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Ew 101 david adamy pdf

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British Library Cataloguing in Publication Data Adamy, David EW a second . Preface EW has been a popular column in the Journal of Electronic. EW a first course in electronic warfare. Responsibility: David Adamy. Imprint : Boston: Artech House, c Physical description: xix, p.: ill. ; 24 cm. EW book. Read 7 reviews from the world's largest community for readers. EW EW has been a popular column in the Journal of Electronic Defense. .

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Ew david adamy pdf. Ew A First Course in Electronic Warfare Artech House Radar Library David Adamy educar desde el locutorio pdf on. Amazon. com. Editorial Reviews. About the Author. David L. Adamy is president of Adamy Engineering and previously worked as senior systems engineer/program manager. EW , a first course in electronic warfare by David L Adamy. EW , a first course in electronic warfare. by David L Adamy. Print book. English. Boston.

Since these antennas are used in combination, the effective antenna gain factor at 10 GHz for the RWR system will be 0 dBi for any direction of arrival. You will note that these bands divide at multiples of three. I just really HATE the way he deals with units. 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. Get A Copy. There are, of course, countermeasures to all of these systems.

Since a jet aircraft engine is very hot, it provides a lucrative target for heat-seeking missiles. Early missiles attacked from behind the aircraft and homed on this high-heat target. Note that small, handheld weapons firing infrared missiles can be lethal to low-flying aircraft. IR missiles are used in airto-air, ground-to-air, and air-to-ground attacks. Modern missile sensors can detect and home on the IR energy from targets at considerably lower temperature than that of a jet engine.

The communication that transfers that information is extremely lethal. Consider a simple example of lethal communication as shown in Figure 2. Artillery has killed more soldiers than any other type of weapon, and cannot typically engage targets without communication. The guns apply nonline-of-sight fire in response to calculated elevation, windage, and powder charge commands from a fire-control center.

The fire-control center modifies its commands to the guns in response to communicated inputs from a forward observer who can see the target and the strikes of the rounds fired. Both communication paths are extremely lethal. Threats 13 Figure 2. If there are multiple targets within the resolution cell, the radar will assume that only one target is present—at the weighted centroid of the individual target locations.

This figure has three different columns, showing the three most common ways that frequency ranges are described. The left-hand column shows the common scientific notation. You will note that these bands divide at multiples of three. This is because each covers an order of magnitude of wavelength. The relationship of frequency to wavelength is given by the formula: The right-hand column shows the electronic warfare bands. The frequencies of threat radars are normally described in terms of these band designations.

For example, D-band covers 1 to 2 GHz. The middle column shows the official radar bands. Note that components antennas, amplifiers, receivers, oscillators are designated in catalogs in terms of these bands.

EW 102: A Second Course in Electronic Warfare

It is also common to describe communications in terms of these bands. For example, satellite television broadcasting is done in C- or Ku-bands. One very important point made by this figure is that it is easy to be confused by commonly used band designations. The best practice is to specify frequencies in megahertz or gigahertz when there is any confusion about band designations. Table 2. A generality about signal frequency is that transmissions become more dependent on line of sight as the frequency increases.

HF and lower frequency signals can propagate clear around the world. VHF and UHF signals can propagate beyond line of sight, but are subject to severe nonline-of-sight attenuation. Microwave and millimeter wave signals are usually considered absolutely dependent on line of sight. A second generality about frequency is that the amount of information carried by a signal is generally proportional to the transmitting frequency.

This is because the amount of information carried depends on the signal bandwidth, and system complexity antennas, amplifiers, receiver performance is a function Threats 15 Table 2. Ground waves circle the Earth. Commercial AM radio. Severe losses beyond line of sight. Line of sight required. Millimeter wave MMW Radars, data links. Requires line of sight.

101 pdf ew david adamy

High absorption in rain and fog. Thus, signals carrying lots of information e. These are active, semiactive, command, and passive. The type of guidance selected for a threat system depends on the nature of the platforms involved and of the typical engagement dynamics. Antiship missiles are an important application of this type of guidance. Once a missile is fired, it travels to the general area of the target ship, turns on its radar, acquires the ship, and guides itself to impact with the ship.

Active guidance has the advantages that: A Second Course in Electronic Warfare target diminishes, and it is very hard to jam at close range because the radar power on the target is an inverse function of range. The transmitter is located remotely, for example, on the launching platform. The weapon then homes on the reflected signals from the target. When the guidance medium is radar, this is a bistatic radar implementation—very common in airto-air missiles.

