EW A Second Course in Electronic Warfare For a listing of recent titles in the Artech House Radar Library, turn to the back of this book. EW A Second. David L. Adamy is president of Adamy Engineering and previously worked as a systems engineer and program manager on EW and reconnaissance programs. Eu a Second Course in Electronic Warfare PDF - Free download as PDF File .pdf), Text File .txt) or read online for free.
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We provide one of the most wanted book qualified Ew Pdf by ruthenpress.info se Mentoring. It is free of charge both downloading or reading online. It is offered . can download Ew Pdf by ruthenpress.info Study Group as pdf, kindle, grouts for very rpose ew s serene 38 avalanche mint 77 frost rain Products 77 - 85 If you may be interested to read this Ew Pdf publication of will certainly additionally discover this e-book in style ppt, pdf, txt, kindle, zip.
Share your thoughts with other customers. Write a customer review. Top Reviews Most recent Top Reviews. There was a problem filtering reviews right now. Please try again later. Hardcover Verified download. Great continuation of the famous EW book. If you liked the first one, or thought it useful, I suggest getting this and the other two in the series 4 total. Excellent book! This book is the logically titled sequel to EW The style is the same.
Namely where very intricate and specialised maths and engineering is deprecated. What Adamy has done is reduce a problem down to the minimum physical model that conveys the essential information. This lets a reader, who might not be an engineer in the electronic warfare field, understand what the various measures and countermeasures are about.
To this ends, there are fascinating instances scattered throughout the text. One nice example concerns a target plane trying to evade a heat seeking missile.
The plane releases a flare, that has more infrared energy than itself. The missile then follows the hotter signal and the plane escapes. One countermeasure is to have the missile's sensor measure and compare the energies at two wavelengths. With enough electronics, it can then discriminate between the flare and the cooler target plane, and track the latter.
Of course, the engineering required to perform this is nontrivial and probably classified. But the basic physical picture is very simple. Another useful feature of EW is the appendix. This has answers to problems presented in EW Plus to problems in EW As far as I can ascertain, none of the problems require a computational device more complex than a calculator.
Equations and drawings consisely describe the concepts and results from various EW disciplines; radar, communications and satellite are just a few areas that are covered. The second book expands greatly on the area of satellite communications - while not an area normally associated with electronic warfare, satellite communications provide the backbone for many countries social, political and military systems.
If there was one downside to this book it would be the price. Find it used if you can, or borrow from a technical library - then decide if it is worth the cost to you. While Mr. Adamy may be an expert, apparently these books were thrown together a little quickly. Read the text and all will be well. Follow the example problems in the back and you're headed for a train wreck.
To determine the detection range of the receiver, we set the received power equal to the receiver sensitivity and solve for the range. Using the previous values for the radar parameters, the detection range is: Thus the transmitter antenna gain is that of the radar antenna side lobes.
It is common practice to assume that the side lobes of a narrow beam antenna are 0 dBi for older radars and up to 20 dB lower for many modern radar threats. A 0 dBi gain means the side lobes are lower gain than the main beam by an amount equal to the main-beam gain. An ELINT receiver typically has a narrow-band receiver, so its sensitivity is calculated from kTB, noise figure, and required signal-to-noise ratio.
Note that most ELINT systems use superhet receivers which have wide front-end bandwidth; however, the bandwidth of each stage of a superhet receiver is typically narrower than the preceding stage. A general rule of thumb for the effective bandwidth of a superhet receiver is that it is approximately equal to the final prediction bandwidth twice the video bandwidth for AM detection. The noise figure and required signal-tonoise ratio should be approximately the same as those of the radar which we set at 10 dB and 13 dB, respectively , so the sensitivity of a typical ELINT receiver would be: This makes the effective range equation as derived earlier for the RWR case: The distance is half of the time that the received signal is delayed from the transmitted signal multiplied by the speed of light see Figure 3.
A very practical problem is to determine when the signal leaves and when the return is received. However, modulation on the signal, which has significantly lower frequency, provides something to measure and compare over the necessary time intervals of the order of milliseconds.
There are many types of modulations used, but they fall into the categories of pulse, linear FM, binary modulation, and noise or pseudonoise modulation. In its basic form, the pulse has fixed radio frequency and is characterized by its pulse width or pulse duration and pulse repetition interval or pulse repetition frequency.
