Choke point bistatic sonar

A sonar system for use in a water body includes a projector on the bottom of a water body for generating and transmitting a direct beam of acoustic energy. A receiving array is immersed in the water body outside the direct beam for detecting reflections from an target in the direct beam. The receiving array may be a horizontal or vertical linear array of hydrophones in the water body. The array of hydrophones may have output signals coupled to an A/D convertor and a multiplexor for digitizing and multiplexing the signals to prepare for transmission and analysis of the signals. A pair of separated projectors on the bottom of the water body may generate parametric acoustic arrays for transmitting direct beams of acoustic energy through the water body towards each other. A receiving array immersed in the water body outside the direct beams detect any reflections from an object in the acoustic beams.

RELATED COPENDING APPLICATIONS 
The following applications, filed concurrently with this, are concerned 
with aspects of bistatic sonar systems using parametric acoustic arrays: 
PLATFORM CARRIED BISTATIC SONAR, Ser. No. 07/887,438, filed May 21, 1992. 
PLANAR AMETRIC SONAR ARRAY, Ser. No. 07/887,204, filed May 21, 1992. 
BISTATIC/MONOSTATIC SONAR FENCE, Ser. No. 07/886,613, filed May 21, 1992. 
BACKGROUND OF THE INVENTION 
The present invention relates to the field of underwater sonar equipment, 
and more particularly, to shallow water sonar. 
Existing sonar systems effectively detect targets in deep water, but are 
less effective against targets immersed in shallow water. Prior to the 
present invention, it had been difficult to cope with the reverberation 
and multipath returns found in shallow water. Reverberation is a narrow 
bandwidth, noise-like signal from a variety of sources in the undersea 
environment, such as temperature and density imhomogeneities in the sea, 
marine life, rough features of the ocean bottom, and reflecting facets and 
bubbles at the ocean surface. Reverberation interferes with the acoustic 
echo from a submerged target, particularly in the case of slowly moving 
targets. Since the sources of reverberation are slowly moving as well, the 
doppler frequency shift associated with echoes from the reverberation 
sources are comparable to those from the target, weakening the 
discrimination between target and reverberation. Reverberation interferes 
with the acoustic echo from a submerged target. 
These problems restrict detection range, heretofore requiring the use of 
more equipment to cover a given search area. Other problems encountered 
with conventional active sonar systems are, severe propagation losses from 
multiple encounters with the ocean surface and bottom; and loss of signal 
coherence due to the multipath nature of the propagation in shallow water. 
SUMMARY OF THE INVENTION 
Briefly, according to one aspect of the invention, a sonar system for use 
in a water body includes a projector on the bottom of a water body for 
generating and transmitting a direct beam of acoustic energy. A receiving 
array is immersed in the water body outside the direct beam for detecting 
reflections from an target in the direct beam.

DETAILED DESCRIPTION OF THE INVENTION 
A parametric acoustic array generates very directive, sidelobe free sound 
beams without the use of large physical arrays. A projector has one or 
more transducers as elements. Two co-propagating beams of sound at 
different but close frequencies ("primary waves") or a single amplitude 
modulated carrier from a projector interact nonlinearly in the water 
column before the projector to generate sum and difference frequency beams 
to generate a virtual array. Only the difference frequency wave is 
considered here as it is much lower in frequency than either primary wave, 
and so propagates to longer ranges. The highly directive beam results in a 
greatly reduced reverberation level and in increased signal coherence from 
reduced multipath. 
The receiver may be a high gain array, with preference in most undersea 
environments being given to a Horizontally Disposed Linear Array (HDLA) 
because its array gain is normally not limited by signal propagation 
conditions, as with a vertically disposed linear array (VDLA). 
Placement of the receiving array outside the main lobe of the transmit beam 
reduces interference between the directly propagating signal and the 
reflected/scattered echo. Such bistatic geometry and a low sonar frequency 
reduces propagation losses and reduces the obscuration of low speed 
targets by reverberation clutter. 
The optimum sonar frequency is that which maximizes the signal to noise 
ratio (SNR), for a particular projector-target-receiver geometry, with 
respect to frequency. The optimum frequency value f satisfies: 
EQU .delta. (SNR(f,p)/.delta.f=0 
where p comprises the set of other parameters on which the signal to noise 
ratio depends. 
