Electronic Watchkeeping Equipment

When contemplating navigation and the correct use of position monitoring and fixing techniques, the OOW needs to consider not just the inherent accuracy of the equipment but also the errors that may be present within it. The accuracy and functionality of electronic aids to navigation have increased considerably in recent years. This realisation needs to be tempered with an understanding that each piece is an aid to navigation and should be treated as such.

There is a danger of over-reliance on the output from a single item of equipment which may lead to an accident. The need to cross-check the vessel’s position using other means is as important today as it ever was, as is the basic requirement under Rule 5 of the COLREGS to maintain a proper lookout. Accidents have occurred with ships equipped with the best of equipment where watch-keepers have been over-reliant on the equipment output and disaster could have been averted by the simple expedient of maintaining a proper lookout.

Radar and ARPA

In the following video, you will gain a greater understanding of how radar and ARPA assist in safe navigation and the errors that can occur with them.

GNSS

Global Navigational Satellite Systems is the generic term for all space-based real-time position finding systems. The most famous of these is GPS, but others such as Gallileo and GLONASS are also in use. They all work on the principle of radio signals travelling from a satellite to the receiver and this taking an amount of time. As the speed of the radio wave is known and the time is measured, the distance to the satellite can be calculated. By using 3 satellites simultaneously, the exact position can be calculated very accurately as the intersection of the three ranges.

GNSS positions are accurate to approximately 20-50m without correction. However, a differential correction signal can increase this accuracy to 5m or less. The differential correct used to be available only in coastal waters where greater accuracy was required, but due to advancements in satellite technology, the differential corrections are now mostly worldwide in coverage and so GNSS positions are considered to be highly accurate at all times.

However, there is often an over-reliance on GNSS, especially when in deep ocean situations that then carries through to coastal and even pilotage navigation. This over-reliance on GNSS can lead to groundings as there are many errors that can creep into GNSS such as multi-path errors and datum errors that are difficult to appreciate as there will not be an alarm. The only way to check the accuracy of GNSS fixes is to manually plot the vessel's position by visual, radar or celestial means to check the accuracy of the GNSS positions.

ECDIS

ECDIS is a real-time navigation and charting system that utilises layers of information to build up a complete picture of the navigation situation around the vessel. The IMO determines only two types of electronic chart systems:

• An approved Electronic Chart Display Information System - approved for use on ships.
• An Electronic Chart System - not approved for use on ships.

To be approved for use, the ECDIS must meet the required standards of S52 and S57 set by the International Hydrographic Office and have integrated alarms and monitoring systems. To operate an ECDIS legally, the OOW must have both a generic ECDIS certificate and a type-specific ECDIS certificate for the particular ECDIS in use on that vessel.  

To comply with the standards and be of an approved type, the ECDIS should allow the operator to:

• Display the chart in a north-up orientation (other orientations are permitted).
• Show true motion mode (other modes are permitted).
• Reset and generate the neighbouring area automatically at a distance from the border of the display determined by the mariner.
• Manually change the chart area and the position of your own ship relative to the edge of the display.
• Plot position fixes (ranges, bearings).
• Measure distances and bearings.
• Record past tracks including time tags and event marks.
• Store tracks to be displayed on request.
• See a description of the electronic chart database.

Most errors in the ECDIS system fall into four categories:

AIS

The Automatic Identification System is a radio-based transceiver that connects the vessel to all other stations and vessels in the area. This allows for the sharing of information between vessels and stations for search and rescue, navigation and collision avoidance purposes. IT works by transmitting short bursts of coded information on a repeating time signature that synchronises with all of the other stations in the area so that each is transmitting in its own unique time-space. The information transmitted is split into two categories; voyage and static data. 

Voyage data is transmitted every 2 to 10 seconds when underway or every 3 minutes at anchor and contains:

• MMSI number.
• Navigation status.
• Rate of turn.
• Speed over the ground.
• Positon and accuracy of the fix.
• Course over the ground.
• True heading.
• Timestamp.

The static data is transmitted every 6 minutes and contains:

• MMSI number.
• IMO number.
• Radio call sign.
• Name of the ship.
• Ship type.
• Ship dimensions.
• Location of the ship data reference point.
• Type of position fixing device in use.
• Draught of the vessel.
• Port of destination.
• Estimated time of arrival.

While it is important for the OOW to keep the information up to date and accurate, the amount of data being sent can be problematic. There is a lot of sensitive and privileged information that could be of use to pirates, terrorist organisations or rival companies. There may well be restrictions on the information input into the machine as part of company orders that limit the information being presented while still allowing for safe navigation.