Navigation Equipment

Everything ARPA gives you starts as a relative-motion plot, and if you can't do it by hand you can't tell when the automation has got it wrong. Two radar observations of a target, taken a known interval apart, give you two points on the target's track as seen from your own ship — the relative track. Join them, extend the line, and you have the whole story: the closest point of approach is the perpendicular distance from your own ship to that line, and the time to closest point of approach is how long the target takes, at the relative speed, to reach the point on the line nearest to you.

That relationship is the one worth holding onto: your own course and speed are known, the relative motion is what the plot gives you, and adding the two vectors — properly, tip to tail — recovers the target's true course and speed. ARPA runs exactly this calculation, continuously, from its own tracking of consecutive returns, and displays true or relative vectors depending on which mode has been selected. Get the mode wrong and a genuinely converging target can be misread as one passing safely clear, because a relative vector and a true vector answer different questions.

A radar picture is a picture of what returned energy, not of what's actually out there. Minimum range is set by pulse length — while the transmitter is still sending, the receiver can't listen, so a long pulse, used for range performance, blinds you close in, and only a short pulse, used near the coast for discrimination between close targets, shows anything a few hundred metres off. Whatever falls inside the minimum range for the pulse in use simply isn't drawn, however solid it is.

The rest of the limitations are about what the beam physically can't reach, or can't tell apart from noise:

• Blind sectors — areas, usually behind the ship's own structure, where the beam is completely blocked; nothing there is shown at all.
• Shadow sectors — areas where a mast or stack attenuates rather than blocks the beam, so a target there is weakened, not necessarily invisible.
• Sea clutter — return from wave crests close to the ship, worst dead into wind and sea; anti-clutter processing controls it but can suppress a genuine small target along with the noise if turned up too far.
• Rain clutter — the same problem caused by precipitation, over a wider area and often at longer range.

Both sectors are chartable and should be marked on the set — relying on memory in the middle of a close-quarters situation is how targets get missed. Clutter suppression is always a compromise: a small wooden or GRP boat, low in the water in a moderate swell, can sit inside sea clutter and simply not paint at all, which is exactly why radar, however good, remains an aid to the lookout and never a substitute for one.

ECDIS will only warn you about what you've told it to treat as dangerous. The safety contour is the single most important setting on the whole system: it's the depth you choose, from the values the ENC's own dataset actually offers, below which the display treats water as unsafe. Cross it, or lay a route through it, and the anti-grounding alarm should fire; leave it at whatever value the system defaulted to on installation, and you may be trusting a depth that has nothing to do with this ship's draught.

ECDIS carries several depth-related settings that look similar and do very different jobs:

• Safety contour — the depth threshold that drives the anti-grounding alarm and the bold shading split on the display.
• Safety depth — a separate setting, also chosen relative to draught, that only controls which individual soundings are shown in bold; it carries no alarm of its own.
• Shallow and deep contours — cosmetic shading bands for a quick visual read of the depth regime, with no safety function at all.

A common examiner's trap is assuming a coloured contour line is doing something it isn't — only the safety contour actually protects you. None of this matters if the underlying route was never checked: a proper route check runs the planned track against the ship's own parameters — draught, air draught, safety contour — and flags every crossing before departure, not as a running commentary once under way. An under-scale or overscale display compounds the risk: zoom too far in on a small-scale cell and the system may show generalised data with a false appearance of precision; zoom too far out and it silently drops detail that was available at a larger scale, without necessarily telling you it's done so.

A GNSS receiver will give you a position, a very precise-looking one, right up until the moment that position is wrong — and it very rarely tells you when that happens. Ordinary accuracy, for a modern multi-constellation receiver with differential correction, sits at a few metres; that's good enough to make officers complacent, which is exactly the vulnerability. Jamming denies the signal outright and is usually obvious, because the receiver loses fix or raises integrity alarms. Spoofing is the harder case: a false signal, stronger than the genuine one, that the receiver locks onto and reports as a valid, high-confidence position — one that can be well away from where the ship actually is, with no fix-quality warning at all.

A wrong datum causes a quieter version of the same problem. Charts and receivers should both be referencing the same geodetic datum, but an older chart, or a receiver misconfigured to output a different one, produces a position that's consistently offset from the chart it's plotted on — sometimes by a distance that matters, right beside a shoal.

