Radar Plotting and Collision Avoidance

A radar display is not a window.
It is a processed interpretation of reflected energy, presented as if it were truth.

Introduction

Every collision investigation report follows a broadly similar arc. Vessels were detected. Targets were tracked — often by ARPA. CPA values were displayed. And at some point, either the data was not believed, or it was believed too completely, or it was never properly assessed at all. The vessel closed. The action was late. The action was wrong. The action was insufficient.

The root cause is almost never equipment failure. It is a failure to plot — in the fullest sense of that word. Not simply to acquire a target and read a number off a screen, but to build a mental and spatial model of what is developing, test it against what the radar is actually returning, and reconcile it with what COLREGs demand. That process has not changed since the first marine radars were fitted. The tools around it have improved enormously. The discipline required has not diminished at all.

This article addresses radar plotting as a collision avoidance discipline. It is not an ARPA operator’s guide. It is concerned with what plotting means, what the derived values actually tell a watchkeeper, where radar returns lie or go silent, and why the competence to build a manual plot remains non-negotiable even on a bridge equipped with the latest target tracking.

Contents

• 1. The Purpose of Radar Plotting Under COLREGs
• 2. Relative Motion and True Motion — What Each Display Actually Shows
• 3. Building a Manual Plot
• 4. CPA, TCPA, BCR, BCT — The Geometry of Risk
• 5. ARPA: What It Does and What It Cannot Do
• 6. The Limits of Radar Detection
• 7. ‘The Radar Shows Nothing’
• 8. Closing Reality

1. The Purpose of Radar Plotting Under COLREGs

Rule 7 of the COLREGs is explicit. Risk of collision shall be determined using all available means appropriate to the prevailing circumstances and conditions. Where radar is fitted and operational, it shall be used — including long-range scanning and radar plotting or equivalent systematic observation of detected objects.

The word systematic carries weight.

Radar plotting is not an occasional check. It is the continuous, structured process by which a watchkeeper determines whether risk of collision exists with any detected target, and if so, its nature and urgency. Rule 7 also states that assumptions shall not be made on the basis of scanty information, especially scanty radar information. This is not advisory language. It is a legal standard, tested repeatedly in Admiralty courts.

The plotting obligation does not begin at some threshold of traffic density or reduced visibility. It exists whenever radar is in use. In practice, this means always. The master who argues that the passage was in open water, traffic was light, and visual bearings sufficed will find little comfort in court once it is shown that radar was available and not systematically used.

COLREGs do not specify the method of plotting. Manual plotting, ARPA, or any equivalent systematic observation may satisfy the requirement. But the operative word is equivalent. Watching an ARPA vector wander across the screen without understanding what generated it, what assumptions underpin it, or whether the acquisition is stable is not systematic observation. It is spectatorship.

2. Relative Motion and True Motion — What Each Display Actually Shows

There are two fundamental display modes. The choice between them is not cosmetic. It changes what the watchkeeper sees and — critically — what can be misunderstood.

Relative Motion

In a relative motion display, own ship is fixed at the centre. All other movement on the screen is relative to own ship. A stationary buoy moves down the screen at own ship’s speed and on a reciprocal bearing. A vessel on a steady bearing with decreasing range draws a line directly toward the centre.

This is the natural mode for collision avoidance. A target whose relative track passes through the centre of the display will collide with own ship. A target whose relative track passes close to the centre has a small CPA. The geometry of risk is immediately visible in the direction and curvature of the relative trail or plotted relative track.

Relative motion is intuitive for assessment of risk. It is less intuitive for navigation. Land, buoys, and all fixed objects move across the screen, which can confuse the picture in confined waters.

True Motion

In true motion, own ship moves across the screen at its actual course and speed. Stationary objects remain stationary. Other moving vessels move at their actual courses and speeds. The display more closely resembles a chart — which is precisely why it appeals to some watchkeepers and precisely where the danger lies.

True motion is better for navigation and for understanding the broad traffic picture. It is worse for assessing risk of collision with a specific target, because the relative geometry — the only geometry that determines whether two vessels meet — must be inferred rather than read directly. Two vessels closing at a combined speed of 35 knots may appear to be moving slowly on a true motion display if both are heading roughly the same way. The closing rate is masked.

Relative motion shows whether vessels will meet. True motion shows how they move.

The competent watchkeeper understands both and switches between them as the situation requires. Over-reliance on true motion for collision avoidance has featured in multiple casualty investigations. In restricted visibility, relative motion is almost always the correct primary display for the plotting radar.

