Radar user blinded by heavy sea clutter

While this phenomenon can and does occur anywhere waves of sufficient height are encountered, the particular experience related here took place off the Pacific coast, where conditions routinely produce large waves. The area generally affected is between 34° and 45° N, east of 132° W. These coordinates, plus or minus a degree or two, may be heard very frequently in Securité messages during the U.S. Coast Guard high-seas weather forecast. It is the area that must be crossed by voyaging vessels from the Pacific Northwest heading south, and it reaches as far south as the latitude of Los Angeles.

I've made the passage from Cape Flattery to Southern California three times in my Valiant 40 New Dawn, at distances of 25, 75, and up to 200 miles offshore, and each time ran into following seas of some 20 feet or more, the last time not far north of the latitude of Pt. Conception. This was during the first week of August. The other two times were in September and October. These three months are well within the window for voyages south from the Pacific Northwest.

As we were heading in toward the Santa Barbara Channel from well offshore that August, we had been running off in 25 to 35 knots apparent wind before seas to match for several days. All this time we were unable to discern anything but wave returns inside six miles on our 24-mile raster scan radar. In other words, the area inside the six-mile ring was completely filled with same-sized radar blips. The radar return of even a large tanker would just be another blip, indistinguishable from the others, and no adjustment was able to alleviate the condition. Since we were between 150 and 200 miles offshore and encountered hardly any ship traffic, this was not a problem. But on our last night out we were crossing the shipping lanes and at any given point in time were tracking not one but several ships.

I can usually pick up a tanker or containership at about 19 miles as it comes up over the horizon. When I have only one blip on the screen I put range and bearing lines on it and then watch its progress over a period of time. When there are a number of blips I try to select the one most likely to pose a potential problem and put the range and bearing lines on it. That particular night, some 70 miles NE of Point Conception, with my crew Barry asleep, I had two or three blips on the screen, their courses not presenting any problem, when another blip showed up at the top at about 35° relative. I put a bearing line on it and waited.

Fifteen minutes later the blip was still on the bearing line and subsequent checks showed it moving right down the line. At eight miles I got concerned because I knew I would lose the blip in the mass of wave returns at six miles, so I called on channel 16. I identified myself, gave my position, heading and speed and asked the other vessel's intentions. A prompt response revealed the ship to be a U.S. tanker that did not have us on her radar. We, too, were lost in wave returns.

With an attempt at humor I reaffirmed that, yes, we were indeed there and, on a more serious note, expressed my concern at the fact that she had been coming down my bearing line for the last 10 miles or so and that we were on a converging course. The response again was that they couldn't make us out in the wave returns and would I repeat position, course, and speed. I gave them the info again and then was asked to stand by on channel 10.

Our invisibility on radar was somewhat unexpected. I've always had full confidence in the superiority of big ship radar and my Firdell Blipper, which is mounted on the mast some 35 feet above deck, well above the wave heights we were experiencing. Also, we were running before wind and seas and did not experience much heel, though we were rolling a bit. Finally, I knew I had been picked up on radar by other commercial ships before and must have given a good return since I could tell by their response when I contacted them by VHF that they were surprised or at least did not know that I was a sailing vessel. So here we were, lost in the sea clutter at eight miles, and the ship would be lost to us at six miles. Time to get Barry up on deck so he could keep a lookout.

About 10 or 15 minutes later the now-familiar voice of the tanker watch came on informing me that they had plotted our respective courses and speeds and that they would pass us about one mile to starboard. Also, they had caught a few glimpses of us on their radar through the waves. Whoever was on the tanker's bridge expressed curiosity as to where we were coming from and our destination. He told us that his vessel was running at less than half speed as she was headed into the wind and seas we were running from, and remarked at several points in the conversation that "It isn't nice out here." After signing off we kept a lookout for lights to starboard, but never were sure whether we saw anything.

My conclusion is that, in the conditions described, a constant radar watch is of primary importance in addition to a lookout on deck. It is essential to identify traffic before it gets close enough to become lost in the wave returns. Relying solely on spotting navigation lights can be chancy at times, regardless of the fact that in the days before radar this was the only option.

The power of waves to hide a vessel can be unsettling. I have had a loaded containership pass me less than a mile to port in broad daylight off the Oregon coast and I only saw him twice for fractions of a minute. I knew from radar that she was there, but still I got more than a start when suddenly he rose from the waves at the same time we did and I was looking at him broadside moving aft, only to have the large ship disappear again less than a minute later.

Leonard D. Ablieter is based in Camarillo, Calif. He wrote "Square Rigger Route" in Issue No. 79, January/February 1997.Chuck Husick comments,

Radar energy is equally happy to reflect from a wave front as from a metallic, man-made structure, such as a ship or a radar reflector. In fact, the multitude of waves and the almost infinite reflecting angles they present will allow the reflections we call sea clutter to totally obscure the target we wish to see.

Your radar set has a means for dealing with this problem to a considerable extent. The sea clutter control, sometimes called the STC (short-time constant) control, is a time-variable receiver gain control. The designers, recognizing that signal returns from close-by reflectors will usually be significantly stronger than reflections from more distant targets, have made it possible to reduce the receiver's sensitivity to returns that arrive soon after the end of each transmitted energy pulse. When this circuit is active, much stronger return signals from close-in targets will be required to provide a response on the display screen than for return signals arriving later, from more distant targets. The degree of desensitization of the radar to early-arriving signal returns is adjustable by the operator.

The effect of this circuit can be seen at any time by operating the radar on a relatively short scale, perhaps one or two miles. Unless the surrounding sea is perfectly flat, with the sea clutter control turned off a considerable amount of intermittent noise will appear as a circular band near the center of the screen. These returns are from the nearby wave surfaces. As the sea clutter control is advanced, this noise will be decreased in amplitude (brightness). The receiver is being told to require stronger signals before placing target data on the screen. Unavoidably, the receiver is also requiring more return energy from legitimate targets. Fortunately, most legitimate targets present somewhat stronger average signal strength returns than those from random reflections from waves. With careful adjustment of the sea clutter control it is usually possible to reduce the signal display created by the unwanted reflections enough to see reflections from more stable targets, such as other vessels. Your comment about expecting to lose the target of the tanker at six miles seems to confirm that the sea clutter control was either not in use or was incapable of dealing with the extreme conditions.

Unfortunately, the reflections from even large targets can be obscured by intense levels of reflection from the sea surface. The problem is made even more severe when the radar is on an unstable platforma small boat in rough seas, where antenna motion adds to the difficulty in capturing the desired target reflections. These motion effects can sometimes be too severe to be overcome even with the use of gimbaled antenna mounts.

You express surprise that your Firdell Blipper radar reflector was unsuccessful in making your vessel visible on the tanker's radar. However, no radar reflector, no matter how capable, can ensure the return of a usable signal to a searching radar. High sea states can make even close objects invisible at short ranges.