TOPIC 2

RADAR SYSTEMS

INTRODUCTION

This topic covers a variety of subjects including the characteristics of radar systems, pulse analysis, methods of modulation, pulse rate generators, telemetry, frequency diversity, electronic attack, and millimeter wave radar systems. The most important characteristics of radar signals are the radio frequency (RF), pulse repetition frequency (PRF), pulse duration (PD), and antenna scan characteristics (discussed in topic 1). The selection of a radar transmitter for a millimeter-wave radar system is governed by many of the same factors that influence the selection of any other radar transmitter. Such factors include peak and average power, PRF, PD, RF stability, and bandwidth.

RADAR SYSTEM CHARACTERISTICS

All radar systems have certain characteristics or parameters that distinguish them from other radars. These parameters can be divided into the following three categories:

MEASURABLE CHARACTERISTICS

Measurable characteristics are those parameters that can be measured and are used to identify a particular radar. Measurable characteristics include RF, PRF, pulse repetition interval (PRI), PD, and scan rate/period.

Radio Frequency

Radio frequency (RF) is a prime factor that controls many capabilities of a radar system. The RF of the radar can determine the radar range, the use of the radar, the size of components, the antenna and waveguide (i.e., the power requirements), and the propagation characteristics. The RF of a signal of interest

(SOI) is the first important characteristic to determine. RF is measured in hertz (Hz), and for a radar is expressed in megahertz (MHz) or gigahertz (GHz). RF is typically measured by comparison with a stable frequency source, called the zero-beat method or by a direct reading from a calibrated dial or frequency counter.

Determining the RF of a radar’s SOI allows the intercept operator to immediately estimate the type and use of an intercepted radar signal. As a general rule, the lower RFs are used for early-warning radar, and the higher RFs are used for other radar systems including missile tracking and control, fire control, navigation or surface search, target acquisition, and precision radars.

The RF of an SOI determines the physical size of an antenna. If the RF is known, the following assumptions may be made:

The designed operating RF range of the radar and corresponding antenna size are prime factors in installation requirements. For example, there are very few airborne radar systems operating at RFs below 3000 MHz because the size requirement of the antenna would be too large for most aircraft. The standard frequency bands that radars operate in are shown in table 2-1.

Table 2-1.— Standard Radar Frequency Bands

BAND

FREQUENCY (MHz)

A

0-250

B

250-500

C

500-1000

D

1000-2000

E

2000-4000

F

4000-5000

G

5000-6000

H

6000-8000

I

8000-10000

J

10000-20000

K

20000-40000

Pulse Repetition Frequency

The pulse repetition frequency (PRF) of a radar system is the number of pulses generated in one second, measured in pulses per second (pps). Once the radar emits a transmitted pulse, a sufficient length of time must elapse to allow any echo signals to return and be detected before the next pulse can be transmitted. Therefore, the longest range at which targets are expected determines the rate at which the pulses are transmitted. If the PRF is too high, echo signals from a target might arrive after the transmission of the next pulse and ambiguities in range might result. Echoes that arrive after the transmission of the next pulse are called second time around (or multiple time around) echoes. The range at which targets appear as second time around echoes is called the maximum unambiguous range (MUR). Therefore, the PRF determines the MUR.

To calculate the MUR of a radar, remember that RF energy travels at the speed of light (about 162,000 nautical miles (NMs) per second). For ease of computation, reduce the velocity of the pulse by half. This compensates for the two-way travel of the RF pulse and equates to approximately 81,000 NMs per second. For example, to find the MUR expressed in nautical miles (NMs) of a radar with a PRF of 400 pps, use the following formula:

                                      81000                    81000
                     MUR =   ----------              ----------    = 203 NMs
                                      PRF                       400

For another example, use a PRF of 3371 pps.

                  81000
MUR =    --------- = 24 NMs
                   3371

Notice the great difference in the MUR in the two examples. This calculation is theoretical while the actual, or effective, range of a radar is determined by a variety of other factors that include weather, radar antenna height, and average power output of the radar. The PRF versus range relationship of radars is shown in table 2-2.

Table 2-2.— Relationship of PRF Versus Range

 

PRF

PROBABLE RANGE CAPABILITY

Low Under 350 pps Long Range
Medium 350 to 1000 pps Medium Range
High Over 1000 pps Short Range

When the system antenna is rotated at a constant speed, the beam of energy strikes a target for a relatively short time. During this limited time, a sufficient number of pulses of energy must be transmitted in order for a signal to return that will be detected by the radar. For example, an antenna rotating at 10 seconds per revolution (SPR) and having a PRF of 800 pps will produce about 22 pulses for each degree of antenna rotation. Therefore, the rotational speed of the antenna determines, in part, the lowest PRF that can be used.

A low PRF has the advantage of permitting very high peak power with reasonably low average power. Another advantage of using a low PRF is that it extends the effective range of the radar. A disadvantage of a low PRF is the ease with which a small return may be lost. With a PRF of 60, the grass (noise level) along the baseline of the scope may be larger than the return. In these instances, small target echoes may be lost.

Pulse Repetition Interval

The pulse repetition interval (PRI) is the reciprocal of the PRF. It is the time between the start of one pulse and the start of the next pulse. It can be measured from any point on the first pulse to a like point on a consecutive pulse and is expressed in microseconds (Ásec). Therefore, it is the length of time that includes the transmit and the receive time of a radar. PRI and PRF have a reciprocal relationship, as shown in the following formula:

                                  1                                    1
                    PRF = ------       and PRI = -------
                                PRI                              PRF

In figure 2-1, three evenly spaced pulses occur in 0.003 seconds, thus one pulse occurs in 0.001 seconds, the PRI. The PRF is the inverse of the PRI and in this example is 1000 pps.

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Figure 2-1.— PRI/PRF relationship.

The PRI provides the electronic intelligence (ELINT) operator with a precise measurement for describing specific modulation and multiple pulse characteristics. For example, we may use analysis equipment to determine a radar is operating with a PRF of 1249.123 pps. The reciprocal of this, the PRI, is 800.5617. As a measurement of time, or in comparing or describing time relationships as they pertain to radar, use the PRI.

Pulse Duration

The pulse duration (PD) is the time, usually measured in m sec, required by the radar to transmit a burst of energy (the pulse). The PD is also considered the "on-time" for the transmitter. The PD is one of the least reliable of all parameters by which a radar is identified. This is due to the bandwidth limitation of the recorder that precludes an accurate representation of the pulse shape. The PD measurement is made at the half power point of the leading edge of the pulse to the half power point on the trailing edge. See figure 2-2. The distance between the trailing edge of a pulse and the leading edge of the next pulse is the receive time, or listening time, for the echo return.

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Figure 2-2.— Pulse duration.

The power output of a radar is proportional to its PD. The pulse contains the power necessary to propagate the RF energy through space. Consequently, the longer the PD, the greater the average power and the farther the RF can travel in space. The PD determines two important parameters—range resolution and theoretical minimum range. The shorter the PD the better the range resolution. The PD versus range relationship is shown in table 2-3.

Table 2-3.— Relationship of PD Versus Range Capability

PULSE DURATION (m )

PROBABLE RANGE CAPABILITY

Narrow

Under 1

Short Range

Medium

1 to 4

Medium Range

Wide

Over 4

Long Range

RANGE RESOLUTION

Range resolution is the ability of a radar to distinguish between two targets that are close together at the same azimuth and elevation. Any smaller separation causes the returns from the targets to combine and appear to the radar as a single return. To find the range resolution, convert the speed of RF energy, 162,000 NMs per second, to 328,000,000 yards per second. RF energy then travels at approximately 328 yards in 1 Ásec, or 164 yards for one-way travel of the pulse. To find the range resolution of a radar, use the following formula:

Range resolution = PD (in Ásec) x 164 yards

For example, determine the range resolution of a radar that has a PD of 3 Ásec.

Range resolution = 3 x 164 = 492 yards

This means that two targets at the same bearing would have to be at least 492 yards apart to appear as two distinct targets on the radar scope. If the two targets were closer than 492 yards, they would appear as one target.

THEORETICAL MINIMUM RANGE

The theoretical minimum range is the minimum distance separating the target and the radar from which energy, reflected from the target, will not arrive at the radar while the antenna is still connected to the transmitter. While the transmitter is firing pulses, the radar receiver is unable to receive echoes. A short amount of time elapses between the pulse being transmitted and the duplexer switching the antenna back to receive echoes. This time is called the recovery time. The total time in which the receiver is unable to receive the reflected pulse is equal to the PD plus the recovery time. Any reflected pulses from a close target returning before the receiver is connected to the antenna will be undetected. Most modern radar systems are designed with very short recovery times. Range resolution and minimum range are closely related parameters, since both are a function of the PD and, for practical purposes, the method for determining the theoretical minimum range is the same as range resolution.

