USA As Represented by Secretary of the Navy

James M Alsup

Harper J Whitehouse

Jeffrey M Speiser

James W Bond

Jack R Williams

Edward C Jelks

Isaac Lagnado

Shelby F Sullivan

A sonar system that includes a new comb-like waveform constructed by modulating the tines of a comb spectrum according to a set of Hermite functions defining a Hermite Function Space (HFS), and a processing method that reduces the sidelobes of the ambiguity function associated with the normally-processed HFS comb waveform. Noise-limited performance remains high, because the waveform is designed to be power-efficient; range ambiguity is superior to the highly-rated sinusoidal frequency-modulation (SFM) waveform; and reverberation-limited performance is equal to or better than that offered by any other waveform designed for this purpose. The full Doppler sensitivity normally associated with pulsed CW can be realized by the application to HFS signals of the constrained, regularized deconvolution method of this invention. The deconvolutionally-processed Hermite-function comb waveform offers better reverberation-limited performance than does the triplet-pair comb waveform or any of its predecessors, while maintaining a noise-limited performance equal to the best of these.

Power-efficient sonar system employing a waveform and processing method for improved range resolution at high doppler sensitivity

An active sonar system with improved noise-limited performance in littoral regions with reverberation. This invention solves the active sonar comb-waveform power-limitation problem by introducing for the first time a system employing a new comb waveform herein denominated the triplet-pair comb waveform. Ambient noise-limited performance of the system of this invention is superior to that of systems employing other Doppler-sensitive waveforms such as the geometric comb waveform. Reverberation-limited performance of the system of this invention is slightly inferior to that of systems employing other Doppler-sensitive waveforms but this invention eliminates much of the range ambiguity problems seen with other non-comb waveforms.

Sonar system and method employing a power-efficient triplet-pair comb waveform

A system is presented for enhancing sonar signals wherein Doppler echoes eived in the time domain from a sonar system are processed by digitizing, shifting to baseband, Fourier transforming to the frequency domain, applying a frequency stretching algorithm designed to expand low Doppler signals, retransforming the signal back to the time domain, reshifting from baseband to original center frequency, converting to analog and outputting the resultant enhanced Doppler time domain signals to either earphones of a sonar operator for audio monitoring or for further processing for visual display.

Aural enhancement of active sonar systems

An apparatus determines the location of a signal source having coordinates x,y). The source may be located as far away as two thousand km and does not require knowledge of ionospheric height or layer structure. The apparatus comprises at least three receivers adapted to receive the signals from the signal source. The coordinates, (x.sub.1,y.sub.1), (x.sub.2,y.sub.2) and (x.sub.3,y.sub.3), of each of the receivers are very precisely known. One receiver is capable of receiving signals from the other two receivers through different means (e.g. a wide-band telephone link). A plurality of at least three timers, one connected to each receiver, measures precisely the time difference of arrival (TDOA) of the signals from the signal source. A plurality of at least three means, connected one to each of the receiving means, determine the vertical angles .phi. at which the signal from the signal source is received by each of the receivers. More precisely, each receiver is in reality a phased array of individual antenna elements whose joint output can be used to measure both vertical angle and azimuth. Means are provided for calculating the coordinates (x,y) of the signal source from the measured vertical angles and the time differences of arrival.

Fix-tracking system

A discrete chirp generator produces discrete chirp signals which correspond o either of two exponential functions, namely the functions exp [j.pi.(m.sup.2 +.alpha.)/N] or exp (j.pi.[m(m+.beta.)+.alpha.]/N), where m=0, 1, 2, . . . , M. Typically, M and N might both have the value 256. Similarly, a discrete chirp filter is described by its impulse response which has one of these same two functional forms. These two new components (discrete chirp generator and discrete chirp filter) can be used to implement

Chirp filters/signals

A sampled speech compression and expansion system, for two-dimensional prssing of speech or other type of audio signal, comprises transmit/encode apparatus and receive/decode apparatus. The transmit/encode apparatus comprises a low-pass filter, adapted to receive an input signal, for passing through low-frequency analog signals. A converter is connected to the low-pass filter for converting the analog signal into a digital signal. A buffer memory, whose input is connected to the converting means, stores the digitized signals. A correlator, having inputs from the A/D converter and the buffer memory, correlates the digital signal received directly from the converter with a delayed signal from the buffer memory. An 'interval-select' circuit, whose input is connected to the output of the correlator, uses the autocorrelation value as a basis for comparison with subsequent peaks in the correlation value which are greater than a specified fraction of the autocorrelation value. The interval-select circuit has an output which is connected to the buffer memory, the value of the fractional peaks and their timing being stored in the buffer memory. A transform circuit, whose input is connected to the buffer memory, performs an even discrete cosine transform (EDCT) of the stored signal. A first modulator, whose input is connected to the output of the EDCT means, differentially pulse code modulates (DPCM) its input signal. A second modulator, whose input is connected to the output of the interval select circuit, differentially pulse code modulates its input signal. A multiplexer, having an input connected to the output of the first and second modulating means, combines the two differentially pulse code modulated signals. A receiver/decoder has circuits which perform an inverse function to those of the transmitter/coder and are arranged in inverse order, from input to output, to those of the transmitter/coder.