For a given bistatic geometry, the Signal to Noise Ratio (SNR) at the 
receiver in decibels (dB) is as follows: 
EQU SNR=SL-(TL1=TL2)+TS-(INL-AG)+PG 
where, 
SL is the Source Level at the parametric difference frequency, TL1 and TL2 
are the transmission losses along each of the bistatic path legs-(TL1 
being the loss associated with the path from projector to target and TL2 
being the loss associated with the path from the target to receiver). 
TS is the bistatic target strength for the submerged object (target) at the 
aspect(s) presented to the projector and receiver. 
INL is the interference level in dB/ 1 microPascal within the receiver 
passband. The interference level is the dB value of the sum of the 
reverberation plus ambient noise power values. 
AG is the array gain of the receiving array; for a Horizontal Linear Array, 
the array gain is approximately 10 log.sub.10 (N) where N is the number 
of hydrophone elements at half-wavelength spacing. 
PG is the signal processing gain for the active waveform employed. 
With most pulsed waveforms, the following approximate rules apply: 
For gain against ambient noise, 
PG.sub.(noise) =10 log.sub.10 (T), where T is the pulse length in seconds. 
For gain against reverberation, 
PG.sub.(reverb) =10 log (W) up to PG.sub.MAX, where W is the bandwidth in 
Hertz (Hz). PG.sub.MAX is an upper limit on reverberation strength where 
the duration of the main lobe of the waveform autocorrelation function 
becomes smaller than the travel time of the pulse across the target. Above 
this limit, the target strength and reverberation level decrease at 
approximately the same rate as individual target echo sources become 
resolved individually in time. 
Several of the above terms are frequency sensitive; the optimum frequency 
is obtained by either using an analytical model for the factors in the 
sonar equation of SNR, and finding the point where the derivative of SNR 
with respect to frequency equals zero; or by using numerical models for 
each effect and searching over frequency to find the frequency which 
maximizes SNR. If the projector area, primary frequency, and input 
electrical power are held fixed, source level (SL) is proportional to 40 
log f (f is difference frequency) in the Westervelt endfire regime, or 20 
log f in the farfield generation regime. The endfire generation regime is 
the regime of operation where the difference frequency is generated in the 
nearfield of the primary beam, where the beam is well collimated, and 
represents the first results treated by Westervelt in his original paper 
on the parametric array; the farfield generation regime is where the 
difference frequency generation takes place mostly in the farfield of the 
primary beam, where the primary waves are spherically spreading. On the 
other hand, projector parameters can be arranged so that the projector 
level is held constant during the optimization. 
A single model of shallow ocean transmission loss given by Urick, 
Principles of Underwater Sound for Engineers, McGraw-Hill Publishers, 
1967, pp. 146-147 is, at long ranges, 
EQU TL=20 log.sub.10 r+.alpha. r++a.sub.T (r/H-1)+10 log.sub.10 
(H)+64.5-K.sub.L 
where r is the range in thousands of yards (Kyds), .alpha. is the 
attenuation coefficient in dB/Kyd, H is a parameter defined by: 
EQU H=[(1/8) (D+L)].sup.1/2 
where D is the water depth in feet, L is the depth of the isothermal "mixed 
layer" or surface duct, in feet, and H is in Kyds. The parameters a.sub.T 
and k.sub.L given by Urick are semi-empirical model parameters derived 
from transmission loss data, and are a function of ocean surface sea state 
number, ocean bottom sediment type, and frequency. 
The limiting noise for detection may be assumed to be the ambient noise of 
the sea, which has a power spectrum dependence on frequency of -6 
dB/octave, corresponding to a functional dependence of -20 log 
(frequency). Assuming the ambient noise is isotopic spatially, the 
dependence of array gain on frequency is given by 10 log f. The optimum 
sonar frequency may be calculated by substituting the preceding formulas 
for each term in the equation for signal to noise ratio, which can be 
solved numerically by calculating SNR over a wide range of sonar 
frequencies and picking a frequency which yields the largest SNR within 
the precision of the search range. For example, the optimum frequency 
calculated for a system configuration with a 600 foot ocean region with a 
sandy bottom is 4.25 KHz, when one desires a 10 nautical mile detection 
range, and the receiver is arranged to achieve maximum detection coverage 
of any submarine crossing through the beam out to a range of 10 nautical 
miles. The beamwidth of the difference frequency wave is relatively narrow 
considering the low frequency, providing high angular resolution. In 
shallow water, the narrow beamwidth results in reduced reverberation as 
the beam can aimed to avoid the top and bottom surfaces of the water body. 
