Abstract: The present invention provides a coherent radar system based on a modification of standard non-coherent radar without Moving Target Indication. Typical radars in this class are Navigation radars which are mass produced with low cost components. These radars utilize a magnetron in the transmitter which is a random phase device. In the present invention, the received signal is extracted just prior to amplitude detection process (where phase information is lost), and digitized using an analogue to digital converter providing coherent detection based on correlation between the transmitted pulse and the received signal.

Abstract: A sonar system and method of use capable of discriminating a direct acoustic signal present at 60 dB or more above the acoustic echo signal.

Abstract: A method for processing data, in particular an image representative of an observation zone in which is situated an object arranged with respect to a reference plane.

Abstract: The present invention refers to a system for detecting obstacles for vehicles, comprising at least one sensor able to transmit a detection signal and to receive an echo signal reflected off at least one obstacle, means of parametric evaluation of said received echo signal which comprise means of comparison between at least two successive echo signals for the determination of a reference echo signal able to define the position of at least one fixed obstacle with respect to said at least one sensor.

Abstract: Provided is a correlation apparatus based on symmetry of a correlation coefficient that can reduce complexity of hardware by reducing the number of adders and multipliers. Accordingly, when values of a real number part and an imaginary number part are exchanged with respect to a middle point of a correlation coefficient sequence and divided into two parts of left and right by the middle point of the correlation coefficient sequence, the values of the real number part and the imaginary number part are even-symmetrical with respect to the middle point of the divided two parts. Therefore, the number of the adders required for forming the correlation apparatus is reduced by at least 20% and the number of the multipliers is reduced by at least 70% in comparison with the 62 adders and the 64 multipliers required for a conventional correlation apparatus.

Abstract: An underwater detection apparatus for detecting a target by transmitting and receiving an ultrasound signal is provided. The apparatus includes a replica memory module for storing the typical amplitude evolution of a seawall echo during a predetermined time period as a template-replica beforehand, a correlator module for determining a correlation between the amplitude evolution of an echo signal reflected from the target and the template-replica, a seawall detector module for detecting a seawall position based on the correlation, and a seawall display processor module for displaying the seawall on its position in an indicator.

Abstract: In a direction detecting device, a transmitting wave generator encodes a transmitting signal with a code having a high autocorrelation so as to generate a transmitting wave. A transmitter transmits the transmitting wave and a plurality of receiver elements receives a reflected wave reflected by an object. A direction calculating unit calculates a direction of the object based on a phase difference between a correlation value calculated based on the reflected wave received by one of the receiver elements and a correlation value calculated based on the reflected wave received by another one of the receiver elements, a distance between the one of the receiver elements and the another one of the receiver elements, and a wavelength of the transmitting wave.

Abstract: The target searching device includes: a sound source which transmits a sound wave into a propagation space; a transducer array placed in an area to receive a forward scattering wave which scatters forward from the target within the propagation space; a subtraction processing device which subtracts a traveling wave directly traveling towards the transducer array from a mixed wave of the forward scattering wave and the traveling wave so as to separate the forward scattering wave; a passive-phase conjugate processing device which performs passive-phase conjugate processing on the forward scattering wave separated by the subtraction processing device so as to generate a passive-phase conjugated signal of the forward scattering wave; an autocorrelation processing device which performs autocorrelation processing on the traveling wave to generate an autocorrelation processed signal of the traveling wave; and a correlation device which judges a similarity between the autocorrelation processed signal and the passive-p

Abstract: An echo detection circuit that detects an echo by detecting the magnitude of a digitally mixed representation of the received acoustic signal and reference sine and cosine signals. That magnitude is then compared against an echo threshold to verify the presence or absence of an echo signal. A low pass filter with a configurable cut-off frequency may be used to define the selectivity of the echo detector.

Abstract: In a direction detecting device, a transmitting wave generator encodes a transmitting signal with a code having a high autocorrelation so as to generate a transmitting wave. A transmitter transmits the transmitting wave and a plurality of receiver elements receives a reflected wave reflected by an object. A direction calculating unit calculates a direction of the object based on a phase difference between a correlation value calculated based on the reflected wave received by one of the receiver elements and a correlation value calculated based on the reflected wave received by another one of the receiver elements, a distance between the one of the receiver elements and the another one of the receiver elements, and a wavelength of the transmitting wave.