The pulse provides a clearly measurable time event in the signal. Radar Characteristics 49 Figure 3. In either case, the time from the transmission of a pulse to the receipt of a reflected pulse is easily measured. Pulse radars have the significant advantage that their receivers are turned off during pulse transmission.
This allows the radar to use a single antenna for transmission and reception and protects the receiver from both saturation and damage. The pulse repetition rate determines the maximum range at which the radar can make unambiguous range measurements as shown in Figure 3. If a second pulse is transmitted before the reflection of the first pulse from a target reaches the radar, the delay time measurement would start with the second transmitted pulse and end with receipt of the first pulse.
A Second Course in Electronic Warfare round-trip propagation time is not accurately measured assuming that the pulses are identical. The pulse duration determines the minimum range at which the radar can detect a signal.
The receiver is not turned on until the trailing edge of the pulse leaves the transmitter plus some guard time. The reflected leading edge of the pulse cannot reach the receiver before the trailing edge has been transmitted.
The pulse width also determines the range resolution of the radar—that is the range difference between two targets which will allow the radar to determine that there are two targets. This is shown in Figure 3.
Consider the pulse in the vicinity of two targets. The round-trip distance between the first and second targets must be greater than the pulse width for the receiver and processor to be able to separate the two returns. The fall time is the opposite for the trailing edge.
There may also be ringing, or other unintentional modulation effects including unintentional frequency modulation. While these effects are important to EW Figure 3. Radar Characteristics 51 systems which perform specific emitter identification SEI , they do not impact the basic function of the pulse to the radar.
The effect of compression is shown in Figure 3. Note that compressed radars are used for long-range detection, so they require high-energy pulses. The peak power is made as high as practical, then the pulse energy is increased by the large pulse width.
The detectability of the radar is a function of the transmitted peak power, but its detection range is a function of the total transmitted energy that is returned from the target.
The long pulse is reflected by the target, but the range resolution is improved by the shortened pulse from the compression function in the receiver. There are two important techniques for achieving pulse compression.
One involves the addition and processing of frequency modulation and the second involves the addition and processing of a digital modulation. The frequencymodulated pulse is transmitted and received just like a fixed-frequency pulse, but in the receiver, it passes through a compressive filter. The compressive filter causes a delay that is a function of frequency—the higher the frequency, the less the delay.
The delay versus frequency function is linear, and matched to the modulation placed on the pulse. The difference between the maximum and Figure 3. However, by compressing the received reflection from the target, the radar performance is as though the transmitted power were greater and the pulse duration shorter.
The function of the compressive filter is shown in Figure 3. Note that the pulse from the compressive filter has all of the received energy concentrated in a time period much smaller than the transmitted pulse width. Chirped radars sometimes have very large compression factors. The radar resolution cell is the area within which the radar cannot distinguish multiple targets.
The detection range for any given target will remain the same because the power reflected by the target remains the same. This may be confusing to EW people who are used to having the intercept range proportional to the square Figure 3. Radar Characteristics 53 root of the transmitted peak power. A way to think about this is that the compressed pulse is narrower, requiring more bandwidth.
The increased bandwidth raises the sensitivity threshold by the amount that the pulse peak power is increased by the compression.
Ignoring losses in the compression process, the detection range for any given target will increase by the fourth root of the compression ratio. Because the received power is a function of the fourth power of range range4 or 40 log[range]. In Figure 3. After decoding, the effective pulse width will be the bit duration rather than the pulse duration.
The tapped delay line assembly is shown in Figure 3. The delay line is tapped with the taps separated in time by the bit period. There are as many taps as the bits in the modulating code, and the delay line is as long as the pulse. The signals from all of the taps are summed to form the output. The bottom part of Figure 3. On the first line, only the first bit has entered the delay line, and on the thirteenth, only the last bit is still in the delay line.
To the right of each bit sequence, the plus and minus values coming from the taps are added to form the output value. Note that only the bits that are in the delay line are summed at the output. Each position has a total value of either 0 or —1 except the position Figure 3. As the pulse passes through the delay line, the sum of the taps is a very low number, unless the pulse exactly fills the delay line, in which case the output has a strong peak.
The range resolution is improved by a factor equal to the number of bits in the code. You will be seeing this again when we talk about low probability of intercept radar modulations later.
