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COUNTERMEASURES
11.1 OBJECTIVES AND INTRODUCTION
Objectives
1. Understand the concept of electronic warfare.
2. Be acquainted with the basic characteristics and requirements of electronic warfare receivers.
3. Understand the basic principles of electronic countermeasures.
4. Be acquainted with the types and classes of ECM.
5. Be acquainted with the characteristics of EW transmitters.
6. Understand the burnthrough equation and its effect on detection range.
7. Understand the following terms and functions: self-screening, stand off, stand forward, noise jamming, deception ECM to include range deception, azimuth deception, and false target generation, CW and doppler deception, and the use of chaff.
8. Understand the principles of target masking and reflectivity modifications. Be acquainted with IR/E-O/Acoustic and other countermeasures.
9. Understand the basic principles of ECCM.
10. Understand the parameters of radars and radar systems pertaining to ECM.
11. Understand the ECM/ECCM interaction and its effect on sensor operation.
Introduction
Countermeasures include all those means of exploiting an adversary's activity to determine his intentions or reduce his effectiveness. Countermeasures may be applied against weapons operating over the entire spectrum from acoustic to gamma wave frequencies. Since the most prevalent use of this spectrum for weapons/sensor's/communications, etc. is in the radio frequency (RF) (i.e. several kilohertz to tens of gigahertz) region, most counter measures and counter-countermeasures also operate in that region. The infra-red (IR) and acoustic portions of the spectrum are also of interest, but since these regions have yet to be fully exploited by weapons/sensors/communications, fewer specific sensing measures and counter-countermeasures exist than in the RF region. Only the countermeasures area has received substantial attention in the IR and acoustic portions of the spectrum.
Besides avoiding detection and identification by an enemy sensor or weapon, countermeasures also includes the means to reduce the effectiveness of his destructive systems. Our own weapons systems (to be addressed in the final 10 chapters of this text) are certainly suited to this task. However, in recent years, the concept of passive defense measures to reduce the enemy's effectiveness has evolved.
A major difficulty in studying countermeasures is how rapidly the subject revolves around classified information. Detection depends on sensors which ultimately depend on a receiver to determine the presence or absence of a contact. Possessing the technical means to disrupt or deceive that receiver is an advantage one would guard very closely.
Hence, our discussion of countermeasures will be generic, emphasizing the principal areas in an expanding field.
11.2 ELECTRONIC WARFARE
Countermeasures in the RF region of the electromagnetic spectrum are collectively grouped under an umbrella termed Electronic Warfare (EW). EW has three branches: Electronic Support Measures (ESM), Electronic Countermeasures (ECM), and Electronic Counter-Counter Measures (ECCM). Figure 11-1 illustrates the relationships and functions of EW's branches.
11.2.1 Electronic Support Measures (ESM)
Electronic support measures (ESM) search the RF spectrum for emissions and analyze the results to exploit the weapons or sensors involved. Exploitation includes tactical early warning, identification for counterweapon selection and recording to support countermeasures development.
11.2.1.1 Basic EW Receivers. The EW receiver is the primary Electronic Support Measures (ESM) equipment and functions as a sensor for, and as a means of identifying friendly, neutral, and enemy electronic emissions. It provides warning of potential attack, knowledge of enemy capabilities, and an indication of enemy use of active countermeasures to manipulate the electromagnetic spectrum. The design of the electronic warfare receiver provides a special challenge to the engineer, in that no single antenna system or specific receiver circuit can cover the entire range of the electromagnetic spectrum. A set of components can be designed to provide maximum efficiency over a range of up to a few thousand megahertz; however, current requirements demand performance from a few kHz to 50 GHz with a wide range of signal strengths and other paremeters such as pulse width, PRF, scan rate, side-band characteristics, and modulation. The solution has been to connect many different frequency-sensitive circuits called tuners and their associated preamplifiers to a common chain of main amplifiers and display or data-storage units. The following are the primary design criteria for EW receivers.
11.2.1.2 ESM Receiver Design Requirements. The basic problem of EW is obviously to obtain the raw data, which in turn is analyzed to determine the enemy threat. To obtain this data, the collection station will have to have dedicated receiving equipment. The EW receiver differs from an ordinary receiver both in its essential design and in the auxiliary equipment associated with it. The essential requirements are:
Wide spectrum surveillance (wide bandwidth capability). The frequency of the enemy radar is not known beforehand. This means, with present technology, that the frequency spectrum must be searched from 30 kHz to 50 GHz. This range is too large for one receiver, so that either several different ECM receivers with different tuning ranges must be used or one receiver must use different tuning units to cover different parts of the frequency range.
Wide dynamic range. The receiver must be able to receive both very weak and very strong signals without changing its characteristics, for the receiver is not always operating at great distances from a radar, but may in fact be very close. It would be undesirable for the resulting strong signal to disable the analysis capability.
Unwanted signal-rejection (narrow bandpass). Many other signals will exist, with frequencies close to the signal of interest. The receiver should discriminate well between the frequency it is tuned to and signals at other frequencies.
Angle-of-arrival measurement capability. This allows for locating the transmitter by taking bearings at different times (different aircraft/ship geographical positions). Plotting these different bearings on a chart will locate the transmitter by triangulation. An airborne or groundbased digital computer can also be programmed to perform the same function. In single ship or submarine operations, a location including estimated range can be determined by plotting successive bearing cuts on a DRT or NC2 plotter and employing Target Motion Analysis (TMA) or Eklund ranging methods.
Signal analysis capability. This provides a means of determining the signal modulation, sidebands, pulse width, and PRF. From this information the signal can be identified and associated with a specific threat or platform. This is mot efficiently performed by a digital computer, but can be done by manual analysis and consulting publications.
Display. The type of display is dictated by the way in which the receiver is used. The varies between the "Fuzzbuster"-type warning light and audio tone devices of Vietnam-era fighter aircraft detection systems to very complex signal analysis scopes and computer-controlled alphanumeric displays on CRTs.
Recording system. There is great intelligence value in electronic emissions of all types, including commercial TV, radio, and computer data. For this reason ships, aircraft, submarines, and Marine units are equipped with magnetic tape and other devices to record emissions for further analysis at major intelligence centers.
11.2.1.3 Signal Processing. Signal collection is a three stage process of warning, sorting and analysis. Warning alerts the operator to the presence of a signal by audio modulation, flashing light or a CRT bearing line. Sorting identifies signals of immediate interest based on the frequency and moudlation of the warning signal. Analysis determines the transmitters specific
characteristics for both immediate and future use.
11.2.2 Electronic Countermeasures
The second major division of Electronic Warfare is ECM, and of the three divisions it is probably the best known. Partly this is because ECM tends to be visualized as "black boxes" that display a visible realization of electronic warfare. Often it appears that if one understands the black boxes, then one has an understanding of ECM, but such an attitude is very narrow because it ignores the two types of ECM: jamming and deception. Thus, the approach in this section will be more general; an attempt will be made to lay down the framework within which the black boxes function.
Of the two types of electromagnetic radiating systems against which ECM may be employed--either sensors and/or communications systems--enemy sensors receive by far the greatest attention. The primary reasons for this fact are: (1) the enemy sensor system produces an immediate threat, whereas the communications system does not, and (2) the sensor system is usually specifically directed toward the friendly forces, and communications are not.