Scan Rate/Scan Period

The scan rate is the angular rate at which the antenna beam is moved through a scan volume while performing various functions. Usually the scan rate is expressed in either Hz or in revolutions-per-minute (RPM). The scan period is the time, usually in seconds, for example, seconds per sector (SPS) and SPR, for one complete scan cycle and is the reciprocal of the scan rate. The particular use for which a radar is designed determines the type of scan it uses. Thus, the scan type and scan rate parameters of a radar are the foremost characteristics that identify the general function or purpose of a radar.

Scan rate or scan period is the speed at which the RF beam is moved about the scan pattern. The slower radar antenna scans provide more pulse returns and improved detection capability. However, a slow illumination (scan) rate means a longer time between observations of the target. This can result in inaccurate target position prediction or tracking. However, if the time between target illuminations is reduced, the energy directed on the target is also reduced, and the probability of detection is decreased.

DESCRIPTIVE CHARACTERISTICS

Descriptive characteristics are nonparameter data that define, modify, or otherwise clarify the measurable characteristics. Some descriptive characteristics include platform, modulation type, scan type, polarization, and user. Descriptive characteristics of a radar are used by the analyst to determine other aspects of the radar. For instance:

Platform

The term platform describes the normal operating environment of an emitter. Table 2-4 contrasts common platform types.

Table 2-4.— Common Platform Type Characteristics

PLATFORM TYPE

DESCRIPTION

Fixed Landbased

Exhibits a steady azimuth and a constant signal strength.

Mobile Landbased

May vary in azimuth and signal strength over several minutes.

Airborne

Associated with aircraft and missiles.

Characterized by rapid changes in the emitter azimuth and fluctuations in signal strength.

Shipborne

Exhibits slow changes in azimuth and signal strength, similar to mobile landbased emitters.

Modulation Type

The modulation type employed by a transmitter can be very helpful in establishing a radar's use and purpose. The overall modulation type of common radars is pulsed. Depending on the complexity of the radar system, any number of different things could be happening to the pulse train. There are two basic types of modulation—interpulse and intrapulse. These will be discussed in detail later. Familiarity with the different modulation types can assist in emitter identification.

Scan Type

Scan type is a good indication of the probable function of the radar. Knowledge of the type of scan employed by a radar provides valuable information about the purpose of a radar. With the application of suitable processing techniques, it is possible to determine the scan type, duration of illumination, and scan mode, and, hence, make a reasonable estimate of the function of the radar.

Polarization

Polarization refers to the orientation of the electromagnetic wave with respect to the earth. There are several types of polarization including horizontal, vertical, circular, or elliptical. Horizontal and vertical polarization is self-explanatory. Circular polarization is an electromagnetic wave whose electric field wave spirals about the magnetic wave. Polarization by itself has little meaning, yet it can be an important parameter. It is sometimes the only means of telling one type of radar from another. Polarization can also be a misleading parameter due to changes that can occur during propagation. Polarization is determined by comparing signal strength from horizontal and vertical antennas. If there is more than 3 decibels (dBs) difference in signal strengths, the signal is determined to have the polarization of the antenna from which the strongest signal is obtained.

User

The term user denotes the country or countries that use a particular type of emitter. In most cases, this can be determined by line of bearing (LOB), signal parameters, or by identification of the platform from which a signal emanates.

ADDITIONAL INTERCEPT PARAMETERS

There are several other intercept parameters that do not come under the headings of descriptive or measurable characteristics. These characteristics are time of intercept, bearing, signal strength, and antenna beamwidth.

Time of Intercept

The time of intercept (TOI) is very useful in correlating intercepts. Collectors often use it to determine if the signals intercepted by different sites represent different signals or multiple intercepts of the same signal. TOI can also be used in an analysis of radar systems supported by several emitters. The up and down times of the signal reveal the relationships between the functions performed by various emitters in supporting the overall operation of the system. TOI is an important characteristic that should be measured with maximum accuracy.

Bearing

The bearing measurement indicates the direction of arrival of the signal of interest. Bearing is the azimuth measurement to a target where the greatest signal strength is detected. There are a number of ways of determining bearing, depending on the measurement equipment available and the radiation pattern of the transmitted signal. A number of bearing measurements, each from a slightly different location, results in LOBs that hopefully intersect in some general area, establishing the approximate location of the transmitting antenna.

Signal Strength

Because so many factors affect signal strength, this parameter can be misleading. If all the factors are known, it is possible to determine the power of the transmitter. To measure signal strength, take a reading of the attenuation setting of the receiver in dBs, or read the field strength meter. Factors that affect these readings and, thus, the measurement of signal strength are as follows:

Beamwidth

Beamwidth (BW) is the area, in degrees, filled with RF energy in both the horizontal and vertical plane. The BW is measured between the half-power (3dB) levels. Radars with wide beams are normally associated with early warning and air search. Radars with narrow beams are normally associated with fire control, weapons guidance, navigation, and surface search. Radar frequency and the size and shape of the radar antenna determine the beamwidth.

There are at least two factors that make BW measurement difficult. One factor is that collectors are usually not within the main beam of the target emitter. It is usually impossible to determine the exact geometry between the intercept site and the main beam. The second factor is that the target emitter is usually scanning. Unless the defining parameters of the scan are precisely known, it is only possible to estimate the beamwidth.

As a radar beam scans across the target, the time that the beam illuminates the target is called the lobe duration (LD). It is timed by a stopwatch and is expressed in seconds. In the case of a circular scanning radar, the BW, if not known, can be estimated by multiplying the LD by 360 degrees, and dividing the result by the SPR. When the BW is estimated in this manner, the result is called the detectable BW. The following formula is used:

                                   LD x 360 degrees
                       BW= ---------------------------
                                         SPR

For example, the BW of a radar with a scan rate of 20 SPR and an LD of 1 second is:

                                 1 x 360
                     BW =  ------------        BW = 18 degrees
                                    20

The quality of the LD measurement depends on many factors, such as operator skill, signal strength, speed of emitter rotation, emitter beamwidth, and whether side and back lobes are present.

Beam Pattern

In addition to the main beam or major lobe, all radar systems possess, to some degree, many smaller beams. These beams are called minor or side lobes. See figure 2-3. The presence of minor lobes can often be detected with intercept equipment, particularly when the radar is located near the intercept platform. These minor lobes may cause the reception of weak signals when the scanning antenna is not pointed at the intercept station. Decreasing the gain of the receiver usually eliminates the reception of minor lobes.

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Figure 2-3.— Major and minor lobes.

In radar design, the choice of radiation patterns to be used is dependent upon the angular accuracy requirements and the volume of space to be searched. The greater the angular accuracy required in any plane, the narrower the beamwidth in that plane must be. Radar systems requiring high accuracy in both bearing and elevation use a narrow, symmetrical beam. Radar systems requiring accurate angular information in one plane and wide coverage in the other plane use a fan beam, narrow in one plane and wide in the other. It is characteristic of air search radar systems to have beams that are fanned in the vertical plane to have wide vertical coverage, while height finders usually have a beam that is narrow vertically and fanned horizontally.

PULSE ANALYSIS

Pulses can be used to convey information, control equipment, or provide a burst of energy that can be used to detect the presence of objects. Without pulses, data communications, radar, and television would be impossible. The pulse is a short burst of energy before and after which there is a rest. Imagine an electrical circuit equipped with a key or switch and supplied by a direct current (DC) power source such as a battery. The switch is open, closed, and quickly reopened. At the instant the switch is closed, the voltage applied across the circuit is suddenly stepped from zero voltage to the full battery voltage. At the instant the switch is reopened, the voltage applied across the circuit is stepped from a full battery voltage back to zero. A sudden, short change in the level of voltage (or current) such as this constitutes an electrical pulse.

The IEEE Standard Dictionary of Electrical and Electronic terms defines a pulse as "…a wave that departs from a first nominal state, attains a second nominal state, and ultimately returns to the first nominal state." The nominal state can be either another amplitude level as seen in the top trace of figure 2-4 or the signal can be switched off entirely as seen in the bottom trace of figure 2-4.

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Figure 2-4.— Possible presentations of a pulse.