Note that the correlation starts increasing linearly when the code comes within one bit of alignment. It reaches a sharp peak when the bits are completely in phase with each other. Then, it ramps down linearly until the signal is again 1 bit out of synchronization. This allows the radar to separate the signal reflected from a moving target from ground reflections.
Being able to Radar Characteristics 55 Figure 3. Since the radar return signal has made a round trip, it has twice the frequency shift. Also, since both the radar platform and the target may be moving, a general expression for the Doppler shift in a radar return is: A Second Course in Electronic Warfare where V is the instantaneous rate of change in the distance between the radar and the target, and all other definitions are the same.
However, it does determine the range rate by measuring the Doppler shift. This is because both the transmitter and receiver are on at the same time. The receiver must use a common frequency reference with the transmitter in order to measure the Doppler shifts—which are very small compared with the transmitted frequency. For example, a GHz radar would see about This modulating signal can either have a fixed-frequency portion or it can be a twodirectional frequency ramp.
First, consider the linear ramp portion of the modulating waveform in Figure 3. It can only determine the range rate to the target by comparing the transmitted and received signal frequencies. Radar Characteristics 57 Table 3. A Second Course in Electronic Warfare amount of time it took the signal to reach the target and return at the speed of light.
Thus, by comparing the transmitted and received signals at any instant during which both the transmitted and received signals are in the linear ramp portion of the modulating waveform, the distance can be measured.
This is shown at the right side of the figure. Now, consider that the difference frequency measured is actually caused by two factors—the round-trip propagation time and any positive or negative Doppler shift caused by the rate of change of distance. If the modulating waveform has a constant frequency portion, the Doppler shift can be measured during that part of the signal and the range measurement adjusted accordingly.
If the radar uses a bidirectional waveform, the range-related frequency shifts will have opposite senses during the up and down frequency ramps— while the Doppler shift will be in the same direction. This will allow the Doppler component to be measured and the range accurately calculated. This pulse train also has an extremely high PRF which makes it challenging to radar-warning receiver processing.
Coherent pulses are formed by interrupting a continuously running oscillator, so each received RF pulse will be in phase with an oscillator that is phase locked to the RF waveform of all previous pulses. This allows the advantages of synchronous detection and also allows Doppler shifts to be measured.
Since the receiver is turned off during pulse transmissions, a single antenna can be used without the very difficult isolation problems associated with CW radars.
The radar can measure range just like any other pulse radar, but it will have significant blind ranges and range ambiguities. These can be resolved by use of multiple FM ranging or the use of other operating modes, and the application of multiple pulse repetition rates implemented in sophisticated processing.
It does this by sensing the Doppler shift of detected targets. MTI radars can either be ground-based or airborne. Radar Characteristics 59 Figure 3. The receiver is off during transmissions, relieving the leakage problem in a single antenna. Range to the target can be determined either by pulse timing or frequency modulation. Range rate is determined from the Doppler shift of return signals. It determines the presence of moving targets in cells as shown in Figure 3.
The angular resolution is derived from the scanning of the antenna beam, and the range resolution is from the return of pulses from everything that reflects the pulses. Like any radar, the range resolution is set by the pulse width—which is typically very narrow.
The pulses may be chirped to improve the range resolution i. If pulse compression is used, the compressed pulse is processed just like the noncompressed pulse.
A Second Course in Electronic Warfare Since the transmitted pulse and any reflected return propagate at the speed of light, the reflected pulse arrives at the radar with a delay from the transmitted pulse of: This sampling can continue during the whole time interval until another pulse is transmitted. Sampling thus looks for return energy from range increments which equal 1m per 6. Since these two digital words represent points on Figure 3. This process is continued for each pulse over the whole range of interest.
This sampling pattern is repeated for each pulse. Then the values for each sample from pulse 1 are subtracted from the equivalent sample values of pulse 2 as shown in Figure 3. The values from pulse 2 are subtracted from those of pulse 3, and so on through pulse m —1 and pulse m. The m pulses illuminate each angular resolution cell during each sweep of the antenna. More complex data subtraction schemes are sometimes implemented to provide better clutter cancellation. The FFT determines the presence of Doppler shifted signals in each range and angle resolution cell.
The Doppler shift is caused by the rate of change of distance between the radar and the target.
