Position sources for ECDIS

Which DGPS receiver is primary and which secondary - and why should I know?

Nowadays ECDIS is becoming a standard on board modern ships, requirements for the compulsory carriage have been set by IMO and these will come into force during the next decade. Apart from the technical issues in the system itself, another aspect is the preparedness of ship’s personnel to face these changes and to show adequate competence while operating ECDIS.
There are lots of assumptions and misinterpretations I’ve seen among system operators, so I let myself to put down few general consideration on equipment use, partly in form of questions and answers.

Why should I know which DGPS receiver is primary position source and which is secondary?
On board there were two DGPS receivers, named as No1 and No2. In ECDIS the primary position source was given as POS 1 and secondary as POS 2. Despite assumption POS 1 could be DGPS receiver No 1 and POS 2 – receiver No2 respectively, that wasn’t the case. In reality the primary position was fed by receiver No2.

What do these primary or secondary position sources stand for?
Primary position source is the first source for position displayed on ECDIS, high-accurate and reliable, normally it’s differential GPS receiver. If for any reason this position source fails or becomes unreliable, ECDIS will change over to use secondary position source.

One vessel I can recall had quite sensitive vibrations due to her design and tight schedule pattern requiring continuous full-speed-run. That led to regular problems with wiring and connections not only for ECDIS. For the last year there were reports on poor alarm presentation within some ECDIS models. I could confirm that having lost connection to primary DGPS, system turned to dead reckoning mode without any audible alarm or specific warning message. The only thing could tell you the real situation was almost invisible status topic, it changed from DGPS to DR.
This wasn’t the newest model; it gives only DR-status enclosed in tiny warning square in the right top of the screen. It must be seen at once and have audible alarm. Fortunately I haven’t met anything similar on the newbuildings.
Here is the obvious solution. When we are dealing with approved ECDIS –the secondary source must be another DGPS receiver.
If there is only one DGPS receiver available, don’t set secondary position source as DR mode, just de-activate it. Except for poor alarm signaling in older models we can’t neglect the human factor – lack of training may stick bridge team to “ECDIS navigation” while it’s switched over to DR-mode. So that’s the safety precaution.

Another case concerned another manufacturer, there two position sources from two separate DGPS receivers were available, but the secondary was deactivated. The system provided good interface and would give warning to the operator if the primary DGPS fails (for instance) but the fact ship’s position becomes ”frozen” on the screen during critical parts of the voyage may disappoint you greatly.
(If the position becomes frozen then there is something wrong with the equipment).
However there will be no surprises if both position sources are properly activated within ECDIS settings, then the secondary position source will give immediate back-up if the primary fails.

What about the alarms for position sources?
Both position sources activated normally do not require setting for alarms in case signal of either of them is lost, that will sound automatically. But for the primary and secondary position divergence the setting is needed. If the limit of discrepancy between positions from primary and secondary position sources is set, there will be still certain delay in alarm output. But there is no requirement in IMO for divergence between primary and secondary

Are there any legal consequences of improper setting and use of the system?
Actually the proper setting should be done for all aspects of system operation. This is the legal issue as the crew is supposed to be trained and competent in use of the ECDIS (or ECS, if the system doesn’t meet IMO performance standard). Otherwise improper using of the system proved will cause liability to owner of the vessel, even more when any navigation accident investigation is in progress.

Which items should the system operator pay extra attention?
Small list of common mistakes I encountered which must be rectified:

•Dead Reckoning Mode chosen as the secondary position source
•Secondary position source not activated
•Alarms not activated (e.g. divergence between primary and secondary positions)
•Display set only to basic layer – wrecks and shoals not shown.
Finally let me to encourage ECDIS operators to become entirely conversant with this really unique and helpful equipment. Users should be familiar with operation of main functions and also should seek answer to unknown, understand principles, know connections of equipment they use daily and know what to expect when this equipment fails, be familiar with alarms and warnings given by system in case of such failure, do utmost to gain knowledge about nowadays electronic aids to navigation.
Even sisterships may have significant differences in equipment fitted on the modern bridge, even being exactly the same manufacturer the components may differ in connections – thus diligent familiarization yourself is what we always do on joining vessel.

Eugene Ternovskiy,

Class 2 Deck Officer