A gyrocompass settles on true north by mechanical and electrical design, not by sensing the Earth's magnetic field, so it's immune to magnetic deviation — but it isn't immune to error. The one you're examined on is speed/latitude error: the compass's own north-seeking behaviour is disturbed by the ship's velocity over the Earth's surface, and the resulting error grows with speed, changes with the course steered relative to north, and grows with latitude — becoming significant for high-speed vessels operating at high latitude. Most modern gyrocompasses apply an automatic speed/latitude correction from log and position input; if that input is wrong — a faulty log, a stale position — the correction itself becomes a source of error.

Magnetic compass error is a different animal, and it stacks: variation is the angle between true and magnetic north at a given place and date, taken from the chart; deviation is the additional angle, caused by the ship's own magnetism, that varies with the ship's heading and is recorded on a deviation card built for that particular vessel. Getting from compass to true means applying both, in the right order, with the correct sign.

The deviation card is only as good as the day it was swung. Cargo, new equipment near the binnacle, a refit, even a heading held for a long time, can shift a ship's magnetism enough to make the card wrong on a particular heading — which is why a competent officer checks the gyro and, ideally, the magnetic compass against an independent true bearing (a transit, an azimuth) on a regular basis, not just when something feels off.

AIS looks like radar on the display and behaves nothing like it underneath. Dynamic data comes from the other vessel's own GNSS and sensors and is broadcast automatically, so it's usually current and reasonably accurate as far as it goes. Static and voyage data is a different matter entirely:

• Dynamic data — position, course, speed, heading, rate of turn — sensor-derived and broadcast automatically; generally trustworthy as far as the sending vessel's own instruments are.
• Static and voyage data — name, type, destination, draught — typed in by that vessel's crew, sometimes once on departure and never updated; treat it as unverified until there's a reason to trust it.

Neither category makes AIS a collision-avoidance tool in its own right: it identifies a target and, used well, resolves ambiguity in a radar plot, but the duty to keep a proper lookout and to assess risk of collision by radar plotting or visual bearing stands regardless of what an AIS symbol claims about itself.

The echo sounder measures the water directly below the transducer, nothing more — no forward look-ahead, and no allowance for the ship's own draught unless the display has deliberately been offset for it.

Depth under the keel is that figure minus the ship's draught at the transducer's location. A speed log, meanwhile, measures speed through the water — against the surrounding water mass — not speed over the ground; the two only agree when there's no current or tidal stream. Mistaking one for the other is a quiet, compounding error in any set-and-drift or dead-reckoning calculation, exactly the kind of thing that stays invisible until the fix disagrees with the plot.

The three examples below run the full working: a radar relative-motion plot recovering CPA, TCPA and a target's true course and speed; an ECDIS under-keel clearance and safety-contour calculation across a tidal passage; and a cross-check of gyro and magnetic compass error against a charted transit.

Worked example 1

Radar plot: relative motion, CPA/TCPA and the target's true course and speed

Your ship is steaming a steady course of 000°(T) at 15 kn in open water. At 0800 you plot a target broad on the port bow; twelve minutes later, at 0812, you re-plot it. Reading the reflection plotter, the target's position relative to your own ship (in nautical miles North and East of own ship) is 10.0 N, 5.0 E at the first plot and 7.0 N, 1.0 E at the second. The target's own course and speed have not changed between the two observations. Find the target's true course and speed, the CPA and TCPA, and state whether the situation calls for immediate action against a company minimum passing distance of 2.0 nm.

Given

Own ship: course 000°(T), speed 15 kn (steady) 0800 plot: target's relative position = 10.0 nm North, 5.0 nm East of own ship 0812 plot: target's relative position = 7.0 nm North, 1.0 nm East of own ship Interval between plots: 12 min (0.2 h) Company minimum passing distance (CPA): 2.0 nm

Required

Find the target's true course and speed, the CPA and TCPA, and state whether the situation calls for immediate action against a company minimum passing distance of 2.0 nm

1. Find the relative motion.

   ΔN=7.0 − 10.0 = −3.0 nm ΔE=1.0 − 5.0 = −4.0 nm Relative distance run=√(3.0² + 4.0²) =√25.0 =5.0 nm in 12 min Relative speed=5.0 nm ÷ 0.2 h = 25.0 kn Relative course=180° + arctan(4.0⁄3.0) =180° + 53° =233°(T)

   The displacement of the target's plotted position between the two fixes gives the relative course and speed directly.