Thus, the MTI can only detect targets which have some component of motion directly toward or away from the radar. For example, an MTI radar with pulse repetition frequency of 6, which samples times-per-pulse repetition interval and digitizes the I and Q values at 12 bits each—generates 30 Mbps of raw data. Then each sample is subtracted from the equivalent sample in the previous pulse, and an FFT is calculated from the resulting differences for all pulses illuminating from the cell.
Since the MTI only reports the presence and magnitude of motion in resolution cells, each target report need only contain the cell location and the magnitude and sense of the movement; 80 bits is typically plenty of data for each target. If there are moving targets detected per second in the covered area, the total target report data rate will be 8 Kbps. Even adding a bit status word 30 times per second yields a total output data rate of less than 10 Kbps. This data rate is easily carried over an audio bandwidth link.
The aircraft in the figure has been deliberately drawn to show that it is not the airspeed of the aircraft, but the ground speed that determines the induced Doppler shift. The aircraft-induced Doppler shift will be: The Doppler shift observed by the MTI radar in each resolution cell must be corrected for this aircraft Doppler before the presence of a moving target is reported. This can be accomplished by shifting the zero frequency point of the Doppler up or down by the above-stated amount.
It can also be implemented by varying the local oscillator in the receiver or varying the transmitted frequency by the same amount. This allows extremely high resolution at long range, with relatively small physical antennas. SARs are used to create maps of large areas, along with vehicles and other objects present in the area. There are combined MTI and SAR radars in which moving objects are identified and, as soon as the object stops moving, a SAR image of that object is made to allow it to be identified.
The SAR creates range and azimuth resolution cells as shown in Figure 3. The required resolution is a function of the smallest objects that must be located or identified. If it uses range compression chirp or phase coding the compressed pulse width determines the range resolution. I and Q samples are collected for each cell since the SAR process requires that the phase be preserved.
The beamwidth is a function of the size of the antenna. For a parabolic dish antenna, the surface of the dish actually a parabolic section reflects all of the energy it receives to the feed which is at the focus of the parabola. The larger the dish, the more narrow the antenna beam. For a phased array antenna, delay lines are used to create a coherent addition of signals received by many array elements antennas when those signals arrive from a single direction—thereby forming a narrow antenna beam.
The longer the array, the more narrow the beam. Radar Characteristics 65 Figure 3. The SAR transmits coherent pulses and creates the effect of a phased array by collecting the returns from each pulse as the platform moves forward. Assuming that the area being mapped by the SAR is much farther from the aircraft than the flight distance over which data is collected, the returns from objects on the antenna boresight can be added in phase—while objects away from the boresight will add up out of phase.
Thus, summing the data in corresponding range resolution cells over several pulses has the same beam-narrowing effect as a phased array. After each integration period, data from a new pulse is added and the data from the oldest pulse is discarded. It should be noted that the azimuth resolution i.
The synthetic array azimuth resolution equation is: The data from the same range bin is summed for several pulses to form the azimuth-resolution distance. For a phased array of real antennas, the azimuth resolution distance is: This requires that the paths from the target to the radar be very close to parallel for all of the integrated pulses.
This limits the length of the synthetic array and thus the azimuth resolution. Much longer synthetic arrays can be formed using focused array techniques. The phase error is: In a focused array, this phase error is corrected before the azimuth data is summed for each range cell. This can require an immense amount of processing, but the processing load can be reduced by use of Doppler filters formed by an FFT. Thus, an LPI radar is one that satisfies this very broad criteria.
Whether or not a radar is LPI depends on what the radar is trying to do, what kind of receiver is trying to detect it, and the applicable engagement geometry. For purposes of this discussion, we will call the intercepting receiver system an ESM receiver. Table 3. One is to make the signal so weak that the ESM signal cannot receive it. This is difficult for the radar because the radar must receive enough energy after the round trip to the target 40 log range in the radar range equation to detect the target.
The receiver encounters only a one-way path loss 20 log range. A second way is to narrow the radar beam thus increasing the antenna gain or to suppress antenna side lobes. This makes it more difficult for a receiver not located at the target to intercept the signal, but does not impact a receiver located on the target. A third way to reduce the interceptability of a radar relative to its performance is to give the radar a processing gain not available to the ESM receiver.