Methods for generating pulses range from the simple to the complex, depending on how closely the transmitted pulse must approximate an ideal pulse. The ability to accurately reproduce a transmitted pulse from a received signal depends on the bandwidth of all devices through which the signal passes. Any exclusion of components will change the shape of the pulse.

Pulses have various shapes including rectangular, square, saw-tooth, double, exponential, stepped, cosine, squared, and bandwidth limited. In fact, about the only restrictions on the variety of pulse shapes are those needed for a given pulse shape, and those imposed by the ability of circuits to produce them. Some of the standard pulse shapes in wide use in electronics and communications are illustrated in figure 2-5.

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Figure 2-5.— Pulse types.

For purposes of illustrating pulse measurement, the square wave is shown in two extremes in figure 2-6. Figure 2-6 view A represents the ideal square pulse. For reasons to be discussed, this pulse is never quite realized in practice. Figure 2-6 view B represents a pulse of a square wave that is left after severe distortion from phase shift and loss of high frequency components. In actual practice, the ideal square wave encountered is not perfectly square. However, it is not as distorted as the one shown in figure 2-6 view B, but it is a compromise somewhere in between. The following terms explain measurement technique:

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Figure 2-6.— Pulse duration measurement.

PULSE AMPLITUDE

Every wave has some amplitude that describes its voltage, current, or power. The peak amplitude of a pulse is the absolute value of the difference between the largest and smallest level of voltage, current, or power. This is illustrated in figure 2-6 view A as the height measured from the base of the pulse to its top.

LEADING EDGE/TRAILING EDGE

The major portion of the rise of a pulse as indicated in figure 2-6 view B is termed the leading edge. The leading edge of a pulse is the portion of the pulse during which the amplitude is increasing. The major portion of the decay of the pulse, as indicated in figure 2-6 view B, is termed the trailing edge. The trailing edge of a pulse is the portion of the pulse during which the amplitude is decreasing.

PULSE RISE TIME

Associated with the leading edge of a rectangular pulse is the rise time. The amplitude of a practical pulse does not rise instantaneously as it would with an ideal pulse. Some finite amount of time is required for the pulse to reach its maximum amplitude. To avoid difficulty in determining the exact points at which a pulse begins to increase in amplitude and ceases to increase in amplitude, the measurement of the rise time of a pulse is taken at points that represent ten percent of rise time to ninety percent of rise time.

PULSE DECAY TIME

Associated with the trailing edge of a rectangular pulse is the decay time. Just as no practical pulse can instantaneously rise to the peak amplitude, no practical pulse can instantaneously drop from the peak amplitude to zero. The decay time of a pulse is measured between the points representing ninety percent of the decay time to ten percent of the decay time.

PULSE DURATION MEASUREMENT

In looking at the ideal rectangular pulses in figure 2-6 view A, notice it would make little difference whether the PD were measured near the top, near the bottom, or somewhere in between. With practical pulses in figure 2-6 view B, it would make a great deal of difference where the pulse duration is measured. PD is the time interval between the first and last instants at which the amplitude reaches a stated fraction of the maximum pulse amplitude. Looking at both the ideal and the practical pulse compared in figure 2-7, the only place at which the duration is identical is at the fifty-percent amplitude point. For this reason, the duration of a pulse is specified as the value measured at a point equal to fifty percent of the peak amplitude of the pulse.

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Figure 2-7.— Comparison of ideal versus practical pulses.

PULSE PERIOD

The period of a pulse is the time required to complete one cycle or until the pulse repeats itself. The period of the pulse may be measured between identical points on adjacent leading edges as indicated in figure 2-6 view B or between identical points on adjacent trailing edges, or between any other pair of identical points on adjacent pulses. Note that the period of a pulsed waveform is also the interval between repetitions of the pulse, or the PRI.

DUTY CYCLE

Duty cycle is the ratio of the active or "on" time to the duration of the specified period, usually the total period of one cycle. This relationship is expressed as the ratio of the PD to the PRI. Duty cycle may be expressed as a rational number, a decimal fraction, or a percentage. The duty cycle is used to calculate both the peak power and average power of a radar system. The formula for the duty cycle expressed as a percentage is:

                        PD x 100
           DC = ----------------
                            PRI

Given a PD of .5 Ásec and a PRI of 780 Ásec, what is the duty cycle?

                       .5 x 100
            DC = --------------    = .64%
                          780

PULSE MODULATION METHODS

Pulses have individual characteristics with respect to height, width or duration, repetition rate, formation time, decay time, shape, and displacement from normal occurrence. The characteristics may be used singly, or in combination, to provide a wide variety of pulse modulation methods adaptable to the transmission of intelligence in many forms and degrees of complexity. The fact that in pulse modulation energy is transmitted only for short intervals, makes possible the use of high peak power in the carrier and the use of certain equipment to generate carriers of high frequencies. The transmission bandwidth depends upon pulse shape, as well as pulse range, and the method and degree of modulation. When high frequency carriers are used, the bandwidth can be much greater.

The various pulse modulation schemes differ from conventional amplitude modulation (AM), frequency modulation (FM), and phase modulation (PM) because modulation is applied to a train of pulses rather than to a sinewave carrier. The pulse train has the same characteristics of amplitude, frequency, and phase just like a sinewave carrier. Modulation of these characteristics provides three basic types of pulse modulation, pulse amplitude modulation (PAM), pulse frequency modulation (PFM), and pulse position modulation (PPM). A fourth parameter, the PD, can also be modulated providing pulse duration modulation (PDM). These four types of modulation, as depicted in figure 2-8, can be used for either interpulse modulation (pulse to pulse) or intrapulse modulation (within a pulse). A fifth type of modulation, pulse code modulation (PCM), is only used with interpulse modulation. Pulse modulation is valuable for applications that require multiplexing for transmission of many types of data, in short periods of time, on a common carrier.

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Figure 2-8.— Pulse modulation methods.

INTERPULSE MODULATION

Interpulse modulation is applied to a radar signal that is transmitted at intervals that are varied from the uniform, fixed PRI of conventional radar, a pulse constant radar. A pulse constant radar is a radar that the nominal PRI value does not vary (remains constant) for significant periods. The pulse constant radar may employ several PRIs, operating on each PRI for a period of time before switching. Interpulse modulation offers several advantages for the radar including reduction of range ambiguities, elimination of blind speeds, and improvement of antijamming characteristics. There are several approaches to this technique.

Staggered PRI

A staggered pulse train is a train of pulses wherein two or more precise interpulse intervals are alternated in a patterned or random sequence. Figure 2-9 illustrates the difference between a staggered train and a nonstaggered train. Staggered PRIs are usually used to reduce blind speeds. Types of staggered PRIs include the following:

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Figure 2-9.— Staggered versus nonstaggered PRI.

Sliding and Stepping PRF

A sliding PRF pulse train has a continuously varying (increasing or decreasing) PRF between minimum and maximum PRF limits. Upon reaching a limit, the PRF will normally jump to the other PRF limit and repeat the variation again. Modern radars do not use PRF sliding but use discrete PRF steps in several sizes. However, the effect is very similar. Sliding and stepping PRF can be used to reduce main bang eclipsing. Main bang eclipsing occurs when a target return arrives at the radar at the same time that a pulse is being transmitted. When this occurs, the antenna is connected to the transmitter and no echo can be received.

Jittered PRI

A jittered PRI is a pulse train where the PRI value is switched randomly within the bounds of a maximum and a minimum PRI value and where there is a variation in the starting time of each successive pulse relative to the time it would start if the pulse train occurred at regular intervals. The PRI is bounded between a maximum and a minimum value but can assume any value between these limits. The two types of jitter are intentional and unintentional. Intentional jitter is divided into two categories, pseudorandom and pulse frequency modulation (PFM), which will be discussed later. Using jittered PRI prevents range gate stealers from locking onto the PRF. Jitter can also be used to resolve range ambiguities.

Pulse Amplitude Modulation

In pulse amplitude modulation (PAM), the pulses vary in amplitude from pulse to pulse in accordance with some type of waveform (figure 2-8 view A). PAM is like AM, with the pulse train being modulated instead of the sinewave carrier.

Pulse Frequency Modulation

In pulse frequency modulation (PFM), the PRF varies with some respect to the modulating waveform. In the example shown in figure 2-8 view B, the PRF is greatest when the amplitude of the modulating signal is greatest. PFM is very similar to FM in many of its characteristics.