These factors are particularly important for operation in shallow water, 
less than 1000 meters in depth, since propagation losses and reverberation 
are high due to repeated interaction of the propagating sonar signal with 
the water body's surface and bottom in contrast to operation in deeper 
water. 
In conventional monostatic operation, the projector and detector are 
located at the same location and the detector receives the direct beam 
from the projector as well as beams reflected from the target, e.g. 
submarines and divers. 
In bistatic operation the projector and detector are at different 
locations. The detector may located to receive beams reflected off the 
target but not beams direct from the projector. Detectors placed outside 
the direct beam of the projector reduce interference by the direct 
transmitted signal with the target echo. Bistatic detectors may detect a 
doppler frequency shift on the received echo, even though the target may 
have a direction of motion perpendicular to the projector beam, thereby 
returning an echo with no doppler shift in the direction of the projector. 
A monostatic sonar which had its receiving array co-located with the 
projector would in such circumstances find significant interference 
between reverberation from the medium, which is at or near zero doppler, 
and the echo from the target. 
Since target detection is based on active transmission, the passive 
signature and quietness of the target are of little interest, resulting in 
a detection sonar effective against a wide array of possible targets. 
Reference is made to FIG. 1. As an embodiment of the invention, there is 
seen a rapidly deployable sonar system for use in choke point applications 
in shallow water. The system includes at least one projector and at least 
one acoustic receiving array. The projector and acoustic receiving array 
is placed on a shallow area of the water body's bottom. 
Power is either cabled from shore, from a nearby ship, or is provided by a 
local power source, such as a battery. The parametric acoustic array is 
constructed to allow either mechanical steering of the sonar beam 
direction, via a pan and tilt arrangement, or electrical steering though 
application of appropriate time delays to phase the signals applied to 
each transducer element of the acoustic array. The direct beam from the 
projector is steered in a direction which results in maximum coverage of 
the water column, accounting for refraction of the beam due to the sound 
velocity profile, and is resteered as the conditions change. 
The receiving array consists of a linear array of passive hydrophone 
elements arranged in a horizontal configuration. Alternatively a vertical 
array may be used. The receiving array is placed outside the direct beam 
to reduce the effects of interference between the direct signal from the 
projector and the echo signal arriving from the target. Placement of the 
receiving array is chosen so that the array lies within the detection 
range of a target within a the beam for all ranges out to the maximum 
allowable detection range. A bistatic propagation loss analysis is used to 
accomplish the array placement according to this criterion. Additional 
siting considerations can include calculations of target echo doppler 
shift; positions further along a perpendicular from the projector beam 
increase the observable doppler shift of a target crossing the beam, 
thereby improving the ability of the system to discriminate moving from 
non-moving targets. 
Reference is made to FIG. 2. For better coverage two projectors may be 
used, facing each other from each side of the choke point of the water 
channel to be surveyed. The direct beams from the two projectors partially 
overlap. Additional receiving arrays may be located outside the beams to 
increase the probability of detection. 
A projector may be a line-in-cone transducer, similar to the International 
Transducer Corporation (Santa Barbara, Calif.) type 5392. This transducer 
has an acoustic diameter of 24 inches, providing a 3 dB beamwidth of 5 
degrees at 30 kHz. It is capable of handling 3.5 kW input power. 
A tuning/matching network may be included in a cavity in the projector 
housing. A cable connects the projector to a topside or ashore power 
amplifier which provides the required drive for the projector. The 
projector is mounted on a gimbaled structure to allow for gross 
orientation with respect to the detector. 
The linear array may have 50 hydrophones/preamplifers mounted in a 
triangular enclosure. Each hydrophone may be similar to Model ITC-4046, 
manufactured by International Transducer Corporation. The array is 
preferably mounted on a gimboled frame to allow for orientation with 
respect to the projector. The total length of the array and enclosure is 
approximately 25 feet. Pre-amplifiers, D/A converters, and a multiplexer 
may be included as part of the array assembly. Individual signals from 
each hydrophone will be brought by cable to the surface or to shorebased 
equipment. 
