Page 241 - Radar Technology Encyclopedia
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231   jamming, self-screening [-protection]                                                   JITTER



           ment by air-defense weapons systems. Self-protection jam-  Stand-forward [stand-in] jamming  is jamming emitted
           ming may use noise jamming  to deny  the victim  radar or  from a platform located between the weapon system and the
           missile seeker range information, or it may employ a reper-  strike vehicle. Since the stand-forward jammer usually is
           toire of deceptive jamming techniques to frustrate the vic-  located within the lethal range of defensive weapon systems
           tim’s angle tracking and homing guidance system. PCH  for a considerable time, the use of remotely piloted vehicles
                                                                as a platform for jammer is practical. SAL
           Sidelobe jamming  is “jamming through  a sidelobe of  the
           receiving antenna in an attempt to obliterate the desired signal  Ref.: Skolnik (1990), p. 9.6; Chrzanowski (1990), p. 82.
           received through the mainlobe of the receiving antenna at  Standoff jamming is jamming emitted from a support plat-
           fixed points.” SAL                                   form (typically an aircraft) orbiting within line-of-sight of a
           Ref.: Johnston (1979), p. 66.                        victim radar  but beyond  the  range of the defensive system.
                                                                When  the protected  vehicles  start their penetration of the
           The  jamming-to-signal ratio  (JSR  or  J/S), is ratio of  the
                                                                combat area, the support vehicle can direct jamming against
           power received from a jammer (or jammers) to the target sig-
                                                                all significant radars in this area. In principle it is a generic
           nal power received, as measured at  a  common  point  in the
                                                                term applied to standoff ECM, escort ECM, and cooperative
           victim radar (e.g., at  the radar antenna  terminals). For  the
                                                                ECM. The term support jamming can be applied to this or any
           general noise-jamming case, this ratio can be expressed as
                                                                other offboard jamming technique. SAL
                            2                                   Ref.: Johnston (1979), p. 67.
                   æ P G A B F ö
                     j
                       j r r j
                   ç ------------------------------ ÷  2        straddle jamming (see two-signal jamming).
                        2
                   è 4p R B L  ø  P G æ F öB æ  4
                                                      1
               J       j  j aj   æ  j  j ö j æ  r ö R öL a öæö
                                                 æ
               --- =  ----------------------------------- =  ----------- ç ------ ÷ ----- ç ------- ÷ --------  ---
                                                     èø
               S            4    è P G ø 4 è B ø 2 è L  ø s     Support jamming is any type of jamming originating on a
                                    t è
                   æ P G A sF ö    t   F ø j è R j  ø aj
                     t
                       t r
                   ç ---------------------------- ÷             platform other than the radar target.
                        2 4
                   è ( 4p ) R L ø
                            a
                                                                surface-bounce jamming (see ground-bounce jamming).
           where the radar parameters are identified in accordance with
                                                                A swept jammer is a jammer that sweeps a narrow band of
           the radar range equation. Here the subscript j applies to the
                                                                electronic energy over  a  broad  bandwidth.  A transmitter in
           jammer parameters, r applies to the victim radar parameters,
                                                                which a narrowband signal can  be tuned over a broad fre-
           F  includes the relative radar antenna gain in the direction of
            j
           the jammer, and  L  represents the signal loss due to atmo-  quency ban and then locked on a particular frequency is
                          a
           spheric attenuation (one-way for the jammer, two-way for the  called a sweep-lock-on jammer. SAL
           radar signal). (See also  barrage noise jamming,  RANGE  Ref.: Johnston (1979), p. 67.
           EQUATION, with jamming.) PCH                         Two-frequency jamming is jamming emitted as two  jam-
                                                                ming carriers separated by the IF of the victim radar. In this
           Skirt-frequency jamming  is  jamming that can be used
                                                                case radar receivers that lack RF preselection are subject to
           against monopulse radar based on the fact that sum and differ-
                                                                generation of spurious outputs. The  effectiveness  of this
           ence channels must be well matched in phase so that the error
                                                                method depends on the jamming level and receiver perfor-
           detector can preserve the polarity of the target error voltage.
                                                                mance, in particular on the nonlinearity of input mixers. Basi-
           An error  in receiver phase matching  causes a  shift  in the
                                                                cally the technique is applied against radar seekers that must
           tracking point, and if error exceeds 90° the tracking loop may
                                                                operate at very short ranges. The main ECCM is to use RF
           settle on the previously unstable null, more than one beam-
                                                                preselection or amplitude comparison after envelope detec-
           width away from the desired tracking axis. A jamming tech-
                                                                tion. DKB, SAL
           nique that exploits this  problem  is  called  skirt-frequency
           jamming, because it produces a signal lying on the skirts  Ref.: Barton (1989), p. 501.
           (edges) of the IF passband. DKB, SAL                 two-point coherent jamming (see cross-eye jamming).
           Ref.: Barton (1989), p. 497.
                                                                Two-signal jamming “is a method of jamming whereby two
           A smart noise jammer is a hybrid type of jammer that incor-  signals are transmitted on two frequencies only slightly sepa-
           porates the features of both noise and deception jamming. In  rated. Effective against certain types of radars where receiver
           this case, repeater-type jammer generates responsive noise in  bandwidth is narrow enough to defeat noise jamming.” SAL
           a transponder mode. A noise burst occurs before and after the  Ref.: Johnston (1979), p. 67.
           actual target return and  covers the true  echo. The span of
                                                                JITTER. (1) The noise-like fluctuations due to target echo
           ranges where the noise is radiated is synchronized to the vic-
                                                                characteristics, propagation, or  receiver thermal  noise that
           tim radar and the jammer may adapt its directive beam to the
                                                                degrade the tracking accuracy and result in zero-mean ran-
           angles of arrivals of radar illumination. SAL
                                                                dom  errors in successive target  position measurements.
           Ref.: Barton (1988), p. 367; Schleher (1986), p. 11.
                                                                Sometimes  is termed also tracking noise. (2) “Intentional
           Spot jamming is “the jamming of a specific channel or fre-  variation of a radar parameter, for example, pulse interval.”
           quency.” SAL                                         SAL
           Ref.: Johnston (1979), p. 67.                        Ref.: IEEE (1990), p. 19; Skolnik (1980), p. 167.
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