Pulse Position Modulation

In pulse position modulation (PPM), the positions of the pulses within the pulse train vary (figure 2-8 view C) with respect to some point of reference (constant point). In PPM, the modulation causes the position of the pulse along the time axis to vary about its unmodulated position. PPM is like PM. PPM uses the narrowest pulses of any modulation scheme. The bandwidth of the PPM spectrum is correspondingly larger than that of any other type of modulation with the same information-handling capacity. However, PPM has considerable noise improvement over PAM as random noise greatly distorts the amplitude of a pulse but alters its timing very little.

Pulse Duration Modulation

In pulse duration modulation (PDM), the duration of each pulse in a pulse train is varied according to the amplitude of a modulating waveform. See figure 2-8 view D. The PD is varied by modulating the position of the pulse trailing edge while the leading edge remains fixed. Thus, all pulses in a train start at fixed intervals but end at times determined by the modulation on their trailing edges. PDM can also occur in conjunction with the changes of the PRF of a signal. If the PRF is changed to a higher rate, the PD becomes shorter.

Pulse Group Modulation

Pulse group modulation is applied to a pulse train. A pulse group repetition interval (PGRI) is a pulse train in which there are groups of closely spaced pulses separated by much longer times between these pulse groups. PGRI is normally defined by the number of pulses per group (PPG) and the number of pulse groups per second (PGPS).

Optimum range resolution of a radar requires short pulses while optimum speed resolution requires a long pulse. One way of optimizing both is to transmit a burst of short pulses instead of a single pulse. The range is dependent only on the average power that is proportional to the number of pulses in a burst. Longer range requires more pulses within a burst. The maximum unambiguous range is determined by the burst repetition interval. Pulse group modulation can be used to eliminate blind speeds in a conventional radar. However, one of the more common uses of pulse group modulation is missile guidance.

INTRAPULSE MODULATION

Modulation on the pulse (MOP) radar uses an intentional change in the frequency or phase of the carrier of a pulse while the pulse is being transmitted. Intrapulse modulation techniques make it possible to simultaneously maximize the target range, the range resolution, and the velocity resolution of the radar set. Thus, instead of the pulse being a burst of RF energy at a given carrier frequency, the pulse is a burst of RF energy at a carrier frequency that varies in phase, frequency, or amplitude. Use of this technique spreads the frequency spectrum making it significantly harder to jam. Intrapulse modulation is the transmitter technique and pulse compression is the receiver technique.

Pulse Code Modulation

Pulse code modulation (PCM) is more complex than those previously mentioned in this section. In PCM there is a train of coded bits of information. Each coded bit represents an instantaneous amplitude of the modulating waveform. In a PCM system, the intelligence modulates a sequence of bits made up of a group of pulses instead of modulating a sinewave or pulse train. These pulses are assigned a number, and they are spaced at regular intervals. The coding function of the PCM system is accomplished through the absence of any combination of these pulses. The information provided by the transducer determines the absence or presence of pulses that, in turn, can be decoded at the receiving end. See figure 2-10.

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Figure 2-10.— Pulse code modulation.

Amplitude Modulation On The Pulse

Amplitude modulation on the pulse (AMOP) is a change of amplitude during the transmission of a pulse and is seen when pulse characteristics are analyzed. There are two types of AMOP, intentional and unintentional. Intentional is used by many identification, friend or foe (IFF) signals as an identification code. It may be used to apply a binary number to the transmitted pulse for the purpose of identification by the receiving set. Unintentional is common and may be seen when two signals "beat" together and the beat frequency produces AMOP. The presence of multipath can also cause unintentional AMOP. Multipath pulses appear to be very distorted and are easily recognized.

Frequency Modulation On The Pulse

Frequency modulation on the pulse (FMOP) is a change of frequency during transmission of a pulse. The two most common forms are linear frequency changes and discrete frequency steps. Additionally, many state-of-the-art radars are being designed with nonlinear frequency changes within the pulse. FMOP has two primary uses for changing the frequency on the pulse—movement of the position of transmitted beam or pulse compression techniques.

Phase Modulation On The Pulse

Phase modulation on the pulse (PMOP) is a signal that has a constant peak amplitude through the duration of pulse, but the phase angle relative to that of the continuous carrier is switched at regular intervals or in a set pattern. This type can have as few or as many phase changes as are necessary for the intended use. Phase changes are used as a coding format in pulse compression systems such as those that use Barker code, Taylor codes, linear recursive sequences (LRSs), pseudorandom sequences, etc. These codes enable the system’s receiver to have a better signal-to-noise ratio (SNR) and a higher probability of target detection at maximum range.

RADIO FREQUENCY MODULATION

RF modulations are characterized as two basic types, intrapulse and interpulse. Regardless of the type of RF modulation, if the receiver’s bandwidth is less than the signal’s RF excursion, part of the signal will be lost.

INTRAPULSE RADIO FREQUENCY MODULATION

Intrapulse RF modulation is intentional change in the frequency or phase of the carrier of a pulse while the pulse is being transmitted. The way in which the frequency changes defines the intrapulse RF modulation technique being used (e.g., linear and nonlinear, frequency stepping, frequency shift keying (FSK)). The primary uses of intrapulse RF modulation are pulse compression and electronic beam steering. Pulse compression is a technique that both increases the radar's detection capabilities and improves the range resolution. Electronic beam steering is a technique used with array antennas (and conventional antennas with array feeds) to control the position of the radar beam as a function of the transmitted frequency. The following are examples of intrapulse modulation:

INTERPULSE RADIO FREQUENCY MODULATION

Interpulse RF modulation is a change in carrier frequency on a pulse-to-pulse or pulse group-to-pulse group basis. This form of RF modulation differs from intrapulse in that the RF modulation takes place between pulses rather than on or during the pulse. Interpulse RF modulation may be used for electronic protect (EP), to support electronic scanning, or to reduce range ambiguities. Common forms of interpulse RF modulation are RF hopping, RF sliding, and interpulse chirp. These forms are explained as follows:

COHERENT CARRIER RADAR

The Doppler frequency shift produced by a moving target may be used in a pulse radar to determine the relative speed of a target or to separate desired moving targets from stationary objects (clutter). A radar that uses the Doppler frequency shift as a means of discriminating moving from fixed targets is called a moving target indication (MTI) or a pulse Doppler radar. The two are based on the same physical principle, but there are recognizable differences between them. The MTI radar usually operates with ambiguous Doppler measurement (blind speeds) but with unambiguous range measurement (no second-time-around echoes). The opposite is true for a pulse Doppler radar. Its PRF is usually high enough to operate with no blind speeds but at the expense of range ambiguities.

A simple continuous wave (CW) radar consists of a transmitter, receiver, indicator, and antennas. By adding a power amplifier and modulator to turn the amplifier on and off for the purpose of generating pulses, the radar becomes a pulsed radar. An MTI coherent radar has a small portion of the oscillator power diverted to the receiver to take the place of the local oscillator where it acts as the coherent reference needed to detect the Doppler frequency shift. By coherent it is meant that the phase of the transmitted signal is preserved in the reference signal.

A coherent carrier radar is a radar, either CW or pulsed, that pays particular attention to the phase or frequency stability of the transmitted energy and where some use is made of the carrier phase information to process target returns. The term coherent carrier means the frequency and phase of a signal have a fixed relationship to the frequency and phase of the reference signal. In general, coherent carrier radar systems incorporate special highly stable transmitter and receiver designs to achieve advanced performance capability. In most radar systems, there is a certain degree of unintentional modulation present on the carrier wave or pulse of the radar signal. In coherent carrier techniques, there is a lower than normal degree of unintentional modulation present. This enables the radar to acquire and track targets in heavy clutter where targets are obscured by nonmoving reflecting surfaces. The degree to which coherent processing techniques are used varies from application to application because the cost of a coherent system is high. Coherent systems can make very accurate speed measurements, reject clutter, and improve the probability of detection.

The basic functioning of a coherent radar system is the same as that of a noncoherent system. However, the requirement for coherency places more stringent requirements on the subsystems and components. Coherent radar typically incorporates more elaborate signal processing schemes. In coherent carrier radar, a portion of the transmitted signal is stored for a time equal to the PRI. This reference signal is the distinguishing feature of the coherent carrier technique. Each target return travels over two signal paths in the receiver. One copy is sent through a delay line circuit and the other copy is sent to the phase detector and canceler unit. The copy sent through the delay line is used to compare with the return signal’s phase. The path through the delay line is timed so that the target return appears at the end of the delay line just as the return from the next pulse arrives at the canceler circuit. The copy sent to the phase detector is forwarded to the canceler circuit for comparison with the previous return signal. Several types of systems that use coherent waveforms are CW Doppler, pulsed Doppler, and MTI.