The power amplifier provides up to 4 kW continuous output. The amplifier 
may be similar to the S11-4 linear switching amplifier manufactured by 
Instruments Incorporated of San Diego, Calif. 
The changing Doppler frequency from maneuvering targets modulates the CW 
pulse. The maximum Doppler bandwidth is estimated at 90 Hz. 
Referring to FIG. 3, digital signal processing is used to detect sonar 
returns. Rejection of the direct wave is accomplished by use of adaptive 
beamform type nulling in the direction of the arriving direction signal 
transmission. The nulling algorithm in most cases is required to perform 
placement of nulls on each raypath from the projector which has sufficient 
level to interfere with an echo signal. For an M element receiving array, 
M-1 such nulls can theoretically be formed, which is more than ample, 
since it is usually the case that no more than a few paths have sufficient 
strength to cause interference with the echo. Each analog voltage output 
of the hydrophone elements is separately digitized at the Nyquist 
frequency of the difference frequency plus the Doppler bandwidth. The N 
digitized acoustic signals are multiplexed into one data stream which is 
sent to the signal processor. There the multiplexed data stream is 
demultiplexed back into the N acoustic channels. Each channel is 
separately complex heterodyned to baseband, passed through an 
anti-aliasing low pass filter, and downsampled to the Nyquist frequency of 
the information signal which is equal to the Doppler bandwidth with 
complex sampling. The N downsampled acoustic channels, with their greatly 
reduced sampling rate, are appropriate for recording for subsequent 
non-real time processing. 
A block adaptive minimum variance distortionless look algorithm is 
employed. In the Minimum Variance Distortionless Look, (MVDL) algorithm, 
an interference covariance matrix estimate is formed by time averaging 
products of complex voltages from hydrophones in the array when the direct 
signal transmission from the projector is received. Averaging of the 
estimate of multiple transmission cycles is performed to improve the 
Covariance Matrix estimate. 
MVDL beamforming is accomplished by premultiplying the complex hydrophone 
voltages by the inverse of the estimated Covariance Matrix prior to 
conventional beamforming. The algorithm produces a beam which has unity 
gain in a selected look direction, and a minimum variance output, thus 
effectively placing spatial nulls on any interfering signals from other 
than the desired look direction. The outputs of each adaptively formed 
beam are processed via a set of matched filters which incorporated 
possible doppler shifts of the echo signal with respect to the transmitted 
waveform. The outputs of each matched filter are enveloped detected and 
thresholded. 
In more detail, the baseband signal is processed with an adaptive digital 
beamforming algorithm which adapts over several ping cycles of the active 
projector in order to minimize interference from the projector with any 
echo signals arriving at the bistatic detectors. 
Coherent combination of all N channels with phase shifts makes the returns 
from one direction add constructively. The beam directions are chosen so 
that the main lobes overlap significantly; this reduces scalloping loses 
when detecting returns form a direction between two beams. For a regular 
array of hydrophones, the formation of all beams can be implemented 
simultaneously as an FFT, thus reducing the processing burden. The beams 
are designed with a compromise between minimizing the width of the main 
lobe and minimizing the power in the side lobes. With the FFT 
implementation, this is controlled by the spatial weighting function for 
the N channel inputs. 
Signal detection is implemented in parallel for all of the beams. If the 
targets will be non-maneuvering, so matched filtering provides optimum 
detection in Gaussian noise. The matched filter for a CW pulse can be 
implemented as an FFT (Fast Fourier Transformation). Detection is 
performed by looking for the maximum FFT bin as seen in FIG. 4. The 
corresponding frequency is an estimate of the Doppler frequency. Zero 
padding is used to increase frequency resolution, and also to improve 
detection performance for Doppler frequencies in between two bin 
frequencies. The length of the non-zero data from the beam is matched to 
the pulse length. Overlapped FFTs are used to avoid the performance loss 
associated with splitting the energy of a pulse return between two FFTs. 
The preferred embodiment and best mode of practicing the invention has been 
described. Various modifications will be apparent to those skilled in the 
art in light of these teachings, Accordingly, the scope of the invention 
is to be determined by the following claims.