Another important case of semiactive guidance is the laser-guided weapon that homes on the scintillation of a laser designator from a ground target. This type of guidance requires that the platform carrying the illuminator be present and within line of sight of the target throughout the engagement. Based on the tracking information developed by the sensor, a weapon is guided to a point at which it will intercept the target see Figure 2.

The weapon has no information about the target location—it just goes where it is commanded. The classic example is a typical surface-to-air guided-missile system.

One or more missiles are targeted and guided by a ground-based radar. The weapon homes in on signals reflected by the target. Threats 17 Figure 2. Examples are: The weapon system does not radiate any type of targeting signal, so there is only one signal path—from the target to the weapon. Thus, the launching platform including individuals using shoulder-fired launchers can leave the area or hide as soon as the weapon is launched.

In the practice of electronic warfare, it is useful to consider the way that the signals transmitted by a radar reflect the mission of the radar. We will consider acquisition radars and tracking radars in ground and airborne platforms, fusing radars, moving-target indicator radars, and synthetic aperture radars. We will consider the radar scan and the modulation on the transmitted signal, relate them to their appearance to an EW receiver, and then relate them to types of threat radars.

Chapter 3 includes more detailed discussions of the signal characteristics themselves. This is caused by the shape of the radar antenna beam and its 18 EW The antenna beam is shown as rotating in that dimension relative to an EW receiver location. Note that the main beam and side lobes all rotate past the EW receiver. The shape of this curve can be analyzed to determine the beamwidth and scanning pattern of the radar. The more accurately the radar must know the location of the target, the narrower the beam.

The radar can determine the actual angular location of a target within its resolution cell by small angular adjustments of the antenna pointing to peak the received signal strength.

If the rotation rate of the antenna is known, the beamwidth of the antenna can be derived from this figure. For example, if it is known that the antenna makes one full revolution in 5 seconds and the 3-dB beamwidth duration is 50 ms, the antenna beamwidth is 3. Threats 19 Figure 2. If the radar is trying to find a target, the beam will be swept over the angular area that might contain a target.

If it is tracking a target it has already found, the beam on an older radar will be moved through a much smaller angular area around the target to enable the tracking function. In the acquisition case, an EW receiver will see the antenna movement as received signal strength versus time.

A monopulse radar beam is received as a constant level signal like that described next for a scan on receive-only radar. This causes an EW receiver to see evenly spaced main beams as shown in Figure 2. The time between main beams is equal to the period of rotation.

A ground-based TWS radar will typically cover an angular segment of many degrees. It tracks multiple targets in its angular range while continuing to look for more.

For example, the SA-2 radar has two fan beams, one measuring the azimuth of every target in its field and the other measuring the elevation of every target in its field.

The receiver sees a sector scanned beam as a power-time 20 EW A typical antenna for an airborne intercept radar in acquisition mode has a raster scan. This scan comprises a series of horizontal traces across a twodimensional angular area—much like the way the beam in a television picture tube covers the face of the screen. As shown in Figure 2. In this example, the radar beam passes through the EW receiver location on the second scan line. A conical scanning radar uses a conical movement of its antenna beam to develop correction data to keep the target centered in its scan.

The high point of the sine wave occurs when the antenna beam passes closest to the target—causing the antenna to rotate in that direction to center the target in the scan. This causes two time intervals between the main beams. A is from the receiver to the right edge of the scan segment and back. B is from the receiver to the left edge and back. Threats 21 Figure 2. A scan-on-receive-only radar uses two antenna beams typically generated by the same antenna.

One is moved in a scanning pattern for example, a conical scan to receive return pulses from the target and calculate beam steering corrections. The second beam does not scan, but is pointed at the target using the correction information from the scanning receive beam. In this case, the EW receiver does not see any antenna scan, but rather the constant illumination of the transmit antenna. A Second Course in Electronic Warfare Modulation Characteristics of Threat Radars The modulation characteristics of a radar signal are dictated by the function of the radar.

In this section we will consider pulse, pulse Doppler, and continuous wave radars in acquisition, guidance, and fusing applications. The pulse width PW is also called the pulse duration PD. The pulse interval is the time from the leading edge of one pulse to the leading edge of the following pulse.

The pulse interval in a signal is usually identified in terms of the pulse repetition frequency PRF or pulse repetition interval PRI , but is also sometimes called the pulse repetition time PRT. The pulse width and repetition rate are the same whether measured at the transmitter output, at the target, or at the receiver as long as the radar and the target are not moving, but the pulse amplitude changes a great deal.

The pulse amplitude in a radiated signal is the signal strength during the pulse. As the pulse leaves the transmitting antenna, this is the effective radiated power ERP. When the pulse reaches a target, the pulse amplitude is the instantaneous power applied to the target.

