When a radar is being jammed during the target acquisition time, there are several signal processing techniques that the radar operator may attempt. Some of these methods are normally installed in radars to overcome natural phenomena such as weather or ground clutter, but they all are counter-countermeasures (ECCM). By strict definition, if an ECM can force a radar tracker into an ECCM mode, the ECM has performed its function, but to be truly effective ECCM should also be negated.
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The most important ECCM is the radar operator -- the man in the loop. Almost all radars have a manual mode in which the operator keeps track on the target and fires weapons by observing his display screen. Manual tracking degrades the accuracy of the SAM system since the operator cannot maintain a smooth, steady track or point to an accuracy better than the calibration error of the display. Target carried noise transmitters greatly aid manual tracking because the target will be located in the center of the noise return; by holding the radar on the center of the noise, range information can be obtained as well as angle data. An experienced operator can locate and track a jamming target quite accurately.
Some radars can use passive tracking (no target illumination by the tracking site) as a very effective ECCM. This approach is particularly useful to those systems in which the missile receives no direct information from the TTR. If the target carries noise transmitters, they are necessarily at the frequency of the TTR receiver and therefore highlight the target for passive track. Noise will still appear in the computer and on the display but the operator can manually track this as described above. Passive track can only be countered by having no target generated radiation (jammers, IFF, comm, LORAN, etc.).
To the RWR, optical track by the TTR in a command guidance SAM system will result in the reception of an MGR PRF which will have no TTR correlation. That is, even with optical tracking, guidance signals must still be transmitted to the missile. This condition is generally defined as an "optical launch".
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Various target sizes and aspect angles have differing RCS. A radar must be designed to produce a strong signal from the smallest target at its maximum range. To prevent large or close targets from saturating the receiver and flooding the display screen, Automatic Gain Control (AGC) is used to vary the sensitivity in high signal areas. RCS is additive in one resolution cell; if there are 20 square meters of noise and a lO-square-meter target in the same resolution cell, the radar will see a 30-square-meter return. If there is a large area of noise (ECM, weather, ground return, etc.) of 20-square-meter RCS, the gain of the radar receiver can be set for a threshold of 20 square meters and only the 10-square-meter target will then be displayed. Jammer return can be greatly reduced in this manner, but if the jamming power is varied rapidly, the AGC will be in a constant state of unbalance which can degrade tracking accuracy.
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When AGC is performed on a pulse-to-pulse basis it is called Instantaneous Automatic Gain Control (IAGC). AGC varies gain based on the return from a broad area while IAGC allows mapping of the high noise area. IAGC subtracts the power of the first pulse from the second; if the noise is uniform it will be "erased" from the receiver and display, leaving any targets that may be in the noise area.
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When a target moves with respect to the radar transmitter, the reflected signal receives a frequency (phase) shift proportional to v/c where v is aircraft velocity and c is the speed of light. At microwave frequencies and fighter airspeeds the Doppler shift is up to 20 kHz. Radars can use this frequency/phase shift as an excellent tracking method and ECCM.
The period (wavelength) of 20 kHz is 200 microseconds. To recover this 20 kHz shift from the original radar signal, the original must be CW or a pulse train with pulses of a period several times longer than 200 microseconds (a "pulse Doppler" radar). On regular pulsed radar, the nominal quarter microsecond pulses will contain very little of the Doppler shift per pulse -- but they will have measurable phase differences. Systems which recover the shift as a discrete frequency are called Doppler radars while those circuits which only measure pulse-to-pulse frequency/phase differences are called Moving Target Indicators (MTI).
Stationary targets return radar signals -- pulse-to-pulse -- of fixed phase. The output of a phase detector is a signal whose amplitude is directly proportional to the phase difference of its inputs. MTI uses a phase detector to provide zero amplitude (no input) signals to the tracking computer and display screens from fixed targets such as weather and ground return. A simpler way to understand MTI is in terms of scintillation. As an aircraft flies through a SAM area, its aspect angle and hence its RCS is constantly changing -- the return scintillates. Note that MTI differs from IAGC in that the pulse-to-pulse comparison is for phase (scintillation) differences instead of amplitude differences.
Another method of MTI is to compare the target location on a pulse-to-pulse basis. If the target return occurs at exactly the same time (range) on two or more successive pulses, it did not move and hence is not applied to the computer and display screen.