The emphasis of ECM employment in this section will be against sensor systems. However, some mention of the theory and practice of employing ECM against communications systems is considered appropriate, particularly in the contemporary Navy, which is so heavily dependent upon communications--including the various computer data links that provide the backbone to the fleet-wide command and control efforts.
From a strategic point of view, using ECM against an enemy communications system is questionable, for by so doing the opportunity to gain valuable information by eavesdropping is lost. Tactically, however, it may be very advantageous to jam the enemy communications system in order to cause a breakdown in his battle plan. This was vividly illustrated during the 1973 Middle East War when the Egyptians successfully jammed the Israeli UHF/VHF radio frequencies, which resulted in a complete disruption of the
Israelis' air-to-ground communications and consequently significantly reduced the effectiveness of their close air support.
Typical electronic sensors against which ECM might be used include long-range passive detectors; radar-warning picket ships; airborne radar patrols (AWACS); long-range early-warning radar sets; ground-controlled intercept radar sets; fighter intercept radar; missiles guided by radar or infrared; radio and radar navigation equipment; electronic bombing equipment; electronic identification equipment (IFF); terrain-following radar; antiaircraft artillery (AAA); fire control radar; and surface-to-air (SAM) control radar, etc. The particular method used will depend upon the tactical situation.
11.2.2.1 Basic Principles of ECM Effectiveness. The basic purpose of ECM is to interfere with the operation of the sensors of the air/surface defense system, and through them to interfere with the operation of the system itself. Briefly, ECM attempts to make the defense more uncertain as to the threat it faces. The greater the defense uncertainty, the more effective the ECM. To state this principle another way, ECM attempts to reduce the information content of the signals the defense receives with its sensors. The objective of ECM, then, is to force the air/surface defense system to make mistakes or errors.
One should always keep in mind that ECM does not have to prevent tracking completely to be effective. In an age where rapid reaction is critical to survival, delaying the establishment of a solid track on a target, causing a moment's confusion, or forcing the decision maker to wait just those few more seconds to be sure of the proper response can enable weapons to penetrate an adversary's defenses.
11.2.2.2 The Two Classes of ECM. Given that we want to interfere with an enemy air/surface defense radar, how may we go about it? In general there are two fundamental ways, jamming and deception and each may be implemented in many ways. (See Table 11-1) We can
1. Radiate active signals to interfere with the radar.
2. Change the electrical properties of the medium between the aircraft/ship and the radar.
3. Change the reflective properties of the aircraft or ship itself.
The first method encompasses most jamming and deception techniques. The second includes such techniques as chaff dispersion. The third way includes applying radar-absorbing materials to aircraft and both electronic and mechanical echo (BLIP) enhancers for decoys.
Table 11-1. ECM Techniques by Class and Type
Class Jamming Deception
1. Active radiators Noise Spot False target generators
radiation barrage Track breakers
swept
2. Medium modifiers Chaff corridors Random chaff
Chaff bursts
Vehicle design
3. Reflectivity modifiers RAM (Radar absorbing
Materials)
Echo enhancers
Corner reflectors
11.2.2.3 Transmitters. The effectiveness of an ECM transmitter
(jammer) depends, among other things, upon the power output of the transmitter, losses in the transmission line, antenna gain in the direction of the victim receiver, and transmitter bandwidth. In addition, the amount of the ECM transmitter emission delivered into the victim receiver is a function of the receiver bandwidth, its antenna gain, and the radar cross-sectional area of the target. In order to be effective, the ECM transmitter must be capable of emitting enough power in the bandwidth of the victim receiver to mask (jam) its intended signal or to simulate a deceptive signal realistically.
In order to meet these requirements, most ECM transmitters are designed to be versatile in operation. When the ECM transmitter is used against only one missile, one radar, or one communications device, or against a few such devices grouped closely in frequency, the transmitter can concentrate its available power output into a narrow spectrum. On the other hand, if the ECM transmitter must operate against several devices separated in frequency, it must spread its available power output over a correspondingly increased spectrum. For example, a 1,000-watt transmitter that emits its energy in a 10-MHz spectrum is developing a power density of 100 watts per MHz. If the same transmitter must spread its energy to cover a spread of 100 MHz, its power density is therefore 10 watts per MHz. As indicated previously, the effectiveness of the ECM transmitter depends upon its power density within the bandwidth of the victim receiver, after considering antenna factors and propagation path losses.
11.2.2.4 Burthrough. Before addressing the individual ECM techniques, the idea of radar burnthrough needs to be discussed--radar burnthrough. Burnthrough is usually stated as the range at which the strength of the radar echo becomes equal to or greater than the ECM signal. This phenomenon occurs for all radars at some range; to see why, the inverse square law of electromagnetic wave propagation must be examined.
As you will remember from equation (2-14) of Chapter 2, the power density or energy reflected from a target per unit area arriving back at the radar becomes:
Per = PtG
(4R2)2
Where:
Per = power density of the echo at the radar
Pt = radar transmitted power
= radar cross section
A simplified equation for the power density of the jamming
signal at the victim radar antenna, disregarding losses, is given by:
PECM = PiGiBr
4R2Bj (11-1)
Where:
Pi = jammer transmitted power
Gi = jammer antenna gain
Bi = jammer bandwidth
Bi = victim radar bandwidth
Note that the jamming signal only needs to travel one way to the victim radar, and therefore the inverse square law 1/4R2 is only applied once rather than both ways as in equation (2-14) for the victim radar's own signal. This advantage is referred to as "leverage."
In accordance with the definition of burnthrough as stated above, the target will be just barely perceptible through the jamming when Per = PECM. If the self-screening jammer is effective at a specific range, then as the jamming platform closes the victim radar and range decreases Per will grow more rapidly than PECM. There will be some separation between the jammer and the radar where Per = PECM and as the separation continues to decrease, Per eventually becomes greater than PECM.
If we set Per then by setting equation (2-14) equal to equation 11-1) and solving for R we can estimate the burnthrough range RB.
If
PtG = PiGiBr
(4R2)2 4R2Bj
then
RB = PtGBi
PiGi4Br (11-2)
Note: We have restricted this explanation to the case of a self-screening jammer, though the principles apply to the stand-forward and stand-off jammer as well.
Usually the burnthrough range is calculated for a standard set of conditions and is used to compare the effectiveness of ECM against various radars. But is should be understood that, in practice, the burnthrough range is never a constant. First, the reflective properties of the aircraft or ship can vary over a range of 1,000:1 (radar cross section) depending upon the particular aspect presented to the radar. Second, the ability of the radar to distinguish its echo from the ECM depends upon its design, its condition of maintenance, and on the signal-processing circuits in use at the time. That is, burnthrough to the operator or automatic detection circuit may occur when the echo is either stronger or weaker than the ECM signal depending upon the radar configuration and condition.
11.2.2.5 Noise Jamming. One way of preventing a radar receiver (or any other receiver) from functioning correctly is to saturate it with noise. Noise is a continuous random signal and is dissimilar to the radar signal. The radar signal or echo is a periodic sequence of pulses. Figure 11-3 shows the radar echo first and then the echo with the jamming superimposed. The objective is to conceal the echo. As figure 11-2 illustrates, this means that the average amplitude of the radar echo to be concealed. This idea can be alternatively expressed by saying that the average power of the jammer must have the same effect as the peak power of the radar echo, or by saying that the noise-to-signal ratio at the input is raised to a level beyond which the receiver can extract intelligence.
