Note: Descriptions are shown in the official language in which they were submitted.
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SYSTEM AND METHOD FOR MEASURING ICE THICKNESS
Back~round of the_Invention
This invention relates to apparatus and method for
measuring ice thickness an~ more particularly to a measure-
ment of ice thickness using sonar located in the water under
the ice layer.
There exists a need to know the thickness of the ice cap
in many applications where the thickness of the ice determines
the limits at which certai~ equipment will function. One of
thase applications involves submarine operations where a
submàrine is ;n water covered by an ice cap. In order to
prevent damage to the submarine when surfacing is desired,
it is necessary that the thickness of the ice cap be known
before surfacing is attempted. If the ice cap is thicker
than a prescribed thickness, the submarine will be unable to
braak the ice cap and the impact may cause damage to the
submarine or its contents because of the severity of the
impact with ice which is tc~o thick to crack.
Therefore, it is necessary that an accurate determi-
nation of the thickness of the ice cap be made prior to
attempting to break through the ice cap.
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62901-705
Summary of the Invention
The foregoing problems of prior art techniques for
measuring ice thickness are obviated or mitigated and other objects
and ad~antages of sonar ice thickness measuring equipment are
provided by a system in accordance with the presen-t invention.
According to one aspect, the invention provides a method
for measuring ice thickness comprising: generating in water a
m~dulated sonic signal having a first carrier frequency and a
modulating frequency; generating in water an unmodulated continu-
ld Oll~ wave transmission sonic signal at a second carrier frequency;
said modulated signal nonlinearly interacting with said water to
generated sonic energy at said modulating frequency t said modulat-
ing frequency propagating through an ice cap of said water and
being reflected from an air-ice interface; detecting the reflected
first carrier frequency with its modulating frequency reflected
from the ice cap-water interface; said seoond carrier frequency
signal also being reflected from the ice cap-water frequency;
said reflected modulating frequency and said reflected second
carrier frequency nonlinearly interacting in said water to gener~
ate a modulated second carrier frequency; detecting said modulated
second carrier frequency to detect the reflected modulating
~requenc~ from said air-ice interface; and measuring a time dif-
ference between reception of said detected modulated first carrier
frequency and said detected modulated second carrier frequency
thereby providing the thickness of said ice.
According to another aspect, the invention provides
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62901-705
apparatus for measuring ice thickness comprising: means generating
in water a modulated sonic signal having a first carrier frequency
and a modulating frequency; means generating in water an unmodulat-
ed continuous wave transmission sonic signal at a second carrier
frequency; said second carrier frequency being reflected from an
ice-water interface produced by ice on the surface of said water
to produce a reflected second carrier frequency; said modulated
si~nal nonlinearly interacting with said water to generated sonic
enex~y at said modulating frequency, said modulating frequency
propagating through an ice cap of said water and being reflected
fxom an air-ice interface; means detecting a time of arrival of
reflected first carrier frequency with its modulating frequency
reflected from the ice cap-water interface; said reflected
modulating frequency from the air-ice interface and said reflected
second carrier frequency from said ice water interface nonlinearly
interacting in said water to generate a modulated second carrier
frequency; means detecting a time of arrival of said modulated
second carrier frequency to detect the reflected modulating
frequency from said air-ice interface; and means measuring a time
2~ ference between said detected first and second carrier frequency
modulations providing the thickness of said ice.
The aforementioned features of the invention are ex-
plained in the following description taken in conjunction with the
accompanying drawings, wherein:
Figure 1 is a pictorial:view showing the invention being
used to measure ice thickness;
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62901-705
Figure 2 is a circuit block diagram of the invention;
and
Figure 3 is a signal ti.ming diagram.
