Jog detector

A shipborne Electric Countermeasures (ECM) system protects ships against an pproaching missile by transmitting electromagnetic jamming signals directly at the missile's radar in order to disturb its range and angle tracking system sufficiently to prevent the missile from intercepting the ship. Various jamming techniques are transmitted until one is found that causes an angle deflection or "jog" in the radar transmitting antenna of the missile. The ECM's receiver looks for a sudden drop in the strength of the radar signal transmitted by the missile's radar, which indicates that a "jog" may have occurred, in order to determine if a jamming technique is effective. Multipath disturbances due to forward scattering interference also cause drops in the strength of a received radar signal but these types of signal fluctuations are a height dependent phenomenon. This ECM system diferentiates between multipath disturbances and actual "jog" by using a receiving antenna array with a number of antennas located at different heights. The vertically separated antenna measures drops in signal strength at different times if these drops are caused by multipath disturbances. However, any drops in signal strength caused by a "jog" reach all the antennas at the same time and are measured simultaneously providing an indication that a "jog" has occurred.

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'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 "jog" in the radar antenna of an 
approaching missile's range and angle tracking system. The shipborne ECM's 
receiver looks for a sudden drop in the strength of a radar signal 
transmitted by the missile in order to detect a "jog". 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 "jog" resulting in false "jog" measurements being 
received. This degrades the jamming performance of these types of ECM 
systems since no reliable indication of whether an actual "jog" has taken 
place can be obtained. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide a reliable system for detecting 
"jogs". A jog detection system, according to the present invention, 
discriminates between radar signal fades caused by multipath disturbances 
and "jog" 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'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.

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 "jog", 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.sup.1 and 10.sup.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.sup.1 is downconverted by mixer 12.sup.1 in Channel 2 
before being applied to detector 16.sup.1 and the signal from antenna 
10.sup.11 is applied to mixer 12.sup.11 whose output is then applied to 
detector 16.sup.11 in Channel 3. Signals from the detectors 16, 16.sup.1 
and 16.sup.11 are processed in separate channels 1, 2 and 3 respectively. 
The signals from detectors 16, 16.sup.1 and 16.sup.11 are first digitized 
by analog-to-digital (A/D) converters 18, 18.sup.1 and 18.sup.11 before 
being differentiated in differentiating circuits 20, 20.sup.1 and 
20.sup.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.sup.1 and 22.sup.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.sup.1 and 22.sup.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.sup.1 and 10.sup.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 "jog", caused by electronic 
jamming techniques created by the ECM system. This will provide a logical 
1 output from AND gate 24 indicating that a "jog" 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.