Multifunction displays and the ‘big picture’

Computer technology in the form of high-speed microprocessors, large capacity, fast-response memory and color display screens is being used with increasing success in marine electronic systems. Chart plotters (integrated GPS navigation, digitized chart database and display/control hardware), raster scan radar and sonar/fish finders are now common equipment on many recreational vessels. The hardware is both effective and relatively low in cost. And the software allows for a great deal of flexibility in displaying and combining data from various sources and sensors.

The challenge, of course, in using all of this capability lies in choosing the optimum combination for the information display. Too much of a good thing can create clutter, reducing the value of the information. Displays that overlay information from two sources, such as radar and a chart plotter, must be done with special care to avoid creating a situation that can confuse the helmsman and lower his effectiveness.

A raster scan CRT or LCD screen can display anything presented in a compatible electronic format: radar, chart information, sonar, engine instrumentation, the status of any monitored vessel system, video and email. The list is endless.

Blocks of data or overlays

There are two primary ways of using the flexibility inherent in a digital display: organizing separate data blocks in on-screen windows of varying size and form, or creating overlays of related data, such as a chart and information from the radar or sonar. Multi-source data displays that subdivide the viewing screen into defined segments for each data source usually work well, provided the viewing screen is of adequate size. Jamming too much information into a small area is clearly counterproductive. Most users will soon find which combinations of data are most useful for each navigation situation. Some systems allow various configurations to be stored as “hot pages” for quick recall.

The greatest display challenge occurs when an image-overlay display is used, for example, combining the data from the radar with the chart plotter. This overlay combination can be very appealing, especially for the navigator new to radar data interpretation. Being able to see that the shoreline on the chart coincides with the radar return greatly simplifies the radar-return identification process. Radar signal returns from charted navigation aids will also help the user gain confidence in using the radar. However, to be of value, the displayed information from the chart plotter and the radar must always be in register, in agreement in both heading reference and scale. The confused image that results from differences in heading reference will diminish the value of both sources of information to the point at which it becomes imperative to turn off either the radar or the chart image. Left in view, the confused images can become a hazard to safe navigation.

The radar/chart-plotter information must coincide both when the vessel is on a steady heading and when turning. Achieving this goal begins with assuring that the radar antenna’s azimuth reference (the heading or lubber line) precisely coincides with the longitudinal axis of the boat. While a few degrees’ difference between the radar’s lubber line and the vessel’s bow rarely causes problems, when the radar image is viewed by itself, even a very few degrees’ difference in alignment can create a confusing picture when combined with the chart image. The confusion is compounded by the fact that radar returns often emanate from the positions offset from that of charted objects and especially from shorelines, where the radar image originates from higher-elevation features well inland.

In addition to this static alignment, the data from the radar and the chart plotter must remain aligned as the vessel maneuvers. This dynamic alignment depends on the performance of the vessel’s heading-reference system. For all practical purposes, the bearings to targets on a radar screen are depicted without time lag or delay. (The typical antenna scanning rate of 24 revolutions per minute, 144 degrees per second, imposes minimal azimuth lag, even during the most aggressive maneuver.) Any change in a vessel’s heading will result in an immediate and obvious change in the relative bearing of all targets. By contrast, the heading information used by the chart plotter and the radar is usually supplied from a magnetic flux detector (flux gate) that is subject to the same magnetic dip and acceleration errors common to the mechanical magnetic compass. Heading information from this source will, at best, lag behind any changes in the vessel’s heading and, under some circumstances, can be initially misleading. The resulting combination of momentarily erroneous chart-plotter heading information with accurate radar data can yield a confused navigation picture. The tug and barge that were the center of the helmsman’s navigation attention are suddenly seen to move outside the charted channel while the radar image of the shoreline appears to have jumped to a new position relative to the chart.

Higher-quality heading data

The dynamic heading-data error problem can be reduced with substitution of a higher-quality source of heading information. The addition of a gyro- to a flux-gate-based heading system will greatly minimize the system time lag. Even better results can be achieved with use of a north-seeking ship’s gyrocompass or a GPS compass. Regardless of how it is accomplished, a successful radar/chart-plotter overlay display requires an accurate and lag-free source of heading information.

Even when a high degree of target coincidence is achieved, the value of data overlay presentations can be compromised by visual clutter, the equivalent of trying to listen to an interesting conversation in a loud cocktail party. The clutter may in part be the result of using a too-small display screen; however, the mass of data contained on marine charts used in in-shore areas can overwhelm even large, 22-inch-diagonal screens. The various data deletion options available for use with vector-based charts can help, but there is always the risk of deleting a needed layer of information. The data content from the radar is usually fixed, and other than using the offset control to minimize the display of unneeded information (say, aft of the vessel), there is not much that can be done to clean up this information source. A further complication occurs when the navigation situation requires frequent changes in radar range. The screen redraw time for a combined chart-plotter/radar image may be prolonged.

Finally, there is the problem of too much information in too small a space. Few yachts carry display screens large enough to deal effectively with the mass of overlaid data a chart plotter and a radar can generate. A small screen may look great in the showroom or when the boat is stationary, but it will look quite different when maneuvering in less-than-perfect weather. Separate side-by-side display of radar and chart-plotter data are often preferable to an overlaid presentation. It is still very desirable for the two systems to share a common, lag-free heading source; however, the result of a small amount of heading lag on the separate chart image will not be nearly as serious as when the overlay image is used. With separate display windows, the range setting of the radar can be shifted when necessary, while avoiding the image blackout that occurs when a data-rich chart image must be redrawn. The navigator’s eye and brain will fuse the two separate images into a clear and useful picture of the overall situation.

By Ocean Navigator