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Recall that to predict the future location of a target (to track) requires that the radar look at areas where the target is not located. When this scanning is accomplished with the radiated beam, large angle targets such as chaff clouds can create complete radar "white-out"; that is, large areas of reflective noise will be seen by the receiver. Also, RWR indications can be obtained before actual target lock-on. These problems can be overcome by scanning only the receiving antennae and using a separate transmitting antenna pointed only at the target. This scheme is called Lobe On Receive Only (LORO).
In a LORO system, a transmitting antenna emits a few "exploratory" pulses along a direction obtained from an acquisition radar. These exploratory pulses are the acquisition mode of the TTR. That is, in its acquisition mode the small beamed TTR must scan the large location segment provided by the acquisition radar. In radars equipped with Fast Time Constant (FTC, see next section) the return pulse is applied to a differentiator of extremely short time constant. When the pulse is received, it is "cut-off" on the eading edge and only that portion is fed to the computer. This allows the radar to effectively track on the leading edge of the target. FTC does not improve the range resolution (and hence the res cell) but it can prevent any countermeasures aft of the target but in the same res cell as the target (such as chaff) from interfering with the radar receiver.
The receiving antennae scan their sector for the target return due to these exploratory pulses; as the power centroid is located, the center of the receiving pattern is brought onto the target. The trasmitting antennae, which is slaved to the receiving antennae, is then pointing directly at the desired target and only that target is radiated during tracking. This approach allows a very small radiated beam, but the resolution cell of the system is still that of the receiving antenna or antennae.
Note that LORO systems are ideally suited for passive tracking of any signals generated onboard the target.
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The range accuracy of a radar is a measure of the time of flight of microwave pulse to a target. Upon transmit, a clock starts and it stops when a return pulse is received. The range is equal to 1/2 this time.
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These were fully discussed in Section 2.2. These are the most commonly used because the S/N ratio inherent in pulsed operation minimizes the need for high average power. However, due to the reduced ECM vulnerability of CW type radars, many of the new threat systems are using CW.
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When a reflecting target moves with respect to the receiver, the returned signal will have a frequency shift proportional to v/c, where v is the target velocity and c is the speed of light. The frequency shift increases for inbound targets and decreases for outbound targets by an amount proportional to the radial range change. That is, crossing targets will have low Doppler shifts while inbound/outbound targets have the maximum Doppler shift. If a target orbits a radar at a constant radius, there is no Doppler shift. At microwave frequencies and fighter speeds, Doppler shift varies up to 20 kHz.
The radar receiver can recover the Doppler shift by mixing the transmitted and received signals. Because of the low frequency of the shift with respect to the transmitted frequency, the transmitter must operate as a continous wave (CW) signal source or in a pulse mode with pulses many times longer than the period of 20 kHz (Pulse Doppler). In the CW case, range resolution is not possible but in Pulse Doppler range can be obtained by transmitting short pulses between the "interrupted CW" pulses. But the change in Doppler shift is directly proportional to range rate (dR) so that the radar can recover dR, a quantity which not only yields antenna slew rates but also precisely locates when R=0 is identical to dR=0 and thereby greatly improve missile miss distance.
Doppler shift from a target can be used as a homing beacon for any guided missile equipped with a Doppler receiver as seen in Figure 12.
In this case, the ground site CW Illuminator (CWI) radar radiates the target. The missile has both forward- and aft-looking antennae so that Doppler is received. By use of a slotted antenna array (for example) the missile can passively track the target Doppler to an intercept point; when dR = O, the missile is at the target and detonates. Thus, no proximity fuze is required. (This approach so improves the probability of a kill, Pk, that direct hits are quite common. Editor's note from actual experience.)
Two examples of fire control systems for these homing missiles are:
1. Target tracking is accomplished by a non-Doppler pulse TTR. A CWI is slaved to the TTR, often sharing the same parabolic antenna. The missile is launched and homes on the reflected Doppler. The TTR in this case does not receive target Doppler so that ECM techniques applicable to pulse radars will defeat the system by denying acquisition and track to the TTR.
2. The TTR itself as well as the missile has a Doppler receiver and tracks the target in frequency. In this approach, the TTR obtains initial tracking data from a pulse radar or from manual operation after which it can auto-track the Doppler. That is, the CWI is the TTR.
CWI TTRs are ideally suited for two excellent ECCM techniques -- coherency and home-on-jam. A continuous wave can be modulated by an ultra-low frequency signal. If an 85 Hz modulation is used, the period for one cycle is about 2000 nm. Thus, at normal SAM racking ranges, the phase of the 85 Hz will be changed very ittle by the round-trip distance; the transmitted and received signals will be in phase -- coherent.
