Radar: target discrimination

What you do on the bridge

1. Long pulse: one echo
2. Short pulse: two echoes
3. S-band: one echo
4. X-band: two echoes

The oral questions, and what a good answer covers

The key points are in our own words. In the course, these questions come once you have done the job.

1. 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

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

2. 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

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

3. What limits the radar's minimum range, and why does it matter on a yacht?

   A good answer covers:

   • Minimum range is the closest a target can be detected: limited by the pulse length, the time the receiver needs to recover, and the vertical beamwidth
   • The vertical beam passes over close, low targets, more so with a high aerial
   • Sea clutter close in hides small targets too
   • So a small boat close in may not show at all: keep the look-out, use the short pulse and the short range close in
4. How do target size and aspect affect what the radar shows?

   A good answer covers:

   • Echo strength depends on size, material, shape and aspect, not only size: a big ship end-on can paint weaker than beam-on
   • Wood, GRP and rounded shapes reflect poorly; flat steel faces square to the beam reflect well
   • A low target is detected later than a high one of the same size, because of the radar horizon
   • So the size of an echo is not a reliable guide to the size of the vessel

How the lesson runs

The tutor's narration from the lesson, step by step. On the bridge, each control lights up as it is named.

1. Brief

   We're stopped, with both radars on the three mile range. Out there are four yachts at anchor, in two pairs, and visibility is two miles.

   I want you to find how many echoes the radar shows for each pair, and what it takes to see all four.

2. Long pulse: one echo

   Look on one two zero, about two miles. You're on the long pulse. How many echoes? One. There are two yachts there, ninety metres apart, one behind the other.

3. Short pulse: two echoes

   The long pulse is a hundred and fifty metres long, so two targets closer than that in range paint as one. Shorten the pulse, and watch them part.

4. S-band: one echo

   Now the other pair, on about zero four one at two and a half miles: side by side, two degrees apart. Select radar two, the S-band. One echo again.

5. X-band: two echoes

   The S-band beam is about two degrees wide, so they merge. Back to radar one: the X-band's beam is narrower, and they show as two.

6. Close in

   Close in there's another limit: minimum range. The pulse length, and the vertical beam passing over low targets near the ship, can hide a small boat altogether.

   And echo size isn't boat size: a wooden or plastic hull, end on, can paint smaller than a buoy. Keep the look-out.