This area of the site provides a tutorial which is intended to provide fundamental definitions and descriptions of the various principles of radar systems with empahsis on target acquisition and tracking plus weapon guidance systems.
A proper understanding of these principles is necessary in order to derive an appreciation for the operational use of Defensive Electronic Countermeasures (DECM) and digital Threat Warning Systems (TWS).
The tutorial concludes with a brief section on the generic system concept of digital Radar Warning Receiver Systems (RWR) with reference to certain specific systems on which more informatin can be provided in an advanced course on Passive EW.
Finally, an extensive glossary of Electronic Warfare terms provides a handy reference to these frequently used definitions.
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Radar (Radio Detection and Ranging) is employed in many forms, from complex air defense networks to simple IFF beacons and altimeters. The primary threat radars for aircraft are the fire control radars associated with weapons, particularly guided missiles. In this section, each radar and radar parameter important to RWR will be discussed in a general manner. Frequent reference will be made to more detailed sources; the reader should pursue these sources in the library.
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Before one can understand electronic warfare one must first know the principles of radar tracking. The emphasis in this tutorial will be placed on pulsed radars since they are the most commonly used. (Continuous wave (CW) radars are described in Section 2.4.2; note, however, that only the techniques change, and the principles are the same.) Basically, a fire control radar system consists of a transmitter, a receiver, an antenna system, a display device, and a computer capable of target tracking (predicting the location of the target at some future time based on its present flight parameters so that the radar can move itself always to point at the target). To perform this function, the radar must measure azimuth, elevation, and range and the rate of change of each. (See Figure 1.)
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The transmitter sends out a high energy signal which is reflected back to the radar whenever it strikes a reflecting object. The amount of energy reflected by an object depends on its physical size and reflectivity, the two parameters which determine the radar cross section (RCS) of an object. When the RCS of the smallest object a radar wishes to track and the maximum range to which track is required are known, the receiver sensitivity and required transmitter power can be determined. A radar determines range to an object by the round trip time-of-flight (at the speed of light) of a transmitted pulse. The uncertainty in range is the distance that the transmitted pulse travels in a time equal to one-half the width of the pulse. Thus, time and range are identical to a radar. For a TTR, the maximum range and range resolution are determined by the weapon associated with that radar. These factors all interact as follows: the transmitter must pulse as often as possible so that the maximum average power is returned to the receiver, but it cannot pulse faster than the round-trip time to a target at the maximum range of the weapon and the pulsewidth must locate the target within the accuracy and warhead size of the weapon.
Example: The weapon has a 40 nautical mile maximum intercept range and its kill radius is 300 feet. The pulse travels about 1000 feet per microsecond so that the time to and from the target is 500 microseconds, and the pulse width must be 0.6 microseconds or less. Therefore, the radar cannot pulse faster than about 2,000 times per second with a Pulse Width of 0.6 microseconds.
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A TTR receives initial range information from an assisting radar as discussed later in the tutorial. Receiver signal-to-noise ratio can be greatly improved by only "opening" the receiver input circuitry when a target echo is expected. This is called "range gating" and the period when the receiver is open is called the Range Gate. The optimum time interval for a range gate is equal to the pulsewidth of the radar.
By using two adjacent range gates, the radar can determine where the target is (equal return in both gates). As the return becomes unequal in these two gates, the radar can measure range rate and direction of change. With this data, the radar computer can automatically range track a moving target. This is known as Range Gate Tracking. Automatic range tracking is accomplished by keeping equal target return in two adjacent range gates as the target moves.
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If the radar pulsed at twice the rate of the example above, a target at 40 nautical miles would reflect two pulses in 500 microseconds and two targets would appear -- one at 20 nm and one at 40 nm -- so that range information is unreliable. This is the most common form of ECM -- for each pulse of the radar, send back one or more pulses from a target carried transmitter to destroy range data. If the ECM pulse repetition rate (PRR) is properly selected, the radar will "see" and display a continuous chain of targets along the radial from the radar to the true target and beyond.
A long line of targets generates a continuous chain of undesirable pulses in the receiver (e.g., noise). Since time and distance are the same for a radar, these noise pulses need not be physically removed from the target but can be generated on board. This is known as noise jamming -- sending random, high rate false target echoes to the radar. (See Figure 2.) If the radar is multiple frequency (RF) or there are several different radars in the area, noise can be generated at all frequencies by "sweeping" the frequency of the noise pulses through all the known frequencies at a rate at least equal to or faster than the pulse rates of the radars.
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A radar determines angle information by using an antenna array to focus the transmitted signal into a well defined beam. Due to the property of antenna reciprocity, signals will be received from the same area defined by the transmitted beam -- a directional transmitter is a directional receiver. When an antenna focuses a beam, it produces a main lobe and numerous side lobes; the more directional the antenna, the greater the number of side lobes. In a perfect antenna, the size of the main lobe is
where A is the angle (in radians), w is the transmitter wavelength and s is a geometrical factor determined by the physical size and shape of the antenna. For a given frequency, the larger the antenna, the smaller the main lobe. This formula defines the entire main lobe (beam) size whose energy distribution has a central maximum and falls to zero at the edges. The points at which the power falls to 0.707 of the maximum are known as the half-power points and the angular size of the beam between these half-power points is the defined beamwidth of the radar. This definition is always understood when discussing radar parameters, but the difference between the full beamwidth and the defined beamwidth becomes important in EW. Outside the defined beamwidth the power drops very rapidly to the outer edges of the full beamwidth.
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Radar beams will also be polarized. Polarization is the physical orientation of the E and H fields which exist in electromagnetic energy. For best efficiency, the transmitting and receiving antennae should have the same polarization.
