Chart Plotters: The Best Is Yet To Come

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Both vector and raster systems can overlay radar to show the radar data for shorelines, ships, aids to navigation and many other targets. It is especially useful to show ships and boats against a nautical chart. Some systems can even show the Automatic Identification System data that ships broadcast.

However, radar doesn’t consistently show an image that matches a chart. Many beaches slope gradually and show radar echoes inshore of the actual shoreline. And radar always shows a point or an island wider than it is, due to the radar beamwidth. Longer radar antennas are better than short ones in this respect, but they all show the effect. Radar cannot show land features over the crest of a hill, since radar uses line-of-sight radio frequencies. It takes some experience to match a radar picture with a chart, but radar overlaid on a chart plotter speeds up the learning process.

The path ahead

As useful as chart plotters are, a series of automatic, accurate position plots is only one part of navigation. A navigator must plan a safe route and cross-check every piece of information with other data. Thus, a navigator needs to compare fixes (of every type) with the depth and the proposed course. Normally this involves plotting a DR (dead-reckoning) position and comparing it with fixes to reveal set and drift of the current, errors and necessary changes. It is most important for a navigator to examine the course ahead of the ship to see if the present track will cross any hazards. As I mentioned, it is possible to program a chart plotter or computer using an ENC to look ahead of the ship to identify hazards by distance, type and time to arrive. This is the age-old problem for navigators and goes far beyond the first-generation equipment that plotted vessel position continuously.

Many ships have stranded when the navigator failed to examine the path ahead for hazards after changing from the planned course to avoid a ship or for another reason. There are many notorious examples of this: the 1995 Star Princess grounding on Poundstone Rock in Alaska, the 1992 Queen Elizabeth 2 grounding in Buzzard’s Bay, Mass., and, of course, the 1989 Exxon Valdez grounding on Bligh Reef in Alaska. These and many other groundings occurred when the officer of the watch concentrated on one thing, such as an approaching ship, and overlooked hazards ahead on an altered course.

In order for a chart plotter to provide the best input for warnings of an impending grounding, it is necessary to adapt the software to account for the ship’s characteristics, including turning radius and stopping distance. Obviously, the system must alert the navigator well before it is too late to take effective action. This is far beyond many of today’s systems, which will merely show what was hit after the ship stops. But some systems today using ENCs offer an early version of this feature.

This isn’t a new problem, and it’s been addressed in the past. The Iotron Company developed an add-on to radar back in the 1970s that did exactly this. It was called Digiplot and was the brainchild of Jack Herther. Digiplot had many advanced features, such as automatic acquisition of radar targets, fully automatic tracking, and consolidation of 3-cm and 10-cm radar information. It was limited by computing power and memory costs.

With today’s ENCs, a system based on this technology could make a significant improvement in ship safety. The great increases in computing power and data storage make this type of system far more affordable than before.

It took about three decades to go from manually plotting radar targets, to determine risk of collision and predict the effect of proposed maneuvers, to the Automatic Radar Plotting Aid that is used aboard ships. The job of developing and implementing a system to predict groundings from chart plotter and radar data is more complex but within reach. I see it as a problem that requires international cooperation and standards to be fully effective.


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