CW DOPPLER RADAR

A CW Doppler radar uses a single antenna since the necessary separation between the transmitted and received signals is accomplished by the frequency difference between the transmitted and Doppler shifted return signals. The transmitter and receiver are coupled through the antenna since the receiver requires a small amount of the transmitted signal for a reference. In the radar receiver, the Doppler shifted return signal is mixed with a portion of the transmitted signal, and the lower sideband, or beat frequency, is bandpass filtered to produce the output, which is the Doppler frequency. Some isolation is required between the transmitter and the receiver, except when the CW radar operates with relatively low power or insensitive receivers.

PULSED DOPPLER RADAR

A pulsed Doppler radar uses PRFs that are high enough to provide an adequate sampling rate for Doppler frequencies and processing techniques that directly measure the target speed. Generally, this system includes the following:

Pulsed Doppler radars have range ambiguities due to the high PRFs. The high PRFs create multiple-time-around echoes; as much as 50 percent of the target echoes may arrive at the receiver while the radar is transmitting. This also results in a reduction in detection probability. Doppler frequencies are also ambiguous and are dependent upon the PRF.

MOVING TARGET INDICATION RADAR

Radar receivers using MTI techniques can distinguish the returns from moving targets and stationary objects. With conventional radar presentations, the returns from moving targets can be masked by unwanted returns called clutter, because the radar display becomes so congested with clutter that the returns from moving targets are impossible to detect. An MTI radar uses a reference oscillator to maintain coherence. The MTI system detects moving targets by comparing the phase of successive target returns. Successive returns from moving targets exhibit pulse-to-pulse phase characteristics. A target moving toward the radar will have a range that contains successively fewer cycles of phase in the round-trip propagation path. Conversely, a target moving away from the radar will have a range that contains successively more cycles of phase in the round-trip propagation path. Returns from stationary objects are eliminated during processing and are not displayed.

PULSE RATE GENERATORS/TIMERS

Radar timers, sometimes called pulse rate generators or synchronizers, are used to define a time relationship between radar components. Pulse stability is essential to the operation of a radar system, and some timers produce pulses with more stability than others. Pulse rate (or PRF) is roughly related to the design range of the radar. In general, a low PRF rate indicates long range while a high PRF indicates

shorter range radars.  Some pulse rates characteristically signify the type of pulse generation device used and, in some cases, indicate the intended use of this distinct characteristic. The function of timing control may be accomplished as a separate "timer" unit or as an integral function of the transmitter. Older radar timers that are not very stable are the power line synchronization, rotary spark gap, and self-excited oscillator. Newer radar timers that are much more stable are the delay line and crystal controlled.

POWER LINE SYNCHRONIZATION

In many of the early varieties of radars, there was no need to use a discrete pulse rate, nor was there a need to maintain a specified stability at this pulse rate. In order to simplify the circuitry, some early radars used the alternating current (AC) power line frequency as a source of timing for pulse generation. European equipment to be operated on 50-cycle power lines often used 50 or 100 pps. U.S. equipment, built with pulse rates derived from 60-cycle power lines, operated with 60 to 120 pps. Airborne applications sometimes exhibit 400 or 800 pulse rates derived from the 400-cycle AC power supply. Wide variations in pulse rate stability can be expected. More stable pulse rates could be expected from fixed stations operating from a large power grid encompassing extensive areas of the country. Mobile units with independent power sources, particularly airborne units, can be expected to be highly unstable in pulse rate.

ROTARY SPARK GAP

The rotary spark gap is outdated and probably not used anymore. This system uses an electric motor that operates a commutating device to produce pulsed signals. The major characteristic by which this system can be recognized is the erratic pulse jitter.

SELF-EXCITED OSCILLATOR

Many radars have been built using variations of the self-excited oscillator. The particular output pulse frequency is determined by the values of inductance and capacitance in the oscillator circuit. The basic operation is similar to the operation of a crystal-controlled timer. However, the self-excited oscillator uses an unstable tuned circuit to generate a timing signal. Some self-excited oscillator timers can be tuned to provide some PRF diversification. This is usually limited to approximately 10 percent variation. For instance, a radar designed for 500 pps might have a spread from about 450 to 550 pps.

DELAY LINE

In this system, pulse interval is determined by the transit time of an acoustical pulse from a transmitting transducer to a receiving transducer. A column of mercury can be used for the medium of this delay; however, it would only be useful as a land-based radar. Often the delay is made of a block of fused quartz, and the delay interval is the transit time of an acoustical pulse in the quartz between transducers. The quartz crystal is carefully designed and cut so that the total delay introduced is equal to the desired PRI. Delay lines require temperature compensation for PRI stability and are used in electronically controlled ovens. The delay line type of pulse generating circuit has a very stable pulse interval as well as a stable stagger ratio. In a system where multiple PRFs are desired, the delay line works very well. Having two or more delay lines allows a radar to have staggered PRFs or to change PRFs as desired.

CRYSTAL-CONTROLLED OSCILLATORS

Crystal-controlled pulse rate generators provide highly stable PRFs and have wide usage as master timing elements in radar systems. The use of crystal-controlled modulators dates back many years. Crystal-controlled PRFs offer stability and provide more precise range determination.

A crystal is a piezoelectric material, such as quartz, which is mechanically stressed, in a vise-like holder. The quartz crystal, cut to precise dimensions, is placed between two metal plates then sealed in a small container. When subjected to an electrical charge, extremely stable sinusoidal oscillations result. Factors that govern the rate of oscillation are the physical dimensions of the crystal, placement of the electrodes on the crystal, and to a lesser extent, environmental conditions such as temperature and humidity.

When a radar set uses a crystal source for a timer, the PRF remains constant for that radar, normally for the life of the crystal. Each individual crystal provides a unique value, unique enough to be traced to the emitter. This property is the basis for radio fingerprinting. By closely measuring all parameters associated with a radar, then associating the radar to a particular location, aircraft, or ship, the emitter can be correlated to the emission platform. Crystal-controlled PRFs also help the intercept operator to identify a particular radar in a dense electromagnetic environment.

Normally crystals fall into three broad ranges of distance measurement: kilometers (kms), nautical miles (NMs), and yards (yds). Emitters using crystals cut to these ranging distances can often be characterized as to their probable function by the category into which the crystal falls. As an example, an emitter using the nautical mile crystal, 80,905 kilohertz (kHz), is assumed to be serving in a navigational role, while an emitter using the 2000 yard crystal, 81,932 kHz, is almost certainly associated with a naval fire-control system. Finally, a kilometer value crystal is generally associated with tactical weapons or nationwide integrated defense systems. An important note to remember is that each crystal will have a different value, deviating slightly from the model frequency.

Crystals are usually designated by the range marks they generate vice the frequency of the crystal. Table 2-5 shows the crystal frequency for some common Commonwealth of Independent States (CIS) crystals.

Table 2-5.— Crystal Frequency For Some Common CIS Crystals

Range Marks

Crystal Frequency (kHz)

.25 NM

323.649

1NM Model A

80.912

1NM Model B

80.882

.125 KM

1198.5971

.5 KM

299.67

1 KM

149.908

2 KM Model A

74.82

2 KM Model B

74.919

2 KM Model C

74.995

2 KM Model D

74.98

2000 yds

81.938

Range Marks

Crystal circuitry is used in radars to provide a stable PRF timing generation and a method of establishing range marks (kilometer, nautical mile, yards). Range marks appear on the target indicator display, usually a cathode ray tube (CRT), of the radar system. They designate specific distances so that when a return is received, the distance to the target can be determined by its relation on the screen to the range markings. Since it may not be practical for a radar system to use a PRF in the range of 80,905 pps, the PRF can be changed. To change the PRF, a radar system uses divider circuits to take the high-frequency crystal and convert it to a usable PRF. This divider circuitry is called the crystal countdown. Before we discuss the crystal countdown, let's first learn how range marks are established.

The first item to consider for a radar using crystal control is the range marks desired. The range marks desired determine the frequency the crystal should produce. To make range measurements, for example in kilometers, the time required for a radar pulse to travel one kilometer and return should be determined. The following formula is used:

                                       C
XTAL FREQ = -----------------------
                            2 x range mark

 

                          299.7925 x 106m/s
XTAL FREQ =--------------------------- = 149896.25 Hz
                                 2 x 1000

"C" is the approximate speed of light or 3 x 108 meters/sec (more precisely 299.7925 x 106). Range marks in kilometers is desired, so use 1000 meters. And to allow for the travel and return time of the pulse, use 2 x range mark.

