Radar Plotting and Collision Avoidance

Prerequisites

You should already be comfortable with chartwork to Yachtmaster Coastal standard, the COLREGs (especially Rules 5 to 8 and Rule 19, covered in the previous lesson), and the basic controls of a small-craft radar. The RYA one-day Radar course is strongly recommended before the Yachtmaster Offshore exam, and an examiner may expect you to use radar for pilotage and collision avoidance on the exam boat if it is fitted. You need a pencil, a plotting sheet (or a reflection plotter) and a Portland or Breton plotter for the exercises.

Learning Objectives

By the end of this lesson you should be able to:

• Explain how a marine radar works and the factors that limit detection range and accuracy.
• Optimise gain, tuning, sea clutter (STC) and rain clutter (FTC) for the conditions.
• Distinguish relative motion and true motion, and head-up, course-up and north-up displays.
• Carry out a manual radar plot to find CPA, TCPA, target true course and speed, and aspect.
• Recognise a constant bearing, decreasing range (CBDR) as the indicator of collision risk.
• Decide on and verify avoiding action that complies with Rule 19.
• Use guard zones, parallel indexing and radar ranges for pilotage and fixing.
• Describe the limitations of ARPA, MARPA and AIS overlay.
• Map your knowledge to the RYA Yachtmaster Offshore syllabus (G158) items on radar and collision avoidance: optimising the radar controls for detection in different sea states and rain, manual relative-motion plotting to find CPA, TCPA, target course, speed and aspect, parallel indexing, and integrating radar with ARPA or MARPA, AIS and a visual lookout, together with the COLREGs Rules 5, 6, 7, 8 and 19.
• Interpret false echoes, shadow sectors and clutter, and know what to do if the radar fails.
• Compare avoiding-action options by calculation, and explain why slowing down is sometimes the wrong answer.

How Radar Works

A radar transmits short pulses of microwave energy from a rotating antenna and measures the time for each echo to return. Range follows from the time (radio waves travel at about 162,000 nautical miles per second, so an echo from 1 mile returns in about 12.4 microseconds). Bearing is the direction the antenna was pointing when the echo returned. Each sweep paints the echoes on a plan position indicator (PPI) around own ship.

Most yacht radars work in the X-band (around 9.4 GHz, 3 cm wavelength). X-band gives good definition and is the band that triggers a SART. Ships also carry S-band (3 GHz, 10 cm), which sees better through rain and sea clutter but with less detail. Modern solid-state "broadband" or Doppler radars give excellent close-range performance and can colour approaching targets, but the principles below still apply.

Range of detection

Radar waves travel in nearly straight lines, bending slightly more than light. The radar horizon in nautical miles is approximately:

Range = 2.2 x (square root of antenna height in metres + square root of target height in metres)

Worked example. Your scanner is 4 m above the sea. A ship's bridge and superstructure stand 25 m high. Range = 2.2 x (2 + 5) = 15.4 miles. A low cliff 9 m high: 2.2 x (2 + 3) = 11 miles. Another yacht whose reflector is 4 m up: 2.2 x (2 + 2) = 8.8 miles at best, and in practice a GRP yacht with a poor reflector may not show until 2 or 3 miles, and may disappear in sea clutter.

What returns an echo depends on size, shape, material and aspect. Steel and flat surfaces facing the radar reflect strongly; GRP, wood, rubber and rounded shapes are poor reflectors. A sloping sandy beach may return almost nothing, so the "coastline" you see on radar may be cliffs well inland.

Bearing and range accuracy

• Horizontal beamwidth smears echoes sideways. A typical 60 cm radome has a beamwidth of about 4 to 5 degrees, so two buoys close together at the same range may merge, and a headland's edge appears wider than charted. Radar bearings are therefore less accurate than radar ranges. Fix by ranges where possible.
• Pulse length limits range discrimination. On longer ranges the radar uses longer pulses, so two targets on the same bearing and close in range may merge.
• Heading accuracy. On a head-up display the bearings are relative to the ship's head; yaw of a few degrees in a seaway gives the same error. Use a heading sensor (fluxgate compass) to give a stabilised north-up or course-up display.

Display modes

Unless the radar is set to true motion (or true trails), the movement you see is relative motion: the apparent movement of the target as seen from your boat, which is the combination of its true movement and your own. The diagram below shows the idea.

