Monopulse radar
The monopulse method is also used in passive systems, such as electronic support measures and radio astronomy.
Modern systems determine the direction from the monopulse ratio, which contain both amplitude and phase information.
Conical scanning is not considered to be a form of monopulse radar, but the following summary provides background that can aid understanding.
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).
One problem with this approach is that radar signals often change in amplitude for reasons that have nothing to do with beam position.
Over the period of a few tenths of seconds, for instance, changes in target heading, rain clouds and other issues can dramatically affect the returned signal.
Since conical scanning systems depend on the signal growing or weakening due only to the position of the target relative to the beam, such changes in reflected signal can cause it to be "confused" about the position of the target within the beam's scanning area.
The jammer simply has to send out signals on the radar's frequency with enough strength to make it think that was the strongest return.
Jamming of this sort can be made more effective by timing the signals to be the same as the rotational speed of the feed, but broadcast at a slight delay, which results in a second strong peak within the beam, with nothing to distinguish the two.
Monopulse radars are similar in general construction to conical scanning systems, but split the beam into parts and then send the two resulting signals out of the antenna in slightly different directions.
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.
Against monopulse systems, ECM has generally resorted to broadcasting white noise to simply blind the radar, instead of attempting to produce false localized returns.
For a high gain antenna, the feedhorn assembly is located facing the reflecting surface at or near the focal point.
An explanation of how real and imaginary parts are used with RADAR can be found in the description of pulse Doppler.
Dynamic calibration is needed when there are long waveguide runs between the antenna and first down converter (see Superheterodyne receiver).
The Cal term is created by injecting a calibration signal into the receive waveguide while the system is not active (sum and delta).
A fixed value may also be stored for systems with long wave-guide runs to allow degraded operation when RF calibration cannot be performed.
Antenna error signals are used to create feedback as part of a RADAR system that can track aircraft.
The monopulse angle error information that is received is used to adjust the position and velocity data for the aircraft.
Conical scan and monopulse antennas are susceptible to interference from weather phenomenon and stationary objects.
Systems with no Pulse Doppler tracking mode may remain aimed at irrelevant objects like trees or clouds.
Monopulse radar was extremely "high tech" when it was first introduced by Robert M. Page in 1943 in a Naval Research Laboratory experiment.
Early uses included the Nike Ajax missile, which demanded very high accuracy, or for tracking radars used for measuring various rocket launches.
These radars played an important part in the Mercury, Gemini, and early Apollo missions, being deployed in Bermuda, Tannarive, and Australia, among other places for that purpose.
[4] The cost and complexity of implementing monopulse tracking was reduced and reliability increased when digital signal processing became available after the 1970s.
The technology is found in most modern tracking radars and many types of disposable ordnance like missiles.
