The COLREGS, however, does not specify exactly the role of VHF and other navigational aids, other than the use of ‘all available means’ when keeping a proper lookout. To this effect, the use of VHF radio equipment for the purposes of anti-collision is strongly discouraged. Anticollision measures agreed upon via VHF radio communications may not always be appropriate and may lead to a catastrophic situation.

In this regard, Transport Malta highlighted the potential safety risks involved in VHF radio communication between vessels and reliance on AIS information, for the purpose of collision avoidance.

Using VHF: Potential risks

All Masters and navigation watch-keeping officers on shall be vigilant against the use of VHF communications as a means of avoiding collisions and shall take note of the following risks:

  • The agreed actions between two or more vessels, made over the VHF radio to avoid collision and without considering the risks of such an agreed action, may not comply with the requirements of the COLREGS.
  • Agreement reached via VHF radio communication between vessels for collision avoidance could be misunderstood or misinterpreted due to language difficulties
  • Uncertainty may exist over the identity of approaching vessels, when navigating in restricted visibility, during period of darkness, and in circumstances when there is more than one vessel.
  • A natural phenomenon known as Tropospheric Propagation may cause radio signals to travel in the part of the atmosphere adjacent to the surface and extending up to some 7,620 metres. Such signals are thus directly affected by weather conditions extending over some miles.
  • Important messages in the conversation through VHF radio could be interrupted or are not being received clearly due to busy radio traffic, squelch control, static noise and interference of radio communication;
  • The loss of valuable time in trying to make contact on VHF radio or else having a lengthy conversation on VHF radio instead of taking appropriate and immediate action, in ample time, to comply with the COLREGS and to avoid a collision.
  • Identification of vessels without AIS information may be difficult during night-time, in restricted visibility or when there are more than two vessels in the vicinity within the VHF radio range.


Using AIS: Potential risks

Not all vessels are fitted with AIS, particularly small craft and fishing vessels. Other floating objects which may appear to be conspicuous on a radar screen may not be displayed by AIS. AIS will, however, sometimes be of a great assistance, when it makes it possible to be able to detect and identify targets which may be within an area of a radar shadow sector.

It is imperative that all Masters and watch-keeping officers always bear in mind the following when AIS is used in the ship to ship mode for anti-collision purposes:

  • The use of AIS does not negate the responsibility of the OOW to comply at all times with the Collision Regulations, particularly rule 7 when determining whether a risk of collisions exists.’
  • The availability and display of AIS data should not be given priority over the data produced by systematic radar target-tracking (e.g. ARPA).
  • AIS target data will only be based on the target vessels’ course and speed over ground whilst for compliance with the COLREG such data must be based on the vessels’ course and speed through the water.
  • The quality and reliability of position data obtained from targets will vary depending on the accuracy of the transmitting vessel’s GNSS equipment.
  • The use of AIS on board ship is not intended to have any special impact on the composition of the navigational watch, which should be determined in accordance with the STCW Convention and COLREGS.
  • AIS positions are derived from the target’s GNSS receiver, usually GPS. This may not coincide exactly with the target as detected by radar.
  • Received AIS data is whatever another vessel transmits and is subject to potential errors.
  • A recent development of AIS is the ability to provide synthetic AIS targets and virtual navigation marks enabling coastal authorities to provide an AIS symbol on the display in any position. Mariners should note that this ability could lead to the appearance of virtual” AIS targets.