Abstract: A method and a system for detection and position determination of chips, which transmit ultrasound signals in a room. The system comprises electronic identification chips, which are attached to objects that have to be monitored. Each chip is equipped with a transmitter and a receiver. The signals are received by a plurality of detector units, which are connected to a detector base unit that registers and interprets the signals transmitted form the identification chips. Detector base units located in different rooms are interconnected in a network and transmit processed information to one or more central units for further interpretation and sorting. The special feature of the invention is that line interference is substantially removed, and it is possible to determine position even though the identification chips are in motion.

Abstract: The invention relates to a method for the visualisation in a single image of the useful information obtained by the operation of an active sonar with line spectrum emission. The method permits an operator to simultaneously view the echoes which may need monitoring and the trace left by the sonar wave reverberation, seabed, surface or volume reverberation. Said method comprises at least three phases: one phase for Doppler processing (71) particularly for differentiation of the fixed echoes from mobile echoes, a step for generation (72) of an artificial sonar image positioning the detected echoes in a direction/distance plane, a step (73) comprising the replacement in the artificial image of a representation derived from the reverberation, in particular of that from the marine bed. Said method is particularly suitable for sonar systems using the Doppler effect to characterise the received echoes.

Abstract: The invention relates to the field of underwater acoustics and more particularly to the field of signal processing in a low frequency (LF) active sonar system. The present invention makes it possible to decrease the false alarm rate while retaining the classification of the objects. The subject of the invention is a method of processing signals received corresponding to a signal emitted comprising by recurrence two pulses, a first Doppler tolerant broadband pulse of HFM type in particular and a second Doppler intolerant broadband pulse of BPSK type in particular, comprising: a step of detecting objects performed on the part of the signal received corresponding to the first pulses and providing an alarm for each object detected, and a step of classifying the objects detected performed on the part of the signal received corresponding to the second pulses for the alarms satisfying at least one predetermined criterion.

Abstract: A system and method for estimating the SNR in a sonar environment and for determining the effect of the estimated SNR on sonar ranging accuracy. The system includes a sensor, a transmitter, a receiver, a plurality of band-pass filters, a cross correlator, and a data analyzer. The transmitter transmits a first signal having a predetermined frequency range through a transmission medium. The sensor generates a second signal corresponding to an echo signal reflected from an object. The first and second signals are provided to the band-pass filters, each operative to pass a respective sub-band of frequencies. The filters provide filtered versions of the first and second signals to the cross correlator, which performs cross correlation operations on the filtered signals. A data analyzer analyzes the cross correlator output data to determine the variability of cross correlation peaks within each frequency sub-band, thereby allowing more accurate SNR estimations in noisy environments.

Abstract: The selection of multiple, suitably designed, demodulation reference signals can enable the measurement of a vehicle's speed using continuously transmitted frequency modulated (CTFM) signals. The unique solution of vehicle speed can be used to resolve the range Doppler ambiguity found in conventional CTFM sonar and radar systems. This can enable continuous range and speed estimates of high-speed vehicles with an attendant reduction in transmit power relative to pulse-echo systems with similar measures of performance due to the 100% duty cycle.

Abstract: Dynamic tuning of a tunable filter within the receiver of a pulse-echo ranging system. A filter tuning module compares a transmit signal and a filtered signal to obtain a phase difference indicative of any mis-match between the resonant frequency of the tunable filter and the transmit frequency. The filter tuning module outputs a phase correction signal corresponding to the phase difference and a controller uses the phase correction signal to determine any adjustments necessary to minimize the phase difference. Adjustments to the tunable filter may be made with each ‘shot’ or transmission.

Abstract: To provide a method of processing output or base signals (S), in particular intermediate frequency output or base signals, from at least one device, in particular at least one radar device, for determining a distance (d), in particular a small distance on the order of magnitude of approximately zero meters to approximately 7 meters, of an object, by which it is possible to obtain from the raw signals, i.e., the output or base signals, distance information with respect to at least one object located in the detection or sensing range of the device, the following steps are provided: (a) Adaptively determining the background signal (S0) by localized filtering of the output or base signal (S) using at least one localized filter having a specified width (B); (b) Correcting the background of the output or base signal (s) by (b.1) subtracting the determined background signal (S0) from the output or base signal (S) and (b.

