With radars, sheer power and long range isn't everything. There's abso­lutely no point in seeing over the horizon if the image you receive is made up of indeterminate blobs. Much of the information we need lies in the finer details. In short it must discriminate, says John Andersson.

Discrimination
GOOD discrimination is the ability of a radar set to display separately two contacts which are close together. As a user, it's this quality which most impresses me when comparing radar sets. Without good dis­crimination the radar set is rather like an artist trying to paint a portrait using a decora­tor's pasting brush!
There are two areas where dis­crimination is important:
Bearing discrimination is determined by the width of the radar beam and, since beam width is inversely propor­tional to the width of the radar scanner, it's clear that the small scanners fitted to most small yachts will suffer from poorer bearing discrimination than large scanners. Generally, this isn't too much of a problem when operating at long range, but at short range - in say a pilotage area - it can be very confusing. The effect of greater beam width is to cause a contact to appear wider than it really is, so that small targets can give the appearance of small arcs circling the screen. The effect on small contacts is most noticeable at long range, when a small but strong contact may give the appearance of a much larger ship. This is why it's important in this context not to rely on the shape of a contact to indicate its aspect or heading. A small contact will often appear to become narrower as the range decreases, at least until it comes under the influence of the side lobes, when a strong contact may paint a complete circle around the centre spot. Larger contacts will also be drawn out in azimuth, so that head­lands will appear to be wider than they are, and bays and other indentations along the coast will tend to be filled in and lose their identities. Beam width accounts for much of the distortion of a coastline as portrayed on the radar screen. A typical example of this type of distortion occurs when trying to locate a narrow entrance into a bay, such as Lulworth Cove.
If the distance between two contacts at the same range is less than the beam width they will paint as one contact. Incidentally, this will often cause an Automatic Radar Plotting Aid to swap vectors when two contacts pass close to each other.
Bearing discrimination can be improved by judicious use of the gain and or anti-sea clutter controls. Turning down the gain control will tend to make the contacts appear narrower right across the screen. This must be exercised with a fair degree of caution as any reduction in gain is liable to result in some weaker contacts being lost. I have, however, found it to be an acceptable technique when operating at short range in pilotage waters. If you maintained the same level of gain when reducing from, say, 12nm range to O.5nm range, the picture would be so strong that all defInition would be lost. An alternative to turning down the gain is to turn up the anti-sea clutter control. This will have a greater effect on reducing the width of short range contacts while minimising the effect on those more distant.
Many radars - for example the Raytheon Pathfmder - now employ automatic clutter and gain controls which almost always give a much better radar picture than the average operator could achieve. In pilotage waters, however, there's a tendency for the automatic option to produce too much suppression, with consequent loss of contacts, so the operator should revert to manual control.

NB
If a radome is mounted on a mast, it may causes excessive side lobes in that direction. However, the radar boffms at Raytheon tell me that this effect can be minimised by fItting a block of wood to the mast directly behind the radome. It might be worth a try if you're experi­encing this problem.

RANGE DISCRIMINATION

This is the ability of a radar to display separately two contacts which are close together on the same bearing. Range discrimination is largely deter­mined by the length of the transmitted pulse, and has the effect of radially elongating a contact. Pulse length is adjusted automatically by the transmit­ter as the range scale is changed, being very short on short range to very long on long range scales. The pulse length, as already mentioned, compliments the number of pulses transmitted per second - ie the Pulse Repetition

Frequency. The latest Raytheon radars offer a combination of eight pulse lengths and PRFs, to give the best pos­sible discrimination on all range scales. Broadly speaking, if two contacts on the same bearing are separated by less than half the pulse length they'll paint on the screen as one. And, since dis­crimination is most important on minimum range scales, it follows that a
short pulse is required. In spite of the very short pulse length capability of modem radars, it'll still be found that, at very short range, small contacts such as buoys and beacons will appear as elongated radial spikes. With your radar on minimum range scale, observe the appearance of a buoyed channel and you'll see that as the range of a buoy decreases the shape it paints on the radar sceen changes from an arc (due to beam width distortion) to a spike caused by pulse length distortion. The situation can once again be improved by the operator, this time by judicious use of the FTC, which has the effect of chopping off the trailing edge of the echo pulse. Care must once again be taken as this could result in loss of weak signals. The adjustment for sea clutter will probably need reassessing at the same time.

NB
You may fmd that some radar sets have a switch marked Rain and another marked FTC. On the Raytheon the Rain switch is used to minimise the effect of rain at the yacht whilst FTC is used to minimise the effect of rain which may obscure a contact at some distance from the yacht. In this case the FTC can also be used to improve contact discrimination at short range. Where only one Rain clutter control is provid­ed, it'll be the same as the FTC func­tion in its effect. The terms Anti-Rain Clutter, FTC and Differentiator are often used to describe much the same type of function, where the control is used to chop off the trailing edge of the echo pulse. It's very important to read the manufacturer's handbook to identi­fY the correct function of the various controls, as the terminology isn't entirely consistent.