Click to zoom

A buoy you are motoring past will "move" down the screen at your speed on a relative motion display. A ship on the same course and speed as you appears stationary. Misreading relative motion as true motion is the root of many radar-assisted collisions.

Setting Up the Radar

Before relying on the picture, set it up in this order:

1. Brilliance and contrast, so that the display is comfortable for the light conditions (night mode at night to protect night vision).
2. Range scale to suit the task: 6 or 12 miles for early warning offshore, 3 miles or less for close-quarters plotting and pilotage. Switch ranges regularly; a target missed on a long range may be obvious on a short one.
3. Gain (receiver sensitivity). Increase until a light speckle of background noise just appears on the longer ranges, then back off slightly. Too low and weak targets vanish; too high and they are buried in noise.
4. Tuning. Most modern sets auto-tune. If manual, tune for the strongest returns from a distant known target.
5. Sea clutter (STC, anti-clutter sea). Reduces the gain close to the boat where wave tops return echoes. Use the minimum that leaves a few clutter speckles visible; excessive STC removes small boats, buoys and people in the water within the clutter area. Auto sea clutter is reasonable in moderate seas but check manually.
6. Rain clutter (FTC). Breaks up large areas of rain or snow so that solid targets inside them can be seen. It also weakens all echoes, so switch it off when not needed.
7. Check heading line, EBL (electronic bearing line) and VRM (variable range marker) against a known target to confirm alignment.

The professional operator readjusts every time the conditions change. Write down the settings that worked in each condition in the boat's radar notes; it saves time when fog arrives at 0300.

Collision Risk: Constant Bearing, Decreasing Range

Rule 7(b) requires proper use of radar, "including long-range scanning to obtain early warning of risk of collision and radar plotting or equivalent systematic observation of detected objects". Rule 7(c) warns against assumptions made on scanty radar information.

The basic sign of risk is shown below: if the bearing of a target stays the same while the range decreases, the two vessels are on a collision course.

With the EBL laid on the target, watch whether the echo moves along the line towards the centre (danger), or drifts off to one side. Equally, a target whose bearing is changing slowly at long range can still pass dangerously close. And remember Rule 7(d)(ii): risk may exist even when an appreciable bearing change is evident, particularly with a large vessel or a tow at close range. Plotting gives a number for "how close" instead of a feeling.

Manual Radar Plotting

A manual plot uses a series of ranges and bearings of a target, taken at fixed intervals, to build up its relative track, from which everything else follows. Plot on a north-up display, or convert relative bearings to true bearings before plotting.

Step by step

1. Note the time and read the target's bearing (EBL) and range (VRM). Plot it on the sheet as O (the first position).
2. Keep a steady course and speed. Wait exactly 3 or 6 minutes (6 minutes is one tenth of an hour, which makes speed arithmetic easy).
3. Plot the next position. Plot at least three positions to confirm that the target is moving in a straight line; if the three plots are not in line, the target is manoeuvring or your observations are poor.
4. The last plotted position is A. Draw the line from O through A and extend it beyond the centre. This is the relative motion line (sometimes called the line of approach).
5. CPA: drop a perpendicular from own ship (the centre) to the extended relative motion line. Its length is the closest point of approach.
6. TCPA: measure the distance from A to the CPA point, divide by the relative speed (the distance O to A divided by the time between them). Add to the time at A.

The diagram below shows the CPA and TCPA construction.

Click to zoom

Finding the target's true course and speed

The relative motion of the target is the combination of the target's true motion and the reverse of your own motion. So the target's true vector equals your own vector plus the relative vector, as shown in the vector triangle below.

On the plotting sheet the triangle is traditionally labelled W, O, A:

1. From O, lay off a line in the reverse direction of your own course, with a length equal to the distance you travelled during the plotting interval. Its end is W ("way of own ship"). WO is your own ship's vector.
2. OA is the target's relative motion over the same interval.
3. WA is the target's true course and speed. Measure its direction for the course; its length divided by the interval gives the speed.

If WA is very short, the target is nearly stopped (perhaps a fishing boat or vessel at anchor). If W and A coincide, it is stationary, so the relative motion is simply your own motion reversed.

Aspect

Aspect is the relative bearing of your own ship as seen from the target, measured from the target's bow, 0 to 180 degrees, Red (port) or Green (starboard). It tells you which way the target is heading relative to you, and therefore which COLREG situation applies if the vessels come into sight of one another.

