Monopulse radar

Monopulse radar is an adaptation of conical scanning radar which sends additional information in the radar signal in order to avoid problems caused by rapid changes in signal strength. The system also makes jamming more difficult. Most radars designed since the 1960s are monopulse systems.

Description

Conical scan systems work by sending out a signal slightly to one side of the antenna's boresight, and then rotating the feed horn to make the lobe rotate around the boresight line. A target centered on the boresight is always slightly illuminated by the lobe, and provides a strong return. If the target is to one side, it will be illuminated only when the lobe is pointed in that general direction, resulting in a weaker signal overall (or a flashing one if the rotation is slow enough) with the maximum return in the direction the antenna should be moved.

One problem with this approach is that radar signals often change in amplitude for reasons that have nothing to do with beam alignment. Over the period of a few tenths of seconds, for instance, changes in target heading, clouds and other issues can dramatically affect the returned signal. Since conical scanning systems depend on the signal growing a weakening due only to the lobe position, such changes in reflected signal can cause it to be "confused" about the position of the target within the lobe's scanning area.

Monopulse radar avoids this problem by always comparing the signal against the signal, as opposed to the mechanical position of the antenna. Monopulse radars split the beam into parts, and then compares the signal strength of the parts when they return. That means the comparison always takes place based on the reflection of a single pulse, and hence the name "monopulse". The signal can then be rotated as in a conical scanning system, changes in signal strength over the rotation period will have no effect because those comparisons are not made.

In effect a conical scanning system displays the strength of the signal "now" as opposed to "then", over time. Monopulse systems compare a single pulse with itself, so there is no time delay in which the signal strength can change. Changes in signal strength during the pulse are possible, but as these are normally only a few microseconds long, simple effects like changes in direction will be far too slow to confuse the monopulse receiver.

Making such a comparison requires that different parts of the beam be distinguished from each other. Normally this is achieved by splitting the pulse into two parts and polarizing each one separately before sending it to a set of slightly off-axis feed horns. This results in a set of lobes, usually two, overlapping on the boresight. On reception, the signals are separated again, and then one signal is inverted and the two are then summed. If the target is to one side of the boresight the resulting sum will be positive, if on the other, negative, a signal that can be amplified and sent directly to the pointing motors.

Monopulse radar was extremely "high tech" when it was first introduced in 1943 in a Naval Research Laboratory experiment. As a result, it was very expensive and generally more difficult to maintain. It was only used when extreme accuracy was needed that justified the cost. Early uses included the Nike Ajax missile, which demanded very high accuracy, or for tracking radars used for measuring various rocket launches. One of the larger installations first appeared in the 1970s as the US Navy's AN/SPY-1 radar used on the Aegis Combat System. Over time the cost of implementing a monopulse tracker has fallen, and the technology is today found in practically all modern radars, even those used in disposable ordinance like missiles.

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