To establish kilometer range marks, a crystal would have to be cut to emit approximately 149896.25 Hz. The reciprocal of this XTAL FREQ would be the XTAL PERIOD, or the time required for the pulse to travel and return from a one kilometer distance.

                                 1
                    ----------------------   = XTAL PERIOD
                         XTAL FREQ

             1
----------------------  = XTAL FREQ
     XTAL FREQ

             1
  ----------------------  = 6.67 Ásec
    149896.25 Hz

Using the XTAL PERIOD, a target at 10 kms would have a 66.7 m sec return time, a target at 20 kms 66.7 Ásec after that, and so on. A crystal-controlled oscillator can be used to introduce artificial calibration pulses into the radar display at the desired ranges (i.e., 10, 20, 30, 40, 50 kms). If the range marks were desired at 5-km intervals, these calibration pulses must be inserted 33.4 Ásec apart.

Crystal Countdown

A countdown is a whole number value that equates to the sum of the prime factor values of a radar's divider circuits used in generating the PRF from the parent crystal frequency. More than one countdown sequence can be employed with a crystal to provide multiple PRFs, a scheme employed in fire control and surface-to-air missile systems.

The PRFs generated directly by this crystal would not be within a realistic range for radar operation. For example, consider the PRF associated with the 1-km crystal, 149896 Hz or 149896 pps. The MUR would be only 1000 meters:

                              299.7925 x 106
               MUR = ---------------------  = 1000 meters
                                2 x 149896

However, to maintain the crystal stability and accuracy and at the same time generate 1000-meter range marks, count down the crystal frequency by the appropriate amount to provide the desired PRF.

There are two fairly simple means of counting down a crystal frequency: one is by amplitude processing and threshold setting, the other, the most common method, is divider circuits. The larger the countdown, the lower the PRF. If the divider circuit is too large, stability problems will creep back into the radar system. The common countdown circuits are count down by 2, 3, 4, 5, and 7. If a countdown is obtained that is other than a factor of these, the countdown is probably done in one large countdown. Countdowns by 3 and 25 are two of the highest known countdowns to date.

To find the countdown, find out the PRF. If an intercepted radar or analyzed tape of a radar reveals a 1249 PRF that is very stable, then it is probably crystal controlled. Then, using common crystals such as 1 km, 2 km, .5 km, figure out the total countdown using the following formula:

                                                                   XTAL FREQ
                   
TOTAL COUNTDOWN = --------------------
                                                                         PRF

                                                                      149900
                   
TOTAL COUNTDOWN = ------------------- = 120.016 = 120 for 1 km
                                                                        1249

                                                                      299790
                     TOTAL COUNTDOWN = -------------------= 240.024 = 240 for .5 km
                                                                        1249

                                                                      74948
                     TOTAL COUNTDOWN = ---------------  = 60.007 = 60 for 2 km
                                                                        1249

NOTE: The total countdown cannot be fractional, it must be an integer.

The signal probably comes from one of these crystal values since each one can be counted down to give this PRF. To obtain a probable countdown sequence, factor each total countdown into component parts. There will be intermediate PRFs with each division. An example is as follows:

For 1 km crystal:

149900 Hz    ¸ 5 = 29980 Hz

29980     ¸ 4 = 7495 Hz

7495     ¸ 3 = 2498.33 Hz

2498.33        ¸ 2 = 1249.16 Hz

If we multiply all of the divisors together (i.e., 5 x 4 x 3 x 2), the total will equal the original dividend or the total countdown. The factoring sequence used is only one possibility, as any total factoring of the total countdown of 120 would work just as well.

A PRF in the range of 1249 pps may not be a desired operating PRF. For instance, if a 625 PRF is desired, add another divisor stage, or divide by 2, and the PRF would be 624.58 pps. Remember that multiplying the divisors equals the total countdown, which would now be 240.

Taking another example, an intercepted radar has a very stable 312 PRF. What countdowns would be possible from a 2 km crystal?

Now factor the 240:

74948 Hz ¸ 5 = 14989.6 Hz

14989.6 ¸ 3 = 4996.5 Hz

4996.5 ¸  4 = 1249.1 Hz

1249.1 ¸  4 = 312.3 Hz

Multiply the divisors by themselves, 5 x 3 x 4 x 4 = 240.

Given the range marks desired (5 km) and the MUR (400 km) of a radar, find the crystal to be used and the possible countdown and the total countdown. First solve for the crystal frequency, which gives the calibration range marks.

                                                                C
                     XTAL FREQ =  -------------------------
                                                  2 x range marks

                                                
                                             = ----------------------- = 29979 Hz
                                                        10000

Then find the PRF that equates to the MUR (desired range):

                                            C
                        Rmur = ----------------
                                       2 x PRF

                                               299.7925 x 106
                         400 x 103 = ----------------------
                                                   2 x PRF

PRF = 800 x 103 = 299.7925 x 106

PRF = 374.74 pps

Now find the total countdown:

                                                                    XTAL FREQ
                    TOTAL COUNTDOWN = ----------------------
                                                                          PRF

                                                                         29979
                                                              = ---------------------  = 79.999 or 80
                                                                        374.74

Factor 80 and find a countdown sequence:

29979      ¸ 5 = 5995.8 pps

5995.8     ¸ 2 = 2997.9 pps

2997.9     ¸ 2 = 1498.95 pps

1498.95  ¸ 2 = 749.475 pps

749.475  ¸ 2 =      374.7375 pps

To check the countdown sequence multiply 5 x 4 x 4 = 80.

One common problem with countdown systems is that if one of the countdown circuits malfunctions, a different pulse rate is generated; however, the new pulse rate will also be very stable. When this happens, the total system divides the crystal frequency less than normal. Be aware of this common malfunction when studying a known radar with new parameters. It is often possible to figure which divisor has malfunctioned. An example of this is as follows. Assume in the previous problem that the first divisor circuit (5) malfunctions and does not divide. What would be the new PRF?

299790     ¸ 4 = 7494.75 pps

7494.75    ¸ 4 = 1873.6875 pps

TELEMETRY

The word telemetry is derived from the Greek words "tele" meaning from afar and "meter" meaning to measure. The modern version of the term is the science of measuring a physical quantity at some remote or inaccessible location (a missile in space) and transmitting the measured data to a more convenient or accessible location (the earth's surface). For the ELINT field, telemetry also means the receiving of this information, the conversion of the signal back into a replica of the original, and a recording of this replica. The science of telemetry came about because of the development of rockets, missiles, and aircraft. Due to the small size of the vehicle and weight restrictions, a method was developed that allows ground personnel to monitor events during the flight of the vehicle. Additionally, with the advent of the jet aircraft there were too many things for a test pilot to monitor to ensure proper operation, so telemetry was added to pass the information to a ground site.

Some physical properties that are measured and transmitted to a ground station by a telemetry system include the following:

Modern telemetry is an extremely important intelligence resource, and, as a result, many current telemetry systems use data encoding to prevent or reduce the risk of hostile exploitation. This is perhaps analogous to the encryption used in secure communications systems. All telemetry systems have certain common elements. These components are found in every telemetry system. The common components are transducers, multiplexers, transmitters, and receivers.

TRANSDUCERS

A transducer is an instrument that converts mechanical or physical variations into corresponding electrical variations. The transducer is the most essential component of a telemetry system since the system is only as good as the device performing the measurements. A number of devices are used as transducers. Some of the more common include the following:

The requirement for small size, lightweight, low power consumption devices, and for these devices to have the ability to withstand the vibrations and acceleration loads of take-off and landing influences the type of transducer used.

MULTIPLEXERS

Multiplexers are used so that a separate transmitter is not required for each transducer placed on a vehicle. In most telemetry applications, it is necessary to transmit more than one type of information simultaneously. Multiplexing techniques will be explained later in this topic.

TRANSMITTERS

Telemetry transmitters are no different than any other transmitter. They operate at a particular frequency and are modulated by information from a multiplexer. The type of modulation used differs with the type of telemetry system.