3. Building a Manual Plot

ARPA has been standard for decades. Automatic tracking is faster, more consistent, and handles multiple targets simultaneously. None of this eliminates the need to understand manual plotting. The principles are identical. ARPA simply automates the arithmetic.

A manual plot requires three things: a bearing and range to the target at regular intervals, a plotting sheet or reflection plotter, and a stable course and speed for own ship during the observation period.

The process is straightforward. At time T0, the target’s bearing and range are marked on the plotting sheet. At T1 — typically six minutes later for a 1/10th speed interval — a second observation is plotted. The line joining T0 to T1 is the relative track. Its direction is the direction of relative movement (DRM). Its length, scaled by the time interval, gives the speed of relative movement (SRM). Extending the relative track forward reveals how close it will pass to the centre — the CPA — and when — the TCPA.

From the relative track, own ship’s course and speed vector is laid off. The resultant triangle yields the target’s true course and true speed. This allows the watchkeeper to determine not only the risk, but the nature of the encounter — head-on, crossing, overtaking — and therefore which COLREGs rules apply.

Manual plotting is slow. That is not the point. The point is that the officer who can build a plot understands what every ARPA vector on the screen represents.

The officer who cannot is reading numbers without comprehension. When ARPA swaps targets, loses a track in clutter, or presents an unstable vector after a course alteration, that officer has nothing to fall back on. The one who plots — even if only occasionally, to verify ARPA performance — retains the ability to see through the automation to the underlying geometry.

4. CPA, TCPA, BCR, BCT — The Geometry of Risk

Four values define the collision avoidance problem. Each answers a different question.

CPA — Closest Point of Approach

The perpendicular distance from own ship’s position (the display centre in relative motion) to the target’s extended relative track. CPA states how close the target will pass if neither vessel alters course or speed.

CPA is not a prediction of what will happen. It is a statement of what will happen if nothing changes. In heavy traffic, where frequent alterations are occurring, CPA values shift constantly. A comfortable CPA at one moment becomes zero after an unexpected alteration by the target.

TCPA — Time to Closest Point of Approach

The time remaining until CPA is reached, derived from the distance along the relative track to the CPA point, divided by SRM. TCPA determines urgency. A CPA of 0.5 miles with a TCPA of 45 minutes is a very different situation from the same CPA at 8 minutes.

TCPA is the primary driver of decision timing. The earlier the action, the smaller it needs to be. Rule 8 demands early and substantial action. TCPA defines what early means.

BCR — Bow Crossing Range

The range at which the target will cross own ship’s bow (or stern). BCR is calculated by dropping a perpendicular from the target’s current position to own ship’s heading line on the relative plot. It answers a question CPA alone does not: will the target pass ahead or astern, and by how much?

BCR is operationally critical in crossing situations. A target with a CPA of one mile may cross the bow at 0.3 miles if own ship maintains course and the geometry is tight. That is a very different risk profile than a target passing one mile down the port side.

BCT — Bow Crossing Time

The time at which the target crosses own ship’s heading line. BCT, paired with BCR, allows the watchkeeper to assess the temporal margin as well as the spatial one. If a target will cross the bow at 0.5 miles in two minutes, the options have narrowed severely.

These four values — CPA, TCPA, BCR, BCT — are not abstract mathematical exercises. They are the quantified answers to the only questions that matter in collision avoidance: how close, how soon, where relative to my heading, and when relative to now.

A watchkeeper who cannot articulate these four values for every target of concern is not conducting a proper lookout.

5. ARPA: What It Does and What It Cannot Do

ARPA acquires targets — manually or automatically — and tracks them by successive bearing and range observations, performing the same triangulation described above, typically at every antenna rotation. It then displays CPA, TCPA, true course, and true speed for each tracked target, usually as a vector overlay.

ARPA does this well, consistently, and for dozens of targets simultaneously. It also performs trial manoeuvre calculations, allowing the watchkeeper to assess the effect of a proposed course or speed change before executing it.

What ARPA cannot do:

• Guarantee that a detected echo corresponds to a single, real vessel. ARPA can track rain, sea clutter, sidelobe returns, and interference as if they were targets.
• Produce a stable vector immediately after acquisition. ARPA needs time — typically one to three minutes — to build a reliable track. During that settling period, displayed vectors are unreliable. CPA values during this phase are meaningless.
• Respond instantly to target manoeuvres. When a tracked vessel alters course, ARPA must detect the change through successive observations. The displayed vector lags reality. The lag can be 30 seconds to three minutes depending on the system and the magnitude of the alteration.
• Discriminate between a genuine target manoeuvre and a target lost and reacquired at a different position — target swap. Two vessels in close proximity can exchange identities in the tracker. The resulting vectors bear no relation to what either vessel is doing.
• Overcome the fundamental limitations of the radar sensor itself. If the radar does not detect a target, ARPA cannot track it.