Abstract: A method for processing echoes in a time-of-flight ranging system or level measurement system. The method comprises applying an entropy filter to the echo profile. The entropy filter comprises determining the information content in the echo profile. The entropy filter determines the occurrences of a pattern in the bit stream representing magnitudes in the echo profile. Repeated occurrences of the pattern are used to characterize the echo signal as having low randomness (i.e. higher information content) indicative of a potential echo pulse or as having high randomness (i.e. lower information content) indicative of noise.

Abstract: A received beam-forming apparatus is disclosed is which echo signals received by multiple ultrasonic transducer elements arranged in a linear form are sampled at a specific scanning frequency to obtain sample data. A receiving beam-forming apparatus includes a memory which stores the sample data derived from multiple scanning cycles; and a beamformer which divides the multiple ultrasonic transducer elements into multiple blocks, reads out the sample data derived from different scanning cycles for the individual blocks from said memory, and forms a receiving beam in a specific direction using the individual sample data which have been read out. A sonar system using the receiving-beam forming apparatus is also disclosed as is a matched filter which selects an arc-shaped part of an ultrasonic transducer element array.

Abstract: A temporal correlation SONAR calculates the velocity and position of a water navigable craft. A first pulse and a second set of pulses are transmitted towards an ocean bottom. A tetrad of hydrophones on the craft receives echoes of the pulses. A correlation between the first pulse received on one hydrophone with the second pulses received on another hydrophone produces correlation points, for which a correlogram is generated. The distance between the hydrophones is divided by twice the time value at the peak of the correlation to calculate the velocity of the vessel. Another correlation is generated for hydrophones pairs that are positioned diagonally from one another, and maximum correlation values, along with hydrophone angles, are used to calculate athwart-ship velocity.

Abstract: In order to optimize detection of targets from a broadband return, a multiplicity of narrow bands are matched-filter processed simultaneously in parallel. The band or bands which provide the best joint combination of target and propagation in response are used for target detection.

Abstract: In order to optimize detection of targets from a broadband return, a multiplicity of narrow bands are matched-filter processed simultaneously in parallel. The band or bands which provide the best joint combination of target and propagation in response are used for target detection.

Abstract: The invention is a digital sounder module and its method for detection. The digital sounder module includes a sonar carrier wave producing means, a band-pass filter, a pre-amplifier for providing a high sensitivity and extending a wide dynamic range, and an analog-to-digital converter for providing a digital implementation of a superheterodyne detector and producing an intermediate frequency. The digital sounder module also includes a programmable logic device for controlling a gain of the pre-amplifier and for digitally filtering the intermediate frequency and a microprocessor. The method implemented by the controlled per-amplifier gain processes the return echo signal by controlled ramp up of the gain over time to compensate for the change in signal strength in proportion to the length of the return path.

Abstract: A system for detecting distances for vehicle and robotic applications includes a transducer for generating a transmission signal to be sent in the direction of an obstacle, and for obtaining a receiving signal corresponding to an echo produced by the reflection of the transmission signal off the obstacle. The transducer is driven by a chaos generator, such as a Chua's circuit. The system also includes a correlator for correlating the transmission signal and the receiving signal so that the distance between the obstacle and the transducer is identified by an instant at which the correlation assumes a high value. The transmission signal may be a square-wave signal selectively generated with one first frequency and one second frequency, and jumps between the two frequencies are determined by the instants of emission of the pulses generated by an analog type pulse generator driven by the chaos generator.

Abstract: A sonar system and method are provided to extract and identify information about a particular target illuminated by an active sonar system. The present invention utilizes a Volterra Series Expansion in conjunction with a least squares procedure to estimate channel and target responses, which may be linear or nonlinear. The system estimates the channel and target responses for different orders of the Volterra Series Expansion and then selects the order for which a minimum error is determined with respect to a measured acoustic return signal. The system requires as inputs only the excitation waveform and the measured acoustic return signal.

Abstract: An error-suppression signal measurement system and method therefor is provided. The system transmits a test signal from a first probe, through a device under test, and into a second probe. The probes extract normalization signals from reference signals therein, exchange specific ones of the normalization signals, and combine the normalization signals with data signals derived from the test signal to form receiver signals. The probes propagate the receiver signals to a receiver, where the signals are gain-ranged, digitized, normalized, and compensated for phase-noise.