2. Recover the target's true course.

   Own ship vector (000°T, 15 kn), per hour: N = 15.0, E = 0.0 Relative vector, per hour: N=−15.0, E = −20.0 Target true vector=sum: N = 0.0, E = −20.0 Target speed=√(0.0² + 20.0²) = 20.0 kn Target course=270°(T) (due west)

   Speed by adding the own-ship vector back onto the relative vector — relative motion is what's left once your own ship's motion is removed, so reversing that removal returns the target's true motion.

3. Extend the relative-motion line to find CPA and TCPA.

   Position at 0812, P=(7.0 N, 1.0 E); relative displacement per 12-min interval, d = (−3.0, −4.0) Projection factor n=−(P·d) ÷ (d·d) =−[(7.0×−3.0) + (1.0×−4.0)] ÷ [(−3.0)² + (−4.0)²] =−(−21.0 − 4.0) ÷ 25.0 =25.0 ÷ 25.0 =1.0 CPA point=P + n×d = (7.0 − 3.0, 1.0 − 4.0) = (4.0 N, −3.0 E) CPA=√(4.0² + 3.0²) =√25.0 =5.0 nm n=1.0 further interval of 12 min → TCPA = 12 min, due at 0812 + 12 min = 0824

   The closest point on that line to own ship is the foot of the perpendicular from own ship onto it, found by projecting the 0812 plot onto the line's direction.

4. Assess against the company minimum passing distance.

   A CPA of 5.0 nm against a 2.0 nm minimum leaves 3.0 nm of spare margin, so on the course and speed observed the target will pass clear without a manoeuvre — but TCPA is only 12 minutes away, so the plot must be kept running and rechecked at the next scan, not filed and forgotten.

AnswerTarget's true course and speed: 270°(T) at 20.0 kn. CPA = 5.0 nm at 0824 (TCPA 12 min from 0812). CPA exceeds the 2.0 nm minimum by 3.0 nm — no immediate manoeuvre required, but keep plotting to confirm the target holds its course and speed.

The trap: judging risk from the closing range alone — the range fell from about 11.2 nm to about 7.1 nm between the two plots, which feels alarming, but closing range with a changing relative bearing says nothing about CPA until the relative-motion line is actually plotted and measured.

Worked example 2

ECDIS safety contour and under-keel clearance across a tidal patch

Your ship, drawing 8.4 m, is due to cross a shallow patch on the passage plan where the charted depth (chart datum) is 9.5 m. At the planned time of the crossing the tide table predicts a height of tide of +2.1 m. Squat at the planned passage speed has been calculated at 0.6 m, and company policy requires a minimum under-keel clearance of 1.0 m at all times. The ENC covering this area offers safety-contour values of 5 m, 10 m, 20 m and 30 m. Find the under-keel clearance expected at the crossing and confirm it meets policy, then determine the safety contour that should be set in ECDIS for this passage.

Given

Draught: 8.4 m Charted depth at the shallow patch (chart datum): 9.5 m Predicted height of tide at the time of crossing: +2.1 m Squat at passage speed: 0.6 m Company minimum UKC policy: 1.0 m Available ENC safety-contour values: 5 m, 10 m, 20 m, 30 m

Required

Find the under-keel clearance expected at the crossing and confirm it meets policy, then determine the safety contour that should be set in ECDIS for this passage

1. Find the depth of water actually expected over the shallow patch at the planned time.

   Depth of water=Charted depth + Height of tide =9.5 + 2.1 =11.6 m

   By adding the predicted tide to the charted (chart datum) depth.

2. Find the under-keel clearance.

   Static UKC=Depth of water − Draught =11.6 − 8.4 =3.2 m Underway UKC (allowing squat)=3.2 − 0.6 = 2.6 m Policy minimum=1.0 m → 2.6 m exceeds policy by 1.6 m — the crossing is acceptable at the predicted tide

   First at rest and then allowing for squat, and check it against company policy.

3. Set the ECDIS safety contour independently of today's tide.

   Required minimum charted depth=Draught + Squat + Minimum UKC =8.4 + 0.6 + 1.0 =10.0 m

   The safety contour is a standing setting, not a one-off prediction — it must protect the ship referenced to chart datum, with no tide assumed, so it is built from draught, squat and the UKC policy alone.

4. Choose from the values the ENC dataset actually offers.

   ENC safety-contour options

   Available contour	Meets 10.0 m requirement?
   5 m	No — shallower than required
   10 m	Yes — exactly meets it, zero spare
   20 m	Yes — with 10 m spare
   30 m	Yes — with 20 m spare

   ECDIS cannot draw a contour at a depth the chart doesn't hold, so the setting has to be the nearest available value at or above the calculated requirement.

