Below is an article reprinted from Windcheck. It gives a great viewpoint for those of us who are taking sailing vessels offshore, gives some insight into how difficult sailing vessels are to recognize and locate and why detection equipment such as Radar Reflectors, AIS and the like are so important to have on board and working properly.
A perspective from 100-foot height of eye and 16,000-ton displacement ship
By: LT Catherine A.B. Reppert, USN
Picture this: 2200-0200 watch. I have the deck. Gusty southeast winds, 7-to-10-foot seas in the Gulf Stream, six U.S. Navy ships are conducting an exercise off the coast of North Carolina. We have a 4W, or four whiskey, grid (a series of boxes) in which we patrol and watch for ‘hostile forces’ infiltrating from the south. SOG: 16 knots. Three officers are on the bridge – myself as the officer of the deck, a junior officer of the deck (in training), and a conning officer. Five enlisted sailors are on the bridge. Each sailor is in his or her own state of wakefulness.
A red light appears on the horizon, slightly off the port bow. I notice it and tell the conning officer to take a look through his binoculars and mark its bearing through the telescopic alidade. I scan the automatic identification system (AIS) display for a contact that is on a similar bearing. Nothing. I walk to one of our radar displays (an S-band radar) and look for the tell-tale green blip. Nothing. I increase the gain until the ocean is a greenish-black sea – a good trick for finding small contacts that I learned while hunting for a submarine snorkeling in the Yellow Sea. Still nothing. The conning officer is unable to get amplifying information through his binoculars. No bearing drift. I ask the operational specialist at the plotting board if his radar scope (a surface search radar, SPS-67) holds a contact at 350 degrees relative – distance unknown. “Nothing there,” he reports. I tell him, “Look up and out the pilothouse windows – see that red light? Find it on your scope.”
At 16 knots a vessel will travel 1,600 yards in three minutes, almost a nautical mile. Or 1.6 miles in six minutes. The red light is still at the horizon. No appreciable bearing drift. No AIS. No radar hit.
I do a quick check of my contact situation. I verify the positions of the five other warships on the radar scope – visually as well – also the two merchant vessels on AIS and radar. I find a vessel on AIS that hasn’t yet popped on to the radar scope and notify my conning officer to expect a contact to appear at approximately 030 relative; distance 18 nautical miles.
A brief scan out the windows: I see bearing drift on the red light. Rapid bearing drift – too fast to make sense. The light appears to be nearly at the horizon but it is moving too fast to be so far away. I go out to the port bridge wing and take a hard look with my binoculars. The moonlight glows gently on something that does not look like the wave tops. Uh oh! It’s a sailboat. Close. It’s much closer than I am comfortable with after standing over 400 watches on the bridge of a United States Navy warship.
This is a true story. I experienced situations like this several times. It is the reason I highly recommend cruisers purchase an AIS transmitter. In general, fishing vessels tend to be lit up like Christmas trees and large ships are easy to spot and have large radar cross sections. But a single tricolor light is deceiving. There is no frame of reference (bow light, stern light) and no significant radar reflection – especially for S-band radars offshore with some wave action.
As both a sailor and a watch stander on big ships, I know both perspectives. And I want to pass on some of my lessons learned from standing watch on a big ship:
• Big ships make decisions about contact avoidance much further out than you think. We go fast.
• Odds are we can’t see you.
• An AIS transmitter will greatly improve the likelihood that you are detected.
There are many technical details that I won’t cover here, but I will note that Class B transponders, designed for carriage by non-SOLAS vessels, have limited transmitter power of two watts and a range of about 5-10 miles. Class A transponders, on the other hand, have an International Maritime Organization (IMO)-specified transmitter power of 12.5 watts and a nominal range of 20 miles. From experience, I’ve picked up AIS Class A signals as far as 45 miles due to surface ducting. From a cost perspective, Class A systems are more expensive and there are many good Class B systems on the market today. Proper positioning of your VHF antenna and verification of your signal can mitigate some of the power difference between Class A and Class B systems.
Many sailors rely on radar target enhancers (“radar reflectors”) to increase the size of their radar cross section. But observation of your vessel from a fellow cruising boat’s X-band radar or, better yet, commercial S-band radar, often shows that most products offer only marginal performance – especially offshore in rough sea conditions. All mariners should be aware of the limitations of their equipment, and a broadcast AIS signal is an excellent means to increase your probability of detection.
Many articles about AIS by and for yachtsmen tout the advantages of AIS from the perspective of a sailor – ship names simplify hailing vessels (and make it much more likely that they will respond), sailors enjoy the situational awareness provided by the ‘big picture,’ etc. The bottom line is, from the perspective of a ship, AIS dramatically increases the likelihood that your sailboat is seen and standard contact avoidance procedures are followed.