When the reflected signal reaches the radar receiver, it is the received signal strength. The duty cycle of a radar is the ratio of the pulse width to the pulse interval.

In regular pulse radars, this duty cycle can be from 0. This low-duty cycle means that the average power output by the radar is significantly less than its peak power. An important trend in radar development is the increasing capability of solid-state amplifiers to replace traveling wave tubes.

Threats 23 The maximum unambiguous range of a pulse radar is determined by the pulse interval. Otherwise, it would not be clear whether the received pulse is from the first pulse reflected by a distant target or from the second pulse reflected by a much closer target.

RMAX 0. RMIN is the minimum range in meters; PW is the pulse width in seconds; c is the speed of light in meters per second. For effective operation, a radar must place adequate energy onto a target. Since the signal strength of a transmitted signal decreases as a function of the square of the distance from the transmitter, a long range radar will typically have a long pulse width to increase the energy applied to the target as shown in Figure 2.

Because of these considerations, a short-range radar will tend to have short pulses and short pulse intervals, while long-range radars have long pulse width and interval.

The range resolution of a radar is determined by its pulse width. The longer the pulse width, the more crude the range resolution. Thus, long-range Figure 2. Threats 25 radars with long pulse widths will have relatively poor range resolution. The digital modulation is binary phase shift keyed BPSK and allows the range resolution to be improved by a factor proportional to the number of digital bits during each pulse.

Both of these pulse compression techniques are explained in Chapter 3. Note that a few modern tracking radars also use longer pulses with pulse compression.

PD radars use coherent signals. Coherent signal pulses are generated by transmitting intervals of a continuous reference signal. With this large duty cycle, the returning echo from a target can be lost because of the transmission of subsequent pulses.

This occurs if the distance to the target makes the round-trip time equal to a multiple of the interpulse interval. The Doppler principle causes the frequency received by the radar to change by an amount proportional to the rate of change of range. They determine the range rate of targets from Doppler shift, and sometimes have frequency modulation which can be processed to determine range. A Second Course in Electronic Warfare Threat Radar Applications Threat radars are often classified as acquisition radars, tracking radars, and fusing radars.

Acquisition radars search large areas to acquire targets. When targets are acquired, they are handed off to guidance radars. A guidance radar forms a track file on a target i. The purpose of a fusing radar is to set off a warhead at the optimum distance from a target.

For airborne targets, the radar determines when the target is located within the burst pattern of the warhead—so the target will receive the maximum number of projectiles when the warhead explodes. Communication signals include both voice communication and digital data transmission. Tracking radar Shorter range, lethal range of associated weapons. Pulse, pulse Doppler, or CW.

Short pulses, high PRF. Modern tracking radars can also have pulse compression. Fusing radar Very short range, a few times the burst radius of warhead. CW or very high PRF pulses. However, most communication stations have transceivers which both transmit and receive allowing one-way propagation in either direction.

This is important to communication intercept systems because only the transmitter can be located by an emitter location capability. In general, communication signals have continuous modulations and tend to be very high duty cycle compared to radar signals.

However, with the increased use of unmanned aerial vehicles UAVs and communication satellites, microwave communication signals have become common. The wider the bandwidth of the signal, the more information it can carry per unit time. The higher the frequency of the signal, the more bandwidth it can have, but the more dependent the transmission path is upon line of sight. In the following sections we will focus on two important types of communication signals as illustrative of the characteristics of communication threats.

These are tactical communication signals and digital link signals. Whip antennas are the most common in ground-based communication stations, and folded dipole antennas are most common for airborne platforms. The use of nondirectional antennas allows communication to take place without knowledge of the location of the other end of the communication link.

These antennas provide more gain and isolation from undesired signals. Tactical communication transmitters typically have from one to several watts effective radiated power, and the links operate over a few kilometers of range. Note that HF links can be much longer range requiring more effective radiated power because of the nonline-of-sight nature of HF propagation. Communication to and from aircraft in VHF and UHF also has extended range because of the greater line-of-sight distances.

The information carried on tactical communication links can be voice or data, and voice can be carried in either digital or analog format. The information can be encrypted, and the signals can 28 EW Directional antennas are used when station locations are known. This involves several transceivers operating at the same frequency, with only one station transmitting at a time. Threats 29 command and several subordinate stations. Most communication takes place between command and subordinate stations, with the command station broadcasting at a significantly higher duty cycle than the subordinates.

A single net e. The nets of subordinate commands are interlocked with those of the higher command as shown in the figure. The two colocated stations one in each net indicate a lower level command location. The lower level commander uses a subordinate station in net 1 and the command station in net 2 which will have a different frequency.