Click to zoom

To find it: take the bearing of own ship from the target (the reciprocal of the target's bearing from you), then compare it with the target's true course from the plot. Example: target bears 045 degrees from you, so you bear 225 degrees from it; target's course is 236 degrees; you are 11 degrees on her port bow, aspect Red 11. She is nearly end-on to you, showing her port side.

Tools for plotting

A plotting sheet with range rings and a compass rose is the standard tool. Some older sets have a reflection plotter: a hood or transparent disc over the screen on which you mark echoes with a chinagraph pencil, using a mirror arrangement that eliminates parallax.

Click to zoom

On modern displays, the electronic EBL and VRM, plus echo trails set to relative, do much of the same job, and a "floating" EBL can be placed on the target's first position to project its relative track directly. The principle is the same; the discipline of writing down times, bearings and ranges remains essential.

Worked Example: Full Plot

You are in fog in the western Channel, motoring 000 degrees True at 6 knots. The radar is north-up, relative motion, 12-mile range with rings at 2 miles.

Relative motion. The three plots are in line. O to A is 1.5 miles in 6 minutes, so the relative speed is 15 knots. The relative track runs 217 degrees. The bearing is changing slowly (045 to 048 degrees) but the range is closing fast.

CPA. Extending OA past the centre, the perpendicular from own ship to that line measures 0.8 miles. The CPA point lies on your starboard quarter (bearing about 127 degrees), so the target is going to cross your track astern of you, about 1.3 miles behind.

TCPA. From O to the CPA point along the relative track is 5.6 miles; from A it is 4.1 miles. At 15 knots that is 16.4 minutes, so TCPA is about 1222.

True course and speed. You travelled 0.6 miles north in 6 minutes. From O lay off 0.6 miles in direction 180 degrees to get W. WA measures 1.08 miles in direction 236 degrees: the target is steering about 236 degrees at about 10.8 knots.

Aspect. You bear 228 degrees from the target at A; her heading is 236 degrees; you are about 8 degrees on her port bow, aspect Red 8. She is coming almost straight towards you.

Assessment. A CPA of 0.8 miles in fog with a vessel of unknown size, nearly end-on, is a close-quarters situation. Many skippers set a minimum CPA of 1 mile offshore, 2 miles for large ships. Action is required now, at 4 miles, not at 2.

Choice of action under Rule 19(d). The target is forward of the beam on the starboard side, so avoid altering to port. Consider the options:

• Slow down or stop. With own speed zero, the relative motion becomes simply the target's true motion. Rerunning the geometry gives a CPA of about 0.6 miles: worse, because the target is crossing astern of you and slowing lets it pass closer.
• Alter 90 degrees to starboard, to 090 degrees. Relative velocity becomes the target's (9 knots west, 6 knots south) minus yours (6 knots east): 15 knots west, 6 knots south. The new CPA from A works out to about 1.45 miles, with the target passing astern. This is a substantial alteration that will be obvious on the other ship's radar.

You alter to 090 degrees, sound one prolonged blast every 2 minutes (power-driven making way), keep plotting, and confirm with fresh plots at 1209, 1212 and 1215 that the new relative track is passing at about 1.4 to 1.5 miles. Only when the target is past and clearly opening do you resume course. Remember that the target may also act: if her new plot shows a changed relative track, start again.

Avoiding Action in Restricted Visibility

Combining Rule 8 and Rule 19:

• Act early. A plot takes 6 to 9 minutes; if you start at 3 miles with a 15-knot closing speed you have 12 minutes in total. Start plotting every target at 6 to 8 miles.
• Make the alteration large: 60 to 90 degrees, or a substantial reduction in speed. A 20-degree alteration may not even show on your own plot for several minutes, let alone on the other vessel's radar.
• Avoid a series of small alterations, which confuse other plotters.
• Do not alter to port for a target forward of the beam (except one you are overtaking). Do not alter towards a target abeam or abaft the beam.
• If a close-quarters situation cannot be avoided with a target forward of the beam, reduce to steerage way or stop.
• Check the result by further plotting. The action is only proven when the new plot confirms the increased CPA.
• Remember that on a relative motion display, after you alter course, all targets will appear to change their motion. Restart the plot from the moment of your alteration.

Multiple targets

In busy waters, rank targets by TCPA and CPA. A target with a CPA of 0.5 miles at 30 minutes is less urgent than one with a CPA of 1 mile at 8 minutes that is still closing on a steady bearing. Choose an action that improves the most dangerous situation without creating a new one with another target; sketch the predicted relative tracks for your proposed new course before you commit. When targets are converging from both sides and no turn helps, slowing down or stopping is often the best answer.

