Electronic Warfare - Antenna Scan types
Scan pattern referring to how radars steer their beam across their field of view to search for targets
A circular scanning radar uses an antenna system that continuously scans 360° in azimuth . The time required for the antenna to sweep one complete 360° cycle is called the scan rate. Scan duration is the number of “hits per scan,” or the number of pulses, reflected by a target as the radar beam crosses it during one full scan. Most pulse radars require 15 to 20 hits per scan to obtain sufficient information to display a target. The factors that determine the number of hits per scan the radar receives include pulse repetition frequency (PRF), antenna beamwidth, and scan duration.
Circular scan radars provide accurate target range and azimuth information. This makes these radars ideal for the roles of early warning and initial target acquisition. To accomplish these missions, the antenna generates a fan beam that has a large vertical beamwidth and a small horizontal beamwidth. Since elevation information will normally be provided by height finder radars, the size of the vertical beamwidth is not a limitation. This antenna scan allows the radar to scan large volumes of airspace for early target detection. Since early detection is the primary goal of early warning radars, accurate altitude and azimuth resolution are secondary considerations.
Circular scan radars designed for early warning transmit a radar signal with a low PRF. A low PRF allows sufficient time for the radar pulse to travel long distances, and return, before another pulse is transmitted. This gives the radar system a long, unambiguous range capability. Circular scan radars with low PRFs generally use long pulse widths in order to increase their average power and long-range detection capability. The scan durations of early warning radars are relatively long to provide the required “hits per scan” for long-range target detection. The plan position indicator (PPI) scope display is normally used with circular scan radar
In order to provide coverage for a large volume of airspace, the beamwidth associated with circular scan radar is relatively wide. This wide beamwidth, coupled with the long pulse width and low PRF, gives the circular scan radar a large resolution cell, especially at long ranges. This limitation can be exploited to mask force size and composition. However, as range decreases, the dimensions of the resolution cell decrease, and circular scan radar will begin to break out target formations.
Circular scan radars provide range and azimuth information for both early warning and acquisition roles. Modified circular scan radars that can also provide elevation information may be used for ground control intercept (GCI) roles. Two modified circular scan radars that determine range, azimuth, and elevation are the V-beam and the stacked beam.
The V-beam radar transmits two fan-shaped beams that are swept together. A vertical beam provides range and azimuth information. A second beam, rotated at some convenient angle, provides a measure of the altitude of the target.
A stacked beam radar employs a vertical stack of fixed elevation “pencil” beams which rotate 360°. Elevation information is obtained by noting which beam contains the target return. Range and azimuth information is determined in the same manner as in an early warning radar.
Linear scan is a method used by some radar systems to sweep a narrow radar beam in a set pattern to cover a large volume of airspace. Linear scans can be oriented in a vertical direction for height finder radars or in a horizontal direction, or raster, for acquisition and target tracking radars. Unidirectional linear radar scans in a single direction then begins its sweep all over again. Generally, linear scans offer excellent single-axis coverage, and the narrow beam offers enhanced azimuth and elevation resolution.
A helical scan is a unidirectional scan pattern that allows a “pencil” beam to search a 360° pattern. The antenna sweeps a 360° sector in a clockwise direction. After each complete revolution, the antenna elevation is increased. This scan pattern is repeated for a specified number of revolutions, in this case, three, 360° sweeps. At the end of the scan pattern, the antenna elevation is reset to the initial elevation and the scan is repeated. A helical scan pattern is commonly used as a target acquisition mode for radar systems with narrow vertical and horizontal beamwidths.
A bidirectional linear scan, such as a raster scan, sweeps both horizontally and vertically. A raster scan uses a thin beam to cover a rectangular area by horizontally sweeping the area. The angle of elevation is incrementally stepped up or down with each horizontal sweep of the desired sector. After the sector has been covered, the angle of elevation is reset to the original value and the process is repeated. The number of raster bars is set by the number of horizontal sweeps in the basic raster pattern. Shows a four-bar raster scan, which is normally associated with airborne interceptor (Al) radar.
