Underwater depth telemetry

An acoustic telemetry system and technique are disclosed for remotely meaing the underwater depth and descent rate of a hydrographic package. Analog signals indicative of hydrostatic pressure exerted on the package and related to its underwater depth are periodically sampled and converted to an N-bit digital word representative of the value of pressure. Each bit of the digital word operates one of a plurality of tone generators of different acoustic frequencies in a narrow band, and the outputs of the tone generators are combined with a reference tone continuously generated to permit resolution of Doppler shifts. Acoustically projected through the water to a remote hydrophone, a composite signal representative of the combined tones is corrected for Doppler shifts by translating the frequency of the signal in accordance with shifts detected in the reference tone. The composite signal is digitally decoded to reproduce the N-bit digital word using a parallel series of frequency detectors each tuned to one of the narrow band acoustic frequencies, and the word is displayed as an indication of underwater depth using a series of latches to ensure a steady data display.

BACKGROUND OF THE INVENTION 
The present invention relates to acoustic wave communications and more 
particularly to an improved electronic system and technique for 
telemetering underwater depth and descent rate data via an acoustic 
frequency link. 
In the field of underwater telemetry, free-falling hydrographic packages 
are commonly employed to retrieve and transmit research data pertinent to 
the nature of an underwater environment. Generally designed to descend in 
the water at a fixed and stable rate for optimum information retrieval, 
these hydrographic packages are typically provided with pressure 
transducers that produce signals indicative of the package's underwater 
depth for correlation with the research data being retrieved. The 
underwater depth and descent rate of the hydrographic packages, as 
signaled by the pressure transducers, are vital to the effective mapping 
of the retrieved information and must be continuously and accurately 
monitored throughout the descent of the packages without disrupting their 
stabilized free-fall through the water. 
Various acoustic wave communication systems and associated techniques have 
been devised and developed to relay sensor data through water for remote 
measuring and analyzation. However, while such communication systems and 
techniques have been generally successful in acoustically projecting data 
underwater, they have not been sufficiently accurate in underwater 
telemetry operations involving moving acoustical projectors, such as the 
free-falling hydrographic packages, due to errors induced by the Doppler 
effect. In addition, existing acoustic wave communication systems have not 
been easily adapted to present hydrographic operations, generally 
requiring interfacing that has interfered with the performance of the 
hydrographic packages and adversely affected stabilization of their 
free-fall descent. 
SUMMARY OF THE INVENTION 
Accordingly, it is a general purpose and object of the present invention to 
provide an improved system and technique for remotely measuring the 
underwater depth of a hydrographic package without interfering with its 
operation. 
Another object of the present invention is to provide an improved acoustic 
wave communication system and technique for telemetering the underwater 
depth and descent rate of a free-falling hydrographic package without 
disrupting its stabilized free-fall. 
Still another object of the present invention is to provide an acoustic 
wave telemetry system that is highly accurate in measuring the underwater 
depth of a descending hydrographic package by eliminating errors caused by 
the Doppler effect. 
A further object of the present invention is to provide an underwater depth 
telemetry system that is easily adapted to and incorporated within 
existing hydrographic operations without adversely affecting their 
performance. 
A still further object of the present invention is to provide an underwater 
depth telemetry system that is reliable in operation and relatively 
inexpensive to manufacture. 
Briefly, these and other objects of the present invention are accomplished 
by an acoustic telemetry system and technique for remotely measuring the 
underwater depth and descent rate of a hydrographic package. Analog 
signals indicative of hydrostatic pressure exerted on the package and 
related to its underwater depth are periodically sampled and converted to 
an N-bit digital word representative of the value of pressure. Each bit of 
the digital word operates one of a plurality of tone generators of 
different acoustic frequencies in a narrow band, and the outputs of the 
tone generators are combined with a reference tone continuously generated 
to permit resolution of Doppler shifts. Acoustically projected through the 
water to a remote hydrophone, a composite signal representative of the 
combined tones is corrected for Doppler shifts by translating the 
frequency of the signal in accordance with shifts detected in the 
reference tone. The composite signal is digitally decoded to reproduce the 
N-bit digital word using a parallel series of frequency detectors each 
tuned to one of the narrow band acoustic frequencies, and the word is 
displayed as an indication of underwater depth using a series of latches 
to ensure a steady data display. 
For a better understanding of these and other aspects of the present 
invention, reference may be made to the following detailed description 
taken in conjunction with the drawing in which like reference numerals 
designate like parts throughout the figures thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, there is shown a cylindrical hydrographic package 
11 commonly employed to gather research data regarding the nature of an 
underwater environment. Dropped into a body of water W from the air or 
from a surface vessel S, the hydrographic package 11 is typically designed 
to free-fall through the water (indicated by the dotted arrow) at a slow, 
stable rate of descent for optimum data retrieval. Attached to 
hydrographic package 11 at its lower descending end is a self-contained 
acoustic transmitter unit 10 which, as hereinafter detailed in reference 
to FIG. 2, continually projects a composite signal comprising a 
combination of digitally-encoded acoustic frequencies indicative of the 
hydrostatic pressure exerted on the package during descent. The composite 
signal projected by transmitter unit 10 also includes a 
continuously-generated reference frequency that is tracked to permit 
correction of Doppler shifts in the frequencies of the tones emanating 
from the moving transmitter unit 10. A hydrophone receiver unit 12, 
described in greater detail regarding FIG. 3, is deployed in the water W 
from surface vessel S at a location remote from transmitter unit 10 to 
acoustically receive the composite signal, correct it for Doppler shifts, 
and decode the signal for display of the hydrostatic pressure data as an 
accurate indication of the underwater depth of hydrographic package 11. 
Referring now to FIG. 2 in conjunction with FIG. 1, the transmitter unit 10 
includes a conventional pressure sensor 14 mounted upon the hydrographic 
package 11 to detect the hydrostatic pressure exerted on the package by 
the surrounding water medium. The pressure sensor 14 is preferably one 
having a piezoelectric crystal element in a resistive bridge network 
balanced at a referenced pressure, typically atmospheric, so that pressure 
variations sensed by the crystal element produce D.C. voltage changes in 
the balanced bridge network indicative of the pressure variations. A 
temperature compensator 16, typically a thermal sensitive resistive 
element, is electrically coupled to pressure sensor 14 to provide 
compensation for resistance changes in the peizoelectric crystal due to 
ambient temperature variations. A differential amplifier 18 of 
conventional design is electrically coupled to receive the 
temperature-compensated, pressure-induced voltage changes, serving to 
detect and enhance their D.C. level so that an analog signal is produced 
that is indicative of the hydrostatic pressure on package 11 and 
appropriate for further processing, as described in greater detail 
hereinafter. To reduce external interference and the errors caused 
thereby, the electrical coupling at the input of differential amplifier 18 
is preferably effected by a twisted cable pair, as illustrated in FIG. 2. 
It should be understood that the analog signal indicative of the 
hydrostatic pressure exerted on package 11 and so produced via pressure 
sensor 14, temperature compensator 16, and differential amplifier 18 is 
further related to the underwater depth of the package, and, in accordance 
with the present invention, is utilized to provde an improved system and 
technique for underwater depth telemetry. The relation of absolute 
pressure to underwater depth is given by the equation: 
EQU P=P.sub.A +.sub.o.spsb.h pg dh 
where P.sub.A is the atmospheric pressure, h is the underwater depth, p is 
the mass desnity of the water media, and g is the acceleration due to 
gravity. For water depths to several thousand feet, sea water may be 
considered incompressible and an approximate expression for pressure in 
terms of weight density (w) is 
EQU P=P.sub.A +pgh=P.sub.A +wh 
If atmospheric pressure (P.sub.A) is in pounds per square inch (psi), 
weight density (w) is in pounds per cubic foot, and h is in feet: then 
EQU P=14.7+0.447 h (psi) 
and for pressure sensors calibrated to read guage pressure, 
EQU P=0.447 h (psi) 
Accordingly, assuming very small errors in neglecting water compressibility 
and the temperature effect on the mass density of salt water, either of 
the two latter equations may be used and incorporated into the presently 
described invention to convert the hydrostatic pressure indications into 
underwater depth measurements for the hydrographic package 11. 
A conventional analog-to-digital converter 20 is electrically connected to 
the output of differential amplifier 18 for converting the D.C. level of 
the analog signal to a multiple-bit digital word. Designed to sample the 
analog signal continually, about every four seconds, the A/D converter 20 
repeatedly outputs a new digital word representative of the hydrostatic 
pressure on package 11. The digital word is produced by A/D converter 20 
at a set of multiple outputs, each output corresponding to a separate bit 
of the digital word. Represented by N, the number of bits in the digital 
word may be any whole number and is dependent upon the maximum pressure 
anticipated during the measurable descent of package 11 and by the desired 
depth resolution for the particular hydrographic application, a greater 
number of bits being required for higher values of these parameters. A 
plurality of digital gates 22, 24 and 26, equal in number to the number of 
bits in the digital word, are connected to the output of A/D converter 20, 
each output bit being directed to a respective one of the gates for 
controlling its on/off state. As described in greater detail hereinbelow, 
gates 22, 24 and 26, triggered by respective output bits from A/D 
converter 20, permit control of frequency synthesization of a composite 
acoustic signal used to telemeter underwater depth in accordance with the 
present invention. 
Acoustic tone generation in the transmitter unit 10 is provided by a master 
clock 28 which produces a high-frequency square wave output connected for 
parallel distribution to a plurality of frequency dividers 30, 32, 34 and 
36 of conventional digital design. The number of frequency dividers 30, 
32, 34 and 36 is one more than the number of bits in the digital word 
(N+1) outputted from A/D converter 20, each divider counting down the 
square wave frequency of master clock 28 to produce a different acoustic 
frequency (f.sub.1, f.sub.2 --f.sub.N, f.sub.N+1). The N+1 acoustic 
frequencies so produced by the dividers 30, 32, 34 and 36 are located 
within a narrow audio band and are frequency-separated by a small 
differential, typically about 10 Hz. 
The outputs of frequency dividers 30, 32 and 34 are connected to respective 
gates 22, 24 and 26 wherein the associated acoustic frequencies (f.sub.1, 
f.sub.2 --f.sub.N) are gated in accordance with the respective output bits 
from A/D converter 20. Typically, a "high" output bit (binary "1") from 
A/D converter 20 turns the associated gates 22, 24 and 26 "on", permitting 
passage of the respective acoustic frequency, while a "low" output (binary 
"0") from the converter turns the gates "off" thereby blocking frequency 
passage. Accordingly, the presence or absence of the gated acoustic 
frequencies (f.sub.1, f.sub.2,--f.sub.N) provides an indication of the 
value of the digital word outputted from A/D converter 20, and thus, a 
digital indication of the underwater depth of package 11 as it relates to 
hydrostatic pressure. 
The output of frequency divider 36 is not gated and is connected directly 
to a summing device 38 thereby continuously providing a reference acoustic 
frequency (f.sub.N+1) that is tracked and used to resolve frequency shifts 
in transmission due to the Doppler effect, as is described in greater 
detail regarding FIG. 3. Summing device 38 is also connected to receive 
the outputs of gates 22, 24 and 26, combining the gated acoustic 
frequencies (f.sub.1, f.sub.2,--f.sub.N) with the reference frequency 
(f.sub.N+1) to produce a composite acoustic signal representative thereof. 
The composite signal provided at the output of summing device 38 is 
amplified via power amplifier 40 and transmitted into the water W via an 
acoustic projector 42, preferably omnidirectional. It should be understood 
that power for the transmitter unit 10 may be supplied by a separate 
battery (not shown), the battery being preferably activated by water 
immersion. 
Referring now to FIG. 3 in conjunction with FIG. 1, the hydrophone receiver 
unit 12 of the present invention includes an acoustic receiver 44 to 
collect broad-band acoustic signals underwater. A conventional band pass 
filter 46 is connected to receive the signals collected by acoustic 
receiver 44 and serves to eliminate all but the narrow band of acoustic 
frequencies (f.sub.1 -f.sub.N+1) of the composite signal projected by 
transmitter unit 10. From band pass filter 46, the narrow band composite 
signal is fed to an automatic gain control circuit 48 of conventional 
design to provide the composite signal with a substantially constant 
amplitude level that facilitates further processing. 
A frequency translator circuit 40, preferably a conventional analog 
frequency multiplier which operates to produce output frequencies based 
upon the sums and differences of its input frequencies, is connected to 
the output of gain control circuit 48 for shifting the substantially-fixed 
amplitude composite signal into a lower frequency band to facilitate 
frequency separation of its components. A reference frequency tracker 52 
also connected to receive the composite signal from the gain control 
circuit 48 is designed to detect and track the continuously-generated 
reference frequency component (f.sub.N+1) shifted in transmission due to 
the Doppler effect. The reference frequency tracker 52, typically a 
conventional phase locked loop detection circuit, is further designed to 
produce a periodic output signal, such as a square wave, having a 
frequency corresponding to the Doppler-shifted reference frequency, and is 
electrically connected to feed its output signal to frequency translator 
circuit 50 to provide the basis for the degree of frequency translation 
imposed upon the composite signal with substantial cancellation of the 
Doppler shift in its components. It should be understood that since the 
acoustic frequency components (f.sub.1, f.sub.2,--f.sub.N+1) of the 
composite signal are in a relatively narrow band, the frequency shifts in 
all components due to the Doppler effect are substantially the same. Thus, 
the Doppler-shifted reference frequency component detected by tracker 52, 
when used, as described, as the basis for frequency translation of the 
composite signal, serves to null out the Doppler shifts in all the 
individual acoustic frequency components of the composite signal thereby 
providing translated components (f.sub.1 ', f.sub.2 ',--f.sub.N ') always 
having the same frequencies to aid in their detection. 
A plurality of frequency detectors 54, 56 and 58, equal in number of bits 
in the digital word produced at the A/D converter 20 of transmitter unit 
10 (FIG. 2), are connected in parallel to the output of frequency 
translator circuit 50. Each of the frequency detectors 54, 56 and 58 is of 
the phase locked loop type and is respectively tuned to the translated 
acoustic frequencies (f.sub.1 ', f.sub.2 ',--f.sub.N '). Based upon the 
presence or absence of the tuned acoustic frequency, the frequency 
detectors 54, 56 and 58 are conventionally designed to output a "high" 
(binary "1") or "low" (binary "0") digital data state, respectively, 
thereby reproducing the depth-related digital word initially produced at 
the output of A/D converter 20 of transmitter unit 10. 
A plurality of digital latches 68, 70 and 72, one each for the number of 
bits in the reproduced digital word, are connected to receive the "high" 
or "low" data states outputted by respective frequency detectors 54, 56 
and 58 for display purposes. To ensure that only steady data states are 
accepted and displayed via latches 68, 70 and 72, digital control 
circuitry is coupled to the latches including an OR gate 60, Schmitt 
trigger 64, and missing pulse detector 66 connected in a series network. 
OR gate 60 is connected to receive the individual data states as outputted 
from frequency detectors 54, 56 and 58, digitally combining the data to 
produce a "high" level output signal when any data state is "high" as 
indicative of the presence of incoming data. Connected to receive the 
output signal of OR gate 60, the Schmitt trigger 64 provides a pulsed 
output signal compatible as an input trigger for missing pulse detector 
66. The output of missing pulse detector 66 is designed to be "low", 
preventing latches 68, 70 and 72 from accepting data as long as 
periodically, typically about every one-half second, the detector is 
triggered by an input pulse from Schmitt trigger 64, indicative of a lack 
of any steady data states. In the event that there is no input trigger to 
missing pulse detector 66, indicating the presence of incoming data, the 
output of the detector is designed to pulse "high" causing the latches 68, 
70 and 72 to accept the data from frequency detector 54, 56 and 58. Once 
accepted by latches 68, 70 and 72, the data is available for feeding to a 
conventional digital display 74 as an indication of underwater depth. 
A conventional pulse generator 62 designed to produce a periodic output 
pulse, preferably about one pulse per second, is connected to feed its 
output to OR gate 60. In conjunction with Schmitt trigger 64 and missing 
pulse detector 66, pulse generator 62 acts as a display reset, causing the 
latches 68, 70 and 72 to accept "low" data states from frequency detectors 
54, 56 and 58, respectively. 
Therefore, it is apparent that the disclosed invention provides an improved 
acoustic telemetry system and technique for remotely measuring the 
underwater depth and descent rate of a free-falling hydrographic package 
without interfering with its operation or disrupting its stabilized 
descent. Furthermore, the disclosed acoustic telemetry system and 
technique provides a high degree of accuracy in measuring the underwater 
depth of the descending package by resolving frequency-shifting errors 
induced by the Doppler effect. In addition, the present invention is 
reliable in operation, relatively inexpensive to manufacture, and easily 
adapted to and incorporated within existing hydrographic operations 
without adversely affecting their performance. 
Obviously, other embodiments and modifications of the present invention 
will readily come to those of ordinary skill in the art having the benefit 
of the teachings presented in the foregoing description and drawings. It 
is therefore to be understood that various changes in the details, 
materials, steps, and arrangement of parts, which have been described and 
illustrated to explain the nature of the invention, may be made by those 
skilled in the art within the principle and scope of the invention as 
expressed in the appended claims.