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Description of the Preferred Embodiment
This invention utilizes a parametric transmitting array
and a parametric receiving array to measure ice thickness
using a single high frequency transducer. The invention is
described in terms of an extension of a high frequency sonar
system 1 used in a submarine top sounder mode which is used
for navigation under an ice cap to encompass an additional
function of providing a measurement of ice thickness. This
invention describes a sonar system useful for measuring ice
thickness where the top sounding sensors 11 on a submarine are
o~ the type where the sensors have significant response for
both transmitting and receiving only at high frequencies,
e.g., greater than 150 KHz. Sound attenuates rapidly in ice
at these frequencies. It is therefore desirable to operate
around 10 KHz to get significant penetration through the ice
whereby the echo from the air-ice interface can be detected.
The parametric transmitting array technique for the generation
of a low frequency signal from the nonlinear interaction of
two higher frequencies having a frequency difference equal
to that of the low frequency signal is well known to those
skilled in the art. This technique of generating a low
frequency signal can be applied in using the top sounding
transducers for generating a low frequency 10 KHz trans
mission signal which can penetrate the ice cap. Ice thickness
~5 is estimated by dividing the measured one-way travel time
through the ice by the estimated sound speed in ice.
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Referring to FIG. 1, there is shown a submarine 10
carrying a top-sounding high-frequency type transducer 11
both of which are immersed in a sea water 12 environment. A
transmitted signal 13 is shown striking the bottom 18 of the
ice cap 19 from which the reflected signal 14 is received by
the transducer 11. The travel time of the transmitted signal
13 as determined by the reflected signal 14 gives the depth
o the submarine 10 below the bottom 18 of the ice cap. The
signal 13 is a high freguency carrier amplitude modulated
with a low frequency signal which will penetrate the ice
cap. The ice cap-water interface 18 is clearly defined by
the backscatter of the high frequency carrier of signal 13.
On the other hand, a nonlinearly (or parametrically) generated
difference frequency 15 (10 KHz in the example) is generated
by the transmitted signal 13 in the interaction region of
the sea water 12 which extends from the top sounder projector
transducer 11 up to the ice cap 19 (an interaction region of
about 100 meters is adequate to produce sufficient amplitude
of the difference frequency to be detectable). The 10 KHz
difference frequency 15 propagates into the ice cap 19 and
is reflected off the upper boundary (i.e. the air 17-ice 19
interface 16). The 10 KHz difference frequency 15' which is
reflected off the upper boundary 16 propagates through the
ice cap 19 and the water 1~ back to the high frequency trans-
ducer 11.
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Detection of the low frequency 10 KHz reflected signal
15' using the high frequency transducer 11 is accomplished
by following the high frequency amplitude modulated carrier
13 with a longer duration continuous waYe transmission 30 at
a high unmodulated carrier frequency. This high carrier fre-
not
quency need'be the same as that used for the initial trans-
mission 13. This second transmission frequency 30 is not
modulated so that no difference frequency components are
generated in its travel through the water 12 from the trans-
ducer 11 to the ice cap-water interface 18. However, the
transmitted unmodulated carrier frequency 30 is scattered
from the water-ice interface 18 back to the top sounder
sensor transducer 11 which is also used as the projector.
This reflected high frequency CW signal 30' propagates back
to the projector along with the 10 KHz difference frequency
components 15' provided by the preceding 10 KHz amplitude
modulated carrier frequency that are scattered by the ice
19 volume and the air-ice interface 16. The 10 KHz dif-
ference frequency components lS' and the unmodulated carrier
frequency 30' interact nonlinearly in the water in such a
way as to generate sidebands at 10 KHz on each side of the
high frequency carrier 30'. These sidebands arise because
the nonlinear effect manifests itself as a frequency modulation
of the backscattered carrier components by the backscattered
10 KHz components. This frequency modulated high frequency
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carrier is received by the high frequency top sounder trans-
ducer 11 and processed by FM discrimination techniques in an
FM receiver to provide an echo signal from the air-ice inter-
face 16. If the ice thickness, determined by the sensitivity
of the ice thickness measuring system 30 of FIG. 2, is less
than some maximum thickness, for example, a thickness of 5
feet, a discernible echo signal 27' will be detected by the FM
receiver 20 from the ice-air interface 16. A very thick ice
cap, where submarine surfacing is not possible, can be identi-
fied by lack of a detected echo from the ice-air interface 16
because of the attenuation of the modulating frequency 15 in
the ice.