Interpreting the Picture: What You See Is Not Always There

An experienced operator learns to read the screen as a picture of the world, with known distortions.

• Shadow sectors. The mast, boom, a wind generator, a radar arch or the sail can block part of the beam, leaving a blind sector on the screen. A target in the sector will not be seen. Find your boat's blind sectors in clear weather by watching a distant ship or a buoy as you slowly turn, and note them on the radar log.
• Side-lobe echoes. A strong echo, such as a nearby steel ship, may produce arcs or a ring of weaker echoes at the same range on either side. They are false returns from energy escaping outside the main beam. Reducing the gain, or the sea clutter, makes them disappear.
• Multiple echoes. A strong target at close range can give repeated echoes along the same bearing as the signal bounces between the target and your boat. The first echo is the true one.
• Indirect or false echoes. Reflections from the mast or from a large structure on your own boat may paint a target at the wrong bearing. They typically appear in a fixed bearing relative to the ship's head.
• Second-trace echoes. Under unusual conditions (ducting), a distant target beyond the maximum range may appear closer than it truly is, usually distorted. They disappear when you change the range scale or the pulse repetition rate.
• Radar interference from another radar on a similar frequency appears as a pattern of dots or spiral lines. It is usually not a problem; changing the range or pulse length, or the interference rejection control, may help.
• Rain squalls and snow appear as large patches of clutter with blurred edges. Ships and yachts inside them will be hidden. Use the rain clutter control briefly and carefully, and anticipate that a squall may hide a ship.
• Sea clutter is strongest down-wind, so a target is more likely to be lost in the clutter if it is directly upwind of you. A small yacht or a buoy in the clutter zone may be seen only briefly on each swing.
• Land and coastline. Low, shelving beaches and mud flats give poor returns, so the radar coastline may be hills inland; headlands and rocks give sharper echoes. The land behind a headland is hidden. Compare the radar picture with the chart to understand what the radar is actually showing.
• Racons respond to your radar with a coded pulse that shows on screen as a Morse letter behind the target. A racon is a positive identification. Not all radars show them clearly; switch to a longer pulse and reduce the clutter controls.

Safety on the radar

Microwave radiation from an operating scanner can be harmful at close range. Mount the radome or scanner above head height, never stand in the beam of an open array, and switch the radar to standby when working aloft near it. The power drawn is modest, 20 to 40 watts for a small radome in operation, but it matters when you are at anchor on batteries. Use standby rather than off if you want a fast restart, and warm-up time on older magnetron sets is typically 2 to 3 minutes.

What a Target's Aspect and Size Tell You

A target's echo strength gives clues to its size and distance. A small, weak echo that appears and disappears might be a yacht or a fishing boat. A strong, steady echo is probably a large ship or a rock. A line of weak echoes may be fishing floats or a net. At night, put the radar and the AIS together: an echo with no AIS target is a small boat, a fishing vessel, a naval vessel or a vessel with AIS off.

Estimate the size by the arc width. A large ship at 6 miles shows an echo several degrees wide on a yacht radome, because of the beamwidth, which can make her look further to one side than she is. A ship broad on the beam shows a long thin echo; a ship end-on shows a small echo, a short arc, and may be missed.

Aspect matters in the plot: an end-on target (aspect red or green 0 to 10 degrees) has a short echo but a big risk, because the relative speed is the sum of the two speeds.

