Monopulse radar
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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
Conical scan systems send 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). This varying signal will reach a maximum when the antenna is rotated so it is aligned in the direction of the target, by looking for this maximum and moving the antenna in that direction, a target can be automatically tracked.
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.
Jamming a conical scanner is also relatively easy. 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. In this case a series of random short bursts of signal will appear to be a series of targets in different locations within the beam. 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. Jammers of this sort were deployed quite early, the British used them during World War II against the German conical-scanning Würzburg radar.
Conical Scan Receive Only (COSRO)
COSRO systems do not modify the transmit signal sent from the antenna. Antenna waveguide includes an RF received feedhorn structure that includes a left/right RF sample and an up/down RF sample. These two signals are multiplexed inside a waveguide device that has a rotating vane. The output of the multiplex device is a single RF signal and two position signals that indicate left/right and up/down.
The COSRO technique does not transmit any signals that indicate the position of the rotating vane.
Antenna Sampling
RF receive signals from multiple transmit pulses are combined mathematically to create a vertical and horizontal signal. The vertical signal is created by adding RF samples when the vane/feedorn is in the up direction and subtracting RF samples when the vane/feedhorn is in the down direction. The horizontal signal is created by adding RF samples when the vane/feedhorn is in the left direction and subtracting RF samples when the vane/feedhorn is in the right direction.
Monopulse basics
Monopulse radars are similar in general construction to conical scanning systems, but add one more feature. Instead of broadcasting the signal out of the antenna "as is", they split the beam into parts and then send the two signals out of the antenna in slightly different directions. When the reflected signals are received they are amplified separately and compared to each other, indicating which direction has a stronger return, and thus the general direction of the target relative to the boresight. Since this comparison is carried out during one pulse, which is typically a few microseconds, changes in target position or heading will have no effect on the comparison.
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. These lobes are then rotated as in a normal conical scanner. On reception the signals are separated again, and then one signal is inverted in power and the two are then summed. If the target is to one side of the boresight the resulting sum will be positive, if it's on the other, negative.
If the lobes are closely spaced, this signal can produce a high degree of pointing accuracy within the beam, adding to the natural accuracy of the conical scanning system. Whereas classical conical scan systems generate pointing accuracy on the order of 0.1 degree, monopulse radars generally improve this by a factor of 10, and advanced tracking radars like the AN/FPS-16 are accurate to 0.006 degrees. This is an accuracy of about 10 m at a distance of 100 km.
Jamming resistance is greatly improved over conical scanning. Filters can be inserted to remove any signal that is either unpolarized, or polarized only in one direction. In order to confuse such a system, the jamming signal would have to duplicate the polarization of the signal as well as the timing, but since the aircraft receives only one lobe, determining the precise polarization of the signal is difficult. Against monopulse systems, ECM has generally resorted to broadcasting white noise to simply blind the radar, instead of attempting to produce false localized returns.
Antenna Samples
Monopulse antennas produce a sum signal and two delta signals. The sum signal corresponds with the center-line of the antenna beam. The delta signals are adjacent to the center-line of the antenna beam.
The sum signal is usually created by the same feed-horn structure used to transmit RF. The delta RF signals are created by pairs of antenna feed-horns located adjacent to the sum feed-horn. The output of each pair of delta feed-horns creates zero output signal when the incoming RF signal is located at the center of the antenna beam.
The angle error signal is created by performing a ratio.
Error = delta/sum
Antenna Positioning
Tracking systems produce constant aircraft position information, and the antenna position is part of this information. Antenna signals are used to create feedback as part of a RADAR system that can track aircraft.
The horizontal signal and the vertical signal created from antenna RF samples are called angle errors. These angle error signals indicate the angular distance between the center of the antenna beam and the position of the aircraft within the antenna beam.
The horizontal signal and vertical signal are used to create a drive signal that create torque for two antenna positioning motors. One motor moves the antenna left/right. The other motor drives the antenna up/down. The result is to move the antenna position so that the center of the antenna beam remains aimed directly at the aircraft even when the aircraft is moving perpendicular to the antenna beam.
Pulse Doppler
Pulse Doppler signal processing provides the means to separate different objects based on speed. This technique is combined with conical scanning to improve track reliability. It is necessary to separate the object signal from the interference to avoid being pulled off the object. This avoids problems where the system is fooled by aircraft flying to close to the surface of the earth or aircraft flying through clouds.
Concal scan and monopulse antennas are susceptible to interference from weather phenomenon and stationary objects. The resulting interference can produce feedback signals that move the antenna beam away from the aircraft. This can produce an unreliable antenna position when the antenna is aimed too near the ground or too near to heavy weather. Systems with no Pulse Doppler tracking mode may remain aimed at irrelevant objects like trees or clouds. This required constant operator attention.
History
Monopulse radar was extremely "high tech" when it was first introduced by Robert M. Page 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. An early monopulse radar development, in 1958, was the AN/FPS-16, on which NRL and RCA collaborated. The earliest version, XN-1, utilised a metal plate lens. The second version XN-2 used a conventional 3.65 meter [12 ft] parabolic antenna, and was the version which went to production. 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. The IRACQ [Increased Range ACQuisition] modification was installed on certain of these installations, certainly the one located at Woomera, Australia was so modified. 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 ordnance like missiles.