Communications: directive radio wave systems and devices (e.g. – Directive – Utilizing correlation techniques
Reexamination Certificate
1974-09-23
2001-10-02
Sotomayor, John B. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Directive
Utilizing correlation techniques
C342S384000, C342S017000, C342S159000, C342S162000
Reexamination Certificate
active
06297772
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to interference suppression systems and more particularly to side-lobe canceller systems which are compatible with MTI.
Generally, interference suppressing systems of the prior art are designed to reduce the presence of undesired signals in a signal receiving system. As is known, in particular systems, such as a radar system, the characteristics of the receiving antennas are such that undesired signals which are received in the side-lobe interfere with the isolation of the target signal received in the main lobe. Accordingly, to isolate the main lobe signals, side-lobe cancellers have been used to cancel the side-lobe interference of the main radar antenna as exampled by U.S. Pat. No. 3,202,990 to Paul W. Howells.
It has been found that while conventional side-lobe canceller systems are highly successful in most instances, problems arise when the system is used to cancel interference in a high clutter environment where normally Moving Target Indicators (MTI) can be used to cancel the clutter. An MTI relies on the clutter return being the same on a pulse to basis in order to allow distinction from a target return which varies due to target movement between radar pulses. When clutter is present simultaneously with jamming interference, however, it has been found that conventional side-lobe cancellers modify the clutter returns on pulse to pulse basis because of cross modulation between clutter and interference signals. As the clutter returns are not the same, an MTI cannot be used thereby preventing clutter cancellation and prohibiting the use of a side-lobe canceller with the MTI.
One proposal for avoiding clutter modification in a side-lobe canceller involves the sampling of the interference signal in the correlator loop for a short time immediately preceeding radar pulse transmissions, where clutter is weak or non-existent, and using the derived weighting signal to cancel throughout the next pulse repetition period. Such a technique, while reducing clutter modification, also introduces time dependent unbalance in the cancelling signals due to antenna scan between samples.
In U.S. Application Ser. No. 05/499,962 entitled “MTI COMPATIBLE COHERENT SIDELOBE CANCELLER” to Bernard L. Lewis, filed in the United States on Aug. 23, 1974 now U.S. Pat. No. 5,296,865 and assigned to the same assignee as the present application, a side-lobe canceller system is disclosed which delays both main and auxiliary channel signals until cancelling weights at the beginning and end of a pulse repetition period can be derived. Interpolation between the two weights as a function of time is then performed over the pulse repetition period to provide an instantaneous weighting function for translating the delayed auxiliary channel interference signal for subtraction from the main channel interference signal. While such a system provides improved cancellation, the technique suffers from the disadvantage of requiring long delays of wide-band signals to be accurately matched in two separate channels.
Accordingly, the present invention has been developed to overcome the specific shortcomings of the above known and similar techniques and to provide an improved predicting coherent side-lobe canceller system for producing reliable interference cancellation in a high clutter environment.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an interference suppression system that is simple to implement yet highly reliable in operation.
Another object of the invention is to provide a predicting side-lobe canceller that does not require long delays of wide-band signals.
A further object of the invention is to provide an improved sampling side-lobe canceller that reduces clutter modification and operates compatibly with an MTI in high clutter environments.
Still another object of the invention is to provide a coherent side-lobe cancelling system which uses predicting weights to prevent time dependent unbalance in cancelling signals due to main antenna scan during each pulse repetition period.
A still further object of the invention is to provide intermediate side-lobe cancellers which use past weighting signals to more closely approximate instantaneous weights needed for cancellation over any pulse repetition period.
In order to accomplish the above and other objects, the invention provides a unique predicting side-lobe canceller which reduces clutter modification and antenna scan error. Main and auxiliary channel signals are coupled to a canceller loop having phase and amplitude correlating circuits. The phase and amplitude correlating weights are coupled through a sampling circuit where the correlating weights are sampled during a time period immediately preceeding a radar pulse transmission where clutter returns are substantially absent. The current weights are stored along with past weights and combined in the sampling circuits to provide, as the correlating weights, the sum of the current weight and a time integrated difference of the past and current weight, for use over the current radar pulse repetition period. The sum is called a predicting weight and both amplitude and phase predicting weights are combined to translate the auxiliary interference signal to produce a cancelling signal for subtraction from the main channel signal to more effectively reduce interference in the main channel. Since the samples of the correlating weights are made during a time period substantially devoid of clutter, the cancelling signals are produced without the introduction of cross modulation of clutter. In addition, since the correlating weights are modified instantaneously over the pulse repetition period to predict changes in the cancelling signal based on past weighting samples, the error in cancelling signals normally introduced by antenna scan during a pulse repetition period, is substantially reduced. The overall effect is to produce a main channel signal with side-lobe interference substantially eliminated and a clutter signal that is unmodified by the operation of the side-lobe canceller, thereby allowing use with an MTI.
Other objects, advantages, and novel features of the invention will become apparent from the following detailed description of the invention when considered with the accompanying drawings wherein:
REFERENCES:
patent: 3763490 (1973-10-01), Hadley et al.
patent: 3881177 (1975-04-01), Len et al.
patent: 3938153 (1976-02-01), Lewis et al.
patent: 3938154 (1976-02-01), Lewis
patent: 3981014 (1976-09-01), Masak
patent: 3982245 (1976-09-01), Soule, Jr. et al.
patent: 3987444 (1976-10-01), Masak et al.
patent: 4044359 (1977-08-01), Applebaum et al.
patent: 4070675 (1978-01-01), Daniel et al.
patent: 4204211 (1980-05-01), Cavelos
patent: 4586048 (1986-04-01), Downie
patent: 4689628 (1987-08-01), Lewis
patent: 5296865 (1994-03-01), Lewis
McDonnell Thomas E.
Sotomayor John B.
Stockstill Charles J.
The United States of America as represented by the Secretary of
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