Rule 19, Rule 6 and Rule 7 on the Radar Screen

• Rule 19(a) applies only to vessels not in sight of one another, in or near an area of restricted visibility. When the target is seen, the steering and sailing rules (Rules 11 to 18) apply instead.
• Rule 19(b) requires every vessel to proceed at a safe speed adapted to the circumstances, with power-driven vessels having the engines ready for immediate manoeuvre.
• Rule 19(c) requires due regard to the circumstances in complying with Rules 4 to 10.
• Rule 19(d) applies to a vessel that detects by radar alone the presence of another vessel: she must determine whether a close-quarters situation is developing or risk of collision exists, and if so take avoiding action in ample time. If the action is an alteration of course, so far as possible avoid (i) an alteration to port for a vessel forward of the beam, other than for a vessel being overtaken; and (ii) an alteration of course towards a vessel abeam or abaft the beam.
• Rule 19(e) applies when you hear the fog signal of another vessel apparently forward of the beam, or cannot avoid a close-quarters situation with a vessel forward of the beam: reduce speed to the minimum at which you can be kept on course, and if necessary take all way off, and navigate with extreme caution until danger has passed.
• Rule 6 lists factors for safe speed. For vessels with operational radar these include the characteristics, efficiency and limitations of the radar equipment, the radar range scale in use, the effect of sea state, weather and other sources of interference on radar detection, the possibility that small vessels, ice and other floating objects may not be detected at an adequate range, and the number, location and movement of vessels detected by radar.
• Rule 7 requires proper use of radar if fitted and operational, including long-range scanning to obtain early warning, and radar plotting or equivalent systematic observation. It forbids assumptions on the basis of scanty information, especially scanty radar information.
• Rule 8 requires any action to avoid collision to be positive, made in ample time and with due regard to good seamanship, large enough to be readily apparent to another vessel observing visually or by radar; a succession of small alterations should be avoided.

Rule 19(d)(i) does not forbid altering to port for a target abaft the beam or ahead on the port side if the geometry is right, and the rule says "so far as possible". The point is to avoid turning into the track of a vessel ahead on the other side of you.

Worked Example: A Crossing Target and the Choice of Action

You are sailing north (000 degrees True) at 6 knots under engine in poor visibility, with radar north-up, relative motion, at the 6-mile range scale. A target appears on the starboard bow. The plot is read at 6-minute intervals.

Relative motion. The three points lie on a line. O to A is 2.64 miles in 12 minutes, so the relative speed is 13.2 knots and the relative track runs towards 225 degrees. The bearing is drifting only slowly (4 degrees in 12 minutes) while the range is closing steadily: this is a near-constant-bearing situation, and risk of collision must be assumed.

CPA and TCPA. Extend OA through the centre. The perpendicular from own ship to the line measures 0.5 miles, on a bearing of about 135 degrees (the target will cross astern, on the starboard quarter). The distance from A to the CPA point is about 3.3 miles, so TCPA is 3.3 / 13.2 = 15 minutes after 1212, at about 1227.

True course and speed. In 12 minutes you have run 1.2 miles north. From O, lay off 1.2 miles towards 180 (south) to find W. WA measures 2.0 miles towards 250 degrees, so the target is steering about 250 degrees at 10 knots.

Aspect. You bear 054 + 180 = 234 degrees from the target at A, and her heading is 250 degrees: you are 16 degrees on her port bow, aspect Red 16. She is almost end-on to you.

Assessment. A CPA of 0.5 miles with an unknown vessel in poor visibility is unacceptable. She is on your starboard bow, 54 degrees from ahead, so Rule 19(d)(i) allows a turn to starboard but not to port. Compare the options by re-plotting from A with your new course and speed. The target's true vector stays the same (250 degrees, 10 knots). Your new vector is subtracted from hers to give the new relative motion.

The table makes two points. First, slowing down is the worst choice here: it holds you in the target's path and brings the CPA down to 0.1 miles, because at 3 knots you lose the benefit of having crossed ahead of her. Reducing speed is only the right answer when you cannot otherwise get clear, or when a target is forward of the beam and close, as Rule 19(e) says. Second, a small alteration (60 degrees) is barely better than nothing; a large one (90 degrees or more) gives a CPA that is readily apparent to the other vessel's radar and a margin of more than a mile.

Action. Alter to starboard to 090 degrees, sound one prolonged blast every two minutes, and plot again at 1215, 1218 and 1221 to confirm the new relative track. The CPA from the plot should come out near 1.3 miles with the target passing ahead of you. Only when she has passed and the range is opening should you return to your course. Log the times, the bearings, the actions and the result.

Notice that the answer is not the one an intuitive helmsman gives. A yachtsman who slows down in fog "to be safe" can make the situation worse unless it has been plotted. Always check any avoiding action on paper first.

If the Radar Fails

An examiner may ask: "your radar fails in fog in a shipping lane: what do you do?"

1. Increase the lookout and listen, with the crew on deck and the engine on. Sound the correct fog signals.
2. Reduce speed to a safe speed (Rule 6 and Rule 19(b)): without radar you cannot detect targets, so the safe speed is lower.
3. Get out of the shipping lane if you can, towards shallower water where large ships do not go, using the GPS and depth sounder, and keeping to depths less than the draught of big ships.
4. Use AIS (if fitted as a receiver or transceiver) for warning of large ships, remembering it shows only vessels that transmit.
5. Switch on the navigation lights and the masthead tricolour, and show the radar reflector.
6. Check the cause: the power supply, the fuse, the cable, the radome and whether the radar has simply gone into standby or the range scale is wrong.
7. Consider waiting at anchor in shallow water outside the channel, or returning to port, if the fog is thick and the traffic is heavy.
8. Tell the coastguard or VTS if you are in a vulnerable position.

