Communications – electrical: acoustic wave systems and devices – Echo systems – Returned signal used for control
Reexamination Certificate
2002-09-05
2004-08-17
Pihulic, Daniel (Department: 3662)
Communications, electrical: acoustic wave systems and devices
Echo systems
Returned signal used for control
C367S904000, C367S088000
Reexamination Certificate
active
06778467
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an underwater sounding apparatus, such as a scanning sonar.
2. Description of the Prior Art
FIG. 12
illustrates a general principles of using a scanning sonar to perform sounding of underwater situations. Referring to the Figure, designated by the letter A is an underwater sounding apparatus installed on a ship S, designated by the letter T is a cylindrical transducer unit of the underwater sounding apparatus A, designated by the letter Bs is a transmitting beam of ultrasonic waves formed by the transducer unit T, designated by the letter Br is a receiving beam for receiving reflected echoes from underwater objects such as fish which reflect the ultrasonic waves, and designated by the letter Z is a water surface. The transmitting beam Bs is transmitted underwater simultaneously in all directions around the transducer unit T, forming an omnidirectional umbrella-like beam pattern directed obliquely downward with a specific tilt angle. On the other hand, the receiving beam Br is a rotating beam having specific directivity. This rotating beam is produced by successively switching elements of the transducer unit T around its outer cylindrical surface, causing the beam to scan over 360° at a high speed in a spiral pattern. The receiving beam Br thus produced receives the reflected echoes from the underwater objects, and the information on the underwater situation, such as distribution and movements of fish schools, is obtained by analyzing received signals.
FIG. 10
is a block diagram showing the configuration of functional units of a conventional scanning sonar, which comprises a transducer unit
51
, a transmitter
52
, a transmit-receive (TR) circuit
53
as well as a receiver section including blocks
54
to
61
. The transducer unit
51
transmits ultrasonic waves into the water and receives return echoes. The construction of the transducer unit
51
is conventional, including a plurality of ultrasonic transducer elements arranged on an outer surface of a cylindrical structure. The transmitter
52
produces a pulsed transmitting signal, which has a specific pulselength, and delivers it to the transducer unit
51
. The TR circuit
53
is a circuit for switching signal paths between transmit and receive cycles. Specifically, the TR circuit
3
allows the transmitting signal to pass into the transducer unit
1
while prohibiting the transmitting signal from entering the receiver section during each successive transmit cycle and permits only the received signals to enter the receiver section during each successive receive cycle.
Designated by the numeral
54
is a first fixed-band bandpass filter provided in the receiver section, which has a fixed passband. This filter
54
is provided to take out signals in only a desired frequency range from among echo signals received by the transducer unit
51
. Designated by the numeral
55
is an amplifier for amplifying the received signals, which have passed through the fixed-band bandpass filter
54
. Designated by the numeral
56
is a frequency converter formed of a mixer for converting the amplified received signals from an original ultrasonic frequency to an intermediate frequency. Designated by the numeral
57
is a second fixed-band bandpass filter having a fixed passband for removing unwanted sideband signal components contained in the frequency-converted signals to further narrow the frequency range of the received signals. Designated by the numeral
58
is an amplifier for amplifying the filtered received signals up to a rated input level of an analog-to-digital (A/D) converter
59
provided in a succeeding stage. The A/D converter
59
converts the amplified received signals into a digital signal. Designated by the numeral
60
is a beamformer for forming a rotating receiving beam which successively scans the received signals incoming from all directions around the transducer unit T. Further, designated by the numeral
61
is a detector for detecting an envelope (or a combined echo signal) from a combined received signal produced by beamforming based on a phased array technique. An output of the detector
61
is sent to an indicator which is not illustrated.
In the scanning sonar thus constructed, the transmitter
52
supplies the pulsed transmitting signal of a specific frequency to the transducer unit
51
through the TR circuit
53
. The transducer unit
51
converts this pulse signal and emits a beam of ultrasonic waves into the water. The transmitted ultrasonic waves are reflected by fish or other underwater objects and the transducer unit
51
receives return echoes. The transducer unit
51
converts these ultrasonic echo signals into electric signals, which are sent to the receiver section as received signals. The first fixed-band bandpass filter
54
in the receiver section removes undesired noise components contained in the received signals, allowing signal components falling within only the specific frequency range to pass through. The received signals which have passed through the fixed-band bandpass filter
54
are amplified by the amplifier
55
and converted into signals of an intermediate frequency (IF) by the frequency converter
56
. Then, the second fixed-band bandpass filter
57
removes sideband signal components from the IF signals. The filtered received signals are further amplified by the amplifier
58
and converted into the digital signal by the A/D converter
59
. The beamformer
6
D forms a receiving beam which is steered over 360° in a spiral pattern. The combined received signal is sent to the detector
61
which detects the envelope of the received signal to reproduce the echo signal. Echo data thus produced is output to the indicator in a succeeding stage. The indicator produces and displays an image of target echoes showing underwater situations on-screen using the data fed from the detector
61
.
In the conventional scanning sonar of
FIG. 10
, the frequency band of the received signal is primarily limited by the first fixed-band bandpass filter
54
, and finally defined by the second fixed-band bandpass filter
57
. In this respect, it is to be noted that the frequency of the received signal does not necessarily remain constant, but can shift due to factors such as ship speed and motion, tide and waves.
FIG. 11
is a diagram illustrating frequency shifts (Doppler shifts) of the received signal caused by a change in ship speed, in which the ship S equipped with the aforementioned scanning sonar is running in the direction of an arrow K. In
FIG. 11
, (a) to (e) show frequency spectra of different signals. Specifically, (a) shows the frequency spectrum of the transmitting signal transmitted by the scanning sonar, (b) shows the frequency spectrum of a received signal obtained from an echo reflected by a target at point P ahead of the ship S, (c) shows the frequency spectrum of a received signal obtained from an echo reflected by a target at point Q on the right side of the ship S, (d) shows the frequency spectrum of a received signal obtained from an echo reflected by a target at point R behind the ship S, and (e) shows frequency characteristics of the fixed-band bandpass filter
57
.
Since the ship S is approaching the point P, the center frequency fa of the received signal from this point P is shifted by f to the positive side from the center frequency fo of the transmitting signal due to the Doppler effect, as shown in (b) of FIG.
11
. Also, since the point Q is situated at the same longitudinal location with the ship S (i.e., along its direction of motion), the center frequency of the received signal from this point Q is equal to the center frequency fo, unaffected by the Doppler effect, as shown in (c) of FIG.
11
. On the other hand, since the ship S is receding from the point R, the center frequency fb of the received signal from this point R is shifted by f to the negative side from the center frequency fo of the received signal due to the Doppler effect, as shown in (d) of FIG.
11
.
As the amount of
Ishihara Shinji
Tokuda Koji
Furuno Electric Company Limited
Pihulic Daniel
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