A conical scan, or conscan, radar is generally used for precision target tracking. A conical scan radar employs a pencil beam of radar energy that is continuously rotated around the target. This circular rotation of a pencil beam generates a cone-shaped scan pattern with the apex of the cone located at the antenna. Thus, the name conical scan.
As the pencil beam rotates, the circular scan patterns overlap in the center. This creates a central tracking area that has a much smaller effective beamwidth than the rotating pencil beam. This results in a very precise tracking solution.
Since conical scan radars are designed for precision target tracking, these radars normally operate at high frequencies, high PRF, narrow pulse widths, and narrow beamwidths. The rotation rate of the pencil beam can exceed 1,800 revolutions per minute. This means that both azimuth and elevation data can be updated about 30 times per second.
The combination of conical scan and raster scan is called a Palmer-raster scan. A Palmer-raster scan uses a thin beam, employing a conical scan searching pattern, for a specific sector of airspace. With each sweep of the sector, the angle of elevation is incrementally stepped up or down. After the vertical sector has been covered, the angle of elevation is set at the original elevation and the process is repeated. The number of bars is determined by the number of vertical search scans.
The combination of a conical scan and a circular scan is called a Palmer scan. Palmer scans incorporate a circular scanning antenna to search the entire horizon while simultaneously performing a conical scan. If the radar antenna is also performing a unidirectional altitude search in conjunction with this scan, it is employing a Palmer-helical scan.
A track-while-scan (TWS) system uses a technique that allows a radar to track one or more targets while scanning for others. Radar systems with a TWS capability must be able to generate two or more distinct radar beams.
A conventional TWS radar employs two antennas that work with each other to perform the scan function . Each antenna produces a separate unidirectional beam. Each beam is transmitted at a different frequency. The vertical antenna generates a beam employing a vertical sector scan similar to height finder radar except the beamwidth is narrower and it scans at a higher rate. The horizontal antenna generates an identical beam employing a horizontal sector scan at a different frequency. The track function is accomplished in the area where the two beams pass through each other. A target that is within this center area is tracked and positional information on range, elevation, and azimuth is updated each time the beams sweep through the area.
The phased array radar is a product of the application of computer and digital technologies to the field of radar design. A phased array is a complex arrangement of many individual transmitting and receiving elements in a particular pattern. Common arrays include linear, planar, curved, and conformal, with linear being the most common. By using a computer to rapidly and independently control groups of these individual elements, a phased array antenna can, in effect, radiate more than one beam from the antenna. Multiple beams and computer processing of radar returns give the phased array radar the ability to perform the TWS function. The most common employment of the TWS capability of the phased array radar is in the air-to-air arena.
The number of individual transmitting and receiving elements is limited by the size of the radar antenna. The number of targets phased array radar can track is limited by the number of independent beams the antenna can generate. Many phased array radars, especially air-to-air radars, do not track and scan simultaneously, but rapidly switch between the two modes to overcome this limitation.
Modern TWS radars employ computer signal processing and complex computer algorithms to simplify the problem of target correlation. Air-to-air radar typically uses a raster scan to search a volume of airspace. In the search mode, the radar simply presents all targets detected in this airspace to the pilot on his radar display. In the TWS mode, the radar employs computer processing to figure out target correlation and update target information. This is done automatically, and the results are presented on the display.
In radar technology and similar fields, track-before-detect (TBD) is a concept according to which a signal is tracked before declaring it a target. In this approach, the sensor data about a tentative target are integrated over time and may yield detection in cases when signals from any particular time instance are too weak against clutter (low signal-to-noise ratio) to register a detected target.