Practice Drills

• Daily plot. At sea, choose one target each watch and plot it three times at 6-minute intervals on a plotting sheet. Compare the CPA you find with the MARPA or AIS figure.
• Controls drill. Switch off the auto-clutter and adjust gain, sea clutter and rain clutter manually in different conditions; write down the best settings for each.
• Blind sector survey. Find and note the boat's shadow sectors in clear weather.
• Alteration drill. Plot a target with a CPA under 0.5 miles and, before turning, work out the new CPA for three possible actions as in the table above.
• Timing drill. Practice a complete plot in under 12 minutes: three observations, relative track, CPA, TCPA, true course and speed.
• Parallel indexing drill. On a clear-weather passage along a coast, set up a parallel index and check it against the GPS cross-track error.

Guard Zones and Alarms

A guard zone is a sector or ring on the display; any echo entering it sounds an alarm. It is useful when short-handed, but it is an aid, not a lookout. Set the zone beyond the clutter (otherwise it alarms constantly) and remember that a small target may not be detected until inside the zone, or may never trigger it at all.

Click to zoom

Guard zones, CPA/TCPA alarms on AIS and radar, and watch alarms help the double-handed or single-handed sailor, but Rule 5 still requires a proper lookout by sight and hearing.

ARPA, MARPA and AIS

ARPA (Automatic Radar Plotting Aid) and its small-craft version MARPA automate the plot: acquire a target and after a few minutes the set displays its vector, CPA and TCPA. On a yacht, MARPA depends heavily on accurate heading data; a poor fluxgate compass or a yawing boat produces wrong vectors.

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Key limitations:

• Processing delay. After acquisition, or after either vessel alters course, the data takes one to three minutes to settle. During that period the vector may be wrong.
• Target swap. When two echoes pass close to each other, the tracker may transfer one target's identity and vector to the other.
• Lost targets and false acquisition in sea clutter, rain or when echoes fade.
• Garbage in, garbage out. Errors in own ship heading and speed input give errors in the target's true vector (though not in the relative CPA, which depends only on the radar echoes).

AIS overlay shows identity, course, speed and position reported by the other vessel's own GPS. It is extremely useful for identification and VHF calls, but: many small craft do not transmit AIS; Class B units transmit less often and at lower power; AIS data can be wrong (wrong position source, wrong heading); and there can be an offset between the AIS symbol and the radar echo. The radar echo is the "real" target. Use AIS to support, never to override, a radar plot.

Radar for Pilotage and Fixing

Fixing by radar

1. Choose conspicuous, unambiguous targets: steep headlands, isolated rocks, piers, racons.
2. Prefer ranges to bearings, because ranges are more accurate. Three ranges give a good fix.
3. Measure ranges from the point on the chart that actually returns the echo: the cliff face, not the low-lying shore in front of it.
4. Racons on buoys and lighthouses respond with a Morse code letter painted radially outward behind the echo, making identification certain.

Parallel indexing

Parallel indexing lets you monitor cross-track error continuously. Plan on the chart: if you want to pass a headland at 0.5 miles on a track of 090 degrees, draw a line parallel to the track at 0.5 miles from the headland. On the radar (north-up or course-up stabilised), set a parallel index line on the 090/270 direction offset 0.5 miles from own ship on the side of the headland. If the headland echo slides along the line, you are on track; if it moves inside the line, you are being set towards it. Unlike a GPS cross-track display, PI checks you against the real land, independent of satellite position errors.

Approaching a SART

A Search and Rescue Transponder in a liferaft responds to X-band radar with a line of 12 dots radiating outward from the SART's position. As you close within about 1 mile, the dots become arcs, and very close they form concentric circles. Reduce gain and sea clutter carefully, or the faint dots may be suppressed.

