The basic radiation source in a radar is the high powered transmitter. These are reasonant cavities so that their primary frequency is determined by their physical size. For a given source (usually magnetrons or klystrons), slight variations in their center frequency or operation at harmonics are possible, but these variants reduce the power output of the radar set.
The frequency (RF) of a radar is that sinusoidal wave chain generated by the transmitter in its "free-running" state. In a pulse radar, the output is turned off/on to generate pulse trains; each pulse in the train has the RF of the transmitter. That is, each pulse is a wave packet of a frequency equal to that of the transmitter.
Selection of an operating frequency is determined by atmospheric transmission windows and the function of the radar. The frequency determines an optimum antenna size, receiver input stages, antenna-receiver-transmitter connections (plumbing), and output power levels. That is, a radar normally must operate at its natural resonance for optimum performance; so-called frequency agile radars operate within the normal tuning range (about the fundamental) of the transmitter or they switch harmonics. Both techniques require time to accomplish and degrade performance of the radar so that pulse-to-pulse frequency agility is more theoretical than practical. Frequency agility is commonly credited to a radar system, but it normally means that several frequencies are available; once the radar is tracking, the frequency must remain almost fixed.
Threat radars can be characterized by their frequencies -- threat radar implies high frequency (2-40 GHz) -- for the reasons previously discussed. As state-of-the-art improves, the threat frequencies go higher. At the present time, an RWR need only consider the frequency regime of about 2-20 GHz.
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Range resolution is at best one-half the distance that the pulse travels in a time equal to the pulsewidth. This limitation is imposed by nature. Threat radars must be able to resolve multiple targets and targets/jamming. Thus, threat radars can be characterized by short pulse widths:
The pulse travels about 1000 feet per microsecond; weapon warhead size reduction requires minimum pulsewidths. State-of-the-art and signal-to-noise ratios determine minimum pulsewidth. An RWR, then, need normally concentrate on pulsewidth regimes within the range:
Radars whose only functions are initial detection and sector location of a target are called Early Warning, Search, or Acquisition radars. Since range resolution is not a requirement (but high average power is), the pulsewidths of these radars are much longer. Theoretical analysis or field surveys will support the generalization:
Since threat radars are required to have narrow beamwidtns, many TTRs have acquisition modes of operation for initial location (acquiring) the target. Though these modes may have pulsewidths (and scans) which violate the above rule, they should not be confused with a true Acquisition radar (see Section 2.4.3.2).
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A radar computes range to a target by measuring the elapsed time between pulse transmittal and target return reception. For unambiguous range measurements, no more than one pulse should be received from the target for each pulse transmitted by the radar. Thus, the maximum required range of the radar determines the maximum pulse rate of the radar. Two interesting corollaries to the maximum unambiguous range condition are:
(1) High PRF radars are short-range trackers.
(2) Short range weapons have high PRF radars.
Range jamming of a radar is easily accomplished by repeater jammers onboard the target aircraft. For each pulse received, the repeater sends back one or more pulses to cause the radar computer to calculate incorrect range. Since the target pulses have the same PRF as the transmitted pulses, the radar can use a PRF filter to receive only that rate. This requires the repeater jammer signal processor to measure the incoming PRF so that the proper jamming rate is used.
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Several adaptive measures may be assumed by a radar to lessen its susceptibility to ECM; one which will make the job of a repeater jammer more difficult is the incorporation of staggered pulse trains. However, the same basic laws of nature apply to exotic pulse train generation (i.e., the elapsed time between any group of pulses cannot be less than the desired maximum range of the radar). The staggered pulse (PRF) repetition frequency also enhances associated radar features such as Moving Target indication (MTI, see Section 2.3.4) by reducing the effects of blind spots in the radar.
A staggered pulse train is fundamentally a basic PRF with this same PRF impressed upon itself one or more times. Each level of impression (stagger) utilizes a different start time or reference which will preclude the generation of concurrent pulses or pulses shadowing one another. The number of levels (or positions) is the number of times the basic PRF/IPP (inter-pulse period) is integrated in the pulse train.
Figure 3 illustrates the time relationship involved in the generation of a 4-level stagger. As mentioned above, each level has the same characteristic PRF and PW, but the Time to First Event (TFE) for each level is different. This has the effect of slewing the masked pulse groups in relation to one another resulting in the desired stagger pattern. The PRF of the radar is the sum of all the pulse trains so that if an RWR operated on PRF, the additional identification inherent in the stagger pattern would not be useful. This problem is overcome by measuring PRI rather than PRF so that the RWR measures the basic PRI a number of times equal to the number of stagger levels.
The example as presented is a 4-level stagger with a basic PRI of 3679. Train number one is initiated at "time-0" +163 and 2, 3, and 4 follow, respectively. The pulse deltas are determined by subtracting the adjacent TFEs.
