Communications: directive radio wave systems and devices (e.g. – Radar ew – Eccm
Reexamination Certificate
2000-02-14
2003-03-25
Gregory, Bernarr E. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Radar ew
Eccm
C342S013000, C342S016000, C342S089000, C342S149000, C342S159000, C342S160000, C342S195000, C342S378000, C342S379000, C342S380000, C342S383000
Reexamination Certificate
active
06538597
ABSTRACT:
STATEMENTS REGARDING FEDERALLY SPONSORED RESEARCH
Not applicable.
FIELD OF THE INVENTION
The invention relates to apparatus and techniques for providing a phased array radar system having a blanker for reducing the effects of sidelobe interference which is capable of effective operation even in the presence of strong sidelobe jamming.
BACKGROUND OF THE INVENTION
As is known in the field of radar systems, jamming is a technique used to interfere electronically with an enemy radar system. The jammer generates and transmits radar signals which are designed to saturate or otherwise effectively disable the enemy radar receiver.
Jammers may generate jamming signals based on knowledge of typical radar operating bands of frequencies of the system to be jammed. The jammer signal may be transmitted over a limited frequency interval based on knowledge or an estimate of the instantaneous frequency of the jammed radar system (this technique is sometimes referred to as repeater jamming). Smart jammers have the capability to determine the frequency of the transmitted radar signals and focus the jamming energy over a narrow band at the determined frequency (this technique is sometimes referred to as spot jamming). Another type of jamming is referred to as barrage jamming in which jamming signals are radiated over a wide band of frequencies
Various techniques are utilized to counteract the effects of interference signals, including jammer signals, in sidelobes of the main, or sum beam. One such technique is referred to as sidelobe canceling, in which the main beam sidelobes are minimized and preferably, nulled. Sidelobe cancelers can also be used to provide a null in a main beam sidelobe in the direction of the jammer. To this end, auxiliary antenna(s) (typically one antenna element per jammer) receive and process jammer signals in order to determine weights necessary for the auxiliary antenna outputs to be added to the main beam in order to provide a null in the sidelobes at the jammer direction.
Preferably, auxiliary antennas used for side lobe cancellation have a relatively wide field of view, as may be provided by an omnidirectional, or isotropic antenna. However, such antennas have relatively low gain such as on the order of 6 dB, requiring significant gain to be introduced in order to effectively null the sidelobes in the direction of the jammer. Consider for example, a main beam pattern in which the peak sidelobe has a gain of +20 dBi, and (which corresponds to the rms error sidelobe level of a well designed antenna array). For a jammer in a −15 dBi sidelobe, the auxiliary channel is attenuated by +6−(−15)=21 dB. However, a jammer in a +20 dBi sidelobe requires 14 dB amplification of the auxiliary channel (20 dBi=+6+14). The introduction of such gain on the auxiliary channels amplifies thermal noise, which couples into the main beam, thereby degrading, the sensitivity of the main beam. Furthermore, nulling one sidelobe tends to increase sidelobes in other directions. Even with these drawbacks, however, the sidelobe canceler can be an effective way of counteracting the effects of sidelobe jamming.
Blanking is another technique used to reduce the effects of sidelobe interference. A blanker utilizes a dedicated receive antenna and processing channel, with the signal processing being matched to the main beam processing. The gain of the blanking channel is greater than that of the main beam sidelobes. The outputs of the main beam processor and the blanker processor are compared in order to determine whether main beam target detections are valid. More particularly, if sidelobe interference is strong, then the signal strength in the blanker channel will be stronger than in the main beam channel and the main beam output is rejected. Alternatively, if the signal strength in the main beam channel is greater than that in the blanker channel, then main beam target detections are considered valid. Interference signals blanked in this manner include strong sidelobe clutter, larger aircraft, repeater jammers, or jammers which radiate radar signals like chirp signals.
However, one problem with blankers is the requirement that its antenna gain be greater than the sidelobe gain. As with the sidelobe canceler, the output signal of the blanker must be amplified to blank a sidelobe having more dBi gain than does the blanker. With such high amplifier gain, system noise levels are amplified which results in degradation of main beam sensitivity. Furthermore, in the event that the blanker antenna is spaced from the main beam aperture, multipath reflections can reduce the strength of incoming signals in the blanker while increasing them in the main beam, thereby negatively affecting blanker performance. As a result of these drawbacks, blankers are sometimes disabled in the presence of strong sidelobe jamming.
Because blankers and sidelobe cancelers are useful to reduce the effects of different kinds of jamming, and also because each is not without its problems, particularly the exacerbation of noise, generally one such technique will be operated at a time. That is, in the presence of sidelobe jamming, the canceler is used to introduce a null in the direction of the jammer and the blanker is turned off; whereas, in the presence of repeater jamming, the blanker is used and the canceler is turned off.
An additional feature of some radar systems is a spoofer used to reduce the effectiveness of smart jammers. Smart jammers are capable of listening to a transmitted radar signal to deduce its frequency and then focusing the jamming energy to a fraction of the radar band at the deduced frequency. A spoofer includes a waveform generator which generates a spoofer signal capable of confusing a smart sidelobe jammer so as to prevent the jammer from ascertaining the frequency of the actual radar signals. That is, the spoofer signal is selected to camouflage the actual radar signal. This may be achieved in various ways, such as by transmitting a noise-like signal or by transmitting a replica of a radar signal. As one example, the spoofer signal may be a swept sinewave having an amplitude greater than the amplitude of the actual radar signal over the entire frequency range of operation. While it is desirable that the spoofer antenna be nearly omnidirectional, it is also desirable that the spoofer power be relatively low.
Spoofers typically utilize a separate transmit antenna spaced from the main transmit antenna. This is because antiradiation missiles can lock on “active decoys” which are radar signals radiated in frequencies surrounding the frequency of the actual radar signal. If the spoofer antenna were integral to the main transmit antenna, its use could cause an antiradiation missile to lock onto the radar system.
SUMMARY OF THE INVENTION
In accordance with the present invention, a radar system includes an antenna for receiving radar signals, said antenna including a main antenna having a main beam pattern and a blanker antenna having a blanker beam pattern and a beam forming network including a nulling circuit, said beam forming network being coupled to said main antenna for forming a sum beam having a null in the direction of a jammer and for forming a blanker beam having a null in the direction of the jammer. The radar system further includes a first signal processor for processing radar signals received in said sum beam; a second signal processor for processing radar signals received in said blanker beam; and a comparison circuit for comparing the level of signals received in said sum beam with the level of signals received in said blanker beam in order to determine when radar signals received in said main beam are representative of a valid target. With such an arrangement, a radar system can operate in the presence of strong sidelobe jamming without the conventional problems attributable to noise amplification.
In accordance with another feature of the present invention, the radar system further includes an open loop ECM nulling map circuit coupled to said bea
Daly, Crowley & Mofford LLP
Gregory Bernarr E.
Raytheon Company
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