Doppler Speed Input - Problem & Response

Problem encountered:

Our Company tankers have been using AIS coupled with and displayed on electronic charting driven by DGPS for a few years now. My strongly held opinion is that the combination of these two systems give the biggest step forward in navigational safety since the introduction of radar. Hopefully, the progress of ensuring worldwide coverage for quality electronic charts can be accelerated.

Nevertheless, we have just recently noted that one of the AIS models fitted to our ships has the ability to use own ship speed data input from both doppler log and DGPS. But, of concern to us is the apparent fact that the AIS defaults to only doppler speed input whenever the doppler log engages ground tracking mode. The process selecting speed data input source is automatic and beyond any control of the user. Consequently, being aware that doppler speed data is frequently unreliable and can often give rise to large errors for transient periods arising from natural operating conditions we have concern that the design of this particular model is flawed as there are obvious adverse implications for traffic management and collision avoidance. This is now under investigation by both our marine, technical sections and the equipment maker.

– Reported by Shipmaster
 

Technical response:

The AIS Class A standard does not cover this issue. If it did it could be in danger of overriding IMO decisions concerning carriage requirements. Although the quoted example is arguably not the best possible design, it is compliant to IMO requirements.

The minimum (AIS) requirement is for manufacturers to provide the digital interface to understand the VBW message, which is the IMO required Doppler log message. In addition, VTG, OSD and RMC can be optionally used. VTG is the common message from GNSS equipment giving SOG and COG.

The manufacturer has decided that if ground tracking data from VBW is available it will always take the data from that message, in preference to data from the external GNSS (VTG message). This is not a stated requirement of the AIS standard but nor does it prevent this..

SOLAS requires the carriage of "A speed and distance measuring device, or other means, to indicate speed and distance over the ground in the forward and athwartships direction" on vessels above 50,000 gross tonnage. The VBW message transfers this data to other systems.Therefore, there is a good argument to say SOG should come from a type approved SDME, (rather than GNSS), if it is available. This is what the manufacturer has probably assumed.

The manufacturer should perhaps incorporate more set-up options to ensure that this can optionally be prevented from happening, if this is what most users want.

Nevertheless, IMO does need to give some attention to SOG from GNSS as it is becoming very reliable, particularly when using differential systems and is probably more accurate/reliable than Doppler systems in most circumstances.