Communications – electrical: acoustic wave systems and devices – Echo systems – Distance or direction finding
Patent
1994-04-11
1996-06-25
Pihulic, Daniel T.
Communications, electrical: acoustic wave systems and devices
Echo systems
Distance or direction finding
G01S 1589
Patent
active
055306802
DESCRIPTION:
BRIEF SUMMARY
This invention relates to the location and display of features, and is primarily concerned with underwater echosounding apparatus.
In underwater echosounding, the location of each feature (for example each point on the seabed) is defined by two parameters; a range, r, from the apparatus and an angle, .theta., from a specified direction to the apparatus. Echosounding apparatus comprises means for transmitting and receiving a pulsed signal, and processing signals derived from the received pulses which have been reflected by a feature to determine r and .theta..
An example of a known feature location technique for use in underwater echosounding is the use of a phased array, in which a multitude of receivers are connected to a processor via variable delay lines. The processor adjusts the delays on the delay lines until the signals from each receiver produced by reflected pulses are in phase. By measuring the time delay on each delay line, the direction of arrival of the pulse, that is the angular direction .theta. of the reflecting feature, can be determined. Such a technique requires a multitude of receivers which occupy a large physical area, and involves complex signal processing to determine the angular direction .theta..
Another technique for use in mapping surveys of the seabed is disclosed in a paper by Cloet et al entitled "A Sideways-Looking Towed Depth-Measuring System" published in the journal of the Institute of Navigation, September 1982, Volume 35, No. 3. In this technique, three receivers with a wide space (relative to the wavelength of the received signal) are used to produce two feature location signals. Each location signal define the range r of the reflecting feature to high accuracy and the angular location .theta. to high accuracy, but does not define a single feature location, that is the absolute position of the feature is ambiguous since there are a number of different angular positions which would result in identical location signals. Each of the two feature location signals produced by the receivers is used to produce a trace. The traces are then compared in the manner of a vernier scale to identify a single position for the reflecting feature. The technique taught by Cloet et al is primarily for use in survey applications, in which the data collected can be processed in slow-time, for example in the laboratory, to produce a map of the seabed. Such a technique is therefore of no practical value for navigation purposes where real-time information on submerged features etc is required. In addition, the use of vernier scale processing means that the technique suffers from errors in the calculated feature position caused by noise.
GB 2197952A teaches a further technique. Two receivers separated by a wide space (relative to the wavelength of the received signal) and two or more transmitting frequencies are used to generate two (or more) location signals, one for each frequency, each location signal defining the range r of the reflecting feature to high accuracy and a number of possible angular locations of the feature to high accuracy. These location signals are combined to uniquely define the position of the reflecting feature. Such a technique, however, requires careful signal filtering and complex signal processing due to the two or more transmitted frequencies.
It is usual to display the processed signals from the receivers on a chart recorder.
One aspect of the present invention relates to a novel apparatus to determine the angular location of a feature. First and second feature location signals are generated. One location signal defines an unambiguous angular position of the feature with low angular accuracy. The other location signal defines possible feature angular locations with high angular accuracy. The location signals are compared to uniquely identify the angular position of the reflecting feature with high angular accuracy.
In one preferred embodiment, as described herein, three receivers are used to generate the first and second feature location signals. First and second rec
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patent: 5070484 (1991-12-01), Mantel
"A Sideways-Looking Towed Depth-Measuring System", Institute of Navigation, Sep. 1982, vol. 35, No. 3.
"A Digital Phase Delay Compensating Beam-Forming Scheme For Ultrasonic Imaging", Japanese Journal of Applied Physics, vol. 27, 1988, Supplement 27-1, pp. 215-217.
Echopilot Limited
Pihulic Daniel T.
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