The TBD approach may be applied both for pure detection when the tentative target displays a very small amount of apparent motion, as well as for actual motion tracking. In the first case the problem is considerably simpler, both in terms of the amount of calculation and the complexity of algorithms
The function of the antenna during transmission is to concentrate the radar energy from the transmitter into a shaped beam that points in the desired direction. During reception, or listening time, the function of the antenna is to collect the returning radar energy, contained in the echo signals, and deliver these signals to the receiver. Radar antennas are characterized by directive beams that are usually scanned in a recognizable pattern. The primary antenna types in use today fall into three categories: parabolic, Cassegrain, or phased array antennas. Additionally, the method radar antennas employ to sample the environment is a critical design feature of the radar system. The scan type selected for a particular radar system often decides the employment of that radar in an integrated air defense system (lADS). The process the radar antenna uses to search airspace for targets is called scanning or sweeping.
One of the most widely used radar antennas is the parabolic reflector.
The parabola-shaped antenna is illuminated by a source of radar energy, from the transmitter, called the feed. The feed is placed at the focus of the parabola, and the radar energy is directed at the reflector surface. Because a point source of energy, located at the focus, is converted into a wavefront of uniform phase, the parabola is well suited for radar antenna applications. By changing the size and shape of the parabolic reflecting surface, a variety of radar beam shapes can be transmitted.
The antenna depicted in Figure generates a nearly symmetrical pencil beam that can be used for target tracking.
Elongating the horizontal dimensions of the parabolic antenna creates a radar antenna called the parabolic cylinder antenna. The pattern of this antenna is a vertical fan-shaped beam. Combining this antenna pattern with a circular scan technique creates a radar system well suited for long-range search and target acquisition.
Elongating the vertical dimensions of the parabola creates a radar antenna that generates a horizontal fan-shaped beam with a small vertical dimension. This type of antenna is generally used in height-finding radar systems.
Another variation of the basic parabolic antenna includes using an array of multiple feeds instead of a single feed. This type of parabolic antenna can produce multiple radar beams, either symmetrical or asymmetrical, depending on the angle and spacing of the individual feeds.
A Cassegrain antenna uses a two-reflector system to generate and focus a radar. The primary reflector uses a parabolic contour, and the secondary reflector, or subreflector, has a hyperbolic contour. The antenna feed is located at one of the two foci of the hyperbola. Radar energy from the transmitter is reflected from the subreflector to the primary reflector to focus the radar beam. Radar energy returning from a target is collected by the primary reflector and reflected as a convergent beam to the subreflector. The radar energy is rereflected by the subreflector, converging at the position of the antenna feed. The larger the subreflector, the closer it can be to the primary reflector. This reduces the axial dimensions of the radar but increases aperture blockage due to the subreflector. A small subreflector reduces aperture blockage, but it must be positioned at a greater distance from the primary reflector.
To reduce the aperture blockage by the subreflector and to provide a method to rapidly scan the radar beam, the flat plate Cassegrain antenna was developed.
The fixed parabolic reflector is made up of parallel wires spaced less than a half wavelength apart and supported by a low-loss dielectric material. This makes the fixed parabolic reflector polarization sensitive. It will completely reflect one type of linear polarization and be transparent to the orthogonal polarization. The fixed antenna feed, in the middle of the moveable mirror, transmits a radar signal polarized to be reflected by the parabolic reflector. The moveable mirror is constructed as a twist reflector that changes the polarization of the radar signal by 90°. The signal from the feed is reflected by the parabolic reflector to the mirror, which rotates the polarization 90°. This rotation makes the transmitted signal transparent to the parabolic reflector, and the signal passes through with minimal attenuation. The radar beam can be scanned over a wide area by rotating the moveable mirror. A deflection of the mirror by the angle Ɵ results in the beam scanning through an angle of 2Ɵ.
The geometry of the Cassegrain antenna is especially well suited for monopulse tracking radar applications. Unlike the parabolic antenna, the complex feed assembly required for a monopulse radar can be placed behind the reflector to avoid aperture blocking.
PHASED ARRAY ANTENNA
The phased array radar is a product of the application of computer and digital technologies to the field of radar design. A phased array antenna is a complex arrangement of many individual transmitting and receiving elements in a particular pattern. A phased array antenna can, in effect, radiate more than one beam from the antenna by using a computer to rapidly and independently control groups of these individual elements. Multiple beams and computer processing of radar returns give the phased array radar the ability to track-while-scanning and engage multiple targets simultaneously.