Sampled speech compression system

A charge-coupled device (CCD) analog and digital correlator comprises identical modules, each of which is a simple analog CCD correlator with digital input and output. Circuits are included:

CCD Analog and digital correlators

Apparatus for performing a discrete cosine transform of an input signal, table for real-time television image processing, specifically for obtaining an acceptable picture when the number of bits of information available for describing the picture and/or the channel bandwidth are severely limited, comprising: two read-only memories, each containing a predetermined number of +1's and -1's arranged in a predetermined manner; two multipliers, each having an input from one of the read-only memories, and another, joint, input connectable to the input signal which is to be transformed discretely and cosinusoidally; two transversal filters, each having N taps, rather than twice the number (2N-1) of taps in the prior art discrete cosine filters, the input to each filter being the output of one of the multipliers; and a single-pole double-throw switch, which switches position at every shift of the samples through the filter, the output of the transform apparatus being connected to the switch arm, each of the free terminals of the switch being connected to the output of one of the transversal filters.

Apparatus for performing a discrete cosine transform of an input signal

An automatic frequency-hop controller for a surface acoustic wave (SAW) shesizer is used with a means for generating two chirp signals, one having a delay with respect to the other. It comprises a SAW device, adapted to receive the chirp signals. The SAW device comprises a substrate and a pair of sets of interdigitated electrodes, disposed on the surface of the substrate, one set at each end, the electrodes receiving the electrical chirp signals or corresponding impulse signals. Another set of interdigitated electrodes, which are output electrodes, are also disposed on the surface of the substrate, the electrodes of all sets being parallel. The output set of electrodes are so disposed as to receive the acoustic waves generated by the pair of sets of electrodes, and transduce them back to electrical signals. For the case when quadrature summation is performed by offset acoustic delays on the substrate, the pair of sets of electrodes and the output set of electrodes are so disposed with respect to each other that the signal received from the top parts of the output set of electrodes has a phase shift of 90.degree., at a frequency f.sub.c, with respect to the phase shift of the signal received from the bottom parts of the output electrodes. The controller further comprises means connected to the output set of electrodes, which selectively controls/mixes the transmission of the electrical signals received from the output set of electrodes. The controlling/mixing means has the capability of transmitting signals from the output set in any sequence, the frequency of the output signal being determined by the particular signal path which is activated. Another embodiment of the invention includes the means for generating the two chirp signals.

Discrete chirp frequency synthesizer

A frequency synthesizer, for use with a means for generating two sequences of electrical pulses, one sequence having a delay with respect to the other, comprises a surface acoustic wave (SAW) device, adapted to receive one of the sequences of electrical pulses from the generating means. The SAW device comprises a substrate, on the surface of which is disposed a first set of interdigitated electrodes. The electrodes receive the electrical pulses and tranduce them to acoustic waves, which traverse the surface of the substrate. The lengths of the electrodes are configured as a function of m, where m is defined by the relation m = R.sup.n mod P, where P is a prime number, R is a primitive root of P, and 1.ltoreq.n.ltoreq.P-1. A second set of interdigitated electrodes, approximately N-1 in number, is also disposed on the surface of the substrate, the electrodes of the first and second sets being parallel. The second set of electrodes is so disposed as to receive the acoustic waves generated by the first set of electrodes, and transduce them back to electrical signals, which are conducted on a set of leads connected to the electrodes. Means, adapted to receive pulses from the electrical pulse generating means, selectively receives the electrical signals from the second set of electrodes. The receiving means has as an output a signal whose frequency is determined by the order in which the signals are received from the second set of electrodes.

Transversal filter prime sequence frequency synthesizer

A carrier-compatible device for computing the discrete Fourier transform of an input signal, using the chirp-Z transform (CZT) algorithm, comprising means for connecting to a real and imaginary part of an input signal g.sub.k. A pulse generator generates a sequence of very short pulses. A surface acoustic wave (SAW) chirp generator, whose input is connected to the output of the pulse generator, generates cosine chirp signals and sine chirp signals. Four input mixers have as their two inputs a real or imaginary part of the signal g.sub.k and a sine or cosine chirp signal from the SAW chirp generator. First and second summers have as their two inputs the outputs from two of the input mixers. A SAW chirp filter, whose two inputs are the outputs of the summers, filters out the higher components from the input signal and passes the lower components. Third and fourth summers are connected to the SAW chirp filter, whose two inputs are components from the SAW chirp filter. First and second delay lines, whose inputs are connected to the output of the sine or cosine SAW chirp generator, delay their input signals an amount of time such that their output signals are coincident in time with the output signals from the summers. Four output mixers have as their inputs the output from the first or second delay lines and the output of the third or fourth summer. Fifth and sixth summers have as their two inputs the positive or negative components from two of the four output mixers. First and second low-pass filters have as their input the output of the fifth or sixth summer, and their output comprising the real or imaginary part of a complex number G.sub.k at zero frequency.

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San Diego, CA, United States of America

James M Alsup