RECEIVERS

The telemetry receiver is an ordinary receiver that receives the range that the transmitter uses, operates with the correct bandwidth, and has the correct demodulation capabilities. Although any receiver that tunes the transmitted frequency can usually pick up the telemetry, the telemetry is usually modulated in a coded format, and often the data is stored on onboard tape recorders until the vehicle is over the country of origin before it is transmitted on a telemetry link.

MULTIPLEXING

Multiplexing is the technique that allows more than one data channel to be transmitted on a single carrier link. A multiplexer is a device that combines a number of data channels into a single output in such a way that one data input does not interfere with another input. The output from the multiplexer is then used to modulate the transmitted carrier. There are two basic methods of multiplexing data, frequency division mulitplexing (FDM) and time division multiplexing (TDM). In FDM, each data source has continual access to the transmitter, but is restricted to a portion of the total bandwidth. In TDM, each data source has access to the full bandwidth of the transmitter, but for only a fraction of the total time.

Frequency Division Multiplexing

Frequency division multiplexing (FDM) combines multiple data channels on a single carrier by assigning a different frequency band to each channel. At the transmitting site, the transducer senses or picks up the data to be conveyed and converts it to a corresponding electrical voltage level. There must be a transducer for each data category to be transmitted. The output voltage level of the transducer is used to frequency modulate a subcarrier oscillator that produces a frequency band. The different frequency bands

of the subcarrier are then combined, or multiplexed, into the composite signal. Because the inputs to the mixer are made up of individual FM signals, the output from the mixer is also FM. Due to the way the signals add together, the output signal will exhibit amplitude variations. The output from the mixer goes to the transmitter where it is broadcast at the selected frequency.

The receiver used by the ground site has a bandwidth wide enough to encompass all of the frequency components of the telemetry signal, but not so wide that a significant amount of noise is admitted to the receiver. Following the receiver, a discriminator removes the information from the carrier signal. The signal is then sent through a series of bandpass filters. The filters separate the signal back into its original components.

An advantage of an FDM system is that each channel provides a continuous read-out of the data being measured. This allows for a real-time simultaneous transmission of multiple data outputs. A disadvantage of the FDM system is the use of multiple frequency bands, making it necessary to use a large frequency bandwidth.

Time Division Multiplexing

Time division multiplexing (TDM) combines multiple data channels on a single carrier by periodically sampling the data in each channel. Telemetry is one of the major applications of TDM. The selection of the various data channels is accomplished through the use of a rotary switch. This switch, more commonly called a commutator, sequentially samples each transducer output for a definite period of time.

Each transducer output is changed into electrical signals by the signal generator. This generator converts the transducer-produced, time-varying voltage into a pulsed signal, some parameter of which is varied in accordance with the information signal from the transducer. In addition to the pulse generators used for the information, there must be at least one additional pulse generator used as a synchronizing pulse. The synchronizing pulses are used to identify the start of each frame of data. The synchronizing pulses do not change with time and are distinct from the data pulses.

The heart of the TDM system is the commutator, which follows the pulse generators. A commutator is a device that sequentially accepts input from multiple input channels and combines the pulses into an aggregate output data stream. The commutator is rotated from one contact to another in a given sequence, sequentially sampling the information provided by the transducers. The entire system is laid out in a row, in time sequence, so that the individual channels of data do not overlap. The communtator does not stop after one revolution but continues until all of the required information is passed.

An important point to remember is that it takes time to go from pulse number one all the way around and back to pulse number one. This time is called the frame time. One complete revolution of the commutator is called a frame. The synchronizing pulse is used to indicate the start of a frame. The number of frames completed in one second is the frame rate, the inverse of the frame time. The number of contacts on the commutator determines the number of data samples or channels per frame. The number of samples or channels per frame, multiplied by the frame rate, is called the commutator rate or sampling rate. If a telemetry signal is broken into parts, it is said to have subframes. Subframe rate is the number of subframes multiplied by the frame rate.

The modulator in TDM performs the same function as the modulator in FDM; it encodes the aggregate information channel onto the carrier for transmission. The modulator uses any one of several types of pulse modulation. The pulse-modulated signal is then fed to the multiplexer. The mixer in the TDM system serves the same purpose as that in the FDM system, to combine the transducer data with other output signals to transmit the desired information.

The first component of the ground portion is the receiver. The output of the receiver is the information baseband signal. The baseband signal passes into the decommutator. The decommutator demultiplexes the information baseband and reassembles the individual data signals from the aggregate signal.

An advantage of the TDM system is that it requires less bandwidth. Since TDM does not transmit all data at the same time, as with FDM (wider bandwidth, more power requirement), higher peak power can be obtained at the TDM transmitter output. The major disadvantage of the TDM system is that information is only sampled and is not continuously monitored.

TELEMETRY TECHNIQUES

When identifying the methods of multiplexing and modulation used in a particular telemetry system, begin where the signal is first generated and work toward the transmitted carrier. The first three letters identify the type of multiplex, the middle two letters identify the type of subcarrier modulation (when a subcarrier oscillator is used). When a subcarrier oscillator is not used, there will be no middle letter designator in the system identification. The last two letters identify the type of carrier modulation used (e.g., PAM/FM/AM). (NOTE: All three-letter groups beginning with "P" in the first position (such as in PAM, PDM, PCM, and PPM) indicate TDM. In this example, the method of multiplexing and modulation is a PAM type of multiplex, an FM type of subcarrier modulation, and an AM type of carrier modulation. Transmission techniques for telemetry systems are as follows:

FREQUENCY DIVERSITY

Frequency diversity is a technique used by some radar systems. This technique uses at least two different radio frequencies transmitted from the same antenna. Frequency diversity techniques were originally designed as an EP feature. The way in which the radio frequencies are transmitted determines which technique is used. Among the most common methods are frequency jumping, frequency tuning, and multiple frequency.

FREQUENCY JUMPING

Frequency jumping is instantaneously changing the carrier frequency by a discrete amount in either a set or a random manner. There are four modes of operation as follows:

FREQUENCY TUNING

Frequency tuning is changing the transmitted frequency mechanically or electromechanically by tuning the power oscillator. The tuning is done at the output frequency and requires the use of tunable high-power output tubes. Frequency tuning requires that either the low-frequency oscillator, the multiplier stages, or both are modified to obtain a change in the transmitted RF. The final frequency can be any value within the tuning range of the RF oscillator. Frequency tuning is discrete; the transmitted RF will be one of several values and no others.

Illuminating a target with multiple RFs can be used to mitigate the effects of phase cancellation from returns reflected from different locations on the target surface. These effects are called scintillation. For example, energy reflected from the nose of an aircraft may be partly canceled by energy reflected from the tail if the distance separating the nose and tail is on the order of a half wavelength. Changing the transmitted frequency (and therefore the wavelength) increases the likelihood that some energy will result if target scintillation is negligible.

MULTIPLE FREQUENCY

Multiple frequency is a technique that is used to increase the transmitted power of a radar system by transmitting more than one frequency, simultaneously, from the same antenna. A separate antenna is required for each RF transmitted. Identical pulse trains are transmitted on each frequency and transmitted simultaneously. If the aggregate power level is greater than the power-handling capacity of the wave guide and antenna, then the pulse trains can be interleaved (transmitted sequentially) and re-phased in the receiver. Multiple frequency techniques produce the same result as frequency jumping with the results available on a pulse-to-pulse basis rather than across several pulses.

ADVANTAGES OF FREQUENCY DIVERSITY

Frequency agility techniques provide antijamming capabilities for radar systems. Some additional advantages include the following:

As a collection operator, the biggest reason for intercepting and analyzing frequency diversity signals is to determine their antijamming capability. This information has a direct bearing on the design of radar countermeasures equipment. Another reason is to analyze them for their possible use by beam steering systems. Frequency diversity radars may operate in any radar band.

ELECTRONIC ATTACK

Operating in a hostile environment, a radar may be subjected to deliberate interference, or jamming. Jamming is noise or a noise-like signal transmitted at frequencies in the receiver bandwidths of radar that obscure the radar return signal. Jamming appears as extraneous responses on the radar display. These responses may be few and resemble real targets or be many and fill the whole display. The purpose of jamming is to create a sufficient degree of deliberate interference that a radar is unreliable or unusable. Jamming falls under electronic attack (EA).

EA signals can be divided into three classes depending on whether the intent is to obliterate, confuse, or deceive the target radar. Additionally, these three classes can be divided into active or passive.

ACTIVE ELECTRONIC ATTACK

Active electronic attack signals involve the creation of jamming signals that are radiated into the electromagnetic spectrum. Active jamming is any interference produced by an EA transmitter specifically intended to disrupt a radar’s normal operations by saturating or masking radar returns or by introducing additional realistic false targets. Active EA is of maximum concern to the operator and analyst.

