Radar Maps, Navigation Lines and ECDIS Overlay

Radar Maps and Navigation Lines

In short: a radar map is a layer of lines and symbols drawn over the radar picture, used mainly to show dangers the radar cannot see.

• What goes on it: shoals, wrecks and reefs (no echo, so radar-invisible), channel limits and traffic separation schemes (TSS).
• Build or reload: maps can be saved and reloaded per area, or built in real time on the display.
• Reference: each element is tied either to own ship or to a geographic position.
• Routes: the planned route can be imported from the GPS / navigation system, so the track appears on the radar.
• Same symbols everywhere: symbols and colours are preset by IMO — first issued as SN/Circ.243, now IMO SN.1/Circ.243/Rev.2. Like buoyage, a wreck looks the same on a radar in Malta as in Australia.
• Danger — clutter: map lines must never clutter the picture. A line can lie on a small or weak echo and hide it. If in doubt, switch “synthetics off” / “maps off” and check the raw radar.

Checking the Map: Sensors, SOG and GNSS

In short: a map is only as good as the position and speed fed into it — a sensor error moves every line and symbol, so check the map against real echoes.

• Sensor errors shift the whole map. A wrong position or a wrong speed over the ground (SOG) gives a convincing but false picture.
• Check it: anchor map elements to features the radar really shows — breakwaters, coastline, buoys — and confirm the echoes sit where the map says.
• Pick the best SOG source available.

• GNSS can fail. Solar flares (space weather) can spoil coverage for days: north-east of Malta, GNSS was poor for several days, equipment behaved erratically and the DSC VHF lost its position.
• When GNSS is suspect, cross-check SOG and position by other means — log, radar fixes, visual bearings (MGN 379 (M+F) 4.5: never rely on one position source).
• Deliberate GPS errors? You may hear that GPS errors are added on purpose “for security”. That was Selective Availability, switched off in 2000. Today’s errors come from space weather, interference, jamming and spoofing.

Electronic Chart Overlay: The Rules

In short: many radars can show the ENC under the echoes. It is optional under IMO MSC.192(79), but where fitted it must never get in the way of the radar.

Reading the Overlay: GNSS Errors, Coastlines and Collision Avoidance

In short: the overlay is a quick position check — if the charted coast sits on the land echoes, the position is good; if not, trust the radar.

A parallel index at a glance

• If the charted coastline sits on the land echoes, the position input is good — like a parallel index you read at a glance.
• The overlay is the best way to cross-check GNSS integrity (MGN 379 (M+F) 5.10: radar overlays can check the ECDIS position input).
• Danger: the chart is placed using the EPFS (electronic position-fixing system, usually GNSS). An EPFS error moves the whole overlay — the known weakness of the method.
• Example: a GNSS error to the south shifts the chart and AIS targets south, but the radar echoes stay put. The radar does not depend on GNSS — trust the echo.

Low coasts never match perfectly

• The radar coastline is not the charted coastline, especially on low coasts.
• What reflects is the land behind the waterline — cliffs, buildings, rising ground — so do not expect a perfect fit there.

Take the chart off for collision avoidance

• For collision avoidance, remove the ENC: it adds clutter, and if it carries a GNSS error you may take a ship’s echo for land.
• Overlaid target data does not replace radar observation — take collision decisions on the radar (MGN 379 (M+F) 5.10).
• Whenever an echo looks doubtful, look at the raw radar with no chart and no synthetics.

Bridges and tides in the plan

• Bridges give strong echoes and stand out clearly on the overlay.
• Note each bridge in the passage plan with the air draught and the height of tide needed to pass under it.
• If clearance is tight, the ship may have to wait for the right tide — sometimes up to two weeks for a spring low tide.
• In ports with a large tidal range (Felixstowe about 5 m), keep adjusting the gangway and tending the mooring lines alongside.

Display Orientation and Motion

In short: for navigation near land use true motion, north-up; avoid an unstabilised head-up picture.

• Head-up (unstabilised): the heading marker is fixed at 0°, so the picture spins when the ship yaws. It also smears echoes, gives relative bearings only (conversion errors), is hard to compare with a north-up chart and makes plotting unreliable during turns.
• True motion, north-up: own ship moves across the screen, fixed echoes stay still, moving targets show their true course. Best near fixed objects.
• Relative motion: own ship stays fixed and the land moves across the screen — fine for collision avoidance, less so for coastal navigation.
• Memory aid: navigation → true motion · anti-collision → relative motion.
• Exam keywords: “fixed objects” + “north orientation” → true motion, north-up.

Radar Alarms

In short: a radar is a set of computers living with heat, dust, rolling and pitching — it can fail, and its alarms tell you when.

Alarms the radar must give

• Picture frozen — the display has stopped updating. Flashing visual alarm with ACK + audible alarm; the visual alarm stays until the cause is cleared, even after acknowledgement.
• Signal source / sensor failure — loss of the THD (transmitting heading device, the gyro), the SDME (speed and distance measuring equipment, the log) or the EPFS (position).
• Voyage-plan alarms — set these too: sounding (depth) and XTE (cross-track error, distance off the planned track).

How bridge alarms escalate

1. Visual — a flashing indication.
2. Audible — a sounder on the bridge.
3. Escalation — as with the BNWAS: backup officer → Master → mess room → general alarm.

• On watch, keep scanning every screen for anything new and flashing. A sensor (often GPS) can drop out while nothing else seems to change.