Since the jammer must transmit continuously while the radar transmits energy in pulses, the jammer requires large average power. This large average power requirement in turn necessitates a transmitter with a correspondingly large size, weight, and power supply, all of which must be carried on the aircraft, ship, or vehicle. Whereas a ship may not be limited by this requirement, an aircraft or small vehicle is limited in the amount of jammer protection it can carry.
Finally, when the radar antenna is pointed toward the jammer, the radar sees signals at all ranges. The effect on a PPI scope is to create a solid line at the azimuth of the jammer. This line, called a strobe, indicates to the operator that a jammer is present and gives its azimuth, but he does not know the range of the jammer if the jamming is effective. Thus, jamming has the bad effect that it can highlight the target's presence and direction and serve to identify it as hostile, but it has the good effect of denying the radar operator the range of the target if sufficient power is used. Figure 11-4 illustrates the idea of a strobe. The left strobe shows the consequence of insufficient jamming power. The target return can be seen "burning through."
11.2.2.6 Major Noise Jamming Techniques. Within the general class of jamming, there are three different techniques for generating the noiselike signal to be used. In spot jamming all the power output of the jammer is concentrated in a very narrow bandwidth, ideally identical to that of the radar. Barrage and sweep jamming spread their energy over a bandwidth much wider than that of the radar signal. Thus, spot jamming is usually directed against a specific radar and requires a panoramic receiver to match the jamming signal to the radar signal. The other two techniques, however, can be against any number of radars and only require a receiver to tell them that there is a radar present.
The difference between barrage and sweep jamming lies in the modulation techniques and size of the frequency band covered. Barrage jamming often uses an amplitude-modulated signal covering a 10-percent frequency band (bandwidth equal to 10 percent of the center frequency). Sweep jamming often uses a frequency-modulated signal, and the frequency is swept back and forth over a very wide bandwidth. Figure 11-5 illustrates these three types of jamming.
It is nearly impossible to match exactly a jammer frequency to that of a radiating radar; therefore, it is usually necessary to broaden the bandwidth of the noise so that it is greater than the radar bandwidth. A barrage jammer has a very wide bandwidth to cover all radars with frequencies in that band, whereas the spot jammer attempts to match as closely as possible a particular radar frequency.
But this broadening of the jammer bandwidth causes the jammer to require more power than one that is exactly matched, because the power that matters for any radar is the power that is accepted by the receiver. This fact is usually accounted for by specifying the spectral power density that a jammer must have to jam a radar. Power density is the power contained in the jammer output spectrum divided by the bandwidth. Figure 11-6 illustrates this idea by showing that a jammer of a given total power is more effective if its bandwidth is decreased. The usual means of specifying jammer power density is in watts per megahertz. (w/MHz).
Since aircraft are limited in the total amount of jammer power they can carry, it is advantageous for the air defense network to use as many widely different frequencies for its radars as possible. This concept is usually called frequency diversity, and it forces the jamming penetrators to either carry a large number of spot jammers or spread their barrage and sweep jammer power in order to cover all the radars. Frequency diversity will also eliminate the mutual interference of integrated forces during large operations. The ability of a single radar to change frequency to counter a spot jammer is called frequency agility.
11.2.2.7 Jamming Tactics. Three standard tactics are used:
Self-screening jammers (SSJ). In this situation a unit carries jamming equipment for its own protection. This results in a trade-off between weight and space reserved for ECM equipment and that reserved for sensors, weapons, and fuel. The trade-off is most critical in aircraft and least critical in ships. The self-screening method results in maintaining efficient jamming geometry between victim radar target and jammer because the jammer and victim radar are always along the same line.
Stand-off jammers (SOJ). The jamming unit remains just outside the range of enemy weapons, providing screening for attacking units that actually penetrate enemy defenses. Usually the jamming unit will have only that task, although it may be involved in command and control. The advantage of SOJ is that the jammer is safe from enemy home-on jamming (HOJ) weapons (a submode of nearly all RF homing weapons). The disadvantage of this geometry is that burnthrough occurs earlier on the attack units because the jammer must remain at very long range, while the attack units close the enemy to very short range.
Stand-forward jammers (SFJ). The jamming unit is placed between enemy sensors and the attack units. While maintaining proper geometry between victim sensors, attack units, and the jammer is difficult, this method allows most efficient use of jammer power by reducing spreading and attenuation losses. This situation is most dangerous for the jamming unit because he is a prime target for all weapons systems and well within the capabilities of Home-on-Jam (HOJ) and Anti-Radiation (ARM) weapons.
11.2.2.8 Deception. The other major type of active ECM is deception. In contrast to noise jamming, deception tries to mimic the radar echo so that the radar will respond as if it is receiving an echo from another aircraft or ship. For a radar to direct a fire control system correctly, it must accurately measure target range, bearing, and elevation. If either range or bearing is misrepresented without the operator's knowledge, the target's location will be incorrectly established. Deception ECM is generally accomplished by repeaters and transponders, and is sometimes also called repeater jamming.
Repeaters. The theory of repeater operation is basically simple. However, actual implementation requires sophisticated circuitry. Basically, the radar signal is received, delayed, amplified, modulated, and retransmitted back to the radar.
Transponders. The transponder differs slightly in that it plays back a stored replica of the radar signal after it is triggered by the radar. The transmitted signal is made to resemble the radar signal as closely as possible. Delay may be employed, but amplification is usually not used. The power requirements for a deception repeater are much lower than for a noise jammer, since the repeater emits its energy in pulses similar to the radar pulses. Its duty cycle is similar to that of the radar.
11.2.2.8 Range deception. If a repeater were to simply retransmit the received pulse as soon as it was received, it would reinforce the return echo and would help rather than frustrate the radar. But if the received pulse (as opposed to the echo that returns to the radar) could be briefly sorted and then transmitted a short time interval later, the radar would first receive the weak natural echo-return followed by an identical but stronger pulse. If a repeater transmitted a series of time-displaced pulses, identical to the radar pulse, it could produce a series of spurious targets, each at different ranges.
In automatic tracking radars, the first step in the process of locking onto a target is for the operator to designate the specific target of interest by positioning the range tracking gate over the target video. Once this is done, the radar's receiver is, in effect, turned off until such time as an echo-return is expected at the approximate range of the designated target, thereby making allowance for the velocity of the target.
This allows the deception repeater to operate in the "range-gate" or track-breaking mode. Initially, the repeater simply repeats back the received radar pulse without any delay, to allow for the radar's automatic gain control to adjust to the stronger signal, which it assumes to be the designated target.
Then the deception repeater begins to introduce increasing amounts of time delay (figure 11-8) before retransmitting back the received radar pulse. Thus, the range-gate circuitry in the radar tracks the stronger pulse and gradually "walks off" from the true target range and makes the target appear to be at a greater range than it really is.
Similarly, the target can be made to appear at a closer range by delaying the received radar pulse long enough such that it can be retransmitted back prior to receiving the next radar pulse. Then the deception pulse will arrive at the radar before the real echo pulse, producing a false target at a closer range.
This false target range information can cause significant aiming and guidance errors for antiaircraft guns and for missiles that require command guidance from ground-based radars.
