Communications – electrical: acoustic wave systems and devices – Distance or direction finding – By combining or comparing signals
Reexamination Certificate
2002-08-08
2003-07-08
Lobo, Ian J. (Department: 3662)
Communications, electrical: acoustic wave systems and devices
Distance or direction finding
By combining or comparing signals
C367S124000, C367S901000
Reexamination Certificate
active
06590833
ABSTRACT:
CROSS REFERENCE TO OTHER PATENT APPLICATION
Not applicable.
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates generally to systems and methods for a cross-correlator and, more specifically, for an adaptive cross-correlator operable for adaptively suppressing high signal to noise ratio (SNR) narrowband interference before the interference enters the cross-correlator process.
(2) Description of the Prior Art
A vessel's sound signature contains both a continuous, broadband spectrum of sound, as well as discrete, narrowband tonals at specific frequencies along that spectrum that rise above the spectrum. The tonals may be caused by specific pieces of rotating machinery within the vessel. For instance, narrow band tonals may be produced by pumps, generators, and gears, whereas the continuous broadband spectrum is caused primarily by flow noise over the hull surface or by propeller cavitation. However, narrowband tonals or narrowband interference may also be produced by self noise related to a loud platform, such as the platform to which the sonar array is attached. Other narrow band tonals or narrowband interference may be produced by other vessels unrelated to the target vessel.
A vessel's broadband signature may resemble background noise in that it contains a continuous spectrum of frequencies within which sound source levels at particular frequencies rise and fall in random fashion around a mean over time. By contrast, the narrowband component of a vessel's signature may generate sound at several specific frequencies continuously. Thus, compared to the background noise generated at these specific frequencies, which will average out over time to x, the signal plus noise received at the tonal frequency will average out over time to x+y, with y being the source level of the signal.
At close range, a simple sonar will detect a vessel's broadband signal simply by pointing the main beam of its array at the vessel. The sonar is measuring all the sound it receives in a given direction, including both signal and background noise, and as it points in the direction of the target, the signal increases. As the range of the opposing submarine is increased, the relative strength of this broadband signature compared to the broadband background noise declines until it is drowned out and the signal-to-noise ratio drops below the detection threshold. Therefore, self-noise is also an important issue for sonar effectiveness. This is true whether one is seeking broadband or narrowband detections.
In some cases, two important tonals in a submarine's signature may be those modulated by the propeller at the rate which its blades turn, and those associated with particular items of rotating machinery. Blade rate tonals are usually slightly lower in frequency than machinery tonals, and both tonals are usually aspect dependent and speed dependent. Taken together, these tonals provide means to detect targets, classify them, and to track them over time.
High signal to noise ratio narrowband interference creates signal distortion and makes peak detection more difficult for received sonar signals. Removal or suppression of narrowband interference is not a new idea. The current methods for removal or suppression of narrowband interference from signals received by an array of sonar transducers utilize what is referred to as the smooth coherence transform (SCOT). More detailed information about SCOT methods can be found in references such as
Coherence and Time Delay Estimation
, by G. Clifford Carter, IEEE Press, Piscataway, N.J., 1992. SCOT works in the frequency domain and essentially applies a filter on the output of the frequency domain correlator implementation. However, SCOT tends to be computationally intensive. The number of floating point operations required per update are of order M×N where N is 1024 or greater and M is the number of beam pairs. It would be desirable to provide a significant reduction in processing throughput as compared to the SCOT method. It has also been shown that in the presence of noise and wide band signals only, SCOT will have reduced performance over a cross correlator without SCOT processing (standard cross correlator). It is would therefore be desirable to provide an adaptive cross correlator that will perform no worse than the standard cross correlator in the presence of wide band signals and noise.
Various inventors have attempted to solve related problems as evidenced by the following patents:
U.S. Pat. No. 5,724,485, issued Mar. 3, 1998, to David Rainton, discloses an adaptive cross correlator apparatus with a first receiving section that receives a signal and outputs the received signal as a first signal, and a second receiving section that receives a further signal and outputs the received further signal as a second signal, wherein the second receiving section is provided at a position different from that of the first receiving section. A first filter filters the first signal with a first changeable transfer function and outputs a filtered first signal, and a second filter filters the second signal with a second changeable transfer function and outputs a filtered second signal. Further, a cross correlator calculates a cross correlation value by using a predetermined cross correlation function based on the filtered first and second signals, and then, an adaptive controller calculates a discriminant function value representing a misclassification measure of the first and second signals, based on the cross correlation value and a true delay between the first and second signals, and adaptively adjusts the respective first and second transfer functions of the first and second filters so that the calculated discriminant function value becomes a minimum.
U.S. Pat. No. 5,899,864, issued May 4, 1999, to Arenson et al., discloses the energy, power or amplitude of Doppler or time shift information signals that is compared to a threshold in order to select a large or small weighting factor for temporal persistence. In the event of a “flash” signal or strong arterial flow signal, a small weighting factor is chosen to reduce the extent of temporal persistence via feedback of the averaged value for the prior frames so that the effect of the “flash” or strong flow signal would quickly dissipate in the imaging of subsequent frames and good temporal resolution preserved for the current frames, while low energy flow signals would cause a large weighting factor to be selected to improve the signal-to-noise ratio of low energy signals. Similar effects can be achieved by clipping the signals to not exceed a certain threshold.
U.S. Pat. No. 6,130,643, issued Oct. 10, 2000, to Trippett et al., discloses an antenna nulling system for nulling a jamming signal having a multibeam antenna, a correlator, and antenna pattern calculator, a sequential updater and a beamformer. The multibeam antenna includes a plurality of antenna elements and is operable to receive the plurality of signals. The correlator is operable to receive at least one sample signal from one of the antenna elements and a composite signal from the plurality of antenna elements. The correlator determines a cross-correlation of the sample signal and the composite signal. The antenna pattern calculator calculates a difference in pattern magnitude of an adapted antenna pattern and a quiescent antenna pattern of the multibeam antenna. The sequential updater sequentially calculates a new weight for each of the antenna elements based upon an existing weight of each antenna element, the cross-correlation and the difference in pattern magnitude. The beamformer is in communication with the multibeam antenna and the sequential updater to combine a new weight for each of the antenna elements with the plurality of signals received from the multibeam antenna to null the jamming signal.
U.S. Pat. No. 5,978,473, issued Nov. 2, 1999, to Jim Agne Jerker Rasmusson, discloses a measure of a degree of convergence in an adaptive filter arrangement that is derived from the comparison
Kasischke James M.
Lobo Ian J.
Nasser Jean-Paul A.
Oglo Michael F.
The United States of America as represented by the Secretary of
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