Abstract: A system for detecting distances for vehicle and robotic applications includes a transducer for generating a transmission signal to be sent in the direction of an obstacle, and for obtaining a receiving signal corresponding to an echo produced by the reflection of the transmission signal off the obstacle. The transducer is driven by a chaos generator, such as a Chua's circuit. The system also includes a correlator for correlating the transmission signal and the receiving signal so that the distance between the obstacle and the transducer is identified by an instant at which the correlation assumes a high value. The transmission signal may be a square-wave signal selectively generated with one first frequency and one second frequency, and jumps between the two frequencies are determined by the instants of emission of the pulses generated by an analog type pulse generator driven by the chaos generator.

Abstract: A distance measuring system is described which may be used as an altimeter for a helicopter or for a load carried by its winch. The system comprises: a transmitting transducer for converting a first modulated electrical signal into a corresponding acoustic signal to be directed towards and reflected by a surface; a receiving transducer for converting the received reflected acoustic signal into a corresponding second electrical signal; and a correlation processor for receiving the second electrical signal, or the first and second electrical signals, and correlating the modulations thereof to produce an indication of a time delay thereof from which the distance travelled by the acoustic signal can be determined.

Abstract: A method and system are provided for measuring water current. An acoustically-modulated beam of radiation is transmitted to a target location on the surface of a body of water. As the beam transits the water, acoustic radiation propagates away from the beam towards the surface and experiences a Doppler shift in frequency relative to the acoustic frequency used for modulation. The Doppler shift is caused by current in the water through which the acoustic radiation transits. The Doppler-shifted frequency is measured as an indication of water current.

Abstract: A method for estimation of Doppler spectrum, its moments, and polarimetric variables on pulsed weather radars which uses over sampled echo components at a rate several times larger than the reciprocal of transmitted pulse length. The variables are estimated by suitably combining weighted averages of the over sampled signals in range with usual processing of samples at a fixed range location. The invention may be Used on pulsed weather radar, lidars, sonars and accoustic flow meters.

Abstract: There is disclosed an improved ultrasonic hologram or other ultrasonic imaging process that accurately forms phase and amplitude information of the hologram in a manner that renders the unit relatively insensitive to environment vibrations, and provides long maintenance free functioning lifetime. Specifically, there is disclosed an improved ultrasonic hologram detector component that forms an ultrasonic hologram on the surface of a detection deformable detector material, resulting from the deformation of the surface. The surface deformation is due to the reflection of an ultrasound (ultrasonic) energy profile of the combination of an “object wave” that passes through an object and that of a “reference wave” that is directed to the surface at an off axis angle from the “object wave”.

Abstract: A distance measuring system is described which may be used as an altimeter for a helicopter or for a load carried by its winch. The system comprises: a transmitting transducer for converting a first modulated electrical signal into a corresponding acoustic signal to be directed towards and reflected by a surface; a receiving transducer for converting the received reflected acoustic signal into a corresponding second electrical signal; and a correlation processor for receiving the second electrical signal, or the first and second electrical signals, and correlating the modulations thereof to produce an indication of a time delay thereof from which the distance travelled by the acoustic signal can be determined.

Abstract: The signal echo monitoring device includes a transmitting unit (5) for broadcasting a transmitted signal (s(t)) which is a group of randomly generated pulses, a receiving unit (6) for receiving an echo signal (e(t)), reflected by an object (12), of the transmitted signal and a combined control and evaluation unit (2) including a device (10) for jointly evaluating the transmitted signal (s(t)) and the echo signal (e(t)) with an integrated adaptive filter (14). The adaptive filter (14) automatically calibrates its impulse response b(t) and may be a digital filter.

Abstract: A method of processing uncorrelated data from at least one multiple-contact tracking system is provided. The method identifies which of a plurality of contacts being tracked that the uncorrelated data is most likely to be associated with. By combining correlation with reassessment processing, correlation errors can be reduced or minimized. A threshold test (e.g., bearing test) is used to screen or coarsely filter the data while a comparison scoring test is used associate the uncorrelated data with one of the contacts. The same threshold test and comparison scoring test are used for both correlation and reassessment processing.

