A N T E N N A. Tracking Radar I E O T R Y & D E G N

Transcription

1 racking adar

2 racking Functions and Parameter stimation adar Parameter stimation Location zimuth ngle levation ngle ange Motion adial Velocity adial cceleration otation ize mplitude (C) adial xtent (Length) Cross ange xtent (Width)

3 racking adar: Functions Parameter stimation tracking radar has a pencil beam to receive echoes from target. tracking-radar system measures the coordinates (r,, ) of a target provides data(f d, v r ) which used to determine the target path predict its future position. used to measure the trajectory of the moving target [x: missile] and to predict future position. ypes: adar M adar Phased rray adar racking W adar

4 [ingle arget racker] adar designed to Continuously track a single target at a high data rate x: Weapon control radar [guided missile targets] [utomatic etection and rack] adar Lower data rate x: ir urveillance adar [Military and Civilian] Phased rray adar igh data rate lectronically steered phase array antenna Used on time sharing basis x: ir-defense weapon radar system [M] W [rack while can] adar Moderate data rate x: ircraft Landing adar (irborne adar)

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6 X M adar ystem Multl-arget racker (M), arget esolution and iscrimination xperiment (X) is a high-power, high-sensitivity instrumentation radar system is unique because it utilizes a large, steered, pencil-beam antenna. designed to detect and track [ > 63 targets] within the beam of the radar. t provides data necessary for determining the angular locations and ranges of all of these targets, as well as signature data necessary for target identification. t automatically processes received signals, reports targets, initiates and maintains target track files, and presents target information to the radar operators through real-time interactive graphical displays.

7 C-band monopulse precision tracking radar [ Wallops sland tation ] t has a 29-ft-diameter antenna with capable of 0.01 mil tracking accuracy.

8 P [Marshall slands] Long-ange racking and nstrumentation adar (L)

9 ngle racking tracking radar has a pencil beam to receive echoes from target. 0 B

10 Methods to extract error signal may be classified as equential lobing Conical scan imultaneous lobing or monopulse

11 ingle beam on time sharing basis. Multiple beam. equential lobing adar and imultaneous lobing or monopulse Conical scan adar adar] impler Complex ne antenna Multiple antennas Less equipment More equipments ot accurate ccurate C scintillation ingle pulse is used to ngle scintillation determine the angular error. o of pulses are mplitude comparison required to extract the Phase comparison error signal

12 Lobe-switching antenna patterns and error signal (one dimension). Polar representation ect. representation of switched antenna patterns equential Lobing [switching the antenna beam between two positions] B

13 B error signal

14 ngle error sensing in one coordinate by switching the antenna beam position from one side of the target to the other, arget located on the antenna axis arget at one side of the antenna axis.

15 Conical can racking (arget angle):angle between the axis of rotation and the direction to the target. q (quint angle):angle between the antenna-beam axis and the axis of rotation (Beamwidth):angular separation two half power points

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17 Block diagram of conical-scan tracking radar

18 Why C is required he echo-signal amplitude at the tracking-radar receiver will not be constant but will vary with time. he three major causes of variation in amplitude are due to (1) target cross section (P r ) (2) range i.e. P r (1/ 4 ) (3) the conical scan modulation (angle-error signal), and

19 Function of C to maintain the d-c level of the receiver output constant to smooth or eliminate as much of the noise like amplitude fluctuations as possible without disturbing the extraction of the desired error signal at the conical-scan frequency. esults in an error signal that is a true indication of the angular pointing error to prevent saturation by large signals. canning modulation and the error signal would be lost if the receiver were to saturate

20 C Block diagram of the C portion of a tracking- radar receiver

21 he angle-error signal voltage vs function ( / B ) (arget angle):angle between the axis of rotation and the direction to the target. q (quint angle):angle between the antenna-beam axis and the axis of rotation B (Beamwidth):angular separation two half power points

22 bservation: reater the slope of the error signal, More accurate will be the tracking of the target. he maximum slope occurs for a value / B slightly greater than 0.4 that corresponds to a point on the antenna pattern (the antenna crossover) about 2 db down from the peak. t is the optimum crossover for maximizing the accuracy of angle tracking. t has been suggested that the compromise value of / B be about 0.28, corresponding to a point on the antenna pattern about 1.0 db below the peak.

23 Monopulse adar Monopulse ntenna pattern verlapping antenna patterns ifference patterns um patterns rror ignal

24 Monopulse adar [since the 1960s] Limitations in conical scan radar: he conical scanning radar compares the return from two directions to directly measure the location of the target. t creates confusion by rapid changes in signal strength.

25 Monopulse radar is a radar system that compares the received signal from a single radar pulse against itself in order to compare the signal as seen in multiple directions, polarizations, or other differences. n this technique, he F signals received from two offset antenna beams are combined so that both the sum and the difference signals are obtained simultaneously. he sum and difference signals are multiplied in a phase-sensitive detector to obtain both the magnitude and the direction of the error signal. o determine the angular error is obtained on the basis of a single pulse; hence the name monopulse is quite appropriate.