The simplest remedy that the tracking radar can use is to have its operators switch to a manual mode of operation. This remedy is effective because a man watching a radar scope can see the cover pulse move away from the aircraft return, and he can therefore track the aircraft.
Even though manual tracking will largely counter a repeater, manual tracking is never as smooth as automatic tracking. Thus, the weapon miss distance will increase and increas the probability of aircraft survival against non-nuclear defense weapons.
11.2.2.9 Angle Deception. Radar and command control systems can be confused by causing the radar to generate incorrect target bearing and elevation information. For this to be successful, the deception device must cause the radar to indicate the presence of a target at some time other than when the radar is at the target's bearing and elevation. There are two primary methods of achieving this effect.
Sidelobe angle deception. First, the sidelobes in the antenna radiation pattern must be evident to the ECM unit. A false target pulse is then transmitted while the ECM unit is at the azimuth of a sidelobe of the victim radar. The radar circuitry is designed to register target angular position in the main lobe only, and therefore displays target video with an angular error equal to the angular displacement between the main lobe and the sidelobe involved. This technique can be applied to any radar with ineffective sidelobe suppression or cancellation. By combining this method with range deception, many false targets at different ranges and bearings can be generated, causing confusion over the entire search volume of the victim radar, with much less required average power than equivalent noise jamming.
Angle tracking circuit deception. Mono-track radar, such as those employing Monopulse, Conical Scan, COSRO, LORO, and Track-While-Scan (TWS) or active tracking radars can be deceived by causing their angle sensitive circuitry to drive in some direction other than that which would correct the existing angular error at any given time. In doing so, the ECM unit will cause errors in solution of the fire control problem and thus induce considerable error and/or delay in weapons employment. Deception of angle-tracking circuits sometimes involves specific equipment tailored to each angle-tracking technique. For instance, the inverse conical scan (inverse gain) method mentioned in Chapter 5 is only effective against the conical-scan tracker and would not work with the others mentioned above. For all angle-tracking methods, the ECM unit must have knowledge of the scanning and tracking techniques employed by the victim radar. For the conical-scan and lobe-switching radar, this may be obtained by monitoring the radar with an ESM receiver. Monopulse, CORSO, LORO, TWS, and active tracking radars reveal nothing to an ESM operator concerning their tracking methodology. Therefore, good intelligence data is required to build deception devices and properly employ them against these systems. Briefly, two of the techniques that can be employed against all of these systems are:
Blinking. Noise or a sample of the victim radar pulse is amplified and retransmitted from various widely separated points on the ECM unit (or several closely spaced cooperating units) in a random fashion, causing enhancement of the usual movement (wander) of the point on the ECM unit that the radar tracks. For smooth tracking and accurate solution of the fire control problem, the radar should track the centroid of the target. The result of this technique is excessive tracking error.
Crosseye (phase front distortion). Two widely spaced locations on the ECM unit are selected (such as the nose and tail or two wingtips in the case of an aircraft) and interconnected transponders installed. Each of a pair of these locations normal to the direction of the victim radar receives the victim radar pulse and triggers the transponder on the opposite side of the unit, which then transmits a copy of the victim radar pulse with a 180o phase shift. The result is a reversal of the sign of the angular error measured at the victim radar. This causes the radar positioning mechanism to drive in the wrong direction. In the case of a TWS radar or active tracking radar, this technique can result in errors in positioning tracking gates in azimuth and elevation; can prevent the establishment of a smooth track, or can cause problems in acquistion gate, tracking gate, and turn detection gate selection logic.
11.2.2.10 Continuous Wave Doppler and Pulsed Doppler Deception. CW doppler and pulsed doppler radars were developed to track high-speed, low-flying aircraft in the presence of ground clutter. The echo-return from these radars that enables the target to be tracked is the doppler shift due to the target's velocity.
The deception of the CW doppler requires that the repeater retransmit the received CW signal with a spurious doppler shift, gradually increasing its magnitude to cause velocity track breaking. This will not only cause errors in the fire control solution, but because of the velocity gate walk-off, it can result in loss of target tracking when the repeater is turned off.
Deception of the pulsed doppler radar is much the same. The repeater introduces a similar spurious doppler shift when it retransmits the received pulses.
11.2.2.11 Echo/Blip Enhancer. Another type of deception repater is the echo or "blip enhancer." This repeater strengthens the retransmitted pulse in order to make a small radar target, such as a destroyer, appear as a large carrier apparently at formation center. This may also be done mechanically by using properly designed reflectors that will make a small target look like a large one.
11.2.2.12 Chaff. The primary way to change the properties of the medium between the radar and the target is by the use of chaff. Chaff consists of small metallic (aluminum) dipoles that are designed to resonate at the radar frequency. Half-wave dipoles make very good radar reflectors. Typical dimensions for use against a 10-GHz radar would be 0.6 inch long, 0.01 inch wide, and 0.001 inch thick. Only 0.1 pound is needed to cause an echo equal in size to a large bomber. Thousands of such dipoles are compressed into small packages. When injected into the aircraft slipstream, the chaff packages burst open and the dipoles scatter to form a radar-reflectie cloud called a chaff corridor.
Each chaff package, dropped independently, can simulate an additional aircraft. A chaff curtain, consisting of thousands of false targets, can be dropped by a small number of aircraft. Such a curtain can so confuse radars that they are unable to locate the real targets within the chaff cloud. Chaff drops so slowly that it normally takes many hours to reach the ground.
When chaff packages are dropped in close sequence, radars, viewing the resulting continuous chaff corridor from right angles have difficulty tracking targets within the corridor. If the corridor is viewed nearer to head-on (dispensing aircraft approaching the radar), the radar range gate can be forced to stay on the first return received. Thus, the lead aircraft can be tracked and the chaff echoes gated out. When viewing the corridor tail on, the radar can also track the lead aircraft if the range gate is forced to select the last part of the echo. If the dispensing aircraft uses rockets to fire chaff in front of the aircraft, the problem of maintaining tracking is greatly increased.
Since the chaff particles have considerable aerodynamic drag, their forward velocity quickly drops to near zero. Because of its low velocity, chaff can be regarded as an airborne type of "clutter". Radars such as CW, pulse doppler and MTI (Moving Target Indicator) that can reject clutter are not seriously affected by chaff. Thus, they can continue to track a target within a chaff cloud as long as the target has a radial component of velocity.
The use of chaff by surface units has greatly increased in recent years. Chaff dispensed by a rocket or projectile can be used to decoy or break the track of a missile with active radar guidance.
Chaff is a particularly effective means of defending relatively slow systems such as surface ships. In this situation there is so little difference in velocity between the potential target and the chaff that CW, pulse doppler, and MTI radars have difficulty in separating the target from the chaff clutter. In shipboard defense, chaff rockets can be fired to burst at a specific location, hopefully within the field of view of the weapon RF seeker, creating an alternate target that is more lucrative than the ship itself. The disadvantage of this situation is that it requires an elaborate fire control system and movable launcher to position the chaff burst precisely. The alternative, employing fixed launchers and no fire control system, is to fire several chaff rockets to burst relatively close to the ship. The chaff cloud combines with the ship to form one very large target with a combined centroid somewhere in the chaff cloud. An RF homing weapon that seeks the centroid of its target will thus fly harmlessly past the ship and through the chaff cloud.