101 pdf ew david adamy

One of the important reasons for the use of precision emitter location techniques is to identify these colocated stations. Many tactical communication intercept systems include a display of frequency versus angle of arrival as shown in Figure 2. Subsequent transmissions by the same transmitter will show repeated hits at the same frequency and angle.

One exception is the frequency-hopping signal which will have a series of frequencies at the same angle of arrival. When this type of display is allowed to integrate for even a few seconds, it will be seen that virtually every frequency in a communication band is used.

This is an important factor in the operation of tactical communication search systems. We will consider the UAV to control station links as typical examples. The uplink signals will typically be encrypted and have a high level of spectrum spreading. This protects the control station from detection and location by hostile emitter location systems, and makes it much harder for an enemy to interfere with control of the UAV or its payload.

It typically has a much wider bandwidth that the uplink signal because it is carrying a great deal of information. The most common UAV payload is imagery television or forward-looking infrared which often requires millions of bits per second. These signals are typically encrypted and have some level of spread spectrum protection. However, the wide data bandwidth limits the amount of frequency spreading that can be applied. Uplink antennas typically have narrow beamwidth, providing gain and making them harder for hostile emitter location systems to intercept.

Downlink antennas are limited in size because of the size of the UAV airframe and aerodynamic considerations. Therefore, the downlink antennas typically have less gain and more beamwidth than the uplink antennas.

They typically operate at microwave frequencies, carrying voice and data over great distances. Most satellites provide simultaneous access to many authorized users, so their signals are many megahertz wide. Some satellites support both commercial and military users.

Typical commercial applications include television broadcast and telephone communication. Military satellites provide basically the same services, Threats 31 but the signal formats can be significantly more varied. Signals can be encrypted if appropriate, and spread spectrum can be employed for antijam protection. We will be looking at various types of radars to determine what they do, how they do it, and what their signals look like when viewed by an intercepting receiver.

We will cover radar processing only to the level necessary to be able to discuss subjects like resolution, detection range, detectability, and vulnerability to jamming.

Appendix C includes several recommended reference books that provide much more detail on radar theory and systems. A radar determines the distance to some object which we will call the target by measuring the time it takes a propagated signal to travel to and from the target at the speed of light see Figure 3. The range to the target is the speed of light multiplied by half of the time from the transmission of a signal to the reception of the same signal reflected from the target.

A radar determines the angular position of the target by means of a directional antenna with a gain pattern which varies as a function of angle from the antenna boresight.

If the boresight of the radar antenna sweeps slowly through the angular location of the target, the 33 34 EW Like most broad, sweeping statements, these are not always literally true. For example, some radars get additional angular information by processing return signals in the time domain. Still, although the process may be quite complex, there is always some underlying mechanism that traces back to those basic measurement functions. Another characteristic of radars is that they look for consistency in the history of measured positions of objects they track.

If a tracked object is moving relative to the radar, the radar processing will expect the tracked object to continue along the same path it has been following in the last few measurements.

Although there are many radar applications, the primary applications of interest to EW are target acquisition, target tracking, altitude measurement, mapping, moving target detection, and fusing. There are also characteristics or attributes of radars by which we make further distinctions. Several of interest to EW considerations are: Figure 3. Pulse radars transmit short, high-power RF signals with a low duty cycle.

Since pulses are being transmitted only a small percentage of the time, the same antenna can be used for both transmission and reception. The modulator generates pulses that cause the transmitter to output a high-power RF pulse. The duplexer passes the transmitted pulse to the antenna and the received reflected pulse to the receiver.

Note that transmitted pulse is significantly higher in power than the received pulse, so there must be some provision to protect the receiver from reflected energy during the time the pulse is being transmitted. The receiver detects received pulses and passes them to the processor. The processor uses the amplitude of the received signal to perform tracking functions to keep the antenna pointed at the target if appropriate.

It also performs range tracking to keep the radar focused on a single target. Information about the target location is output to displays. Control inputs include operating modes and target selection. It differs from the pulse radar in that its signal is present all of the time.

This means that it must have two antennas, since it must receive a much weaker return signal while it is transmitting. The two antennas must provide sufficient isolation to keep the transmitted signal from saturating the receiver. The receiver compares the frequency of the received signal with the transmitted frequency to determine the Doppler shift caused by the relative velocity of the target.

Modulation can be placed on the Figure 3. The processor performs target tracking and antenna control functions, and interfaces with the controls and displays as for the pulsed radar. It differs from the pulsed radar in that the transmitted pulses are coherent. This means that the transmitted pulses are a continuation of the same signal and thus have phase consistency.