Barrage Jammers

Barrage jammers (sometimes called confusion jammers) radiate a noise signal over a relatively wide band of frequencies. The frequency bandwidth is designed to cover emitters with frequency agility, several emitters simultaneously, or cases where only the approximate RF of a radar is known. The total power available to a barrage jammer is spread over a large bandwidth. The victim radar can burn through by increasing the transmitter power. The best barrage jamming signal is white Gaussian noise (wideband noise) covering the bandwidth of the radar receiver to be jammed.

Spot Jammers

Spot jammers radiate in a relatively narrow band compared to barrage jammers yet wide enough to cover a radar receiver bandwidth. See figure 2-11. This requires high power to provide an interference signal strong enough to mask the real target returns. Spot jamming may be avoided by changing the radar frequency, ideally on a pulse-to-pulse basis to avoid the jammer tracking the new frequency.

fig2-11.gif (8462 bytes)


Figure 2-11.— Barrage versus spot jamming.

The victim radar could also use a notch filter to remove the interfering signal from the video signal path or if the signal is AM or FM modulated, a low-pass filter can remove the interference. There are two types of spot jammers—multiple and sequential. These are explained as follows:

Swept Frequency Jammers

Swept frequency jammers sweep the carrier frequency of a tunable transmitter over the radar band. A single swept jammer can simultaneously jam many different radars operating at different frequencies.

Seek And Lock-On Jammers

Seek and lock-on jammers are essentially a swept jammer that has the capability to follow the victim radar that changes RF or search for and lock-on to the RF of another radar should the first stop operating. An enhancement to jamming has been the addition of a look-through capability. The look-through system colocates a receiver with the jammer. The jammer is switched off at intervals and the receiver monitors any changes in the victim or in the electromagnetic environment.

Deception Jammers

Deception jammers are designed to present jamming signals that are indistinguishable from the real target echoes. Deception jamming degrades a tracking or homing radar once it begins to track a target. It is almost always used for self-protection and generally uses a radar repeater. Three types of deception jammers are repeater jammers, transponders, and range gate stealers.

REPEATER JAMMERS

Repeater jammers generate false targets on the radar display by delaying the received radar signal and then retransmitting it at a slightly later time. This delay causes the retransmitted signal to appear at a range and azimuth different from that of the actual location of the jamming platform.

TRANSPONDERS

When triggered by a radar signal, transponders play back a stored replica of the radar signal or a noise pulse. The transponder can be programmed to remain silent when illuminated by the main radar beam and to transmit only when illuminated by the sidelobes, creating spurious targets on the radar display at directions other than that of the true target.

RANGE GATE STEALERS

Range gate stealers cause a tracking radar to "break lock" on the target. See figure 2-12. A typical tracking radar determines the target range by generating a pair of range gates. The radar receiver adjusts the position of these gates to keep the target return centered between them. When the target return is centered between the two range gates, the tracking radar is "locked on" the target. The range gate stealer initially transmits a single cover pulse for each pulse received from the radar. The cover pulse is transmitted with no delay and therefore arrives at the radar at the same time as the return pulse causing the combined return to be stronger. The stronger return causes the radar to reduce the amount of amplification during processing. The repeater slowly shifts the timing of its own pulse to cause an apparent change in the tracking range. The radar tracking circuits will follow the stronger false signal from the jammer and ignore the weaker echo from the target. Also, the repeater can be turned off, leaving the tracker without a target and forcing it to revert to the search mode.

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Figure 2-12.— Range gate stealer.

PASSIVE ELECTRONIC ATTACK

Passive electronic countermeasures do not generate or amplify electromagnetic radiation; they act in a passive manner to change the energy reflected back to the radar. Passive EA is intended to confuse the radar as to the true location of the target. Examples of passive EA are chaff, decoys, and radar cross-section reduction.

Chaff

One of the oldest methods of confusing enemy radar, chaff first appeared in WWII when metallic strips were used over Hamburg, Germany, in 1943. Chaff is a collective name for a family of reflective materials. Chaff consists of a large number of reflectors, usually in the form of metallic foil strips packaged in a bundle. The length of the foil determines the optimum reflected frequency. Chaff bundles usually consist of a variety of strip lengths for multiple frequency coverage. The objective of chaff is to reduce the effectiveness of the radar by providing a very large, bright target return that masks the returns from targets of interest.

Chaff is used to either deceive or to confuse a radar. Chaff is dispensed from aerial rockets and fired ahead, behind, above, below, or simply dropped from aircraft. Chaff shot forward can deceive the range and velocity tracking gates of tracking radars. Chaff is usually effective for a period of about 30 minutes.

Decoys

A decoy is a small aircraft-like vehicle that appears to a radar operator as a larger target. Decoys are usually designed to incorporate radar signal enhancement devices such as corner reflectors, Luneburg reflectors, or active repeaters. The decoy may use a small active jammer to make it resemble the target aircraft. A defense system can be overloaded by a sufficient number of decoys. Decoys can confuse or saturate an air defense network to reduce their effectiveness against approaching aircraft.

Radar Cross-Section Reduction

Modification of the aircraft radar cross section incorporates the use of radar-absorbent materials and airframe structural changes. The target radar cross section can be reduced by material that absorbs electromagnetic energy. One type of radar-absorbing material is a quarter-wavelength thick. The energy reflected from the front surface of the material cancels energy that entered the material and was reflected from the inner surface. This material is the same as coatings applied to optical lenses and is narrowband. Another type internally dissipates the incident energy. This absorber is much thicker but has the advantage of being relatively broadband.

MILLIMETER WAVE RADAR SYSTEMS

Millimeter waves are radio waves that range in frequency from 30 GHz to 300 GHz. The millimeter wave band gets its name from the wavelength of millimeter wave signals. A radio wave with a frequency of 30 GHz has a wavelength of about 10 millimeters (mm), and a 300-GHz radio wave has a wavelength of about 1mm. Millimeter wave radio signals have the shortest wavelength of any signals commonly associated with the radio spectrum.

The major attributes of the millimeter wave region are the large bandwidth, small antenna size, and the characteristic wavelength. Large bandwidth means the high range resolution can be achieved. The short wavelengths allow narrow beamwidths of high directivity with physically small antennas. Narrow beamwidths are important for high-resolution imaging radar to avoid multipath effects when tracking low-altitude targets. The short wavelengths are useful for exploring scattering objects whose dimensions are comparable to the millimeter wavelengths such as clouds. Another advantage of the short wavelengths is that a Doppler frequency measurement of fixed accuracy gives a more accurate velocity measurement than at lower frequencies.

Interaction with the atmosphere is greater at millimeter waves than at microwaves. Because millimeter wavelengths are comparable to the size of raindrops, fog, snow flakes, and suspended water vapor, they cause significant absorption of all millimeter wave transmissions. This attenuation would normally be considered a disadvantage, but it also makes it very unlikely that the signal is intercepted at some considerable distance by an unwanted listener. It is also unlikely that any nearby listener could intercept the signal sidelobes. For these reasons, millimeter wave systems are classified as low probability of intercept (LPI). Despite the advantages, atmospheric absorption remains as the largest obstacle to the use of millimeter wave systems. The large amount of attenuation can be used to advantage in those special cases where it is desired to reduce mutual interference or to minimize the probability of the radar being intercepted by a hostile receiver at long range.

Millimeter wave systems use smaller components than lower frequency systems because of the smaller wavelengths. The extremely high operating frequency also permits the use of very wide bandwidth signals. The smaller wavelengths allow very narrow beamwidths that are produced by small antennas resulting in high angular resolution. The energy reflected from a target is also a function of the wavelength. Smaller wavelengths produce more information about the target (e.g., shape, size, and relative motion). The greater available bandwidth permits more information to be transmitted in a communication system and greater resolution in a radar system.

The millimeter wave radio band is the last to see significant development as a communications medium. Millimeter wave communication is considered ideal for satellite-to-satellite communication because it is less accessible to eavesdropping, jamming, interference, or unauthorized manipulation. Automobile manufacturer Daimler-Benz is experimenting with technology for collision-avoidance systems for automobiles. In the military realm, millimeter wave microcircuits for missiles are less expensive and more reliable. They work well on the battlefield because, unlike traditional optical or infrared systems, they can form images with millimeter wave cameras that "see" through fog and thick battlefield smoke.


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