11.2.3 Electronic Counter-Countermeasures (ECCM)
Electronic counter-countermeasures is the art of reducing the effectiveness of an EW threat with the objective of making the cost of effective EW prohibitive for the enemy. As in ECM, ECCM includes both radar design and operator training. The radar ECCM designer must understand the various forms of ECM that his radar is likely to encounter, hence he is very interested in intelligence about the ECM threat. Likewise, the radar operator would like to know what ECM he will be facing. But in both cases detailed intelligence will probably be lacking. Therefore, the designer must provide a variety of options to be used against the expected threats. And the operator must be trained both to recognize the various countermeasures that might be used against him and to select the appropriate combination of options against each of them. The most effective measure to combat ECM is an up-to-date piece of equipment operated by a well-trained operator. Radar design for ECCM can be broken down into three areas: radar parameter management, signal processing techniques, and design philosophy.
11.2.3.1 Radar Parameter Management. The basic radar parameters are those characteristics that influence the radar's performance. These are: power, frequency, PRF, pulse length, antenna gain, antenna polarization, antenna scan, and antenna sidelobe characteristics. These values, and the means with which they can be manipulated in service, are established in the design phase.
Power. For a ground or surface radar, power is often considered the fundamental ECCM parameter. With this view, ECM becomes a power battle, with the outcome going to the stronger, more powerful opponent. Airborne jamming equipment is limited in size and weight and therefore has a power limitation. Thus, the power advantage lies with the ground or surface radar. In the case of one surface unit versus another, both operate under the same constraint, and the outcome is not obvious.
Frequency. Frequency agility is a significant ECCM design feature. Using components such as frequency synthesizers (something like those employed in radio scanners) instead of conventional crystal-controlled oscillators, some radars are able to change frequency within one pulse repetition time (PRT). This makes deception and jamming very difficult. The radar can be designed to change frequency automatically within a certain range, or this can be done manually.
A second way of using frequency as an ECCM tehcnique is the doppler radar, including radars designed for MTI signal processing. The actual ECCM advantage is gained from signal processing in the receiver, but the intention to use the doppler frequency shift must be reflected in the transmitter design. For example, in a pulse-doppler radar the transmitter must often be designed to radiate a very stable frequency. In a pulse compression radar the transmitter must radiate a pulse with an FM slide, sometimes called a chirppulse due to the change in "tone" as the pulse is transmitted.
Pulse repetition frequency (PRF). In general, high PRF radars are more resistant to ECM because their average power is greater. Changing the PRF in a random fashion is an effective counter to deception because deception ECM depends on predictability of the radar. However, because PRF is related to the basic timing of the radar, this technique results in additional complexity and expense. Random PRF has been employed as a very effective ECCM feature in some radars for many years and has the additional benefit of elimination of MTI radar blind speeds.
Pulse length. An increase in pulse length will increase average power and thus increase detection probability. The trade-off is increased minimum range and degradation of the radar's range resolution capability. This problem can be compensated for by including a pulse compression capability; however, due to receiver blanking during the transmit cycle, the minimum range will stay relatively long. Some modern radars compensate for these difficulties by employing the pulse compression and varying their pulse width depending on mode of operation and expected target range.
Antenna design. Antenna design, as reflected in low sidelobe levels, is an ECCM design technique, because it prevents a jammer or deceiver from affecting the radar at many azimuths. Low sidelobe levels also make the job of antiradiation missiles more difficult, since there is less chance of the missile homing in on the radar unless the radar is pointing at the missile. Sidelobe patterns can be compensated for by employing techniques such as sidelobe cancellation and sidelobe suppression.
Scan pattern. The radar scan pattern can influence ECCM capability because it influences the amount of energy directed toward the radar target. An active tracking phased-array radar is quite ECM resistant because of its ability to rapidly scan its radar beam in a random fashion than in the regular circular or sector scan pattern of conventional radars. This irregular beam positioning would give the opposing ECM system little or no warning and make it impossible to predict where and when to transmit false signals. In systems where scanning is performed in the receiver rather than in the transmitted beam, such as those mentioned in the section on angle deception, ECM has no direct access to the radar scan pattern and thus has difficulty using that information to interfere with the radar system operation.
Into this class fall the passive detection and home-on-jam techniques where the "radar" does not transmit, but uses the ECM energy emitted by its victim to determine the victim's location--often by triangulation from two or more separate locations.
11.2.3.2 Signal-Processing Techniques. These are usually functions that are incorporated into the radar receiver. Although certain signal-processing techniques may place constraints on the transmitter, many of them have been added to the receiver after the radar has been built. These techniques are called ECCM or anti-jamming (AJ) fixes, since they were initially developed as retrofits to improve existing equipment. Radars now tend toward a more sophisticated design concept in which the AJ devices are included in the basic radar system.
Doppler radars, including radars with moving target indicator (MTI) signal processors, although not designed specifically for ECCM purposes, are quite ECM resistant. Since doppler radars (pulse and CW) operate on the frequency shift caused by a moving target, they automatically filter out returns from nomoving targets and consequently eliminate many unwanted singlas, such as those from chaff. They will even discriminate between returns from objects of different velocities such as an aircraft in a chaff cloud. This technique can also make deception more difficult, since the deceiver must imitate the pr proper frequency shift.
In radar with automatic threshold detection (in which the target is said to be present when the receiver output crosses a preset threshold), the presence of a jamming signal can increase the rate of false alarms (false targets) to an intolerable extent. If the radar output data is processed in an automatic device such as a computer, the device might be overloaded by the added false alarms, due to jamming. Thus, it is important that the receiver present a constant false-alarm rate. Receivers designed to accomplish this are called CFAR (constant-false-alarm-rate) receivers. Their disadvantage lies in the likelihood that some weak targets will remain below the threshold and be lost.
If an operator were monitoring the radar output, the effect of the additional false alarms could be reduced by having the operator turn down the gain of the receiver during the presence of jamming, or else he might be able to ignore those sectors containing ECM. In an automatic threshold detector, the same effect may be obtained by using the average noise level to provide an automatic gain control, much as an operator would by adjusting a manual gain control. Because the automatic CFAR circuits react faster, they are superior to an operator in keeping the false-alarm rate constant, especially when the radar is subject to noise jamming from only a few azimuth sectors.
A CFAR receiver, no matter whether it is an automatic device or an operator controlling the receiver gain, maintains the false-alarm rate constant by reducing the probability of detection. When the threshold level is raised to maintain a constant false-alarm rate, marginal echo signals that might normally be detected do not cross the higher threshold and are lost. Therefore, CFAR does not give immunity to jamming; it merely makes operation in the presence of jamming more convenient by making the receiver less sensitive. If the jamming were severe enough, the CFAR, for all intents and purposes, could produce the same effect as turning off the receiver.
Other ECCM techniques employ track history such as those discussed in the chapter on track-while-scan radars, to reject false returns. The addition of computerized processing of radar video can perform this function as well as echoes returned in serveral successive radar transmissions as a means to counter the deception jammer.
11.2.3.3 Radar Design Philosophy. A general rule of thumb for ECCM radar design is to incorporate unpredictable operating parameters. The more orderly a radar is in its operation, the easier it is to predict what the radar is going to do or how it is going to operate; consequently, the job of applying an ECM technique effectively becomes simpler. ECM becomes more difficult, however, if characteristics of the victim radar are constantly changing. The parameter that may most easily be varied to confuse the ECM operator is the frequency. The capability for operator variation of pulse length, PRF, modulation, and antenna characteristics is commonly built into radars to make ECM more difficult.
