AIS and ECDIS offsets.

3rd March 2019

AIS and ECDIS offsets.

Initial Report


We have received several reports which outline position anomalies between a vessel’s AIS and ECDIS, and positions obtained from a PPU or by visual / radar position fixing.


What the Reporter told us (1):

Prior to getting underway, there was no error on the ECDIS displays. However, once moving, an error was evident. The position displayed on the ECDIS was observed to be lagging behind the visual and radar positions, with the lag increasing as the speed of the vessel increased. The AIS position replicated the ECDIS and both indicated a position approximately 160m behind the vessels actual position. This is not the first vessel with this issue.

Further dialogue:

CHIRP offered to contact the ISM managers of the vessel, but the reporter advised that the port had been in contact with their national administration who had flagged the report for a PSC visit should the vessel return. They had also forwarded the issue to the PSC authorities for the country of the next port of call.


What the Reporter told us (2):

Recently, I noticed that the AIS position of a vessel was out by approximately 20 metres. Once offsets were checked, the independent Portable Pilot Unit (PPU) showed the difference visually on the screen. AIS data indicated that the antenna was forward on the bridge front and 3 metres in on the starboard side.

Upon inspection of the antenna plan, it became obvious that the error was due to differences between the offsets for the AIS GPS antenna and No 2 GPS antenna.  No 1 GPS antenna was situated close to the AIS GPS antenna. The Second Mate showed me a selector switch, and the Captain requested that it be switched to No1. Within seconds, the vessel’s AIS position on my PPU changed and aligned with the independent PPU position. The Captain agreed to post a notice on the AIS to require the AIS external GPS input to be sourced from No1 GPS.


 What the Reporter told us (3):

Upon arrival in port, I observed that the AIS data was showing the vessel behind the data presented on the PPU. Whilst passing a beacon, I asked the master to tell me where it was from looking at the ECDIS, “On the port bow” was the answer, but it was actually on the beam. After the vessel was secured alongside, there was no error.

I asked the Captain if it was possible to look at the settings on the GPS. We found under GPS SETUP, a section called GPS SMOOTHING which allowed for data entry. The “Position” was set at 20 seconds, “Speed” at 30 seconds and “Average Speed” at 120 seconds. The master reset them all to zero and confirmed that the result had been effective with accurate positions displayed following departure.


PPU showing GPS smoothing on the left with vessel “lagging” and smoothing removed on the right


This type of error has been reported before, but at those times there was no assistance from the Captains involved.


CHIRP Comment:

The Maritime Advisory Board commented that these reports raise several issues including some apparent common failings such as an over reliance on ECDIS. Clearly it is vital that the correct data inputs are always utilised.

Of note, the antenna height must be input correctly, and the bridge team must be aware of which GPS is the master unit. From the second report, it is admirable that an antenna plan was produced in short order and that the issue was both identified and rectified.

In addition to the above, accurate positioning depends upon correct speed inputs, and any WGS84 offsets being input to the master equipment.

With respect to GPS smoothing, CHIRP issues a note of caution. The removal of all smoothing may well have solved the position lag in the third report, but smoothing does have a purpose. It can affect course over the ground (COG), course made good (CMG) and time to go to an alteration point (TTG). Thus, it is important to build in the necessary checks and balances for this vital equipment, as reliance on ECDIS and other electronic equipment can and will only increase – this factor will become increasingly important with the advent of autonomous shipping.

Equally there is a responsibility of those installing the equipment to provide advice and warning. For ship managers signing off an installation, there is a need for due diligence to ensure equipment has been correctly tested and that thorough operator familiarisation has taken place, perhaps enhanced with manufacturers training courses, commissioning engineers’ instructions and demonstrations – because from this initial point, information can be progressively lost to subsequent operators as successive handovers omit small items of information.

Every ECDIS system has the facility to input manual positions, visual bearings and radar distances, and every manufacturer advises carrying out cross checks/comparisons with other methods of position fixing. In confined waters the Mk1 human eye is a very effective tool. If ECDIS shows a beacon on the bow but you can see it is on the beam something is not right, question it – don’t always assume that the beacon is out of position.

Finally, CHIRP notes that ECDIS is a very clever and useful tool, but it is only one of many tools in the mariners’ toolbox. A regular check on GPS, ECDIS, AIS etc., versus visual and radar positions should always be maintained. Overlay the radar with ECDIS and any discrepancy will become apparent. It should also be noted that AIS is not intended for position reference but for vessel identification.


A question for our bridge watchkeeping readers, when was the last time you entered a manual position into the ECDIS on your ship?


Report Ends………




Comment (Your email address will not be published)

Up next: