Homing device for underwater divers

This invention is in the field of underwater ultrasonic communications systems, more particularly, underwater acoustic homing devices suitable for diver direction finding. This invention utilizes a single omnidirectional sound source and a simple receiver, located in close proximity to a scuba diver's torso and equipment. The acoustic discontinuity of the diver's torso and equipment provides the required directional sensitivity, thus allowing the device to incorporate the desirable characteristics of flexibility in range in combination with a simple receiver housing.

FIELD OF THE INVENTION 
This invention is in the field of underwater ultrasonic communication 
systems, more particularly underwater acoustic homing devices suitable for 
diver direction finding. 
DESCRIPTION OF THE RELATED ART 
The related art is rich with systems of varying complexity and capabilities 
that allow divers to determine the direction, while underwater, to a fixed 
or moving source of pulsed or continuous sound. In particular, Cappel, et 
al., U.S. Pat. No. 3,475,721, Massa, U.S. Pat. No. 3,489,993, Watson, U.S. 
Pat. No. 3,505,638, and Massa, U.S. Pat. No. 3,587,038, teach various ways 
of specifically realizing this function. 
Ideally, an underwater directional system will satisfy two distinct 
requirements: (1) the system must in some manner modify the inherent 
omnidirectional response of the receiver transducer so the receiver's 
response reliably and unambiguously indicates the direction to the source 
and, (2) the system must have sufficient flexibility in range to be of 
utility to divers in shallow to deep water, and in clean to dirty water. 
To quantify, the system should be operable in depths typically from 10 m 
to 150 m, and in visibility conditions typically from less than 1 m to 30 
m. Sport diving utilizing self-contained underwater breathing apparatus 
("scuba") is typically in 10 m to 40 m depths, while commercial and 
military divers can go 150 m and deeper. Likewise, sport diving is done in 
water having good visibility typically 10 m to 30 m. Commercial and 
military diving, in contrast, is often in water having very limited 
visibility, at times less than 1 m to 3 m. When diving at night, 
visibility is effectively zero. These two requirements of directional 
sensitivity and flexibility in range will ideally be satisfied by a device 
that is inexpensive to produce and easy for the diver to use. 
Directional Sensitivity. The related art demonstrates two well known 
methods for satisfying the first requirement of directional sensitivity. 
One method, used in Massa '993 and Massa '038, is to enclose the receiver 
transducer in a complex and expensive horn housing. This modifies the 
receiver transducer's response from omnidirectional to that shown in FIG. 
3 of Massa '038. Horn housings and enclosures are well known for 
substantially reducing the intensity of sound coming from any direction 
other than directly toward the "horn." To illustrate, consider a well 
known device used in the early part of this century as a hearing aid, the 
"ear horn." This device was horn shaped, the small end (output) of which 
was inserted into the ear canal, and the large end (input) of which was 
pointed toward the source of sound the listener wished to hear. Sound 
entering the horn's input undergoes an increase in intensity at the output 
due to the reducing cross section of the horn. Sound from a direction 
other than that in which the horn is pointed does not efficiently enter 
the horn's input, and thus experiences a substantial decrease in intensity 
at the output. This characteristic furnishes the horn with the required 
directional sensitivity. Both acoustic and radio frequency literature 
disclose numerous devices using horns for enhancing their directional 
response. 
The disadvantage of this first method is that the device housing is 
necessarily complex and expensive. A further disadvantage is that 
implementation of the device will be limited by the strict requirements of 
the specific housing technology, so, for example, it may be impossible to 
integrate such a device efficiently and easily into the standard diver's 
equipment console. 
The second method for realizing the required directional response, used in 
Cappel '721 and Watson '638, does not specifically modify the response of 
the receiver transducer, but rather uses the well known "difference in 
time of arrival at two or more separate receivers" technique. In this 
technique, the difference in time of arrival of a single sound pulse at 
two or more closely spaced receiver locations is used to indicate the 
direction to the source of the sound pulse, relative to the locations of 
the receivers. Use of this technique to indicate direction requires a 
minimum of two separated receivers. Again, this technique for 
determination of the direction to a sound source is well known in the 
acoustic and radio frequency arts. It is, in fact, the backbone of 
directional array design technology. Both acoustic and radio frequency 
literature disclose numerous devices and systems that use this technique 
to realize both direction finding and directional response. 
The disadvantage of this second method of realizing directional sensitivity 
is that it requires expensive and complex electronics, including multiple 
receivers. 
Flexibility in Range. The second desirable characteristic of an underwater 
directional system is flexibility in range. The advantages of a system 
allowing significant distance are obvious. It is equally important for 
commercial or military diving, however, to have a system that functions 
effectively at very close distances. Commercial and military diving often 
takes place in very limited visibility, and it is desirable to have a 
system that is functional at close range in such circumstances of limited 
visibility. The related art fails to accommodate effectively this 
requirement. 
A further elaboration needs to be made at this time on the receiver 
transducer's relative response curves. Both FIG. 5 of the present 
invention and FIG. 3 of Massa '038 show essentially full response in one 
direction, hereinafter referred to as "front," and no response in the 
opposite direction, hereinafter referred to as "back." It is well known in 
both the acoustic and radio arts that receiver transducers and antennas do 
not have such a response. Massa '038 states: "The response throughout the 
region outside the main lobe beam angle falls off to at least 25 dB below 
the level of sensitivity on the main axis of maximum response." Massa '038 
at Col. 4, lines 1-3. This means that the response to a signal coming from 
the "back" is 25 dB less than the response to signal coming from the 
"front." In common antenna terms, this would be stated as a 25 dB front to 
back ratio (F/B). The front to back ratio, F/B, is a critical parameter in 
determination of range capability. 
Both the maximum and minimum range capability of the receiver are 
determined by two quantities, signal intensity and signal to noise ratio 
(S/N) at the receiver. Signal intensity is usually the dominant factor in 
determination of maximum range, whereas signal to noise ratio, S/N, 
dominates at short ranges. The front to back ratio, F/B, is a major 
component of the signal to noise ratio, S/N. 
For quantification of the maximum and minimum range capabilities of the 
related art as given in Massa '038, one can utilize its stated values. 
These are: 
F/B =25 dB 
Full scale deflection (minimum range)=20 m 
Nominal max range (10% full scale deflection)=200 m Massa '038 at Col. 4, 
lines 65-75. 
Analysis of the art disclosed in Massa '038 reveals that, utilizing such 
technology, as the diver closes to ranges less than the minimum design 
range, the indication becomes ambiguous. The signal coming from behind the 
receiver (noise) begins to cause the indicator to deflect. At 2 m the 
diver will have difficulty determining the direction to the source, under 
the best of conditions, and at 1 m the diver cannot determine the 
direction to the source. This situation will usually occur in conditions 
of poor visibility or at night when the diver needs a clear, unambiguous 
indication at short ranges. 
There are two ways this minimum range situation may be resolved utilizing 
the existing art. The first is to reduce the source level. This has the 
disadvantages of either reducing the maximum range capability or requiring 
multiple sources or a multiple level source to maintain maximum range. The 
second is to reduce the sensitivity of the receiver, but again this will 
reduce maximum range capability. 
Neither Cappel '720 nor Watson '638 address the issue of maximum or minimum 
range. 
Accordingly it may be seen from the foregoing that the cited art does not 
have the range capability in the cited configurations to meet the 
requirement for flexibility in range. 
SUMMARY OF THE INVENTION 
In view of the failure of the related art to provide an underwater acoustic 
homing device with sufficient flexibility in range in combination with a 
simple housing and a single omnidirectional source, the object of the 
present invention is to provide an improved diver homing device with 
sufficient flexibility in range, using a small, relatively inexpensive, 
single transducer receiver, a single omnidirectional source, and the 
diver's equipment and torso as an acoustic discontinuity. 
A first object of the present invention is to provide a simple, single 
transducer acoustic receiver to aid divers in determining the direction to 
a fixed or movable source of continuous, pulsed, or spread spectrum sound 
while they are in deep to shallow, clear to dirty water. 
A further object of this invention is to make this receiver small enough so 
the diver is not encumbered or in any way restricted in movement or 
activity. 
A further object of this invention is to make a receiver that may be housed 
in a variety of housings with no requirements on these housings other than 
protection of the transducer and other parts of the receiver from the 
water environment, as required by the components that comprise the 
receiver. 
A further object of this invention is to make a receiver significantly less 
expensive to produce than those cited in the related art. 
A further object of this invention is to make the interpretation of the 
output of the receiver, the direction to the source, simple, unambiguous, 
and reliable. 
Devices in the cited related art satisfy some of the above objects, but 
only the present invention will satisfy all simultaneously. For example, 
Massa '993 and Massa '038 must be housed in the prescribed housing and 
will by nature of this housing, a large cylindrical container, encumber 
and constrain the diver to some degree. Likewise, because of the required 
complexity of the housing, the device will be expensive to produce. 
Watson '638 will not be responsive to other than pulsed sound sources, 
Cappel '721 can only be used in "deep" water and does not meet the object 
of housing simplicity and variety. Additionally, both require multiple 
transducers and both would be expensive to produce. 
In the related art discussion, two techniques were discussed for 
modification of the omnidirectional response characteristics of the 
receiver transducer, or transducers, to give the required "front only" 
directional response. This invention uses a third technique, also well 
known in the acoustics and radio arts, of introducing a discontinuity in 
the path between the source and the receiver. If the discontinuity is 
large enough, essentially no sound from the source can reach the receiver. 
It is clear then that the required "front only" response of the receiver 
transducer is realized if a discontinuity is placed between the receiver 
and source when the diver is not facing toward the source. In this 
invention, this discontinuity is realized using the diver's equipment and 
torso. To determine the effectiveness of this discontinuity and its 
capability to meet the requirements of the invention, consider the 
following analysis. 
In the absence of an acoustic discontinuity, the signal level at the 
receiver decreases as the range from the source to the receiver increases. 
The signal level at the receiver can be calculated as follows: 
RSL=SL-20 log R 
Where RSL is the signal level at the receiver in decibels, SL is the source 
level of the source in decibels, and R is the range in meters from the 
source to the receiver. 
An acoustic discontinuity changes this relationship. It is well documented 
in acoustic texts that when a sound wave passes from one medium into 
another, or in effect passes through a discontinuity, its intensity, or 
proportionally its pressure, will decrease. In simple terms, the most 
common reason for this decrease is a reflection of the incoming energy 
from the discontinuity back toward the source. Energy reflected back 
toward the source is no longer available to propagate onward, so the 
overall effect is a decrease in signal intensity. 
The relative change in magnitude of RSL caused by an acoustic discontinuity 
is proportional to the ratio of a single characteristic of the two media, 
the characteristic acoustic impedance. As an example of how this quantity 
may be used to approximate the loss in RSL, consider the interface between 
air and water. Regardless of the direction the sound wave travels through 
the discontinuity (i.e., whether it goes from air to water or water to 
air), the ratio of acoustic impedance is the same, so the loss in RSL is 
the same. The ratio of acoustic impedances for air-water is approximately 
0.0003. As acoustic quantities are customarily expressed logarithmically, 
a loss of 35 dB can be calculated, based solely on the ratio of acoustic 
impedance. A more exact analysis puts this loss at 30 dB. However, the 
simple approach of using the ratio of the acoustic impedances results in 
only a 6 dB (2 to 1) error in the actual magnitude of the loss in RSL. 
The discontinuity created by the diver's equipment and the diver consists 
of the diver's air-filled tank or tanks, the diver's buoyancy compensator, 
and the diver himself. Each of these will be considered separately to 
illustrate their combined extreme effectiveness in forming the required 
acoustic barrier. 
FIG. 1 shows a diver equipped with scuba gear, which generally includes a 
tank for compressed air, a regulator, and a buoyancy compensator. When the 
acoustic receiver is oriented in close proximity to the diver and his 
equipment (as shown in FIGS. 1 and 5), a sound wave coming from behind the 
diver first impinges on the diver's tank. This tank is usually an aluminum 
or steel cylinder about two feet long by 7 inches in diameter. It is, of 
course, hollow and filled with air. In traversing this barrier, the sound 
wave encounters the following interfaces: water to steel or aluminum 
(water to tank), steel or aluminum to air (tank to air), air to steel or 
aluminum (air to tank), and steel or aluminum to water (tank to water). 
Based again on the ratio of acoustic impedances of these different media, 
the loss in RSL on passing through the diver's air tank can be calculated 
to be 120 dB for aluminum and 130 dB for steel. 
The next discontinuity encountered is the diver's buoyancy compensator, a 
device in almost universal use among sport and commercial scuba divers. A 
detailed analysis of this device shows that in compensating for buoyancy 
changes experienced by a diver at 60 ft due to volume changes in his 
thermal protective suit (wet suit or equivalent), an attenuation of 60 dB 
can be expected due to the buoyancy compensator. 
The final discontinuity encountered is that of the diver himself. In 
orienting the receiver in the position depicted by FIGS. 1 & 5, the 
diver's chest cavity is placed between the sound source and the receiver. 
Again, a detailed analysis of this interface yields a loss of 60 dB in 
traversing same. 
The cumulative effect on the source level at the receiver, RSL, of the 
multiple discontinuities discussed above results in a total attenuation of 
240 dB. It must be pointed out that this is the attenuation in the back 
arriving sound, or as earlier referred to, the noise, and thus is in 
actuality the front to back ratio, F/B. This means the front to back ratio 
of the receiver in the present invention, when positioned in close 
proximity to the diver and his equipment (as shown in FIGS. 1 & 5) is 240 
dB. Even if the above analysis is off by a factor of 10, the front to back 
ratio is still 220 db, a formidable F/B. 
Accordingly, the present invention will clearly and unambiguously indicate 
to the diver when he is facing the source at ranges of 1 m to 500 m. This 
is accomplished with a single 160 dB source. The clear and unambiguous 
indication will function as effectively in dirty water and at night, as it 
will under "ideal" diving conditions. From the analysis given, it may 
safely be concluded that the discontinuity characteristics cited are 
suitable to give sufficient flexibility of range to accomplish this object 
of the invention. 
The preferred and alternative embodiments discussed below will make it 
abundantly clear that all the objects of the invention can be realized 
using the discontinuity technique to modify the receiver transducer's 
response. The above cited and other objects of the invention will become 
more readily apparent from the ensuing specification and drawings when 
taken in consideration with the appended claims.

DESCRIPTION OF PREFERRED EMBODIMENTS 
FIG. 1 shows the three essential elements of the invention, the diver (D) 
equipped with scuba gear including a tank and regulator, the acoustic 
receiver (10), and the sound source (30). As illustrated, the indicator 
(17) is illuminated, indicating the front (20) of the acoustic receiver is 
directed toward the source (30), and the back (21) of the acoustic 
receiver is directed away from the source, thus informing the diver that 
he is facing in the direction of the source. 
The preferred embodiment for the source is a fixed level, omnidirectional, 
ultrasonic pulsed sound source, in the approximate frequency range of 20 
kHz to 80 kHz. This frequency range allows the physical dimensions of the 
receiver to remain small, even at the lower end of the frequency range, 
and minimizes the attenuation loss in water as the frequency becomes 
higher. Many sources that meet this requirement are commercially 
available. Additionally, a source may be designed and fabricated 
specifically to meet particular frequency, source level, size, lifetime, 
or any of a number of other requirements, as system use dictates. The 
flexibility of this invention, however, lends itself to many variations on 
the preferred embodiment of the source. The source may either be pulsed or 
continuous wave as the receiver will work with either. Additionally, if a 
low probability of intercept (LPI) sound source is required, for example 
for covert or military operations, a spread spectrum source may be used. 
This type of source will require modification of the receiver electronics 
for reception of the spread spectrum source. Still other usable 
embodiments of the source can include frequency and output level control. 
This control can be manual, automatic, or programmed from an internal or 
external programmer. The receiver electronics will require modification to 
accommodate these additional embodiments. These modifications are well 
within the current electronic art, however, and do not change any objects 
or essential elements of the invention. 
FIGS. 2 and 3 show the preferred embodiment of the second essential element 
of the system, the acoustic receiver (10). The body (11) may be any of a 
variety of materials from injection molded plastic to precision machined 
metal or composites. The transducer (15) is held within the body with 
potting compound (14) as are the electronics (16), indicating Light 
Emitting Diode (LED) (17), switch (19) and battery housing, including 
battery plug (12), battery seal (13), and battery (18). The battery (18) 
may be any of a wide variety of currently commercially available 
batteries. The electronics (16) may include an amplifier, to amplify the 
signal received by the transducer, a filter as needed to restrict the 
bandwidth of the signal, and a simple amplitude detector to detect this 
signal and illuminate the LED (17) when the diver, is facing in the 
direction of the sound source (30), as shown in FIGS. 1 and 5. All these 
elements of the electronics are well known to practitioners of the art and 
there are no requirements on any of them unique to the invention. 
