Work the radar, then explain it.

Why it matters in the oral

The examiner will put you on a radar in your head: what range, which presentation, what the echo is doing and what you would do about it. Candidates who have only read about plotting tend to lose the thread halfway through.

Here the targets are real ships in the scenario. You plot them, acquire them and judge the risk while they close, then answer the questions that go with each lesson.

Oral questions

Radar and ARPA questions, and what a good answer covers.

Six of the most-asked. In the course, each one follows the lesson where you have just done it.

1. Set the radar up from cold for navigation and collision avoidance. Talk me through it.Often asked

   A good answer covers:

   • Check the scanner area is clear of people and obstructions before transmitting
   • Switch on, let it warm up to standby, choose a sensible range before transmitting
   • Brilliance and palette for the light, then gain, then tune, then rain and sea clutter
   • Gain until speckle just shows, then back off until it has just gone
   • Least sea and rain clutter that clears the picture, without losing small targets
   • Check the performance monitor and the heading line
   • North-up, relative motion, sea-stabilised for collision avoidance; speed from the log

   Practised in the lesson Radar: setting up from cold.

   Show me on the bridgeBoth sets are cold and left as the night watch had them.

2. What does a SART look like on the radar, and what would you do? And a racon?Often asked

   A good answer covers:

   • A SART answers only an X-band (9 GHz) radar
   • At range it shows as a line of 12 dots outward along its bearing; the nearest dot can be up to about 0.6 miles beyond the SART, and within about 150 metres when close
   • As you close to about a mile the dots become arcs, then circles round the centre very close in
   • Treat it as distress: tell the master, report to the rescue coordination centre, and go to assist
   • A racon paints a Morse identity letter outward from behind the beacon, so it can be identified on the chart

   Practised in the lesson Radar: setting up from cold.

   Show me on the bridgeA SART is transmitting about 7 nm ahead. Look on the 12 mile range.

3. What is bearing discrimination, and what decides it?Often asked

   A good answer covers:

   • The ability to show two targets at the same range, close together in bearing, as two separate echoes
   • Decided mainly by the horizontal beamwidth: two targets closer in bearing than about one beamwidth merge
   • A longer aerial gives a narrower beam; X-band gives a narrower beam than S-band for the same aerial length
   • Each echo is smeared by the beamwidth, so at long range two ships or a ship and a buoy can paint as one

   Practised in the lesson Radar: target discrimination.

   Show me on the bridgeTwo yachts side by side on 041°: compare the S-band with the X-band.

4. What is range discrimination, and how can you improve it?Often asked

   A good answer covers:

   • The ability to show two targets on the same bearing, close together in range, as two separate echoes
   • Decided by the pulse length: two targets closer than about half the pulse length in range merge (150 m for a one microsecond pulse)
   • A shorter pulse improves it, which is why the short pulse goes with the short ranges

   Practised in the lesson Radar: target discrimination.

   Show me on the bridgeTwo yachts one behind the other on 120°: long pulse, then short.

5. What is the difference between a blind sector and a shadow sector, and what causes them?Often asked

   A good answer covers:

   • Blind sector: an arc where the beam is blocked and nothing is detected
   • Shadow sector: an arc where the beam is partly blocked and detection is reduced, so weak targets are lost
   • Caused by masts, funnels, other aerials, cranes and structure near the scanner
   • The nearer and wider the obstruction, the wider and deeper the sector

   Practised in the lesson Radar blind and shadow sectors: find them and record them.

   Show me on the bridgeLook astern in the sea clutter for the darker wedges.

6. How would you find a radar's blind and shadow sectors?Often asked

   A good answer covers:

   • In moderate sea clutter, look for dark or thinned wedges where the clutter should be
   • Set the sea clutter so it only just speckles: too much or too little hides a shallow shadow
   • Measure the edges as relative bearings with the EBL
   • Or swing the ship slowly past a small isolated echo, such as a buoy, and note where it fades and returns
   • Record them on a diagram by the display, and repeat the survey after any change near the scanner

   Practised in the lesson Radar blind and shadow sectors: find them and record them.

All 46 Radar and ARPA questions

Preparing for the OOW 3000 oral? See the whole syllabus, topic by topic. Other topics: ECDIS and electronic aids, Position fixing, Passage planning, COLREGs, Watchkeeping, Pilotage, GMDSS and distress, Emergencies, Ship handling, Weather.