Jog detector
FIELD OF THE INVENTION 
     The present invention is directed generally to shipborne Electronic Countermeasures (ECM) systems of the type designed to transmit electromagnetic jamming signals directly at a radar transmitter of an approaching missile in order to disturb the missile&#39;s range and angle tracking system by an amount sufficient to prevent the missile from intercepting the ship. 
     BACKGROUND OF THE INVENTION 
     In general, these types of ECM systems attempt to sequentially apply various jamming techniques until one is hit upon which is effective in producing an angle deflection or &#34;jog&#34; in the radar antenna of an approaching missile&#39;s range and angle tracking system. The shipborne ECM&#39;s receiver looks for a sudden drop in the strength of a radar signal transmitted by the missile in order to detect a &#34;jog&#34;. However, multipath disturbances caused by forward scattering interference results in signal fluctuations which can degrade the performance of a shipborne ECM system equipped with this type of jog detector. The difficulty is that multipath disturbances cause received radar signals to fluctuate in a manner similar to that expected from a &#34;jog&#34; resulting in false &#34;jog&#34; measurements being received. This degrades the jamming performance of these types of ECM systems since no reliable indication of whether an actual &#34;jog&#34; has taken place can be obtained. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a reliable system for detecting &#34;jogs&#34;. A jog detection system, according to the present invention, discriminates between radar signal fades caused by multipath disturbances and &#34;jog&#34; fades based on spatial diversity and the first-order time derivative response of received radar signals. 
     An ECM system, according to the present invention, is provided with a receiver having at least two vertically separated antenna wherein radar signals received by each antenna are downconverted by a mixer in separate channels for each antenna, each mixer having one input connected to an antenna and another input connected to a local oscillator with outputs from each mixer being connected to separate detectors, each detector&#39;s output being connected to a differentiator, an output of each differentiator being connected to threshold detection circuit which provides a signal output on a channel when the output from the differentiator for that channel falls below a predetermined threshold value, outputs from all the threshold detection circuits being connected to an input of an AND gate which provides an output signal only when an output signal from each threshold detection circuit exist on all channels simultaneously. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will now be described in more detail with reference to the accompanying drawing wherein the single figure is a block diagram that illustrates a receiver for an ECM system with circuitry to discriminate between multipath disturbances and &#34;jog&#34;fades according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The radar signal detector of an ECM system, according to the present invention, makes use of the fact that signal fades caused by maritime multipath disturbances are a height dependent phenomenon. In other words, multipath disturbances caused by forward scattering interference result in signal fluctuations that vary over different heights and these fluctuations can be measured by a receiver. This provides a basis for a receiver antenna array that can discriminate between radar signal fades caused by multipath disturbances and those caused by an angle deflection, i.e. a &#34;jog&#34;, of a radar transmitting antenna in an approaching missile. 
     The figure shows one type of receiver for discriminating between signal fades caused by multipath disturbances and jogs. The ECM receiver has 3 radar signal horn receiving antenna 10, 10 1  and 10 11  which are separated vertically from each other by a distance of between 0.125 and 1 meter with the received signals from each antenna being processed in separate channels. The signals received by each antenna are downconverted to baseband by separate mixers which are supplied with a signal from a single local oscillator 14. The signal from antenna 10 is downconverted in mixer 12 and applied to a detector 16 in channel 1. Similarly, the signal from antenna 10 1  is downconverted by mixer 12 1  in Channel 2 before being applied to detector 16 1  and the signal from antenna 10 11  is applied to mixer 12 11  whose output is then applied to detector 16 11  in Channel 3. Signals from the detectors 16, 16 1  and 16 11  are processed in separate channels 1, 2 and 3 respectively. 
     The signals from detectors 16, 16 1  and 16 11  are first digitized by analog-to-digital (A/D) converters 18, 18 1  and 18 11  before being differentiated in differentiating circuits 20, 20 1  and 20 11  respectively. These differentiated signals will indicate when there is a change of signal strength received by an antenna in any one of the Channels 1, 2 or 3. The differentiated signals in Channels 1, 2 and 3 are applied to threshold detector circuits 22, 22 1  and 22 11  respectively which supply a logical 1 output signal in a channel when the derivative of a signal received in that channel falls below a predetermined threshold value. This creates a logical 1 at the output of a channel when a predetermined change in signal strength occurs in that channel. The outputs of the threshold circuits from all the channels are then applied to inputs of an AND gate 24 which provides a logical 1 output signal only when a logical 1 output signal appears from all the threshold detectors 22, 22 1  and 22 11  simultaneously. 
     When a signal received from a radar transmitter of an approaching missile drops in strength as a result of multipath interferences, each of the Channels 1, 2 or 3 will measure a signal fade at different times as the missile approaches the ship. This is a result of a receiving antenna for each of the channels being at a different height from the antenna for other channels and that signal fades caused by multipath interference are a height dependent phenomenon. Therefore, logical 1 output signals will occur in each of the channels at different times for signal fades caused by multipath disturbances which result in no output being obtained from AND gate 24. However, the three antenna 10, 10 1  and 10 11  will measure approximately the same signal fade simultaneously when a radar transmission from an approaching missile fades due to a deflection of the radar signal transmitting antenna, i.e. a &#34;jog&#34;, caused by electronic jamming techniques created by the ECM system. This will provide a logical 1 output from AND gate 24 indicating that a &#34;jog&#34; has occurred and inform operators that a jamming technique then being used has been effective on that missile. 
     Various modifications may be made to the preferred embodiment without departing from the spirit and scope of the invention as defined in the appended claims.