This modulation is called the COHO signal. Any signal, including jamming signal, must be coherent to pass the radar receiver. Since the COHO phase can be easily randomly switched, the active countermeasure is almost negated as an operational system.
The homing missile receives the transmitted signal -- with COHO -- in the aft antennae and the reflected Doppler signal -- with COHO -- in the forward antenna. When the two COHOs are in phase, the missile has identified the correct target. (The correct radar is identified by a modulated code frequency at the aft antenna.) The missile can now fly to the target by its own steering computer, needing no other commands from the radar.
If the target attempts to jam the TTR, the missile will see this jamming in its forward antenna which is locked on the target. If the jamming is not coherent, COHO lock will reject it. Alternatively, the missile can divert to a home-on-jam (HOJ) mode and track the jamming signal to the target. That is, due to the COHO capability of a CWI, target-generated ECM can actually be a highly directional homing beacon for the missile.
It should be noted that a pure CW beam conveys very little intelligence to the missile. As already discussed, anti-jamming signals can AM the CW, radar-missile identity codes can FM it, a range approximation can be determined from a ramp function which FMs the signal and phase modulation can also be used as an ECCM device. Thus, a spectrum analyzer display of an actual SAM CW signal would show a complex AM - FM - FM - PM continuous wave. For Pulse Doppler, such as airborne interceptor pulse Dopplers (AIPDs), this same signal would be interrupted periodically for transmission of several ranging pulses or pulse groups (i.e., stagger, jitter or both).
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Any air defense network will be composed of many radars other than those designed for weapon fire control.
Except for AAA and AI, these additional radars are generally characterized by low frequency, large beams, and no auto-track capability. Some of the radars in this group are Acquisition, Early Warning, Height Finders, GCI, and GCA.
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Because fire control radars require very small beams for location accuracy, they must depend on other radars for initial target detection and location. The Early Warning (EW) radar is typically a low frequency (100-1000 Hz), large beam (6-16 degree), long range (200 or more nautical miles) system capable of searching a full 360-degree Az for initial target detection and heading.
Therefore, any ECM which does not make the target disappear will only assist in the EW mission due to the beaconing effect of jammer transmitters. Although these radars normally employ AGC and MTI (see Sections 2.3.2 and 2.3.4), they represent no real threat to aircraft since they cannot accurately direct weapon fire.
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After the EW radar detects the target, the Acquisition (Acq) radar will further localize the position for the small beam trackers. These radars are characterized by medium (3-6 degree) beams of medium (800 kHz to 8000 kHz) frequencies and no auto-track capability. They generally search an Az segment determined by the EW radar.
Because these radars are very similar to fire control systems, they can be jammed by the same techniques and tactics as those for fire control if the appropriate frequency device is carried. Denying Acq radar coordinates to a SAM radar forces him into a manual target acquisition mode which, due to the small beam SAM radar, can greatly increase minimum acquisition time. With some systems, loss of acq results in denial of track.
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Height Finder (HF) systems are used to provide El data on the EW and Acq Az target data. These radars have characteristics very similar to Acq radars except that the smallest dimension of their beams will be vertical for best El resolution.
For maximum El uncertainty, then, the aircraft formation should be "stacked", but since this system also has no autotrack or associated weapon, it presents no real threat. These radars are primarily used for vectoring airborne interceptors.
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Ground Controlled Intercept (GCI) systems are usually composed of acquisition and height finder radars. They are used to vector interceptor aircraft to an intruding force.
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Ground Controlled Approach (GCA) radars have parameters very similar to those of GCI, Acq, and HF. They differ from those systems primarily in their display units; GCA scopes are premarked with the appropriate glide angle for the site. ECM can easily be used against these radars to force interceptor aircraft to use visual approaches.
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Anti-Aircraft Artillery (AAA) fire control radars operate much the same as missile TTRs in that, after target acquisition, auto-track is accomplished by the radar computer and some sort of scanning method. Figure 13 shows a typical AAA Battery layout. To maintain the high mobility inherent in a simple gun system, the radars have small dishes with medium beams (1-5 degrees) and wide frequency ranges (800 MHz to 20 GHz) with conical scanning (Con Scan).
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Airborne Fire Control (AI) systems are used for Airborne Interceptor Missiles (AIM) guidance. The cockpit operator manually acquires the taret by training the antenna; auto-track is then usually accomplished by some scanning method or frequency (Doppler) track.
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Terminal defense radars are the fire control systems for SAMs and AAAs. As such, they were discussed earlier in this work under those headings.
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