FIG. 4 shows the relative response of a typical receiver transducer without 
any acoustic discontinuities to alter its response characteristics. FIG. 5 
shows the relative response of the receiver transducer of this invention, 
when fixed in close proximity to the front of the diver and his equipment. 
As in the case of the source, the flexibility of the invention lends itself 
to many alternative embodiments of the receiver. For example, the receiver 
may be packaged in a large number of configurations. To illustrate this 
versatility, consider the omnidirectional receiver transducer mounted on 
the back of the diver's tank. Addition of a simple invertor to the 
detector element of the receiver electronics would then cause the 
indicator to turn off, rather than on, in response to a signal. Thus, the 
indicator will still come on when the diver is facing in the direction of 
the source, the desired direction. While the transducer element's response 
is not modified, its position and a simple modification to the electronics 
element of the receiver, make its overall response equivalent to that 
shown in FIG. 5. 
Among the many configurations available to package the receiver is 
integration of receiver elements with another piece of diving equipment, 
such as the tank example given above. As a further example, the indicator 
LED could be integrated into the diver's mask. A final extension of this 
receiver integration would be to totally integrate the receiver with 
another item of the diver's equipment, such as for example, a 
decompression computer. 
Alternative embodiments for the transducer are also easily realized. As 
shown in the preferred embodiment, it is a single disk. It could also 
consist of a plurality of omnidirectional transducers arranged to enhance 
a particular characteristic (or characteristics) such as directional 
sensitivity, sensitivity, shape, or others as determined by particular 
system requirements. It is important to note here that another requirement 
on any receiver embodiment is that the receiver transducer have a small 
cross section when compared to that of the discontinuity. The 
discontinuity item with the smallest cross section is the diver's air 
tank, at a diameter of 7 inches nominal. Since the tank is a cylinder, its 
"effective" cross section can be taken as approximately one half this 
diameter, a nominal 31/2 inches. A transducer with a cross section 
two-thirds of 31/2 inches, or 2 1/3 inches, will adequately meet the small 
cross section requirement. Larger transducer cross sections will not 
necessarily degrade performance, however. Disk transducers 21/3 inches in 
diameter can easily be shown to meet all sensitivity and directional 
requirements for the receiver. 
Still other alternative embodiments of the receiver are related to the 
indicator (17). As shown it is a single LED that illuminates when the 
diver is facing the source. It may be replaced by a plurality of LEDs 
configured such that the number illuminated is proportional to the 
received strength of the source. The single LED indicator may also be 
replaced, or used in conjunction with, a liquid crystal display. 
CONCLUSION 
It has been demonstrated that the invention, consisting of the described 
device positioned in close proximity to the diver's equipment and torso, 
will accomplish all the objects of the invention. It was also demonstrated 
that the related art, particularly that described Cappel, et al., U.S. 
Pat. No. 3,475,721, Massa, U.S. Pat. No. 3,489,993, Watson, U.S. Pat. No. 
3,505,638, and Massa, U.S. Pat. No. 3,587,038, fall short of accomplishing 
at least two of these objects. The major advantages of the present 
invention over the related art are those of flexibility of range, 
simplicity with respect to the number of transducers and electronic 
processing of received signals, flexibility with respect to housings, and 
significant reduction of the cost to produce the device. 
Practitioners of the electro-acoustic arts will recognize many simple 
variations on the cited embodiments of the invention. Among these can be 
included adjustable sound sources that will support increases in maximum 
range of the device; an almost unlimited range of housing configurations 
for the transmitter and receiver; manual, automatic, or programmable 
controlled gain, operating frequency, pulse width, and bandwidth; 
indicators that will give the diver some indication of range to the 
source; and use of multiple transducers in array configurations to support 
enhanced sensitivity or directional sensitivity. Still others may be 
apparent to practitioners of the diving art in the form of multiple tank 
configurations to increase the cross section of the barrier, buoyancy 
compensation devices that give enhanced barrier performance, and other 
techniques for housing and mounting the receiver device to present even 
less encumbrance to the diver. Still others may occur from time to time 
and still remain within the body of the invention.