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The job of the jammer can be made more difficult by the radar's use of a jittered PRI. In Jitter mode, the time between successive pulses, is allowed to vary in a totally random manner over a series of set intervals as long as the maximum range condition is met.
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As long as the maximum range condition is met, an infinite number of PRI patterns can be generated by combining stagger and jitter. The PRI can be modulated by a well-defined function: (a) a sliding PRI very slowly increases/decreases the PRF, (b) a Ramp PRI decreases the interval with a cyclic ramp function, and (c) a modulated PRI varies the intervals in a sinusoidal or triangular manner. Some combinations seemed to be designed to foil processors which use digital analysis.
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Guided missiles are not guided after a target; they do not pursue or chase an aircraft. Instead, the fire control computer predicts an intercept point on some future part of the target flight path based on the known flight parameters from the target tracking radar (TTR) and the known maneuverable envelopes of both the target and the missile. Missiles are like guns in that both are fired at a "lead-angle" point. The missile is accelerated (boosted) for the brief initial phase of its flight after which it can never again speed up--it is accelerated toward the predicted intercept point after which it is only capable of slight course corrections to keep it centered on the intercept point.
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For a guided missile to intercept its target, it must know at all times where the intercept point is in relation to the missile itself. The simplest method for the missile is a separate transmitter, located at or near the TTR, which sends coded guidance commands (fly left, fly up, etc.) to the missile. That is, the missile is radio controlled just as are model airplanes. This approach has the advantage of a cheap expendable (the missile) and a guidance signal (the "up-link") almost immune to target jamming since the missile receiving antenna can be highly directional, aft-looking which allows guidance of the missile by manual mode and optical target tracking when the primary tracker is jammed or otherwise inoperative. It has the serious disadvantage that the ground site must track the missile in order to generate the uplink (error correction) commands; as the missile and target approach the intercept point the missile tracker (the MTR) must point directly at the target and hence is highly susceptible to any jamming source on the target. A second weakness of this system is that since the missile itself never sees the target, some sort of self-fuzing device must be carried on the missile to reduce miss distance. Therefore, this system is vulnerable to countermeasures at three points (1) the TTR, (2) the MTR, and (3) the fuze.
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A variation of command guidance is widely deployed. The MTR is replaced by a high power continuous wave illuminator (CWI) radar which is slaved and boresited to the TTR. The missile homes on the Doppler return from the target (see Section 2.4.2). This approach is still vulnerable in three places; the major difference is that no guidance commands are transmitted. Since the CWI is not an MTR, RWR terminology uses Missile Guidance Radar (MGR) to designate all radars used by an RWR to resolve identifications.
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The third method of guidance is the "beam rider" in which the SAM flies up the beam of the TTR. An onboard flight computer keeps the missile centered in the tracking beam by use of aft-looking antennae. Since a target tracking beam must be quite small to ensure track accuracy, the ground site normally uses a broad-beamed, low-power radar to "capture" the missile during the initial flight stage and guide it into the tracking beam. (This system requires the missile to be in a constant turn as it flies up the tracking beam to the target -- a maneuver which becomes quite severe during the terminal flight stage and may exceed the physical limitations of the missile, particularly if the target "jinks".) The capture beam is immune to target jamming since it has no receiver and the missile antennae can be highly directional aft. Miss distance improvement of this system also requires an onboard fuzing device. Thus, this approach simplifies the ground site by making the expendable more costly and it has fewer jamming points: (1) the TTR and (2) the fuze. The most serious disadvantage to beam riding is that the TTR must be on the air for missile guidance, even if tracking is accomplished by alternate means -- no TTR, no guided missile.
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Both command guidance and beam riders are susceptible to tracking radar and fuze jamming. The simplest fuze is the radar proximity type which sends out a rather broadbeamed signal and measures the power in the target echoes. For a given target size and fuse, transmitter power returned from the target when the missile is within the kill radius of the warhead can be well calculated. By using a simple threshold detector in the fuze receiver the warhead is detonated when the kill radius is reached. This system can be jammed by making the target return much larger than normal so that the warhead is detonated prematurely, outside the kill radius.
In countermeasures terminology, fuze jamming is an "end game reaction" -- a last ditch attempt. End game can be avoided in both these guidance systems if the tracking radars can be defeated either completely or by accuracy degradation. Most ECM systems are dedicated to the track radars since target carried fuze jammer transmitters can act as fuze homing devices.
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Of the guidance methods, command guidance is traditionally the most commonly encountered in a threat scenario. In the case of pulsed TTR and MGR, it should be noted that synchronization of the two radars and the missile correction commands requires that some relationship exist between the PRF of both radars. Thus, it is possible in the case of an all-pulse system to determine if a TTR has entered the missile launch (ML) state by testing time correlation between the TTR and MGR pulse trains.