Phased array radar uses the principle of radar phase to control the individual transmitting and receiving elements. When two transmitted frequencies are in-phase, their amplitudes add together, and the radiated energy is doubled.
When two transmitted frequencies are out-of-phase, they cancel each other.
Phased array radars use this principle to control the shape of the transmitted radar beam.
Phase relationships and antenna element spacing determine the orientation of the transmitted beam. For eg, antenna elements A and B are separated by one-half wavelength and are radiating in-phase, that is, when one is at the positive peak, the other is also at a positive peak. Since the elements are one-half wavelength apart, when the positive peak radiated by A reaches B, B will be radiating a negative peak. As the peaks propagate along the X axis, they will cancel each other out. The total radiated power along that axis will be zero. Along the Y axis, however, the positive peaks from A will add to the positive peaks from B, causing the total radiation along this axis to be at its maximum value. This type of array is called a “broadside array” because most of the radiation is in the direction that is broadside to the line of the antenna array.
The computer controlling the phase of the signal delivered to each transmitting and receiving element of a phased array antenna controls the direction and shape of the radiated beam. By shifting the phase of the signals between 0° and 180°, the beam sweeps. This is the basic means of producing an antenna scan. In addition, the amplitude, or power, of the signal applied to each element can be varied to control the sidelobes. This alters the shape of the beam which affects the range capability and angular resolution of the radar.
Depicts a variation of the phased array antenna, known as a planar array antenna. A planar array antenna uses transmit and receive elements in a linear array, but, unlike the phased array radar, the elements are smaller and are placed on a movable flat plate. The ability to simultaneously track several targets is one advantage of this type of radar.
The most important characteristic of any type of antenna is antenna gain.
Antenna gain is a measure of the ability of an antenna to concentrate energy in the desired direction. Antenna gain should not be confused with receiver gain, which is designed to control the sensitivity of the receiver section of a radar system. There are two types of antenna gain: directive and power.
The directive gain of a transmitting antenna is the measure of signal intensity radiated in a particular direction. Directive gain is dependent on the shape of the radiation pattern of a specific radar antenna. The directive gain does not take into account the dissipative losses of the antenna. Directive gain is computed using Equation
GD = Maximum radiation intensity / Average radiation intensity
The power gain does include the antenna dissipative losses and is computed using Equation
Power gain = max radiation intensity/ Radiation intensity of an isotropic antenna
The term isotropic antenna describes a theoretical spherical antenna that radiates with equal intensity in all directions. This results in a spherical radiation pattern. The power density for any point on an isotropic antenna is the radiation intensity and can be calculated by dividing the total power transmitted (PT) by the total surface area of the sphere,
Power density (Isotropic Antenna) = PT (watts)/ 4*Pie*r2 (Cm2)
The radiation pattern of an isotropic, or spherical, antenna would provide neither azimuth nor elevation resolution and would be unusable for radar applications. To provide azimuth and elevation resolution, a practical antenna must focus the radar energy. The power density of a practical antenna differs from the isotropic antenna only in terms of antenna gain (G).
Power Density (Practical antenna) = PT*G/ 4*pie*r2
The actual power gain (G) of a practical antenna can be calculated by using Equation
Ae= effective area of aperture
Lamda = Wavelength of the radar
The power density and gain of an antenna are a function of the antenna pattern of a radar system. Figures illustrate the antenna pattern of a typical parabolic antenna. Most of the power density of the radar is concentrated in the main beam. However, since the radar is not a perfect reflector, some radar energy is transmitted in the sidelobes. In addition, there is spillover radiation due to the energy radiated by the feed that is not intercepted by the reflector. Finally, the radar has a back lobe caused by diffraction effects of the reflector and direct signal leakage. Sidelobes and backlobes are all undesirable radiations that adversely affect the maximum radar range and increase the vulnerability of the radar to certain jamming techniques.