The most common way to introduce unpredictability into radar design is through frequency diversity. Early radars were all designed to operate in a few specific frequency bands, where narrow-band jamming would render them all ineffective. New radar systems are designed so that each different radar type operates in a different frequency band. The use of a much greater portion of the spectrum, from VHF to SHF (A to J Band), forces ECM operators to cover this total spectrum if they are to be effective. This usually results in being able to put less ECM power against a single radar because the airborne platform is limited in its total power capability.
Another aspect of ECCM design philosophy is the relationship between automatic equipment and the human operator. The trained radar operator fulfills a useful and necessary role in a countermeasure environment and cannot be completely replaced by automatic detection and data processors. An automatic processor can be designed to operate only against those interfering or jamming signals known beforehand; that is, any capability against such signals must be programmed into the equipment beforehand. New jamming situations not designed into the data processor might not be readily handled. On the other hand, a human being has the ability to adapt to new and varied situations and is more likely to be able to cope with, and properly interpret, a strange new form of interference than can a machine. Therefore, a skilled operator is the most important counter-countermeasure for maintaining radar operation in the presence of deliberate and clever countermeasures.
11.2.3.4 Anti-ESM Countermeasures. Radar, fire control and weapon sensors tend to dominate the electronic counter-countermeasures area. There are, however, certain tactical approaches that should be considered when trying to counter an opponent's EW capability: RF emission control (EMCON); and evasion of the opponent's ESM or active sensors (See Fig. ll-1). Application of these tactics must be tailored to the mission objective since each requires giving up information and possibly some operational flexibility.
EMCON is a situation of degree as opposed to no emissions at all. The price of complete EMCON in terms of absence of communications and lack of tactical data may considerably inhibit mission completion. Limited use of surveillance sensors, especially those that are common enough not to identify a ship or ship class can provide adequate tactical information without divulging force size or capability. Use of capability in other parts of the electromagnetic spectrum, such as passive IR or passive sonar, are alternatives to active RF sensing. A combination of the long range capability of passive RF sensing with the high accuracy of passive IR can provide early warning, identification and precise angular location data on platforms of interest.
Line-of-site communications (i.e. VHF, UHF) are far less prone to intercept then HF communications which follow the earth's curvature. If directional communication capability is available it may be used to further increase intercept resistance. Even though platform location/presence may be known to opponents, intentions may be concealed by restricting fire control sensor emissions and minimizing message traffic until such emissions are absolutely necessary. EMCON may also be exercised by limiting the time and regularity of key active sensor operation. Regular repetition of emissions is of great benefit to the ESM identification process and deceptive countermeasures. Toward this end, LPI or low probability of intercept communications will become the rule rather than the exception.
Evasion, although limited by mission characteristics, is still a mission planning consideration. Aircraft may use terrain or radar horizon shielding as well as the advantage of the reduced radar detection performance against low elevation targets. Non-direct flight routes by aircraft may well be worth the flight time, fuel and reduced weapon load-out in terms of possible aircraft losses due to an alerted opponent. Radar horizon protection is also available to surface combatants.
11.2.4 The Effectiveness of ECM/ECCM.
The interaction between friendly and hotile electronic warfare approaches is analogous to a ladder of sequential increases in technological capability that alternate between two opponents (see figure 11-9). Fielding of an electronic system by one side results in the development of a countering system by the opponent; which in turn causes the original side to develop a counter-countermeasure. The process continues to escalate through a series of counter and counter-countermeasures (a "ladder"). Some of the implications of this process follow:
(1) The ECM effectiveness of a force depends on its "ladder" position, training, morale and the uncertainties of combat.
(2) ECM techniques have only a finite time of superiority. Eventually an adversary will develop a counter technique and the superiority will pass to him.
(3) The real advantage of ECM, then, is that it gives relative superiority while an adversary is developing and deploying a countermeasure.
(4) Technological superiority and constant development in the EW area are required to be consistently able to counter enemy advances.
(5) Techniques should not be discarded because they have simple counters; there will always be enefits from the inevitable delay between the time the enemy is certain that the technique is being used and the time he can make his countermeasure operational.
11.2.5 Stealth Technology and Low Observables.
Recent emphasis in the weapons field reaffirms the notion that "you
can't hit what you can't see." A contact that remains undetected until its terminal phase carries the highest probability of successfully completing its mission. Toward that end, future weapons systems, and the platforms that carry them will have to significantly reduce their distinguishable (and hence, detectable) characteristics. The class of weapons so engineered are termed "low observables" and they make use of the latest techniques and materials for reducing their characteristic signature-stealth technology. Because stealth technology has its primary focus on avoiding radar detection it is included here but it applies to IR/EO and acoustic detection as well. Techniques for modifying aircraft, ship, and submarine radar cross sections (a variable in the radar range equation) have existed since World War II. Radar cross section is a function of frequency, target size, shape, aspect and composition. Reduction of radar cross section can take many ways.
11.2.5.1 Alteration of Target Shape. The configuration of the target must be modified according to the principles of geometrical optics such that the large reflections are diverted to unimportant regions of space (i.e., not back to the radar). The designer should avoid flat, cylindrical, parabolic, or conical surfaces normal to the direction of the the radar illumination. These shapes tend to concentrate the energy and provide a large radar return. The target design should include the use of doubly curved surfaces that result in low radar cross section. Unfortunately, in many cases these principles conflict with other important engineering requirements in aircraft and ship design, resulting in increased expense and slow development.
11.2.5.3 Destructive Interference. The object to be protected is coated with material that causes a partial reflection of incident radar energy from the coating surface. If the coating thickness is /4, then the total additional distance traveled between the coating surface and the object's skin surface is /2 or 180o
In this way we achieve destructive interference between the radar energy reflected from the coating surface and that reflected from the skin of the object. Thickness of the coating is not excessive at short wavelengths; however, it is impractical to install coatings thick enough to deal with low-frequency search radars.
11.2.5.4 Radar Absorbent Material. In this case the object to be protected is given a coating of successive layers of magnetic composition material such as Ni-Mn-Zn sandwiched with dielectrics that convert 95% of incident RF energy to heat. This material can be made as thin as 1.75 cm, which is practical for aircraft use; however, the weight penalty of 24.9 kg per m2 is excessive. This would not eliminate their use aboard ship or at ground-based facilities. Another approach, involving continuing research, consists of a phenolic-fiberglass sandwich material. This structure again converts 95% of incident RF energy to heat by using a resistive material consisting of carbon black and silver powder. This material is effective over the range of 2.5 to 13 GHz, which encompasses many fire control and weapon-guidance radars. The disadvantage of this approach is that while it is lightweight and relatively thin, it is not able to handle the high temperature and erosion processes at supersonic speeds. These methods, though promising, still cannot deal with some of the lower radar frequencies.
11.3 ACOUSTIC COUNTERMEASURES
Early sonar operators rapidly discovered that they could passively detect active acoustic devices at ranges greater than the range that the active devices could detect the passive platform. Tacticians reasoned that this counter-detection situation could be exploited much the same as with ESM and radar. The initial underwater threat detection and evaluation capability employed the listening capability of the installed attack sonar set. This was limited to the bandwidth of the sonar receiver until the development of separate interconnected hydrophone arrays for active sonars. By the 1950s, dedicated underwater intercept receivers such as the AN/WLR-2 were developed.