Common Mistakes

• Failing to readjust settings as sea state, rain or range scale changes; too much STC hides small vessels close to you.
• Confusing relative and true motion. A target "moving" slowly across your screen may be stationary, and one that appears motionless may be on your course and speed.
• Deciding on scanty information. One observation, or a target seen only on AIS, is not a plot.
• Acting too late or too small. In fog with a 15-knot closing speed you have only minutes. Start plotting early and make big alterations.
• Altering to port for a target forward of the beam.
• Trusting ARPA/MARPA vectors immediately after acquisition or after altering course.
• Forgetting that after your own course change, the old plot is invalid. Begin a new plot.
• Using bearings rather than ranges for fixing, or ranging off the wrong part of the coast.
• Assuming you have been seen. A GRP yacht may give a weak or intermittent echo on another ship's radar.

Summary

• Radar measures ranges well and bearings less well. Detection range depends on antenna and target heights, target size, shape and material, and on clutter.
• Set up and continually adjust gain, tuning, sea and rain clutter to suit the conditions.
• Unless set to true motion, the display shows relative motion. Plot targets at regular intervals (3 or 6 minutes), at least three times.
• From the relative motion line find CPA and TCPA; from the W-O-A triangle find the target's true course, speed and aspect.
• A constant bearing with decreasing range means collision risk; a changing bearing does not guarantee safety, so calculate CPA.
• Take early, substantial action that complies with Rule 19, and verify it with new plots.
• Guard zones, ARPA/MARPA and AIS are aids with real limitations; manual plotting and a visual and listening lookout remain essential.

Check Your Understanding

1. Your scanner is 9 m above the sea. Roughly how far away will a 16 m high cliff first appear on radar?

Answer: 2.2 x (3 + 4) = 15.4 miles, assuming it is a good reflector.

1. Why is a radar range generally more accurate than a radar bearing?

Answer: Bearings are affected by horizontal beamwidth (several degrees on a yacht radome), heading errors and yaw, while range is measured from the echo's timing, which is precise.

1. What does a target do on a relative motion display if it is on exactly the same course and speed as you?

Answer: It remains stationary on the screen.

1. A target's bearing stays at 320 degrees while its range decreases from 6 to 4 to 2 miles. What does this indicate and what should you do in fog?

Answer: Constant bearing, decreasing range: collision course. The target is forward of the beam on the port side; under Rule 19(d) do not alter to port. Make a substantial alteration to starboard and/or reduce speed, and verify by continued plotting.

1. In the W-O-A triangle, what do WO, OA and WA represent?

Answer: WO is own ship's course and speed over the plotting interval; OA is the target's relative motion; WA is the target's true course and speed.

1. Plots at 6-minute intervals show a target moving 1.2 miles along its relative track per interval, and the CPA point is 3.6 miles from the last plot. What are the relative speed and the TCPA?

Answer: Relative speed is 12 knots (1.2 miles in 6 minutes). TCPA is 3.6 / 12 hours = 18 minutes after the last plot.

1. Name three limitations of ARPA or MARPA.

Answer: Any three of: processing delay after acquisition or a course change; target swap when echoes pass close; lost tracking or false targets in clutter; dependence on accurate heading and speed inputs; small targets not acquired.

1. What does a SART look like on your radar screen at about 5 miles, and what changes as you close?

Answer: A line of 12 dots extending outward from the SART's position along its bearing. Within about a mile the dots become arcs, and very close they become concentric circles.

1. You altered course 60 degrees to starboard two minutes ago. Your old plot shows the target still closing. What should you do?

Answer: Disregard the old relative track, because your own course change alters the relative motion of every target. Start a new plot from the moment of the alteration, and allow the picture time to settle before judging the result.

1. How does parallel indexing help when passing a headland in fog?

Answer: A line set on the radar parallel to your intended track, at the planned passing distance, lets you see immediately whether the headland's echo is following the line (on track) or crossing inside it (being set towards danger), independent of GPS.

11. Why can slowing down make a collision situation worse on a radar plot?

Answer: If the other vessel is crossing ahead of you and you reduce speed, you remain in her path longer, the relative track moves towards the centre and the CPA can fall (in the worked example from 0.5 to 0.1 miles). Slowing helps only when the geometry has been plotted and shows it works, or when Rule 19(e) applies and avoiding action is impossible.

12. A shadow sector on your radar lies between 160 and 185 degrees relative. What is it likely to be and how do you deal with it?

Answer: A blind sector caused by an obstruction such as the mast, a radar arch or a wind generator in the beam. Survey the sectors in clear weather, note them in the radar log, and keep a visual and manual watch for targets in that sector, altering course slightly when needed to check them.