For an RWR to detect the ML state on a homing guidance missile system, the CWI must be received. This detection requires a superheterodyne receiver input to the RWR. On a pure CW system, microwave detection of an ML state may not be possible.
Determining ML from the proximity fuze signal is questionable since fuze power is so low that no real warning will be obtained. That is, fuze power is 100-200 watts broad-beamed. Detection of a Mach 2 (2000 feet per second) missile at one-half mile would give a one- to two-second warning. The aircrew would only be able to "die tense".
One of the most useful features of radar is the ability of a radar set to continuously predict the next location of its target from the information being received from the target and to align itself to continuously point at that predicted location. When this is occurring, the radar set is said to be "tracking the target". To make this prediction, the radar measures the returned target power from several positions slightly offset from the target as well as the power returned directly from the target. That is, to track a target, the radar must also "look" where the target is not. When the returned power moves into one of these offset locations, the radar can say that the target has moved; the elapsed time between looks tells the radar how fast the target is moving. This movement of the radar beam around the target location is called the "Scan Pattern" or the "Scan" of the radar. Several types are shown in Figures 4, 5, 6, 7 and 8.
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Radar systems can be categorized by their Scan patterns. The most commonly used today is the Conical Scan, or Con Scan pattern (see Figure 2). In this method, the radar rotates its beam about the circle described by the half-power points of the beam when the beam is boresighted on the target. The beam, when received at the target or at the radar, will be a sinusoidal waveshape whose amplitude is proportional to the distance the target is away from the boresight. By monitoring the exact location of the scanning beam, the location of the target can be determined from the location of the maximum power received. Note that the more accurately the radar tracks the target, the smaller the amplitude of the sine wave, until zero amplitude implies that the radar is exactly boresighted on the target.
Con Scan systems require a minimum amount of hardware and therefore are commonly used on inexpensive, mobile systems such as AAA or mobile SAM sites. They suffer the serious disadvantage of not being able to see a target outside their narrow scan patterns. This means that not only is a second radar required to help it find the target (to "acquire the target") but also the tracked aircraft can easily "escape" if it is successful in breaking track since the Con Scan radar cannot see the target, except in the track mode.
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Con Scan problems can be overcome with Track-While-Scan (TWS) radars. TWS radars scan their beams over relatively large areas. The radar computer still measures returned power as a function of beam location to provide tracking but the large scanning area enables the radar to still see the target even if track has been broken or lost. However, this large scan area makes the TWS highly vulnerable to ECM jamming. An illustration of a TWS radar is shown in Figure 9.
TWS radars require special consideration during the design of RWR systems. Since many receivers time-share the frequency bands, it is possible that the receiver may not be "looking" at the TWS frequency band when the TWS is illuminating the aircraft and vice versa. The probability of these missed intercepts increases as range from the TWS increases because scan areas have angular divergence. To overcome this problem, the RWR must be programmed to display the TWS on its first intercept; likewise, it is programmed to not erase the TWS symbol until after a set number of missed intercepts. Of these two factors, missed intercepts is the more troublesome to the aircrew since it requires the symbol to remain on the scope even after the TWS is, in fact, no longer tracking.
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Scan can also be accomplished by sequentially pulsing several antennae or sections of a large antenna. This is illustrated in Figure 10. While this technique can yield much higher scan rates, the additional hardware requirements normally exclude it from a threat scenario. It will, however, be encountered on shipboard systems.
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Con Scan, TWS, and monopulse radars will cause an RWR to receive pulses with superimposed sinusoidal waveforms. The Con Scan case is shown in Figure 11. The processor identifies these scan patterns by counting the maxima of the sine wave envelope; these maxima are the scan rate of the radar. When a given scan pattern is counted, the Identity Word is updated with this information.
Some radar systems do not scan their transmitted beams. Instead, the receiving antennae scan an angular section while the transmitter remains on the target at all times. To the radar receiver, the signal returns have the same sinusoidal waveform as normal scan, but to the RWR there will be NO scan pattern. This lack of scan can be used by the RWR processor since it characterizes certain types of radars just as well as an actual scan pattern. However, since lack of modulation on a Con Scan beam means that the radar is boresighted on the target, lack of a scan pattern does not unambiguously identify a radar type.
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To summarize, the use of scan by radars to track a target can assist in the identification of a particular radar type. Scan imposes the following considerations on the RWR processor:
(1) The scan envelope must be counted to determine scan rate.
(2) To ensure the validity of the display, TWS radars must be displayed upon first intercept and must remain displayed for several processor "look cycles" even after the radar appears to be shut down.
(3) Lack of scan data can sometimes be used by the processor to identify radar types.
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