Inverse monopulse seeker

An inverse monopulse seeker is a type of semi-active radar homing that offers significant advantages over earlier designs. The system requires electronics that can compare three signals at once, so this design did not become practically possible until the early 1970s. One of the first such examples was the RAF's Skyflash missile, an adaptation of the AIM-7 Sparrow that replaced the original Raytheon seeker with a monopulse model from Marconi, followed by a very similar conversion by Selenia for the Italian Aspide. The USAF adopted similar technology in the M model of the AIM-7 Sparrow, and such designs are universal in semi-active designs today.

Conical scanning
In order to home in on a target, a semi-active seeker relies on the reflection of radar signals being provided by the launching aircraft. One can visualize such a signal as a cone-shaped reflection off the target, and the missile will see this signal if it is anywhere within that area. In order to approach the target within its warhead's lethal range, the missile needs some way to distinguish where the target is within that cone-shaped area.

The traditional solution to this problem is to use conical scanning. In this system, the receiver is connected not to a single receiver antenna, but two, pointed slightly on either side of the missile's centerline, or boresight. They are arranged so the signal will be stronger if the target is located directly along one of these two lines of shoot. If the target is to one side, say the right, the signal from the right antenna will be stronger than the left.

The missile can guide itself by turning towards the stronger of the two signals, and when it is pointed directly at the target, the two will become equal. To guide in two dimensions, the antenna is spun. At a given instant the two antennas might be horizontal and the control output will be adjusted the flight left or right; an instant later they will be vertical and adjust the flight up and down. In this fashion, the missile seeks its target in a rapid circular motion.

There are numerous problems with this method of tracking. For one, it relies on the difference in signal strength between the two antennas to being due only to the position of the target within the beam. However, there are a number of reasons this might not be the case, for instance, while the target flies through rain. This becomes a serious problem during the terminal approach when reflections from different parts of the aircraft create rapidly changing signals known as glint. This effect limits the accuracy of this method to about 10 m at the absolute best, demanding that missiles with such a seeker have very large warheads.

A more serious problem is that the seeker cannot tell the difference between a signal reflecting off the aircraft and one reflecting off other objects. This is not a major problem in one-on-one combat at high altitudes, but if the missile is shot at a target below the launch aircraft, it will eventually approach a point where it can no longer distinguish between the reflections from the aircraft and the ground around it. Additionally, the target aircraft can release random pulses of signal that will have the same effect, confusing the seeker which sees both the reflected signal and the ones from the jammer with no way to distinguish them.

Inverse monopulse technique
One way to avoid many of these problems is to use the monopulse radar technique. In these systems, the radar signal is split in two before it is sent to the antenna. The two paths include some form of encoding that remains intact after reflecting off the target. Polarization is a common solution. Filters then split the signal back into two components, and a comparison of relative strengths can be made as before. However, if the signals are directional, as in the case of polarization, there is no need to spin the antenna - the difference between the signals can be used to determine the directionality.

The major advantage of this technique is that the comparison is essentially instantaneous, completed within the length of one pulse. This allows the seeker to reject signals that are received at different times, making it highly resistant to random jamming pulses, as well as being able to reject reflections off the ground. Also, because the signals that cause glint take place over a period longer than one pulse, this problem is largely eliminated; in testing, the majority of Skyflash missiles hit the target aircraft directly, compared to the original AIM-7's conical scanning solution which brought the missile to within 20 to(-). Additionally, it was able to attack aircraft flying at 1000 feet altitude, a limit selected to allow tracking cameras to see the target. These tests demonstrated there was no practical lower altitude limit to the technique.

The downside to the inverse monopulse seeker is twofold. For one, it requires the radar on the launch platform to have monopulse encoding, or there will be no directional signal for the seeker to process. This links such missiles to their aircraft more tightly than the more generalist conical scanning systems which can be used with any radar the seeker can tune in. More importantly, the seeker is more complex and requires more electronics, which was not possible in the era of vacuum tube electronics and only became practical in the 1970s. For instance, the Skyflash receiver had a single fixed antenna, but required four receivers, one for each "channel", as well as comparitor electronics to generate three signals, one with the sum of all the signals, and two with the differences.