A monopulse antenna is a type of antenna used in track radars. Track radars are the radar systems that are used to precisely determine the location of a target, in order for you to be able to determine the fire control solution, and engage the target with your weapon system(s).
The point of a track radar is to pick up the target, and continue to track it, following the target as it moves. This is different from your typical search radar, which continually sweeps the horizon, and refreshes the position of the target on each sweep. When we are trying to fire a weapon at something, we really do not have the luxury of waiting the 0.25 – 3 seconds that it might take for a radar to sweep across and get a new fix on the target. We need to know, in very precise terms, exactly where it is, and how it is accelerating, changing course, etc. Keep in mind that we may be talking about a missile that is travelling at Mach 2, and manoeuvring quickly enough to pull a few gs.
So, when a track radar is pointed at a target, it needs to be able to follow the target. Obviously. In order to do that, the radar system needs to be able to figure out how far off it is when it looks at a target. The way that most modern radar systems do so is through the use of a monopulse antenna.
The basic setup of a radar system will have a transmitter / receiver on one end, which sends the pulses of energy towards the antenna via a waveguide. The energy will often exit the waveguide via a feedhorn. (The concept is much like how the fluted end of a trombone directs and spreads the energy coming out of the brass tubing.) This energy will then be bounced off some kind of reflective dish, that is used to direct and concentrate the energy the way we want it. For search radars, we want the energy to be more spread out, in order to cover move volume. In track radars, we want a much tighter beam, concentrating all the energy in one direction, allowing us to precisely determine the bearing of the target, relative to the platform that is housing the track radar.
Monopulse antennas are like that, except they split the energy up into four separate feed horns, grouped together. When looking at them face on, it’ll look something like this:
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| a | b |
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| c | d |
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Each of these feedhorns will be ever so slightly misaligned from the other, each directed slightly away from the centre. Thus, if the boresight of the antenna is pointed dead ahead, the energy from feedhorn "a" will be directed slightly up and to the right. As well, any signals that "a" recieves will be slightly stronger if they are recieved from that same direction. Antenna gain is, after all, reciprocal. If an antenna transmits energy stronger in a particular direction, it will also recieve energy more easily from that direction.
We can use this in order to figure out where a target is, relative to the boresight of the monopulse antenna. If the target is directly up, relative to the boresight of the antenna, then the signal from both a & b will be stronger than the signal from c & d. If it is to the left, then b & d will be stronger than a & c.
So, in order to perform the calculations that can tell us how far we are off, and how the antenna director needs to move the antenna in order to stay on target, the track radar system takes the signals from the four feed horns, and produces three signals. The first signal is the vertical error signal, and consists of (a + b) - (c + d). The second signal is the horizonal error signal, and is (b + d) - (a + c). And finally, we find out the sum, which is the total of a + b + c + d.
It is the ratio of the vertical or horizontal error signals to the sum that allows you to calculate how many degrees (or more likely, milliradians) off target you are, allowing you to steer the antenna director to continue following the target.
Of course, in order to do so, you will also have to have a detailed knowledge of the antenna radiation pattern, and some extensive testing. But, if you are designing one of these systems, you likely work for a company that has an R&D annual budget of many millions of dollars, so this shouldn't be a problem.
Monopulse antennas were first designed by the U.S. Naval Research Laboratory, in 1943. Previous methods of tracking, such as sequential lobing, and conical scan, were much more prone to jamming. It is very difficult for a jammer to "fool" a monopulse antenna, at least in angle. It is just as suseptible to jamming efforts designed to decieve it in range as any other radar system is. Other methods need to be employed to defeat this type of jamming.
It is also faster, as the system can determine angular offset on each pulse, whereas previous systems required the comparison of multiple subsequent pulses in order to determine how far off track the system is. This capability is what gave the monopulse antenna its name.
As well, monopulse antennas have fewer moving parts than previous systems (which typically had a spinning reflector within the antenna). This cuts down on maintenance and possible down time due to mechanical failure, although obviously there is still the requirement to move the director in order to keep the antenna "on track" with the target.
Monopulse antennas are used in the vast majority of modern track radar systems. They are what allows a warship's radar to follow an aircraft as it flies in the sky, and more importantly, what provides the information necessary to allow the ship's guns to follow the plane as well.
Payne, Craig. "Principles of Naval Weapon Systems," Naval Institute Press.
2006. Pages 71 - 74.