Referring to FIG. 2, there is shown a block diagram
of a sonar system 30 of this invention. The transducer 11
referred to in the discussion of FIG. 1 is connected to a
transmitter 21 and a dual AM/FM receiver 20 capable of
providing an output signal for either an AM or an FM received
signal. Transmitter 21 is amplitude modulated by the 10 KHz
amplitude modulator 24 whose output is connected to trans-
mitter 21 through gate 25. Gate 25 is activated by a pulse
rom pulse source 26 during which time the output of trans-
mitter 21 is amplitude modulated with a 10 KHz signal.
Typically, the duration of pulse 260 of FIG. 3 is 1 msec
which is the approximate time for the 10 KHz signal 15 to
travel through a 5-foot thick ice cap 14 and ~or the 10 KHz
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signal 15' reflected from the ice-air interface 16 to travel
back through the ice cap to the water~ice interface 18. The
duration of the pulse 260 and hence the duration of the AM
transmission signal 13 is controllable by control 2~1 to
S cause the transmitted signal 13 to be optimally turned off
at the time that the reflected 10 KHz signal 15' begins its
return travel through the water 12.
At the termination of the pulse 260 of source 26, the
pulse source 22 is activated to provide an energization
` pulse 22n to transmitter 21 which causes it to provide an
unmodulated CW output signal 30, which need not have the
same carrier frequency as the original transmission 13t to
transducer 11 for the duration of a pulse 220. The amplitude
modulated reflected signal 14 is detected by the AM portion
lS of receiver 20 to provide the signal 14' at the output of
filter 23. For reception of the FM signal 27 resulting from
the interaction of signals 15', 30', the FM portion of
receiver 20, connected to the transducer 11, provides an
output signal 27' from a bandpass filter circuit 23 whose
function is to narrow the bandwidth of the received signal
to 2 KHz in order to filter the detected one millisecond 10
K~z modulated FM return signal resulting ~rom the one milli-
second duration of the amplitude modulated transmission
signal 130 The distance below the ice cap 19 of the trans-
ducer 11 is assumed to be 100 meters which is sufficient
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distance to provide the interaction region for nonlinear
interaction of the modulated signal 13 to parametrically
produce adequate 10 KHz signal 15. The 10 KHz modulation
frequency has been chosen somewhat arbitrarily with the
following considerations. The parametric transmitting
signal source level varies as 40 log W where W is the modu-
lation frequency. Thus, the higher modulation frequency is
desirable in order to increase the power level of the modu-
lation (difference) requency W as it enters the bottom of
the ice cap 19. However, the attenuation in ice increases
as the fre~uency W increases. Therefore, the 10 KHz modu-
lation frequency has been chosen as typical in what is
believed to be the optimal range of modulation frequencies.
The power level of the transmitter 21 need only be in the
vicinity of 40 watts in order to provide a detectable signal
27' from the ice-air interace 16 of ice cap 14.
The thickness D of the ice cap 19 is determined by the
elapsed time, TT, between the detected signals 14' and 27'
divided by the product of the velocity of sound in ice, CICE~
(1.6 milliseconds/meter) and two (to account for TT being
the round-trip time through the ice), (D = TT/2CICE).
FIG. 3 illustrates the transmit signals 13, 30 from
transmitter 21, reflected signals 14, 30' and 27 received by
the receiver 20, the detected AM signal 14', and the detected
~S FM signal 27', all as a function of time. The depth of the
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transducer 11 below the water-ice interface 18 is represented
by the elapsed time 50 from the time of transmission of
si~nal 13 to the time o reception of the AM signal 14. The
thickness of the ice cap 19 is proportional to the time TT
between the received signals 14' and 27' by the relationship
given in the preceding paragraph.
Having described a preferred embodiment of the invention,
it will be apparent to one of skill in the art that other
embodiments incorporating its conce~t may be used. It is
believed, therefore, that this invention should not be
restricted to the disclosed embodiment but rather should be
limited only by the spirit and scope of the appended claims.
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