During a cruise to South Brittany it was noticed that AIS data was not being received from several ships sighted. A basic AIS receive-only set is fitted. The AIS antenna is a standard VHF antenna mounted on a short mast on the transom and 3 metres above sea level. All the vessels noted were ‘within sight’ between 6 and 0.5NM according to Radar, with the most distant having superstructure visible over the horizon and therefore its AIS antenna presumably in line-of-sight. Discussions with the manufacturer resulted in no explanation for this.
As an example, three large tankers either anchored or approaching the anchorage off the Loire did not show at all although several other vessels within a similar distance appeared on the screen. On another day during a Channel crossing, at least four ships of significant size and a tug towing a large barge failed to show although other vessels were received.
I feel it unlikely that all these vessels were failing in their duty to transmit AIS information and wonder whether low cost AIS receivers may be susceptible to installation problems where they work partially but do not receive all the transmission.
On return to port in the UK, the set was monitored whilst on my mooring and received transmissions from fourteen ships at anchor or in port. Five of the fourteen showed as ‘steaming’ although registering zero knots.
AIS receivers are a useful tool for slow sailing vessels crossing shipping lanes or other busy traffic areas where most of the shipping will be carrying AIS transceivers. Having speed and course data available makes the avoiding action or stand-on decision easier to make earlier.
Lessons Learned: Electronic aids to navigation are not totally reliable. Traditional methods must be kept up with and used in conjunction with modern technology!
We agree with the reporter that it is highly improbable that so many large vessels would have been failing to transmit AIS and therefore it appears that the problem lies in the AIS receiver and/or antenna on the yacht.
AIS may provide additional information that enables the mariner to make a better interpretation of the traffic situation. However an AIS receiver should not be relied upon to detect other vessels in restricted visibility. There are two reasons for this:
First, not all vessels are required to transmit AIS. Fishing vessels and leisure craft are generally exempt, so an AIS receiver provides no warning of a potential collision with them.
Secondly, as this report illustrates, not all AIS receivers can be relied upon to detect signals with a high degree of reliability.
As to the vessels whose AIS indicates that they are underway when they are clearly at anchor or in port, there does appear to be a significant incidence of failure to input the correct data. This does probably not in itself pose a major danger to navigation but may nevertheless be interpreted as a symptom of less than meticulous attention to bridge procedures.