ARPA is a tool. It automates arithmetic. It does not automate judgement, and it does not automate lookout.

6. The Limits of Radar Detection

Radar detects objects by receiving reflected microwave energy. Anything that attenuates the transmitted pulse, absorbs the reflected energy, or reduces the target’s radar cross-section degrades detection. This is not a marginal effect. Under certain conditions, large vessels can disappear from the display entirely.

Rain Clutter

Precipitation returns energy across a broad area. Heavy rain cells can completely obscure targets within them. The rain clutter control (or automatic rain processing) suppresses returns from precipitation — but in doing so, it suppresses returns from everything within that rain, including ships. An over-adjusted rain clutter control creates blind sectors. The operator sees a clean display and interprets it as a clear sea. It may be neither.

Sea Clutter

Sea returns dominate the inner ranges, particularly in heavy weather. The sea clutter control reduces close-range returns — and with them, small targets, low-freeboard vessels, and anything whose echo is comparable in strength to the sea surface around it. A fishing vessel at two miles in a Force 7 sea may never emerge from the clutter. A loaded barge, a yacht, a man-overboard target — all can be lost.

The interaction between sea clutter suppression and target detection is not linear. Too little suppression, and the centre of the display is an unreadable mass. Too much, and targets vanish. There is no single correct setting. The only correct practice is continuous adjustment, combined with awareness that the settings are a compromise.

Small Targets

A GRP yacht, a kayak, a survival craft, a container awash — these present radar cross-sections orders of magnitude smaller than a merchant vessel. Detection ranges may be measured in hundreds of metres rather than miles, even in calm conditions. In any sea state above moderate, detection becomes intermittent and then nonexistent. Radar reflectors improve the situation but do not transform it. A vessel relying on radar alone to detect small craft in anything above a flat calm is relying on a capability that may not exist.

Shadow Sectors and Blind Arcs

Every vessel has structural shadow sectors — arcs where the mast, funnel, cranes, or containers block the radar beam. These are often documented during commissioning and then forgotten. A target approaching from within a shadow sector will not be detected until it emerges, at which point range may already be short and CPA small. On container vessels with high deck stowage, forward shadow sectors can be extensive.

7. ‘The Radar Shows Nothing’

This phrase has appeared in the narrative of nearly every collision where radar was operational. It is offered as exoneration. It functions as an indictment.

A radar display showing no targets is not evidence of an empty sea. It is evidence that the radar, at its current settings, in the current conditions, with its current limitations, has not detected returns above the threshold set by the operator and the system’s processing. That is a fundamentally different statement.

The absence of a target on radar means one of several things: there is nothing there; there is something there but it is too small to detect; there is something there but it is masked by clutter; there is something there but the radar settings are suppressing it; there is something there but it is in a shadow sector; the radar has a fault that has not been identified.

The radar shows nothing is never proof of nothing.

Rule 5 requires a proper lookout by all available means. Rule 6 requires safe speed, taking into account the limitations of radar equipment. Rule 7 prohibits assumptions based on scanty radar information. The regulatory framework explicitly anticipates that radar has limitations and demands that watchkeepers account for them. A master or OOW who treats a clear radar display as clearance to proceed without caution has departed from the COLREGs before any other vessel even appears.

In restricted visibility, the correct mental model is not “I can see everything on radar” but “I can see some things on radar, and I do not know what I cannot see.” Speed, lookout posture, readiness of the main engine, and the decision to sound fog signals all flow from that model. The alternative model — that the radar provides comprehensive surveillance — has sunk ships.

8. Closing Reality

Radar plotting is not a historical skill preserved for examination purposes. It is the intellectual foundation of collision avoidance. Every vector on the ARPA display, every CPA alarm, every trial manoeuvre output is the product of the same geometry that a plotting officer works through with a chinagraph pencil and a parallel ruler. The automation has changed the speed of the calculation. It has not changed the nature of the problem.

The problem is this: two objects are moving in space. Will they meet? How close? How soon? What must change to prevent it? And underneath that: is the data I am looking at true, or is it an artefact of processing, clutter suppression, target swap, or sensor limitation?

The officer who can answer both sets of questions — the geometric and the epistemic — is conducting a proper lookout. The officer who can answer only the first is trusting a machine to answer the second. The machine does not know the question exists.