ECDIS and electronic aids: oral questions and answers.

A good answer covers:

• It times a sound pulse from the transducer to the bottom and back, using an assumed speed of sound in sea water
• It measures below the transducer; below the keel and below the waterline come from the draught and transducer depth entered
• Set the draught, the datum you want, the range and the gain, and a shallow alarm
• Check the reading against the charted depth plus the tide

Show me on the bridgeThe echo sounder as the last officer left it. Is the depth below the keel right?

A good answer covers:

• A wrong draught or transducer depth entered gives a wrong depth below the keel
• Aeration under the hull, in heavy weather or going astern, can lose the bottom
• Mid-water layers, fish or plankton can give false shallow readings, especially with high gain
• Multiple echoes, and second-trace echoes from a deep bottom on a short range
• On a slope it shows the nearest bottom, not the depth straight down

A good answer covers:

• Water track measures speed through the water, from a layer just under the hull
• Bottom track measures speed over the ground, while the bottom is in range
• Water track for collision avoidance: the radar is then sea stabilised and target aspects are true
• Bottom track for berthing and anchoring, where speed over the ground matters, often with sideways speeds at bow and stern

A good answer covers:

• Electromagnetic log: speed through the water from the voltage the flow induces; needs calibrating
• Impeller or pitot logs on small craft: fouling and wear make them under-read
• Doppler log: can give speed over the ground in shallow water, and through the water anywhere
• GNSS gives speed over the ground only, and fails with the GNSS

A good answer covers:

• Start from the draught, add the under keel clearance the company or master requires, and allow for squat and the tide relied on
• Safety depth: soundings at or shallower than it are shown bold, as a warning
• Safety contour: the line between safe and unsafe water; the anti-grounding alarm and the route check work from it
• If the chosen value is not in the chart data, the ECDIS uses the next deeper contour and says so

A good answer covers:

• The default safety contour is in use: nobody set it for this ship
• Base display: soundings and many dangers are hidden
• A one minute look-ahead gives no time to act
• Audible alerts are off

Show me on the bridgeThe settings as the last user left them. What is wrong?

A good answer covers:

• It is the area ahead the ECDIS searches for the safety contour, dangers and special areas
• Long enough to take action in time at the speed you are making, set by time or distance
• Wide enough to cover the yaw and the set, but not so wide it alarms all the time in a channel
• Too short or too narrow and the alarm comes too late; switched off it never comes at all

A good answer covers:

• It is only as good as the chart data, the position input and the settings
• Cross-check the position by another means: radar, visual bearings, the echo sounder
• Watch for overscale, wrong datum, missing cells and the CATZOC of the data
• Keep a proper lookout and a radar watch: the screen is not the window
• Know the back-up arrangements and practise them

A good answer covers:

• Standard: what the ECDIS shows by default for route planning and monitoring, adding buoys, lights, traffic schemes and more
• All: everything, including spot soundings and cables
• Use Standard as the normal display, with soundings switched on near shallow water; remove clutter with care
• A customised display is flagged on screen

A good answer covers:

• Each leg with its cross-track corridor is checked against the safety contour, isolated dangers and the area categories you have chosen
• Results must be read one by one, and the route changed where it is unsafe, not just acknowledged
• It only knows the chart data and your settings: a wrong safety contour gives a wrong answer
• A visual check of every leg on a large scale is still needed

Show me on the bridgeA route left unchecked in route planning. Check it and read the results.

A good answer covers:

• XTD from the safe water each side of the track: narrow in a channel, wider in open water
• Wide enough for normal yaw and set, so the alarm means something
• Turn radius from the ship's turning ability at the planned speed and the room in the turn
• Wheel-over points follow from the radius; check them on the chart and with parallel index lines
• The route check sweeps the whole corridor, so a wide XTD can bring in dangers a narrow one would not

A good answer covers:

• Cross-track error and the off-track alarm; correct early for set
• Approach to waypoints, wheel-over and critical points
• The anti-grounding and area alarms from the look-ahead
• Cross-check the position by radar and visual means; do not trust GNSS alone
• Keep track control and heading mode clear in your mind, and know how to take hand steering

A good answer covers:

• Static data, set at installation: MMSI, name, call sign, IMO number, length, beam, type and antenna position
• Dynamic data from the sensors: position, course and speed over the ground, heading, rate of turn
• Voyage data, entered by hand: draught, destination, ETA, hazardous cargo and persons on board
• Check it at the start of every voyage, and whenever anything changes

A good answer covers:

• Not every vessel carries it, and it can be switched off
• The data is only as good as what the other ship entered, or her sensors give
• Positions come from her own GNSS, which may be wrong; her heading and COG may differ
• Updates are not instant, so a turning ship may lag on the display
• It does not replace radar, plotting and a proper lookout, and the collision regulations still apply

A good answer covers:

• The time and reason go in the log
• It goes back on as soon as the threat has passed
• In port, follow the port's rules on AIS

A good answer covers:

• Status shows moored, and is not changed to under way using engine
• Draught 2 metres, the shipyard figure, not her present draught
• No destination
• No ETA
• Persons on board shown as none

Show me on the bridgeThe AIS unit on the after bulkhead, as the last officer left it.