Frequency agile radar Each pulse or groups of pulses are transmitted at different frequencies. Random signal radar A radar which uses a waveform that is truly random for example, noise. Binary phase coded CW radar A radar which has a pseudorandom phase coded modulation on a transmitted CW signal. The ability of a receiver to detect the radar signal depends on its noise figure and its bandwidth. In the following analysis, we generally assume that the noise figure of the radar receiver and the intercepting receiver are the same, and that the intercept receiver bandwidth can be optimized to its function.
Here we use ESM receiver as a general term to cover aircraft radar-warning receivers, shipboard ESM receivers, and ground-based warning and targeting receivers. The typical ESM processor has a threat identification TID table with a set of parameters for each expected threat signal type in each of its operating modes. The processor also tries to discriminate against friendly radars and Radar Characteristics 69 Figure 3.
Before the processor can identify a signal, it must first isolate that single signal from the many signals present. A Second Course in Electronic Warfare involves measurement of frequency, modulation parameters, and direction of arrival along with the sorting of data by parametric values. Once an individual signal is isolated, the processor compares its parameters against the TID table to find a match to a threat or nonthreat signal. Then the ESM receiver reports the presence, operating mode, and location of the identified threat type to cockpit displays.
If a radar uses parameters similar to a friendly type of radar, the ESM receiver is likely to identify it as such, and thus not report the presence of the threat even though it was clearly received. Another approach is to introduce parametric agility. It is far easier for an RWR to identify threat signals with fixed parameters.
Agile signals, particularly if that agility causes random parametric changes, require additional analysis time even if the parameters are known. The shortcomings of the LPID approach are that ESM processing is becoming more sophisticated, and that a radar needs certain information from its modulation to perform its mission.
The increasing processor power in modern ESM receivers allows them to more effectively deal with agile parameters and to perform functional and pattern analysis against signals that do not fit the TID choices. More sophisticated processing and precision emitter location techniques will also allow future ESM receivers to perform location correlation and motion analysis to separate friendly signals from imitation friendly signals on hostile platforms.
The range at which a receiver can detect a radar signal is given by the formula: These equations apply in both Figures 3. By selecting some values to plug into these formulas and assigning bandwidth and processing gain values which drive the sensitivities we will be able to investigate LPI radar performance in realistic cases.
It can detect a target at the same range that a receiver on the can detect the radar. It also increases as the ratio between the sensitivity level of the receiver and that of the radar decreases.
To avoid confusion on the sensitivity issue, remember that the sensitivity level is the lowest signal that a receiver can accept and still do its job. Thus as the sensitivity improves, the sensitivity level decreases.
The sensitivity numbers in both of the range equations earlier are large negative numbers sensitivity level —therefore, as each sensitivity improves, the corresponding detection range increases. The second is that several factors impact the sensitivity.
Thus, the radar range can be expressed as a function of average power and time on target. Another constraint on the radar is that its ability to resolve the range to the target is determined by the pulse width.
Range resolution is commonly defined as: Modulations on the signal allow better range resolution for any given signal duration. This modulation can be frequency modulation chirp or phase reversals binary phase shift keying , as explained in Section 3. Since the detecting receiver depends on the peak power of the radar signal to detect the radar, the radar can gain an advantage in detection range by using a lower power, longer duration signal along with some modulation which will allow it to achieve adequate range resolution see Figure 3.
A Second Course in Electronic Warfare noise ratio in decibels. In radar analysis problems, the required signal-to-noise ratio is usually set to 13 dB and kTB is usually taken as: However, there is another factor that is useful to consider in the context of LPI signals; that is processing gain. Processing gain has the effect of narrowing the effective bandwidth of the receiver by taking advantage of some aspect of the signal modulation.
A radar achieves bandwidth advantage over an intercept receiver because it can match its receiver and its processing to its own signal—while the intercept receiver must accept a wide range of signals and must typically make detailed parametric measurements to identify the type of signal it is receiving.
For example, a pulse radar need only determine the round-trip pulse travel time—and can integrate several pulses to determine that time.
The intercept receiver, on the other hand, must determine the pulse width. This requires a pulse with definable leading and trailing edges—which, in turn, requires a bandwidth of about 2. When there is randomness in the signal modulation, this becomes even more pronounced. The most extreme example of this effect is the use of true noise to modulate a radar signal. The RSR uses various techniques to correlate the return signal with a delayed sample of the transmitted signal as shown in Figure 3.
