WAYPOINT - Making the most of multiple data sources
Dr Andy Norris, an active Fellow of The Nautical Institute and the Royal Institute of Navigation, discusses the importance of considering all available data when navigating and repeatedly comparing and analysing it to gain as full a picture as possible
The navigation of ships remains one of the most difficult transport tasks to perform safely, even when compared with civil aviation. A particular difficulty is that the highly important underwater scene is almost always totally invisible to direct sight. This means that navigators must constantly assess the available clearance by comparing the relevant data from many different sources.
These sources include charts, tidal information, onboard depth sensors, radar/ AIS data from buoyage, position sensors, radioed information, etc, not to mention maintaining skilled human optical views of the situation. In shallower waters, the human view especially includes looking out for all relevant physical markers and for areas that have differences in their sea-state.
Of course, safe navigation in any situation requires the navigation team to repeatedly compare and correlate all available relevant information. This must be performed at a rate that ensures safety in the particular situation. Significantly, even lower accuracy sources of data, when intelligently used, can provide crucial indications of potential problems from sources that are generally more accurate.
A consistent picture
Reviewing and evolving a best estimate of the actual situation is essential for safe navigation. The team can only assume that they really have a good knowledge of the immediate circumstances when all data sources give a consistent ‘picture’ of the situation.
In reasonable viewing conditions, the optical scene (‘the view out of the window’) provides considerable assistance in understanding of the total situation. However, a full 360-degree view is not readily available from the bridge on many vessels. Also, there are difficulties in optically viewing hazards that are very low in the water, particularly if they are very close to own vessel. Unfortunately, on many vessels it is still not a particularly quick and easy task to transfer sight-derived information onto navigational displays. Even using an ePelorus is not as common as perhaps should be the case in this modern age.
Safe navigation in any situation requires the navigation team to repeatedly compare and correlate all available relevant information
Data from both radar and AIS-detected targets can vary from being incredibly accurate to being highly inaccurate or even totally missing. However, the inconsistencies between radar and AIS are rarely linked. If they match up, this greatly increases the probability that the positional information of that particular target is correct. False tie-ups can also randomly occur, so never completely rely on this information without confirmation from other sources of data, such as the visual scene.
All available data
Of course, the actual navigational situation greatly influences the amount of effort required to fully maintain safety. Even in deep-water oceanic areas, it must never be assumed that any single source of data is correct and available, such as the vessel’s current position from installed GNSS. Fortunately, any temporary issues with absolute positional accuracy in such areas should not decrease the vessel’s safety – so long as radar data and visual data are well monitored and correlating appropriately.
When the visual scene is heavily influenced by poor weather, such as dense fog, you have to rely totally on all other available data, including the acoustic scene (what you can hear). Of course, this greatly affects the basic parameters that must be chosen to ensure the safe navigation of own vessel, such as the vessel’s track and speed through the water.
The mental skills and energy needed by navigators for continually collecting and assimilating all necessary data in busy situations are demanding. Never assume that you have a 100% correct understanding of an evolving situation and keep looking out for consistencies and inconsistencies at a rate that adequately meets your current circumstances.
Contact RIN at: www.rin.org.uk | 1 Kensington Gore, London, SW7 2AT | Tel: +44 (0)20 7591 3134