A good answer covers:

• Check each cell's edition and the last update applied against the latest available, from the weekly update and the chart agent's list
• Check permits are in date for every cell needed, for the whole voyage
• Make a chart status report and check it, and record updates in the chart log
• Check the updates were applied properly: review the changed features on the chart
• Check temporary and preliminary notices and navigational warnings are on the chart, as manual updates or notes if needed

Show me on the bridgeThe ECDIS Charts tab: one cell behind, one permit expired, one about to expire.

A good answer covers:

• ENCs are encrypted; the permit is the key that lets this ECDIS decrypt and display a cell, for a set period
• Permits come from the chart agent and are loaded on the ECDIS, usually as a permit file
• An out-of-date cell does not meet the carriage requirement and should not be relied on
• Check expiry dates before sailing, cover the whole voyage, and renew in good time

A good answer covers:

• Load the new permits first, or the update cannot be applied to that cell
• Then apply the updates, and check every cell has taken them
• Review the changes on the chart for the area of the passage
• Make and keep a chart status report, and tell the master if anything needed for the passage is missing

A good answer covers:

• Satellite geometry: a high HDOP means a poorer fix
• Ionospheric and tropospheric delays, multipath near large structures, and receiver noise
• Jamming and spoofing, which can give no fix or a wrong one with a good status
• Datum: the position must be on the same datum as the chart, usually WGS 84
• Watch the status, number of satellites and HDOP, and always cross-check by other means

A good answer covers:

• The two receivers disagree, or the ECDIS position no longer matches the radar picture or visual bearings
• The echo sounder disagrees with the charted depth, or speed and course over the ground look odd
• Change to the good receiver or to dead reckoning with radar and visual fixes
• Do not let the autopilot or track control follow a false position

Show me on the bridgeWatch the secondary difference in the Sensors tab as GNSS 1 starts to walk away.

A good answer covers:

• A reference station at a known position measures the errors and broadcasts corrections
• Differential GNSS sends them by radio beacon; SBAS sends them from geostationary satellites
• Better accuracy, and integrity warnings when a satellite is unhealthy
• Corrections only help while they are received: check the receiver shows a differential or SBAS fix, not a plain one

A good answer covers:

• A low frequency terrestrial radio navigation system, independent of satellites
• Ground wave propagation, so useful as a back-up if GNSS is jammed
• Errors from the signal travelling over land, which slows it, and from sky waves at night
• Corrections from reference stations (differential eLoran) improve it; coverage depends on the stations in service, and several national services have closed

A good answer covers:

• Not every vessel carries AIS, and it can be switched off: small craft, fishing boats and warships may not show
• The data are only as good as the other ship's sensors and what her crew entered: position from her GNSS, heading from her gyro, a wrong navigational status
• Class B transmits less often and at lower priority, so its picture lags more
• Reports can be lost in a busy area, and positions are not checked against radar
• So AIS is an aid to the look-out and to identification, never a substitute for radar and a proper visual look-out

Show me on the bridgeCompare the AIS targets with the radar echoes. Is there an echo with no AIS?

A good answer covers:

• Class A is carried by SOLAS ships: higher power, reports every 2 to 10 seconds under way depending on speed and turning, and every 3 minutes at anchor
• Class B is for small craft: lower power and a slower reporting rate, every 30 seconds above 2 knots and every 3 minutes below (Class B CS)
• Class B sends less voyage data: no navigational status, destination or draught
• Both show on the ECDIS and radar, often with a different symbol or label

A good answer covers:

• A sleeping target shows only its position and heading: no vector and no CPA alarm
• An activated target shows its vector, its data on selection, and takes part in CPA and TCPA alarms
• Targets can be activated by hand, or automatically by range or CPA limits that the officer sets
• Leaving the activation range too short means a closing ship stays asleep until it is close

A good answer covers:

• AIS helps identify the ship, but VHF agreements are a known cause of collisions: the wrong ship may answer, or the agreement can break the rules
• Act in line with the COLREGs; do not rely on an agreement in place of them
• If you do call, identify her positively, and still check that the action taken is working
• AIS safety-related text messages are not a substitute for a proper call when urgent