Page 207 - Radar Technology Encyclopedia
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197   filter, Wiener                                                                   frequency agility



            S =  x +  n  ,  where  n   is the noise vector. The  impulse  metric line of sight. In the troposphere, these fluctuations are
           response of the Wiener filter is derived from the solution of  primarily caused by irregularities in the water-vapor content,
           the Wiener-Hopf integral equation:                   especially in the vicinity of clouds but also in the clear air.
                      t                                         The rms value of angle-of-arrival fluctuations in clear air is a
                                                                few tens of microradians, increasing to over a hundred micro-
                     ò (   t ) B t–  t ' ) t=  B ( t t )
                                      '""
                      h t ' +
                                              +
                               (
                                    d
                                           sx
                                                 e
                              x
                            e
                                                                radians on paths through clouds. These fluctuations are inde-
                      0
                                                                pendent of  radar wavelength. Ionospheric  fluctuations also
           where t  is extrapolation time.                      occur, and these vary as the square of wavelength. DKB
                 e
               The impulse response of the Wiener filter is     Ref.: Barton (1969), App. D, (1988), pp. 309–313.
                                 æ  h t() t0                    antenna-scanning fluctuation (see MTI, limitations to per-
                                      ,
                                        ³
                            h  =  ç
                             w
                                 è  0 t 0                          formance).
                                       <
                                     ,
               The frequency response is                        Path-length fluctuations occur along atmospheric paths as a
                                ˜                               result of random variations in refractive index of the tropo-
                                Bsx f ()
                              =
                                       (
                          Hf () ---------------exp j2pft )      sphere  and ionosphere. Tropospheric fluctuations  are inde-
                                            e
                                 ˜
                                Bx f ()                         pendent of radar wavelength and typically amount to a few
                 ˜       ˜                                      millimeters when observed  over periods of minutes,  with
           where  Bsf f ()  and Bx f ()
                              are Fourier transforms  of  B (t) and
                                                     sx         greater drifts over longer periods. Ionospheric fluctuations are
           B (t). The Wiener filter is a filter-extrapolator. IAM
            x
                                                                in the order of centimeters for 0.3m wavelength, varying with
           Ref.: Farina (1985), vol. 1, p. 83; Korostelev (1987), p. 219.
                                                                the square of wavelength. DKB
           The yttrium-iron-garnet (YIG) filter uses a monocrystal of  Ref.: Barton (1969), App. D, (1988), p. 313.
           iron-yttrium garnet, which possesses ferromagnetic properties
                                                                FOLLOWER
           and low losses. They are used as dispersion and directional
           filters (see also ferrite filter).                   A cathode follower is an electron tube amplifier with a load
               Dispersion properties arise in the presence of an external  in a cathode circuit, which repeats the shape and phase of the
           nonhomogeneous magnetic  field in electromagnetic, spin,  input voltage. The cathode follower has a high input resis-
           and acoustic waves having different types  of propagation.  tance, low input capacitance, low output resistance, and high
           The required linearity of dispersion is obtained through ratio-  stability. The gain coefficient of the cathode follower is less
           nal distribution of intensity of the constant magnetic field  than one.
           along the length of the crystal, through control of propagation  The cathode follower is used in pulsed amplifiers (video
           of the acoustic wave using  toroidal plates of aluminum-  amplifiers)  operating  with a low-ohm  active-capacitance
           yttrium garnet, by changing the tilt of the edges of the crystal,  load; and also for matching of a high-ohm source with a low-
           and so forth.                                        ohm load; for example, when a transmission line matched at
               YIG filters are used for analog matched filtering of linear  the terminal is connected. IAM
           frequency-modulated waveforms with bandwidths  of hun-  Ref.: Terman (1955), p. 357; Popov (1980), p. 171; Fradkin (1969), p. 70.
           dreds  of megahertz, providing  a  compression  factor in  the
                                                                An emitter follower circuit is a transmitter amplifier having
           hundreds. IAM
                                                                the common-collector configuration. The performance  and
           Ref.: Shirman (1974), p. 149.
                                                                usage is analogous to the cathode follower. IAM
           FLIP-FLOP (see TRIGGER).                             Ref.: Zherebtsov (1989); Fradkin (1969), p. 184.
           FLUCTUATION. The term fluctuation is applied to a unsys-  FREQUENCY is the rate at which a periodic phenomenon is
           tematic (typically random) deviation of any physical quantity  repeated. In radar, one considers first the radio frequency of
           from its nominal value. In radar it is applied to radar echoes to  the transmitted  carrier, which  is the rate at which the sine
           describe the changes in RCS of complex targets (RCS fluctu-  wave representing the electromagnetic field is repeated. Other
           ations) and effects of irregularities in atmospheric refractive  frequencies used to describe radar operation are the interme-
           index (angle-of-arrival fluctuations). Other types of fluctua-  diate frequency, referring to the downconverted signal in a
           tions can include instability of  carrier frequency, antenna  superheterodyne receiver; the video frequency components,
           scanning, rotation of the polarization plane, and so forth. The  which represent the baseband pulse; and the pulse repetition
           main issue is the RCS fluctuation, usually described in terms  frequency, which is the rate at which transmitter pulses are
           of the Swerling target models (see RCS fluctuation). These  repeated. DKB
           lead to degradation of probability of detection and to errors in
                                                                Frequency agility refers to the capability of a radar to change
           radar measurement. SAL
                                                                its  transmitted carrier frequency on a  pulse-to-pulse or
           Ref.: Skolnik (1962), p. 50; Popov (1980), p. 457; Vasin (1977), pp. 84–88.
                                                                burst-to-burst basis, as opposed to frequency diversity, which
           Angle-of-arrival fluctuations of radar signals are the result  refers to a radar’s use of several complementary transmis-
           of  irregularities in the  refractive index of the  atmosphere,  sions at different frequencies. Frequency agility and diversity
           which cause the path of the echo wave to vary from the geo-  are basically ECCM techniques that force a potential jammer
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