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When a radar attempts to locate a target (scans a small sector of its total tracking envelope) the target receives a large number of pulses, each from a slightly different orientation of the antenna. The radar computer measures each pulse and generates a power plot in which the maximum point is called the power centroid. The accuracy of the radar is a measure of its ability to locate the power centroid and align its antenna so that the centroid is on the antenna axis. Automatic Angle Tracking is accomplished by keeping the power centroid centered on the antenna axis as the target moves.
To track the centroid of power, the radar must "look" at antenna angles where there is no return from the target -- it must look where the target is not. This looking is also called scanning and can be the same scan used for acquiring (locating) the target as in a track-while-scan (TWS) radar. Note that this implies that for best tracking the beamwidth should be larger than the target so that no target exists in adjacent beamwidths. If the target is bigger than the beamwidth of the radar, the power return will be about equal in several antenna orientations so that the power centroid will be broad in angle and thus degrade tracking accuracy. If the target is much larger than the beamwidth, the power centroid will be so broad that the radar will not be able to track but instead will "walk" over the target due to the scan while looking for some point of higher return.
Resolution is the ability to distinguish multiple targets. When the computer generates the power plot, any pulse whose value is less than .707 of the power centroid is assumed to be from a different beamwidth due to the definition of beamwidth. Therefore, to resolve two targets there must be a point between them where the returned power is down to the half-power points. But that, by definition, is a separation equal to the beamwidth of the radar. The resolution cell of the radar, then, is the solid volume described by one beamwidth and the range resolution; multiple targets within one res cell will appear as one target whose power centroid will be located somewhere between all the targets to the accuracy of the radar.
Some radar systems use separate, large-beamed transmitters for Azimuth (Az) and Elevation (El) tracking. This scheme allows the system to track one power centroid while scanning (TWS) its full acquisition sector. The resolution for such a dual beam system is often given as the inter-section of the smallest dimension of each beam, but this is not to be confused with the resolution cell. For a dual beam TWS system, each transmitter has a res cell in which the power centroid of the target or targets will be located to within one beamwidth. The TWS computer can then locate the two power centroids to within the size of the intersecting area of the two beams. This difference between the computed resolution (which is often the published resolution) and the resolution can be important to ECM tactics. The two beams, due to their different physical orientations, may receive differing amounts of jamming. Since every radar requires three coordinates for accurate tracking -- Az, El, and range -- jamming only one beam can be useful if ECM resources become limited.
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Due to the directional nature of the receiving antenna, angle jamming by target carried noise transmitters is not possible since the jammer will only serve to highlight the target like a microwave beacon. Side-lobe jamming is possible from transmitters not carried on the target if these transmitters have enough power to overcome the side-lobe attenuation designed into the antenna. For example, if the first side-lobe is 16 dB down from the "main-bang", the jammer must be capable of returning 16 dB more power to the radar than the target normally returns. Side-lobes are spaced about one beamwidth apart, but since the computer and display are synchronized to the antenna, side-lobe jamming actually creates a false target in the main lobe of the radar. If side-lobe penetration is successful, range jamming can be performed by noise as already discussed.
A highly reflective (large RCS) target can cause side-lobe return in the main beam of the radar. That is, if the return from the target when it is illuminated by the side-lobes can overcome sidelobe attenuation, the radar will "see" false targets due to the synchronization accountability which radars must use. This effect causes the target to appear larger than its actual physical size. Chaff clouds have been observed to create this "side-lobe jamming".
A second effect caused by large targets is called Effective Beam Broadening. In this case, the portions of the target within the full beamwidth but outside the defined half-power point beamwidth return power to the antenna which equals or exceeds the half-power return. The radar, due to accountability, must credit this to an adjacent orientation of the antenna with the effect being that a defined two-degree beam can actually have a three- or four-degree resolution.
Active Deceptive ECM (DECM) also can effectively create angle jamming with ECM transmitters carried on a single aircraft. When a TTR scans a target during track the target return will be modulated at the scan frequency. DECM determines the scan pattern on the target and transmits a stronger signal of opposite modulation. This will cause the radar to track in the wrong direction for one beamwidth. The radar will then see the target in a second beamwidth, but track has probably been "broken" and must be reacquired. DECM systems are much more complex than simple noise jammers.
Angle jamming can also be performed by proper flight tactics. If a formation of aircraft maintains a separation of one beamwidth, then the radar will "see" a large target which appears on the display scope as one target as big as the total beamwidths that the formation occupies. This is particularly effective against systems which use separate Az and El tracking radars and then have computer matched tracking coordinates because the computer must examine every combination of Az and El returns to obtain a match. If the individual aircraft then maneuver within their assigned beamwidth, their power centroids will be constantly shifting, merging and separating so that Az-El correlation will be difficult. When the aircraft are carrying noise jammers, the target cluster becomes two dimensional and automatic target tracking accuracy is degraded. This "cooperative jamming" can be continued to the point that the entire radar display indicator suffers "white-out". For example, in a 2-degree radar with a 16-degree display scope, eight aircraft at 2-degree separations with noise jammers will fill the entire scope with noise (false targets). However, the centroid of the jamming power is located on the target aircraft so that track, and especially manual track, is still possible.
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To summarize to this point, the important concepts for RWR designers are:
1. Determination of unambiguous range places stringent PRF requirements on the radar.
2. Antenna size is inversely proportional to the radar frequency. Since mobility is a prime consideration for air defense systems, most threat radars will be in the higher frequency bands.
3. High accuracy target location requires small transmitted beams and narrow pulsewidths. These small beams must search an angular segment when first acquiring a target. These beams must also "look where the target is not" in order to track the target. These two effects are called Scan.
4. Determination of angle information/error requires well defined scan patterns.
5. Best radar reception requires proper antennae polarization.
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