Acoustic countermeasures began with the Pillenwerfer used by German U-boats when the Allied hunter-killer groups began to take their toll in late 1943 and 1944. This device, composed of lithium hydride, acted like a giant Alka-Seltzer tablet, creating thousands of gas bubbles that returned a solid echo similar to a submarine. This was reasonably effective as long as the attacking units didn't see the submarine and the decoy at the same time. The decoy did not cause a doppler shift; therefore, a trained operator could tell the difference.
Dealing with acoustic countermeasures requires the same basic approach as ECCM. Again, the best countermeasure is an experienced and well-trained operator; nevertheless, improvements such as computer signal processing, various filters and delay lines, and automated doppler detection capability are of great help.
11.3.1 Acoustic Intercept Receivers.
Initial attempts at producing underwater intercept receivers were less than satisfactory because the equipments had a high false-alarm rate due to ambient noise. Also, separate receivers were required for initial intercept and for determination of the azimuth of the noise source. Later receivers employed a triangular-shaped array of three hydrophones that determined azimuth by measurement of the time difference between arrival of the signal at each hydrophone. Noise reduction was accomplished by using delay lines and filters that allowed only signals longer than the delay to reach the display. Short noise pulses resulted in no output. A receiver designed in this way could display frequency and azimuth simultaneously after receiving one ping. With the development of active acoustic homing torpedoes, this type of response is required of all acoustic intercept receivers in order to have time to employ torpedo evasion tactics. Modern receivers categorize intercepts and assign priorities according to the potential threat. In this situation torpedoes would have the highest priority, with other intercepts having lesser priority, the least being search sonars in a long-range mode.
11.3.2 Acoustic Countermeasures. The initial approach to acoustic countermeasures had to do with the control of active emissions from sonars and the reduction of self-noise. As with electronic emissions, acoustic emissions should be kept to a minimum and, if possible, only after counter-detection has occurred. However, if analysis of the FOM problem indicates that passive detection of an enemy is not likely then we may be forced to go active. Active operation must take into account the tactical situation. Power levels must be kept down, if possible, to prevent counter-detection. During active sonar attacks, a change to short-range mode will shorten the time between pings and let an adversary know that the ASW unit is in contact. This could result in an unsuccessful attack, with no second opportunity.
Countermeasures for use against underwater acoustic sensors, homing devices, and fuzes developed as these systems evolved. A mine that could be activated by the sounds emanating from a power-driven ship underway was developed by the British in the late stages of World War I, but the first operational use of acoustic mines was by the German Navy against the British in the fall of 1940. There were cases of mines detonating ahead and to the side of ships too far away for the ship's magnetic influence field to have caused the activation. Work started immediately once these phenomena were noticed and the mechanism identified, to provide a noisemaker that would project noise far enough ahead of the ship to detonate the mine before the ship was close enough to be damaged by the explosion.
Explosives, compressed air, or electrically operated hammers striking the inside of a watertight case suspended over the side, and parallel pipe devices towed through the water causing the pipes to bang together as the pressure between the pipes was reduced by the faster motion of the water through a constrained space (Bernoulli's Principle) were among the earliest methods developed to counter the acoustic mine. There is a considerable similarity between the early attempts and the present-day U.S. Navy acoustic sweeps. Other early sweep gear consisted of vanes or propellers attached to noisemaking attachments, not too dissimilar from a New Year's Eve whirling ratchet. When towed through the water, the propellers turned and the ratchet device produced noise. Devices similar in principle are still used; for instance, the U.S. Navy's airborne acoustic sweep device depends on a water turbine to produce a turning motion for its noisemaking capability. The early noisemakers were turned on and were kept operating at one intensity while the sweeper was in the field. Subsequently, controllers were added that enabled the intensity of the output to be varied to simulate the approach of a ship target. The same approach can be employed as a means of decoying homing torpedoes. There are noisemakers that can be towed from ships or submarines, providing a lucrative alternate target when the torpedo-target geometry is such that there is wide azimuth separation between the noisemaker and the target. Modern acoustic towed decoys, such as the AN/SLQ-25 NIXIE and the older T-MK6 FANFAIR, employ electronic or electromechanical means to produce the required signals. Submarines employ small torpedo-like mobile decoys that are equipped with transponders capable of returning a realistic signal to active sonars. With noisemaking capability they can simulate submarine passive signatures to some degree. This same technology has been used to provide small expendable training targets (figure 11-10) for ASW forces at much less cost than that for a real submarine providing target services. Though small (about the size of a sonobuoy), the transponders in these targets make them look as large to the detecting sonar as a real submarine.
Acoustic jammers are available that provide random noise or false targets to a hostile active sonar. With the power of today's sonars, a jammer must have an output SL of at least 200 db (re.1) pa) to be effective when mounted aboard a submarine. The rules governing jammer output, "burnthrough," jammer bandwidth vs. sonar bandwidth, and sonar frequency diversity/agility are similar to those dealing with radar and ECM devices.
Air bubbles can be employed to mask potential targets or to provide alternate targets. The large difference in characteristic impedance (c) between the air bubbles and the surrounding water make them very efficient as reflectors of acoustic energy. Very little sound will penetrate a curtain of air bubbles, making them very efficient as masking for noise sources. This is the basis of the Prairie Masker system installed aboard frigates and destroyers.
(Described in Chapter 8).
11.4 IR/EO COUNTERMEASURES
With the advent of infrared heat-seeking weapons and their increased use, the Department of Defense has been backing an active program for the development of IR Countermeasures Systems. Several countermeasures have been available for years, such as shielding high IR sources from possible detectors, using special nonreflective paints to reduce IR levels radiated, IR decoys, and the tactic of ejecting infrared flares to cause false lock-ons of IR weapons. Recently, however, these techniques have received renewed efforts to improve and refine them. In addition, several new countermeasures systems have been developed, primarily for aircraft.
Electro-optical or laser and TV-guided weapons are also coming into wide use. The uses of lasers for countermeasures vary from range deception to the use of a laser beam to actually blind the operators of visually aimed weapons. Conversely, lasers are being developed to jam enemy range-finding and weapon-guidance lasers. Countermeasures against TV-guided weapons and TV-directed tracking systems are much more difficult to develop, although research is on going in this area.
Various categories of optical and IR countermeasures are illustrated in figure 11-11. Thus, the primary tactical implication is that further payload weight will need to be sacrificed for electronic warfare devices carried aboard our platforms and those of the enemy.
11.5 OTHER COUNTERMEASURES
11.5.1 Magnetic Influence Countermeasures.
The effect of large ferrous metal bodies such as ship and submarine hulls on the earth's magnetic field has been employed in sensors and fuzes for many years. All ships act much as magnets with surrounding magnetic lines of force that have both vertical and horizontal components. Magnetic mines can be made to fire on either or both of these components or on the total resultant field. Magnetic minesweeping produces simulations of the ship's magnetic fields. Countermeasures have for the most part consisted of using electric current to simulate a change in the earth's magnetic field.