26 mplitude-comparison Monopulse adar Block diagram of amplitude-comparison monopulse radar (one angular coordinate)

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28 Block diagram of amplitude-comparison monopulse radar (wo angular coordinate)

29 (a) ngle error information contained in the envelope of the received pulses in a conical-scan radar. (b) eference signal derived from the drive of the conical-scan feed.

30 Phase-Comparison Monopulse adar

31 (b) Block diagram of a phase comparison monopulse radar (one angle coordinate). (a) Wave front phase relationships in a phase comparison monopulse radar Phase-Comparison Monopulse adar

32 Limitations to racking ccuracy Major effects that determine the accuracy of a tracking radar: lint or angle noise or angular scintillation: which affects all tracking radars especially at short range. eceiver noise: affects all radars and mainly determines tracking accuracy at long range. C scintillation or mplitude fluctuations of the target echo that bother conical scan and sequential lobing trackers but not monopulse. ervo noise

33 CK Most popularly used antennas are: Parabolic eflector ntennas Planar Phased rrays lectronically steered Phased array antennas

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36 dvantages of Beam Forming ncreases coverage and reduces the number of antennas. mproves mproves signal quality ncrease in system capacity LCC BMFM C PV WC/FX Multi Beam rrays Butler Matrix Fixed elector

37 Components of Beam Forming ystems lement rray: Consist of antennas. fficient transmission through the array of elements is the primary design aim. Phase hifters: n electronic phase shifter feeds each element and each value is set so that the array radiates a plane wave of wavelength λ₀. Feed ystem: t collects or distributes the energy from the elements and phase shifters. Methods for Beam teering ime delay delay lines/buffers Phase hifts phase shifters he Butler Matrix hybrid junctions and static phase shifters igital Beam Forming use of signal processing to form beams

38 Corporate Feed eries Feed

39 Parabolic eflector ntennas / ish ntennas emands of reflectors for use in adar pplication adio astronomy, Wireless communication deep-space communication, such as in the space program and especially their deployment on the surface of the moon. eflector antennas take many geometrical configurations, some of the most popular shapes are the plane, corner, and curved reflectors (especially the paraboloid). t provides pencil beam optimizing illumination over their apertures so as to maximize the gain.

40 Parabolic eflector ntennas / ish ntennas he surface of a paraboloidal reflector is formed by rotating a parabola about its axis. he parabolic surface is illuminated by a source of radiated energy called the feed, which placed at the focus of parabola. t generates pencil beam.

41 urface eometry wo-dimensional configuration of a paraboloidal reflector

42 From the geometry f y x f z 4 ' ' ' / tan z d 2 / d f multiply where z 0 is the distance along the axis of the reflector from focal point to the edge of the rim

43 z' f x' 2 4 f y' 2

44 nother form

45 perture fficiency he aperture efficiency is generally the product of the fraction of the total power that is radiated by the feed, intercepted, and collimated by the reflecting surface (known as spillover efficiency: ) uniformity of the amplitude distribution of the feed pattern over the surface of the reflector (generally known as taper efficiency ) phase uniformity of the field over the aperture plane (generally known as phase efficiency ) polarization uniformity of the field over the aperture plane (generally known as polarization efficiency ) blockage efficiency irecivity ain random error efficiency over the reflector surface

46 ypes of Feeds xial or front feed Feed antenna located in front of the dish at the focus, on the beam axis, pointed back toward the dish. disadvantage: Block some of the beam, perture efficiency : 55 60%. ff-axis or offset feed he reflector is an asymmetrical segment of a paraboloid, so the focus, and the feed antenna, are located to one side of the dish. Purpose of this design: does not block the beam Widely used in home satellite television dishes.

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48 Cassegrain eflector ntenna by annan

49 Cassegrain arrangement Larger (main) reflector is parabola econdary reflector is hyperboloid (Convex) perture efficiency: 65 70% Primary reflector is parabola econdary reflector is ellipsoidal (Concave) ver 70%

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51 ain increases as aperture becomes electrically larger (diameter is a larger number of wavelengths)

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53 P [Marshall slands] Long-ange racking and nstrumentation adar (L)

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55 rray Controls

56 eometrical configuration Linear, rectangular, triangular, circular grids lement separation Phase shifts xcitation amplitudes For sidelobe control Pattern of individual elements sotropic, dipoles, etc.

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59 igh ain ntenna- tructures 59

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61 ain increases with increase in superstrate resonant height and number of superstrate layers.

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64 irectional-antennas M array Low efficiency due to dielectric and line losses igh cross-polar radiation due to the feed-line network. 64

65 1. M.. kolnik, ntroduction to adar ystems, Mcraw hill, M.. kolnik, adar andbook, Mcraw hill, 2nd edition, K. en and.b. Battacharya, adar ystems andadar ids to aviation, Khanna Publications, 1988.

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