Abstract: An improved bearing estimation sonar system provides high resolution imaging of extended multi-highlight targets at long ranges. The complex outputs of a pair of offset-phase centered beams are first processed by segmented replica correlators to achieve pulse compression, followed by cross-correlation to provide high fidelity bearing estimates for target imaging. The technique exploits space-time coherence properties of the target receive signal to extract high resolution target information. The processing employed in the system also includes data editing schemes resulting in enhanced discernability of submarine-like targets which is essential for target classification. Substantial processing gains are realizable by the system of this invention.

Abstract: The present invention, generally speaking, provides an acoustic railway crossing detector. The present invention provides a means of not only detecting the presence of a train, but it can determine speed and direction of travel. This allows a more adaptive control of the railway crossing gates and may reduce the nuisance factor of having the gate close too far in advance of the train reaching the crossing. It is also much less susceptible to false activation by natural causes or acts of vandalism. In accordance with one embodiment of the invention, an acoustic railway crossing detector includes at least one acoustic emitter; at least one acoustic detector; multiple reflectors arranged to reflect acoustic energy produced by an acoustic emitter toward an acoustic detector, and control circuitry coupled to an acoustic detector for producing an indication of whether or not a train is present.

Abstract: A high resolution ranging method is described utilizing a novel modulated waveform, hereafter referred to as coherent burst modulation. In the coherent burst method, high frequency modulation of an acoustic or electromagnetic transmitter, such as a laser, is performed at a modulation frequency. This modulation frequency is transmitted quasi-continuously in the form of interrupted bursts of radiation. Energy from the transmitter is directed onto a target, interacts with the target, and the returning energy is collected. The encoded burst pattern contained in the collected return signal is detected coherently by a receiver that is tuned so as to be principally sensitive to the modulation frequency. The receiver signal is processed to determine target range using both time-of-flight of the burst envelope and phase shift of the high frequency modulation.

Abstract: A method and apparatus for preventing the occurrence of range ambiguities and Doppler ambiguities in both a radar and sonar environment. A series of N pulses are produced, each of which contains a number of contagious subpulses. Each of the subpulses exhibit a different frequency than the remaining subpulses in that particular pulse. Furthermore, the order of appearance of the subpulses in each of the pulses is unique with respect to the remaining pulses in the series. A matched filter receiver and Doppler processor are used to provide auto correlations and cross correlations to prevent the range ambiguities and Doppler ambiguities.

Abstract: In echo-location and imaging systems, interference suppression is accomplished by shaping the transmitted signal waveform. Interference suppression permits more accurate range estimates or image formation, respectively. These specially shaped waveforms can also be used in communication systems to secure more accurate message transmission in the presence of interference. Shaping of the transmitted signal may be accomplished in two ways: (1) by convolving an elemental waveform with a multi-level pseudorandom sequence (i.e., binary, ternary, Poisson, etc.); or (2) by transforming nonlinearly the amplitude of the elemental waveform to give it high skewness. Interference suppression in the former case is achieved through matched filtering (replica correlation or pulse compression), and in the latter case it is accomplished by bispectrum-bicoherence processing.

Abstract: A digital signal processor wherein a bit stream containing correlation inmation for a large sample size is converted into several basic correlation functions, each of which covers a small number of samples within the large input sample. Each basic correlation function is examined to determine whether it contains a valid sonar signal, and the original bit stream is then integrated over those time periods for which the corresponding basic correlation functions indicate the presence of a valid sonar signal. The resulting optimized digital correlation function is converted into an analog signal.

Abstract: A high resolution ranging method is described utilizing a novel modulated waveform, hereafter referred to as coherent burst modulation. In the coherent burst method, high frequency modulation of an acoustic or electromagnetic transmitter, such as a laser, is performed at a modulation frequency. This modulation frequency is transmitted quasi-continuously in the form of interrupted bursts of radiation. Energy from the transmitter is directed onto a target, interacts with the target, and the returning energy is collected. The encoded burst pattern contained in the collected return signal is detected coherently by a receiver that is tuned so as to be principally sensitive to the modulation frequency. The receiver signal is processed to determine target range using both time-of-flight of the burst envelope and phase shift of the high frequency modulation.