Magnetic mines, which were first developed and used in World War I by the British, were used extensively by the Germans in the early days of World War II against the United Kingdom. The Royal Navy created the first magnetic sweeps, both for surface craft and aircraft. The German Navy responded to British magnetic mines with their own sweeps, and whole families of magnetic mine countermeasures were developed. These included the familiar electrode and closed-loop electric current sweeps that we still use today: ships converted to huge magnets to set off mines at some distance from the ship; towed solenoids of several types; and arrays of large magnets towed astern of a sweeper. Although several types of sweeps were built or developed in the United States, main reliance was placed on the straight-tail, open electrode sweep that is common today (figure 11-12). Closed-loop versions of the electrode sweep were also used. The earlier ships relied on large banks of wet-cell batteries for electric power, but the U.S. sweepers are now equipped with powerful generators to provide the electric current. Besides using the electric power to clear mines, the magnetic field of a ship can be reduced considerably by Degaussing cables ringed around the hull.
The same principle can be employed to deceive magnetic anomaly detection equipment. In this case a trailing wire is towed behind a mobile decoy, such as the acoustic decoys described previously. Electric current is applied, resulting in an indication on MAD equipment at close range. The general arrangement is similar to that in the mobile training target in figure 11-10.
11.5.2 Pressure Influence Countermeasures.
Ships and submarines cause unique pressure changes in their vicinity while in motion. These pressure changes can be used to trigger mines and cause torpedoes to detonate with a specific geometric relationship to ship and submarine hulls.
Pressure mines were first used operationally in the later stages of World War II by both Axis and Allied forces. The Normandy invasion forces faced the German "oyster" pressure mine in 1944, and the U.S. made liberal use of pressure mines during Operation Starvation against the Japanese homeland in 1945. Despite the many years of scientific effort since their introduction, no operationally acceptable method of sweeping pressure mines is available today. As with any mine, the pressure mine is susceptible to minehunting.
As in any influence sweep, the object is to produce a spurious influence in the vincity of a mine, which will cause the mine mechanism to recognize the spurious influence as a ship and detonate the mine. A ship moving in relatively shallow water creates an area of reduced pressure under the keel, which extends some distance to either side of the ship's track. This phenomenon, an example of Bernoulli's Principle in operation, is illustrated in figure 11-13. A pressure mine takes advantage of the pressure reduction to actuate a firing meachanism. But because a pressure mine is susceptible to actuation by the pressure flunctuations from waves and swell, the pressure influence will (in most cases) be combined influence mine.
There has been no way found of displacing as large a volume of water as a ship displaces in order to simulate the pressure reduction caused by a ship. Consequently, much effort has been spent and continues to be spent on the use of ships as pressure sweeps (Guinea Pig Principle) and in attempting to make the ship resistant to the explosion of the mines that it sweeps.
Only one method has been devised to counter the pressure mine, the safe-speed technique. It is based on the principle that reducing the speed of a ship reduces the amplitude of the pressure field of the ship. This technique is very effective for small ships. For large ships, however, the reduction in speed required may be so great that the ship cannot maintain steerageway. In this case, steerageway may be maintained with the assistance of tugs. The safe speed technique comes under the category of self-protection.
11.6 PASSIVE DEFENSE MEASURES
This chapter has focused on methods of avoiding detection and, barring that, reducing the effectiveness of an enemy's fire control solution. If successful, we needn't worry about the destructive effects of his weapons. However, prudence requires that we take at least minimal actions to defend against enemy weapons effects.
11.6.1 Armor
Since earliest time, the most prized possessions have been protected by the best available method. For warships that meant armor - mainly the gun turrets, mounts, and bridge spaces. On early 20th century ships, anti-torpedo nets and armor belts were also installed. But in the main, the steel structure of the ship itself was considered adequate to reasonably protect personnel and to preserve mobility and seaworthiness. The shift in the second half of the century to non steel superstructures (the weight reduction intended to increase fuel economy) has two negative aspects. First, the protection afforded by aluminum (of the same thickness as steel) is considerably less and the probability of spalling is much greater. Second, aluminum superstructures have proven much less resistant to fire damage. Further development of lightweight armor and an overall reduction of superstructure area itself (with its concomitant benefits of smaller radar cross section and lower crew imperilment) seem to be the best solutions for ships.
For tanks, a passive defense against shaped charge warheads exists in reactive amor. This protective coating detonates instantaneously in the direction of a shaped charge jet, thus negating its incoming energy.
11.6.2 Nuclear, Biological and Chemical Defenses.
After nuclear weapons tests in the 50's, the Navy experimented with one destroyer modified to keep out NBC contaminants. Deemed effective but unlikely to be necessary, it was determined to be not cost-effective to install such a system on all ships and consequently forgotten. Further analysis and the realization that European and Warsaw Pact navies had ships with such "citadel" systems, drove a decision in the early 80's to introduce it into U.S. Navy ships. Newly designed ships, such as the Arleigh Burke (DDG 51) class, and the Supply (AOE 6) class are being constructed with a collective protection system (CPS) installed. This system is already installed in LHD1 and LSD 44, providing "air-tight" ships designed with a positive pressure capability. Selected spaces are provided with filtered, contaminant-free air, thus defeating the possibility of infiltration by chemical agents exterior to the ship.
11.6.3 Double Hull Designs
With the advent of more accurate and powerful torpedoes and mines, the controversy over double hulled warships (both surface and submarine) is renewed. Use of double hulls reduces the probability a single sub-surface hit will sink the unit. But the added construction and operating (fuel economy) costs are enormous. Research into reducing the underwater hull form (like hydrofoils) and making it less susceptible to torpedo or mine damage are potential solutions. To do this, the outermost underwater hull could be filled with a styrofoam-like substance (relatively impervious to blast) or be super-compartmentalized.
11.7 SUMMARY
This chapter has provided the reader with an overview of the major methods of countering detection and impairing the effectiveness of an enemy's fire control solution. Barring success in those two functions, a brief review of passive defense considerations was presented.
Most countermeasures revolve around electronic warfare and its three branches, ESM, ECM, and ECCM. Acoustic, magnetic, and IR/EO countermeasures must also be addressed by a commander. He must continually assess the trade off in his requirement for information and the probability his forces will be detected and identified because they radiated to get that information to him.
11.8 REFERENCES/BIBLIOGRAPHY
Aviation Week & Space Technology. Vol. 102, No. 4, 27 January 1975.
Bachman, Christian G., Radar Sensor Engineering. Lexington, Mass:
D.C. Heath & Co., 1982
Boyd, J. A., D. B. Harris, D. D. King, and H. W. Welch, Jr.,
Electronic Countermeasures. Los Altos, CA: Peninsula
Publishing, 1978.
Burns, R. L., CDR, USN, Unpublished Notes on Mine Warfare, 1983.
Fitts, Richard E., Fundamentals of Electronic Warfare, Colorado
Springs, CO: U.S. Air Force Academy, 1972.
Leydorf, G. E., ed. Naval Electronic Systems, Vol. 4, Annapolis,
MD: U.S. Naval Academy, 1971.
Nathanson, Fred E., Radar Design Principles, New York: McGraw-
Hill, 1969.
Sippican Ocean Systems, Inc., The Expendable Mobile ASW Training
Target, Marion, MA: Sippican Ocean Systems, Inc., 1983.
Skolnik, Merrill I., Introduction to Radar Systems, New York:
McGraw-Hill, 1980.
Van Brant, Leroy B., Applied ECM, Vol. 1, Dunn Loring, VA:
EW Engineering, Inc., 1978
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