Communications: directive radio wave systems and devices (e.g. – Radar ew – Ecm
Reexamination Certificate
1975-08-26
2002-08-06
Sotomayor, John B. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Radar ew
Ecm
C342S015000
Reexamination Certificate
active
06429800
ABSTRACT:
SCOPE OF THE INVENTION
This invention relates to radar jamming systems and more particularly to the use of a primed oscillator in combination with an antenna and a keying circuit as an expendable decoy ejected from a target which produces suitable jamming signals as the decoy moves away from the target.
BACKGROUND OF THE INVENTION
One of the frequently occurring situations necessitating radar jamming is a situation involving an incoming missile which homes on a “target” via a CW radar signal. This signal, when returned from a “target”, is analyzed as to doppler frequency shift which provides the relative velocity of the “target” with respect to the missile, and as to modulation which provides the angular bearing of the “target”. In the past, jammers for this tactical situation have involved the generation of a CW signal by the target in which the frequency of the signal is made to vary in a predetermined fashion to convey false doppler frequency shift information. This is commonly called “stealing the speed gate” of the “enemy” radar. Additionally, the jamming signal is modulated to convey false angular bearing information. In the prior art jamming devices, it is usually necessary to detect the frequency of the incoming radar signal and to synthetically produce a signal having this frequency. Having generated this signal, the frequency of this signal is slowly varied for the required doppler shift and is modulated to give the false bearing information.
By way of background, there have been basically three approaches to generating the above described CW jamming. The first is a CW amplification approach in which the incoming signal is amplified by a chain of amplifiers and reradiated. In this approach, the false doppler and bearing modulations are applied during amplification and two antennas are required. This approach suffers from two disadvantages. 1) A very high antenna isolation is required to prevent a ring-around type feedback between the antennas. On many installations sufficient isolation is physically impossible to achieve. 2) A separate CW power amplifier tube is required, in addition to the pulse tube which is normally carried for pulse jamming.
The second approach eliminates the antenna isolation problem by using an oscillator to generate the jamming signal. The oscillator is set on frequency by a receiver which briefly samples the received radar signal at periodic intervals. Thus it will be appreciated that the jamming must be interrupted during these “look-thru's”, so that received radar signals may be sampled. This approach suffers from the disadvantage that the receiver for the jammer is either complex or has a slow response. In addition, a separate CW power tube is still needed.
The third approach makes use of the pulsed power amplifier and simulates CW by a train of pulse bursts. The switching-off between pulses solves the aforementioned ring-around problem, and the time between bursts is available for pulse jamming other threats. However, this approach suffers a severe power/efficiency limitation because the effective power is reduced from the average power by the duty factor. By way of example, typical pulse tubes are limited to a duty factor of about 10%; therefore the effective power is only about 1% of the peak power. It will be appreciated that all of these jammers are carried “on board” the “target”.
Instead of “on-board” apparatus, doppler jamming may be accomplished by, for instance, the ejecting of a repeater or transponder at an angle from an aircraft. It is the function of the repeater or transponder to duplicate the incoming signal, with false doppler information being introduced by the different velocity of the ejected decoy with respect to the “enemy” radar, and with the false bearing information being automatically provided by virtue of the angular difference between the ejected decoy and the true “target”.
In the subject invention, an expendable jammer is ejected as a decoy from a target for countermeasuring CW signals from “enemy” radars, in which the jammer includes a single port, keyed, high power oscillator which is primed with the low level signal received from the “enemy” radar. The priming signal is injected directly into the tank circuit of the oscillator which is rapidly turned on and off (keyed) to produce a rapidly pulsed signal which tracks the frequency of the incoming signal. The keying is accomplished either by removing the power from the oscillator, by shorting the tank circuit, or by gating the negative resistance element in the oscillator.
It is important to the understanding of the subject system to distinguish the subject priming system from traditional injection locking systems. In injection locking the injected signal is a relatively high level signal strong enough to lock the oscillator frequency to the injected frequency. The signal from the “enemy” radar normally does not reach injection locking levels at the “target” and thus injection of a received signal into a remote oscillator circuit will not lock the remote oscillator to the frequency of the received signals. In priming, the injected signal need only be strong enough to force the oscillator to start up in phase with the priming signal at the start of each pulse. In priming there is no locking or changing of the oscillator. Rather the oscillator is allowed to run at its natural frequency and is rapidly keyed so that the oscillator is turned off before its signal can accumulate a significant phase error with respect to the incoming signal. The result is that the output signal from the oscillator tracks the frequency of the priming signal to the extent that the phase difference between the priming signal and the natural frequency signal from the oscillator is small. Since this phase difference can be made very small by rapid keying, the frequency of the output signal from the keyed primed oscillator approximates or tracks that of the priming signal.
The resultant signal from the oscillator is coupled back to the same antenna used for receiving the “enemy” radar signal and is transmitted back to the receiving section of the “enemy” radar which cannot distinguish the individual pulses of returned energy due to the high PRF (pulse repetition frequency) of the returned signal. Thus, the signal from the jammer is detected as a CW signal. By virtue of ejecting the jammer, the enemy radar is given false bearing and doppler information causing the enemy radar to either lose track or to home on the expendable jammer. The expendable jammer also has a multiple threat capability in which simultaneously arriving signals from different “enemy” radars result in a jammer output at each of the different frequencies of the incoming signals with sufficient jamming power at each frequency.
The primed oscillator is therefore a very simple broadband jammer which can follow the incoming frequency by virtue of the rapid keying or chopping. This rapid keying or chopping while permitting the oscillator to follow the frequency of the incoming signal also results in a pulsed signal with a sufficiently high PRF that the “enemy” radar receiver cannot resolve the pulses of the returned signal and thus “sees” only a CW signal. Due to the availability of IMPATT diode oscillators which have outputs exceeding 100 watts, amplification stages are unnecessary. Moreover since the duty cycle can be close to 100%, the effective power of the oscillator can be very high. Since the subject system utilizes only one antenna there is no isolation problem. It is also an important feature of the primed oscillator that it can be used in its chopped or keyed mode for handling simultaneous multiple incoming signals. This comes about as follows. When two or more CW signals are present, at different frequencies, their resultant is a single signal of varying phase and amplitude. Each time the oscillator is keyed on it is primed by that resultant. Its phase on successive pulses will therefore faithfully follow the phase of the resultant signal, thereby in effect reproducing all the incoming frequencies.
Thus the u
BAE Systems Information and Electronic Systems Integration Inc.
Long Daniel J.
Sotomayor John B.
Tendler Robert K.
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