Thus, the receiver can detect the return pulses coherently. As previously discussed for communications signals, coherent detection generally gives significant sensitivity advantages. It also allows the measurement of Doppler shifts so that the relative velocity of the target can be measured. A common form of this equation is shown here. However, in electronic warfare, it is usually more useful to consider the received power in the radar receiver.

It can be called the radar received power equation, and is often called incorrectly the radar range equation. It has the implied assumption that the transmitter and receiver are colocated and have the same antenna gain. The most common form of the equation is shown here: A slightly different form of the equation uses the gain of the antenna in the transmit mode and the area of the antenna in the receiving mode which makes the equation independent of frequency.

In this form, the equation is written as: A form of the radar power equation frequently used in electronic warfare applications is derived by converting the first of the earlier equations into decibel form with the received power expressed in dBm, range in kilometers, and frequency in megahertz. Then the constants and conversion factors are combined: The radar power equation then becomes: You will note that the constant in the decibel form of the radar power equation is reduced to This is appropriate if the calculation need not be Radar Characteristics 39 more precise than 1 dB.

Otherwise, the It is also common usage to shorten log10 X to log X in decibel equations.

Please note that this type of decibel equation is only correct if exactly the proper units are used. If any other units are used for example, nautical miles versus kilometers for range , the constant will need to be modified. The rest of the power is absorbed. The formula for RCS is: It is typically very irregular with aspect angle and varies with the frequency of the radar.

The radar cross section of a target can either be measured in an RCS chamber or determined from a computer simulation. The RCS chamber is a specially instrumented anechoic chamber which measures the radar returns from actual targets, parts of targets, or scale models of targets.

Computer RCS models are developed by representing the target by a number of reflecting surfaces cylinders, plates, and so forth and calculating the overall RCS from the phase adjusted combination of reflections from all of these surfaces. As shown in Figure 3. The equation for this gain is the following: The units of the charts are dBsm [i. Note that these RCS diagrams will vary widely with the types of aircraft and ships involved. That is the sensitivity of the radar receiver.

The sensitivity is defined as the minimum signal level which the receiver can receive and still perform its specified functions see Figure 3. To determine the detection range, set the received power in any form of the radar range equation equal to the sensitivity and solve for the range. If we use the decibel form of the range equation, this is: It is larger from the sides because of the larger cross section of the fuselage and the angles between the wings and the fuselage.

Both of these effects are reduced in modern aircraft designed to Figure 3. Its detectability range is the range at which its signal can be received and detected by an EW or reconnaissance receiver. Both of these range numbers are very situation dependent. Is the receiver at the target or away from the target? What are the parameters of the receiver system detecting the radar? The radar either tracks the target to keep it at the peak of its main beam or scans the beam through the target location.

This means that the radar detection range equation, as described in Section 3. Setting the received power equal to the sensitivity, we can solve for the range at which this occurs: Then we find the range d from: The sensitivity is defined as the lowest signal a receiver can receive and still do its job.

High sensitivity means that the receiver can accept a very low signal level. A typical value for the noise figure might be 5 dB.

Let the sensitivity be —96 dBm, and let the other radar parameters be: Plugging these values into our expression for 40 log d: We will take two cases. One is a radar-warning receiver RWR that is located on the target. Both of these cases are shown in Figure 3.

It must detect a wide range of Radar Characteristics 45 Figure 3. Since the peak of the main beam of the radar antenna is pointed at the target, the RWR will see the peak gain of the radar antenna. Because the RWR cannot be optimized to any specific radar, its bandwidth must be wide enough to accept the most narrow pulse widths expected. The RF bandwidth is typically 4 GHz, so if there is RF gain some have it, some do not , the noise bandwidth will be a few hundred megahertz, determined by the formula: Without RF gain, the video bandwidth is the effective receiver bandwidth.

Since the signals can come from any direction, the RWR uses antennas with wide beamwidth. The antennas also have wide frequency coverage. The combination of these two factors dictate that RWR antennas have low gain from about 2 dBi at the highest frequency to about —15 dBi at the lowest frequency. Since these antennas are used in combination, the effective antenna gain factor at 10 GHz for the RWR system will be 0 dBi for any direction of arrival. The radar to RWR link is shown in Figure 3.

The received power in the RWR is: 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.

Formats and Editions of EW : a first course in electronic warfare [homeranking.info]

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.

EW 101: A First Course in Electronic Warfare (Radar Library)

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.

Adamy D.L. EW 101: A First Course in Electronic Warfare

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. Serving as a continuation of the bestselling book EW Adamy presents the information in an.

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