertical movement, but even so, commercial aircraft only have to be flown close to the ground during take-off and landing – which is only permitted in highly regulated areas. Many aircraft are also able to utilise automated landing systems, although these need constant monitoring by the pilot.
Additionally, landing areas are well lit, smooth and readily inspectable by groundbased staff. Approaches and take-offs are regulated by ground controllers, who also ensure that the airport-based electronic systems that assist the landing are working well. All this means that, for commercial aircraft, it is very much easier to avoid any ‘grounding’ problems compared to the maritime world – except in some very exceptional circumstances, which generally result in tragedy.
THE SEABED IS A COMPLEX SURFACE THAT IS LIKELY TO DIFFER IN DEPTH AT ALL POINTS UNDER THE ENTIRE HULL
Ships spend a lot of time in situations where the keel is extremely close to the seabed, sometimes with less than a metre or two of estimated Under Keel Clearance (UKC). In comparison, if a vessel approaches this close to any above-water obstacle, it becomes an emergency situation, unless the vessel is manoeuvring very cautiously in known conditions, e.g. when berthing.
The obvious problem with the underwater scene is the general lack of high-precision information available to the navigator. We have to rely on integrating information from a host of sources, including charteddata, onboard depth sensors, tidal data, maritime notices, radioed information, physical markers and the current sea conditions, in order to get the best concept of the actual situation.
Also, we need to be constantly aware that no single figure can possibly describe the actual detailed situation under the hull. The reality is that the seabed is a complex surface that is likely to differ in depth at all points across the entire hull. These differences can be quite major when the ground is sloping, such as near the edges of a dredged channel, or if debris has fallen into the channel. The actual manoeuvring of the vessel can also cause major changes in depth along the vessel even if the seabed is completely flat.
Laser-based range equipment, known as LiDAR, could be used as the basis to give navigators a much improved real-time depiction of the situation directly under the keel – and of the surrounding area, below and even above the water surface. LiDAR has been used for some years for underwater surveying. LiDARbased systems directed towards the navigation of commercial vessels are now beginning to emerge, not least because of current thinking towards the use of autonomous vessels.
The existing international requirements for the performance and fitting of depth sensors to ships have not changed since 2001. It will be interesting to see how they evolve as LiDAR technology becomes increasingly available for appreciably assisting the safe navigation of vessels. the maritime world – except in some very exceptional circumstances, which generally result in tragedy. Ships spend a lot of time in situations where the keel is extremely close to the seabed, sometimes with less than a metre or two of estimated Under Keel Clearance (UKC). In comparison, if a vessel approaches this close to any above-water obstacle, it becomes an emergency situation, unless the vessel is manoeuvring very cautiously in known conditions, e.g. when berthing.
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