The amount of delay required to peak the correlation determines the range to the target. Since the transmitted signal is completely random and the intercepting receiver has no way to correlate to the transmitted signal, it can only determine that the radar is present through energy detection techniques rather than detecting modulation characteristics.
This is a much less efficient process than that available to the radar receiver. Radar Characteristics 75 Figure 3. With bandwidth greater than the inverse of the pulse width, the pulse parameters can be measured. Controlled Delay Line Figure 3. It determines the range to the target by correlating return signals with a delayed sample of the transmitted signal. There are also several such systems under current development, and random signal radars are described in technical literature.
Again, engagement parameters must be specified e. It is easy to fall into the trap of considering only the radio frequency part of the electromagnetic spectrum, but there is a significant amount of EW effort in the IR, visible light, and ultraviolet parts of the electromagnetic spectrum. In this chapter, we will deal with the general nature of these parts of the spectrum, the systems that operate in this range, and the nature of the countermeasures against those systems.
Although we typically use frequency to define the RF portions of the spectrum, it is more common to use wavelength at the higher frequencies. Note that wavelength and frequency are related by the speed of light in the formula: Frequencies below GHz i.
This includes the rear aspect view of a jet engine looking up into the engine. Objects in the normal range of temperatures i. The IR emissivity of real-world materials is defined in terms of a percentage of blackbody radiation at a given temperature. Typical examples of emissivity are: The blackbody radiation versus wavelength is a function of the temperature of the emitter.
As shown in Figure 4. The total energy varies as the fourth power of temperature. Also, the peaks of the curves move to lower wavelengths as the temperature increases.
Figure 4. An interesting point is that the surface of the Sun is about 5,K, which causes its radiation to peak in the visible light spectrum convenient for those of us with eyes that operate in that range. Note that there are absorption lines from various atmospheric gasses, but there are major transmission widows in the near-, mid- and far-infrared ranges.
In IR transmission, the spreading loss versus range is calculated by projecting the receiving aperture from its range onto a unit sphere around the transmitter as shown in Figure 4. The spreading loss is then the ratio of the amount of the surface of the unit sphere covered by the image of the receiving aperture to the whole surface area of the sphere.
This is actually the same way we calculate spreading loss for RF signals. However by assuming isotropic antennas we get range and frequency terms in the RF equation. Examples of these systems and threats are: There are, of course, countermeasures to all of these systems.
Sensors can be blinded temporarily or permanently and IR-guided missiles can be defeated by flares or IR jammers. Some of these EO devices operate in the infrared spectral range. EO systems and applications and their countermeasures discussed in this chapter include: They are primarily air-to-air missiles and surface-to-air missiles, and include small, shoulder-fired weapons.
An IR missile detects the IR signature of an aircraft against a cold sky and homes on energy in one of the three IR bands. Early IR missiles required high-temperature targets, so they needed to see the hot internal parts of jet engines to achieve good performance. Therefore, they were usually restricted to attacks from the rear aspect of jet aircraft.
More recent missiles can operate effectively against cooler targets the plume, the tailpipe, heated leading edges of wings, or the IR image of the aircraft itself. This allows them to attack all types of aircraft from all aspect angles. This type of missile suffered from considerable solar interference and severely restricted air-to-air tactics. More modern seekers use sensors of lead selinium PbSe , mercury cadmium teluride HgCdTe , and similar materials that operate in the mid- and far-IR bands.
While these seekers allow all aspect attack, they require that the sensors be cooled to about 77K with expanding nitrogen.
The nose of the missile is an IR dome. This is a spherical protective covering for the seeker optics, made from a material that has good transmission of IR energy. A seeker senses the angular location of the IR source and hands-off error signals to the guidance control group, which steers the missile toward the target by control commands to the rollerons.
There are two mirrors a primary and a secondary reflector that are symmetrical around the optical axis. They focus energy through a reticle onto an IR sensing cell.
Not shown in the diagram is the filter that limits the spectrum of signals passed through the reticle—and the sensor cooling if required. A simple, spinning reticle pattern is shown in Figure 4. The top half of this reticle is divided between very low and very high transmission segments.
An IR target is shown in one of the high transmission segments. This reduces the dynamic range required of the IR sensor. As the reticle rotates, the IR energy from the target onto the IR sensor will vary in the partial square wave pattern shown in Figure 4. The square wave portion of the waveform starts as the upper half of the reticle starts to pass the target.
