A monopulse radar is a type of radar that includes additional information in the signal. This allows the radar to be less subject to difficulties caused by sudden changes in the strength of the signal. Monopulse radar is also more difficult to jam than conical scanning radar, which was in use until the 1960s.
Conical scanning radar transmits a single signal through a feed horn that is slightly off center. The lobe of the radar always illuminates the target when the radar is centered on it. The signal will be strongest when the radar is pointing directly at the target, allowing the conical scanning radar to determine the direction of the target. The primary difficulty with conical scanning radar is that other factors, such as the weather, can affect the strength of the signal.
The primary difference between monopulse radar and conical scanning radar is that monopulse radar splits its beam into two signals and transmits each signal in a different direction. The signals reflect off the target and are received by the radar, which then compares the two signals to determine which one is stronger. This allows the radar to determine the direction of the target with greater accuracy than the conical scanning radar. The radar performs this comparison during each pulse, hence the term “monopulse” radar.
A monopulse radar must be able to identify the different parts of the beam to compare the two signals. The radar typically polarizes each signal separately and transmits each signal through feed horns that are slightly off center. The signals are then received back from the target and separated again, using the difference in polarity to differentiate between the signals.
The display of monopulse radar typically consists of two overlapping lobes. This provides a high degree of targeting accuracy when the lobes are close together. Conical scanning radar generally has a targeting error of 0.1 degrees, and monopulse radar generally has an error of no more than 0.01 degrees. Advanced systems typically are accurate to within 0.006 degrees.
The fact that monopulse radar produces signals with different polarities makes it much more difficult to jam than conical scanning radar. The jamming radar must duplicate the polarization and timing of the signal. This generally is impractical, so electronic countermeasures for this type of radar typically consist of transmitting white noise rather than generating a false signal.