Advertisement

Using Radar Effectively

Recognizing false radar echoes, targets that aren't really there, can improve safe boating in poor visibility conditions.

October 4, 2007
Share on facebook
Share on twitter
Share on pinterest
Share on email
Share on facebook
Share on twitter
Share on pinterest
Share on email

If forced to choose one electronic navigational device from the panoply of gadgetry available, many seasoned coastal navigators would unhesitatingly surrender all in order to keep their radar set. There are good reasons for treating radar as a firstborn. Most electronic navigational equipment serves to refine and render more convenient information that can be obtained by other, albeit more tedious, means. For instance, a depth sounder can be a treasured friend at times, but a brick tied to the end of a string will tell you just as much, and without the danger of misinterpretation due to the fact that your chart is in fathoms but your sounder is reading in feet or meters. Similarly, while most of us are quite happy to worship the god of GPS, the fact is, bearings, soundings, and dead reckoning are still the bedrock of navigation. Moreover, numerous vessels have run aground in close quarters because the navigator was busy sharpening pencils and fiddling with parallel rules trying to plot a GPS fix which, at best, will only tell you where you were two minutes ago. Radar, however, is a different animal altogether. Radar enables us to see in the dark. Radar enables us to see in the fog. Radar allows us to see where we are, not just where we were. Lastly, radar allows us to see other vessels and other objects that don’t appear on the chart.

For all its virtues, radar has some limitations, hence the mantra, “The prudent navigator will not rely on any one source of information, etc. One of these limitations is a tendency to produce false radar echoes under certain conditions. Targets may appear on your scope that aren’t really there, or aren’t what they appear to be. The problem for the navigator is if he takes avoiding action for something that isn’t really there, it could place the vessel in a different predicament unnecessarily. On the other hand, assuming that a particular echo is false invites a potentially disastrous interaction should the echo prove to be real. Whole chapters have been written about false echoes, and there are many types, but here is a look at a particularly insidious one that is not so rare on our coasts: the false echo generated by overhead power cables.

My first memorable encounter with this phenomenon occurred some years back while ascending the Potomac River to Washington D.C., in the topsail schooner Pride of Baltimore II. About 20 miles below the city, just before Mount Vernon, a rather substantial power line crosses the river in an area where heavy fog is a common occurrence. It was just such a day when the watch officer called me to say that he had a down-bound vessel in the vicinity of the power lines. I was well aware of the power lines because, with a 107-foot mainmast, one must be. The officer explained that he had twice altered course to starboard to give the target vessel more room but the other vessel appeared to still be dead ahead. By the time I came on deck we were on the extreme edge of the navigable channel and no longer aligned with the area of maximum overhead clearance. If we returned to the deep water we risked collision, but if we altered more to starboard we risked grounding or striking the power lines.

Advertisement

I asked the watch officer if he had hailed the downbound vessel by radio. When he replied that he had tried without response, I realized that we might be dealing with a false echo. I ordered that we slow to bare steerageway and eased back into the channel. Lo and behold, the target eased back toward mid-channel also. While there is nothing conclusive in determining a false echo from receiving no reply on the VHF, especially when navigating with difficulty in fog, I also observed that the target was approaching at a rate identical to our own speed. In other words, the target was dead in the water. Additionally, there was no reply to our sound signals. As we passed under the cables with bare steerage way, it became apparent that we had been dancing with a ghost, and had nearly gotten into trouble in the process. Why was the officer so sure that this ghost was down bound? That was just a trick played by his assumptions, but that’s another story.

False echoes from power lines are not generated by the cable itself, but by the electromagnetic field surrounding it. Electronic pulses emanating from the radar are reflected by this field back to the scanner, producing a target on the scope. Instead of a solid, linear target spanning the waterway, as a bridge appears on radar, the echo from a power cable resolves into an isolated blob, much like the echo coming from a vessel. The false echo from a power line will tend to stay between you and the nearest portion of the cable, and it will appear at the same distance as the cable. Where power lines cross a waterway at right angles, the false echo will hover out there, dead ahead, waiting for you. If you move to starboard, so will it, and vice versa. This is the relatively straightforward scenario that I experienced on the Potomac River. But when the cable crosses at an angle to the waterway, the echo it emits can appear like a vessel getting under way from one side of the river bound for the other on a collision course with you, which just happens to replicate the path of the cable.

With a target approaching from port the natural response is to alter to starboard, thus adhering to “your side of the waterway, as well as to the Rules of the Road. Inevitably, however, your course will continue to converge with that of the mysterious target, forcing you farther to starboard. Watching this on radar, you might eventually opt to alter course radically to port to pass under the stern of what is assumed to be a relentlessly oncoming vessel. Instead of passing ahead, however, the echo from a power cable will appear to stop and abruptly reverse course, thus confounding the final, desperate attempt to stand clear. Instead of the actions of a bizarrely prescient and contrary mariner, you might be witnessing the tendency of a false echo from a power cable to stay between your vessel and the cable. If this were merely an amusing phenomenon there would be no reason to address it here. But, in the process of avoiding this ghost, all manner of other complications can arise with regard to water depth, other vessels, buoys, and your flexibility to avoid hazards, not to mention general disorientation in what are already challenging circumstances.

Advertisement

Now that you know about the potential for overhead cables to generate these ghost ships, what can you do to interpret the phenomenon correctly? If passing under a power cable is part of a passage you expect to make again, take a moment on a clear day to switch on the radar and observe any false echoes and how they behave so that, come some foggy, rainy night, you can picture what is going on out there. Go ahead and annotate the chart to alert yourself, or whoever may be navigating with you, as to what to expect. Recognize that a false echo of this particular type is essentially dead in the water-you will approach it at a rate equal to your own speed. This does not eliminate the possibility that a vessel could be sitting out there in the fog under a power cable, so the radio call, the appropriate sound signals, and a safe speed are necessary measures. If the target behaves as described above, then the possibility of a false echo clearly exists. But since there is ultimately no way to guarantee the falseness of an echo in poor visibility until you are occupying the same patch of water, slowing down is always a good idea when in doubt. At sea all echoes should be treated with caution even when your suspicions of a “ghost are well justified.

Advertisement

More Electronics

Advertisement
Advertisement