Since the sensor knows the angular position of the reticle, it can sense the direction to the target from the timing of the square wave portion Figure 4. This allows the guidance group to make corrections to steer the missile toward the target. When the target is near the center, the high-transmission segment does not admit the whole Figure 4. As it moves farther from the center, more of the target is passed. Once the whole target is passed, the peak energy level to the IR sensor does not increase more.
This means that the sensor only provides proportional correction inputs when the target is quite near the center of the reticle. It also means that the seeker has no way to discriminate against a high-energy false target near the outer edge of the reticle. To generate steering information, the energy entering the seeker is nutated to move it around the optical axis.
If the target is on the optical axis, it will cause a constant amplitude square wave of energy to reach the sensor. However, if the target is off center, its image will move in the offset circle shown in the figure. This causes the irregular square wave form shown at the bottom of the figure.
The control group can then determine that the missile must steer in the direction away from the narrow pulses. Figures 4. Since there are different numbers of segments in each of several rings, the number of pulses seen by the sensor changes as a function of the angular distance from the optical axis to the target.
This supports proportional steering. To avoid extremely high g forces on a missile as it reaches its target, missiles use proportional navigation as shown in Figure 4. If the aircraft and the Figure 4. If either is accelerating e.
It is mounted on a manned aircraft or unmanned aerial vehicle UAV which flies a fairly low-level path over an area of interest. The IRLS makes a two-dimensional image by scanning an IR detector over an angular increment across the ground track of the vehicle while the second dimension is provided by the movement of the platform along its ground track.
This approach to mine detection is practical because buried mines will gain or lose heat at a different rate than the surrounding soil or sand. Thus, the mines will be at a different temperature during times of temperature change, for example, right after sunset.
However, the resolution of the IR sensor must be adequate to differentiate the temperature of the mine from that of the soil, and it must have adequate angular resolution to differentiate mines from other buried objects, for example, rocks. This high resolution will be required because the soil can have a relatively wide temperature range and post mission analysis will probably be required to find the relatively narrow temperature difference between the mine and the soil anywhere in this range.
The altitude required is: It is traveling at knots, 1, feet above the ground. Ground resolution distance versus sin sensor aperture angle At a 0. The vehicle can fly at any speed, but the sweep rate must be fast enough to make one cross-track sweep every 3 inches along the flight path. At our chosen speed, knots, the vehicle travels over the ground at feet per second: Infrared and Electro-Optical Considerations in Electronic Warfare 89 One sweep per 3 inches requires 4 sweeps per foot or sweeps per second at knots.
The sampling of the IR sensor must also be performed for every 3 inches of movement of the sensor over the ground in the crosstrack sweep. The width of the cross-track ground coverage the swath width is: At 8 bits per sample, this produces a data rate of It is stated in radians per second.
To understand this choice of units, consider observing the aircraft from a fixed point below it on the ground. Remember that a radian is the angle observed from the center of a circle for one radius along the circumference of a circle.
Thus the subtended angle-per-unit time, converted to radians, would be equal to the velocity divided by the radius i. As you can see from this typical example, the detection of buried mines requires an airborne platform that flies low and slow, and the collection and analysis of a great deal of data. Detection of larger objects for example, tanks in underground bunkers would require less angular resolution. This will allow operation at higher Figure 4. However, a high-data rate should always be expected for IRLS applications because of the fine temperature resolution and large temperature range required.
If the vehicle is unmanned, or for any other reason the data is linked to a ground station, a wide data link will be required.
This can be in the visible light wavelength range television or it can be in a nonvisible wavelength range. Our concern here is imagery at infrared wavelengths. For all electronically implemented imagery, the displayed picture is divided into pixels.
A pixel is a spot on the screen; there must be enough pixels to create the picture to the required quality. The system captures and stores the brightness or the brightness and color to be displayed in each pixel—then displays the appropriate values at each pixel location on the screen.
The screen display can be generated with a raster scan as shown in Figure 4. If an imagery system is mapping the ground, the relationship between the pixels and the resolvable distance on the ground would be as shown in Figure 4. If the system is looking level or up, the same relationship applies, but the resolvable distance is a function of the range to the individual objects being observed.
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