Abstract: A process for locating a transmitter in which the signal is received by first and second receivers and then processed by respective analog to digital converters and filtered to yield first and second digital signals. There is relative motion between at least one of the receivers and the transmitter. A compression process local to the first receiver compresses the first digital signal. The result of the compression process is transmitted to a cross-correlation unit which is local to the second receiver and coupled also to receive the second digital signal. Locally to the first receiver, correction factors for the solution to the cross-correlation function are determined and transmitted to a correction unit at the second receiver site which applies the correction factors to the solution of the cross-correlation function. The location of the transmitter is determined based on time difference of arrival and frequency difference of arrival results output from the correction unit.

Abstract: A method and apparatus for imaging underwater targets utilizing sidelooking onar. A sonar pulse consisting of a concatenated series of subpulses is transmitted. The echo return is match filtered against each of the subpulses to produce a series of range-doppler maps, each providing information in a different direction. The range-doppler maps are normalized, thresholded, and intersected, thereby yielding data points which describe the target outline. Information as to the target length, width and orientation can be computed from the target outline data.

Abstract: Intervals of timing pulses generated by a timing pulse generating means are changed by an interval changing means, interference signals from a second vibrator having the same intervals of the timing pulses being eliminated by changing the interval of the timing pulses by the interval changing means.

Abstract: A signal processing system of reduced complexity over conventional banks of matched filters for each Doppler velocity of interest is responsive to a transmission which is in the form of a train of repeated Doppler invariant signals which are preferably phase continuous. The system utilizes a head end zero Doppler correlator and tapped delay line summer with summing nodes in cascade for each Doppler velocity of interest. The summer provides coherent sums indicative of the Doppler velocity of the target.

Abstract: The invention discloses an apparatus for and a method of adaptively processing a sonar power spectrum for improving the display of narrowband line structure characteristic of a target in a background of broadband interference which is rippled due to multipath propagation arrival structure. The processor employs a first Fourier transformer to convert the sonar frequency data into a complex correlation format. The autocorrelation is then time difference sampled to separate the broadband ripple interference from the broadband trend interference. The separate time difference samples are then transformed back to their real frequency format to form a broadband ripple estimate and a broadband trend estimate of the interference background. The ripple estimate is subtracted from the sonar signal to remove the ripple component and the resulting difference is normalized by a division by the interference trend estimate.

Abstract: A multi-dimensional acoustic data processing and display system arranges acoustic data in a three-dimensional matrix. The three-dimensional matrix is compressed using singular value decomposition into singular vectors and singular values. A historical database is created and maintained and is also concatenated with the three-dimensional data. This database allows reverberation and noise to be diminished and other, weaker features in the data to be enhanced. Once the data is compressed, the data can be analyzed efficiently. The singular vectors are partitioned into one or more groups on the basis of their singular values or other criteria. Certain of the compressed data elements are enhanced or diminished by modifying the singular values within each of the groups of singular vectors. Selected singular vectors are processed further by other techniques for further enhancement, detection, isolation, feature extraction and classification.

Abstract: An apparatus including a first light modulator for modulating a first light beam with a signal, a second light modulator for modulating a second light beam with a reference signal, first lens means, positioned in an optical path of the second light beam, for introducing into the second light beam horizontal magnification/demagnification which gradually varies with respect to a vertical direction, a beam combiner for combining the first and second light beams, and photodetecting means for detecting the combined light beam received from the beam combiner. The apparatus, thus constructed, is capable of processing the 50 to 100 billion operations per second required for true Doppler (time compression/expansion) processing of SONAR signals.

Abstract: An ultrasonic detecting apparatus which includes a sequence generating means and a transmission signal generating means for generating a transmission signal corresponding to the sequence. The detecting apparatus also includes a transmitting means to transmit waves, which may be ultrasonic waves, to an object and a receiving means for receiving an echo corresponding to the transmission signal. A correlation operation means performs a correlation operation with respect to the echo. In another general aspect, the sequence generating means may generate more than one sequence, some of whose auto-correlation functions produce substantially zero sidelobe in the summed result. In a further general aspect, the sequence generating means generates two or more sequences, with each of the sequences being different from the others and having substantially the same auto-correlation function as the others.

Abstract: An ultrasonic detecting apparatus which includes a generator which generates more than three transmission signals. The detecting apparatus also includes a transmitter/receiver which transmits waves, which may be ultrasonic waves, to an object and receives echoes corresponding to the transmission signal. A correlator performs correlation operations with respect to the echoes. The results of the correlation operations are processed to obtain a signal with substantially zero side lobes.