This note aims to help sailors and motor cruisers decide which of the aids to visibility may be most useful to them. In considering the problem it should be remembered that the systems discussed are only aids - they are not substitutes for keeping a good lookout.
The International Regulations for the Prevention of Collisions at Sea (ColRegs) makes it clear that it is every vessels duty to avoid a collision and defines which is the stand-on vessel in all circumstances. For vessels to be able to meet their obligations under ColRegs they must be able both to be seen by other vessels and to see them. The aids available and in use fall into these categories as shown in the following table:

AIDS TO BEING SEEN	AIDS TO SEEING
Radar reflector (active or passive)	Radar
AIS Class B	AIS Class B
	"AIS" receive only

Radar Reflectors. Leisure vessels are generally poor reflectors of radar signals and need to take steps to improve their reflecting efficiency or Radar Cross Section (RCS) by fitting a radar reflector. Passive reflectors are the most common and there are many in the market. As a general rule size matters - the bigger the reflector the bigger its RCS. Active reflectors, also known as Radar Target Enhancers (RTE), almost invariably have a bigger RCS and can warn the user when he is within radar range of a radar. The rule should be to fit the best you can.

Radar. All vessels over 300 tons have to carry X Band radar and all those over 3000 tons have to carry S Band radar as well.

In addition many vessels under 300 tons have this equipment, normally of the X Band type. Used by a trained and experienced operator they are a very useful aid to seeing and are the prime aid used by shipping.

AIS Class B. AIS is a system which transmits and receives data via VHF. Ships over 300 tons must carry Class A whilst Class B has been defined for smaller vessels for which it is not mandatory. Class B is compatible with Class A and so will receive data transmitted by Class A. A vessel with a Class B system will be able to see on his display, at a minimum, the position of all vessels with operating AIS, A or B, which are in range. Whether they will be able to see him to see where he is is not so clear. Ships only have to carry what is known as a "minimum keyboard and display". This is a stand-alone device which is not integrated with the rest of the navigational suite. There will be no requirement for AIS to be integrated with the radar display until 2008 and then the requirement will only apply to new ships and to new radar sets. It will not apply to repaired radar systems. The minimum keyboard and display may not be under continual observation.

It must also be remembered that not all vessels carry AIS in any form. Leisure vessels, fishing boats (under 300 tons), warships and government-owned vessels are all exempt. Further, AIS may be switched off at the discretion of the master.

SOLAS Chapter 5 makes it very clear that AIS is not, yet at least, an anti-collision device. It is simply an aid and acts in support of radar and the eyeball.

It follows from all the above that possession of an AIS Class B (transmit and receive) by no means guarantees that you will be seen by other vessels with AIS. As regards the "be seen" part of the equation it is thus second order and is not a substitute for a good radar reflector. The receive part of a Class B suffers from the problems given below.

"AIS" Receive only. These low cost systems are not really AIS as they do not meet the specification for either class. However they are capable of receiving and displaying data from Class A and Class B systems. It must be noted that not all vessels carry AIS and that they can be switched off. "AIS" receive only systems will almost certainly therefore not give the full picture of what is around and are certainly no substitute for keeping a good lookout. They are a help to the "seeing" part of the problem but are not the complete answer.

Before making a decision as to which equipment to fit it is perhaps worth reflecting on the Ouzo tragedy. In August 2006 the sailing yacht Ouzo was run down by the ferry the Pride of Bilbao with the loss of all 3 crew members. The Ouzo may well have thought that she had given way to the Pride of Bilbao but the latter, which could not see the Ouzo on either her X Band or her S Band radars, made a course change which took her onto a collision course with the Ouzo. At perhaps 5 knots the Ouzo had no chance of getting out of the way and so the tragedy occurred. The Ouzo was not visible to the ferry?s radars because she was not carrying an effective radar reflector. It is also worth noting that a receive only "AIS" would not have helped avoid disaster.