5. Decide, and record the margin.

   The 10 m value satisfies the calculation exactly, but exactly is not comfortably — any shortfall in the squat estimate, or a lower tide than predicted, erodes that margin to nothing. Set 10 m as the working safety contour for this ship and treat it as a hard limit, not a comfortable one; if the passage plan allows flexibility, timing the crossing for a larger tide restores real margin.

AnswerUKC at the crossing = 2.6 m, meeting the 1.0 m policy with 1.6 m to spare — but only at the predicted tide. ECDIS safety contour set to 10 m (the lowest available value at or above the calculated 10.0 m requirement), with zero spare margin against the ENC's own dataset.

The trap: calculating the safety contour using today's height of tide instead of chart datum — the contour is a standing safety setting that must hold even if the tide is lower than predicted or the ship arrives early or late, so it is built from draught, squat and the UKC policy alone, never from a single tidal prediction.

Worked example 3

Cross-checking gyro and magnetic compass error against a charted transit

Approaching harbour, you line up a charted transit — two beacons in line, charted true bearing 128.0°(T). At the instant they come into line, the gyro repeater reads 130.5°, and the magnetic steering compass reads 121.0°. The ship's head at the time is 045°(C), and the deviation card gives 2.5°E at 040°(C) and 3.5°E at 050°(C). The chart's compass rose shows a current variation of 5.0°W. Find the gyro error, the magnetic compass's total error, the deviation the transit implies on this heading, and how that compares with the deviation card.

Given

Charted true bearing of the transit: 128.0°(T) Gyro bearing of the transit: 130.5°(G) Magnetic compass bearing of the transit: 121.0°(C) Ship's head at the time: 045°(C) Deviation card: 040°(C) = 2.5°E, 050°(C) = 3.5°E Charted variation: 5.0°W

Required

Find the gyro error, the magnetic compass's total error, the deviation the transit implies on this heading, and how that compares with the deviation card

1. Find the gyro error by comparing the gyro's bearing of the transit with its charted true bearing — a transit is a fixed line between two charted objects, so its true bearing never changes and gives a direct check with no calculation beyond a subtraction.

   Gyro error=True bearing − Gyro bearing =128.0° − 130.5° =−2.5° Gyro reads 2.5° more than true → Gyro error = 2.5° High

2. Find the magnetic compass's total error the same way.

   Compass error=True bearing − Compass bearing =128.0° − 121.0° =+7.0° Compass reads 7.0° less than true → Compass error = 7.0°E (add 7.0° to any compass bearing to get true)

   From the compass bearing of the same transit at the same moment.

3. Split the total compass error into variation and deviation.

   True=Compass + Deviation + Variation (each signed, East positive, West negative) 128.0=121.0 + Deviation + (−5.0) Deviation=128.0 − 121.0 + 5.0 =12.0°E

   Using the variation printed on the chart for this position and date — variation is a property of the place, deviation is a property of the ship's heading, and only the second can be checked against the deviation card.

4. Compare with the deviation card for the heading actually being steered.

   Deviation card (extract)

   Compass heading	Deviation
   040°	2.5°E
   050°	3.5°E
   045° (interpolated)	3.0°E

   045°(C), interpolating between the two bracketing entries.

5. Judge what the discrepancy means.

   The card predicts 3.0°E on this heading; the transit shows the compass is actually 12.0°E out — a 9.0° gap far too large to be reading error. The gyro, by contrast, checked out close to its normal, steady 2.5° High. That pattern points at the magnetic compass, not the transit or the gyro: something has changed the ship's magnetism since the card was last swung — new equipment near the binnacle, magnetic cargo nearby, or a heading held so long the compass has been re-magnetised — and the card can no longer be trusted on this heading.

AnswerGyro error = 2.5° High. Magnetic compass total error = 7.0°E, implying an actual deviation of 12.0°E on 045°(C) against a card value of 3.0°E — a 9.0° discrepancy. The gyro checks out and should be used as the primary reference; the magnetic compass and its deviation card need re-checking, and likely re-swinging, before being trusted again.

The trap: trusting the deviation card without a live check against an independent true bearing — a card is only valid until something on board changes, and comparing both compasses against the same transit is what actually catches that, not comparing one compass against the other.