GB2128331A

GB2128331A - Acoustic underwater antenna with synthetic aperture

Info

Publication number: GB2128331A
Application number: GB08325867A
Authority: GB
Inventors: Karl-Alwin Gudesen; Heinz Urban
Original assignee: Fried Krupp AG
Current assignee: Fried Krupp AG
Priority date: 1982-09-30
Filing date: 1983-09-28
Publication date: 1984-04-26
Also published as: NO833420L; SE457913B; DE3236209A1; NO160166B; GB8325867D0; SE8305101L; GB2128331B; NO160166C; SE8305101D0

Abstract

A synthetic aperture acoustic underwater antenna comprises an assembly (10) of electro-acoustic transducers which is supported on the sea bed and rotates at constant angular velocity in a fixed circular path. The assembly is connected by a cable (23) to an above-water signal processing unit (22). In a further embodiment rotation is achieved by selective switching of a circular array of transducers. The device may be used in offshore technology e.g. drilling site supervision and sea-bed prospecting. It may also be used for harbour and sea-lane traffic control. <IMAGE>

Description

SPECIFICATION Acoustic underwater antenna with synthetic aperture The invention relates to an underwater antenna with synthetic aperture of the kind set out in the introductory part of claim 1.

Lateral view radars with synthetic aperture have long been known (see [1] and [2]). They achieve a high resolution transversely to the direction of transmission and reception, the movement of an aircraft carrying the radar being used to produce a large synthetic aperture angle.

Such lateral view radars are used primarily for investigation of the earth, measurement of sea waves and military observation.

In [3] are described ideas for applying the synthetic aperture techniques to sonar systems, particularly lateral view sonar systems for cartographic study of the sea bottom. Such a lateral view sonar has a physical underwater antenna which is so mounted on a travelling ship that it transmits and receives transversely to the direction of travel. The positions of the antenna elements of the resulting synthetic underwater antenna of large aperture are the positions of the physical antenna at the moments of transmission and reception.

The advantages of the synthetic sonar system over the conventional lateral view sonar are set out in [3]. They reside primarily in improvement by a factor of 2 in the azimuthal resolution of the underwater antenna as compared with a real linear antenna of the same length or aperture, the problem-free focussing of the antenna on each reception range and the resulting achievement of a constant resolution, independent of the range and transmission frequency as well as a substantial improvement in signal/noise ratio with otherwise identical parameters.

Synthetic aperture sonars have however for a long time found no practical application, in particular for the following reasons:- The carrier platform, e.g. a ship, for the synthetic aperture antenna may not deviate from its rectilinear path by more than A/8, where A is the wavelength of the sound frequency. As this cannot be ensured for ships or towed underwater bodies, measures must be taken to apply calculated compensations for the deviations. To this end the acceleration forces on the carrier platform, which cause the deviations from course must be measured with sufficient accuracy and the resulting phase errors taken out in the signal processing. The cost required for the necessary measurements and calculation is extremely high.

Also to achieve the desired high resolutions with large display range necessitates a very low speed of the carrier platform which increases the problem of constancy of path.

The invention is directed to the problem of providing an acoustic underwater antenna with synthetic aperture such that, at acceptable cost and with use of standard components the known advantages of a synthetic aperture sonar can be realized in practice.

This problem is solved for an acoustic underwater antenna with synethetic aperture of the kind defined in the introductory part of claim 1 by the features in the characteristic portion of claim 1.

The underwater antenna according to the invention has the advantage, despite small dimensions, of high resolution portrayal of the surroundings. In contrast to preveious antennae the resolution remains high even at the greatest range of portrayal targets. Due to the defined constant path and the spatially fixed scanning positions problems and costs, such as those with towed synthetic aperture sonars, are avoided.

While it is true that the underwater aperture according to the invention is confined to a fixed position, this limitation nevertheless renders it possible at acceptable cost to make the synthetic underwater antenna with its superior properties competitive with conventional sonars. The underwater antenna according to the invention can be constructed of standard components and involves no high calculation costs like those of synthetic aperture radars.

The underwater antenna according to the invention is particularly suitable for use in offshore technology, e.g. in supervising drilling sites and for prospecting on the sea bed, and also for protection of underwater objects and for supervision of harbours and sea lanes. The power requirements of the underwater antenna according to the invention are very small. The average transmission power, for example, in an embodiment embracing a semi-circular surrounding area of 6.5 ha is only 46 mW.

An advantageous embodiment of the invention is the subject of claim 3. There, measures result in a very compact configuration of antenna providing a panorama display of the surroundings of high resolution. The technical realization of the underwater antenna is extremely simple. Owing to the synthetic display production it is unnecessary to display the entire circular surface.

Portrayal of selected rings of the surroundings is also possible, the inner and outer radii of the rings being determined only by the number of evaluable or portrayable targets.

Another advantageous embodiment of the invention is the subject of claim 4. These measures yield a synthetic underwater antenna at minimum cost. By slow rotation of the carrier a very high resolution is attained. A slow speed of movement of the carrier in its path is preferred when only stationary objects in the surroundings are to be detected. Moving objects are then not displayed sharply. Sharp display can be attained by faster movement of the carrier and consequent more rapid traversal of the moving object. The resulting decrease in resolution can be compensated by increase in the radius of the path.

Another advantageous embodiment of the invention is set out in claim 10. The resolution of the underwater antenna according to the invention increases with decrease in its aperture, i.e. increase in its aperture angle. A maximum practically attainable aperture angle is 180".

Another advantageous embodiment of the invention is set out in claim 11. By spatial separation of the part of the antenna providing signal preparation from the signal receiving part the costs due to loss of components is minimised.

The spatial separation is made possible in particular by the small band width of the signals to be transmitted and the low power requirement of the underwater antenna.

Certain embodiments of the invention will now be described in detail with reference to the drawings, in which: Figs. 1 and 2 are schematic perspective views of first and second embodiments of underwater antenna, and Fig. 3 is a schematic view of the antenna geometry in three scanning positions.

The underwater antenna shown in Fig. 1 includes a transducer assembly 10, consisting of a plurality of electroacoustic transducers 11, which is fixed to a carrier 12 which moves at constant speed in a horizontal plane. The carrier 12, which is approximately half-spherical in shape, is fixed to an arm 13 which is carried by a vertical shaft 14 rotated at constant angular velocity by an electric motor 15, mounted on a framework 1 6 supported on the sea bottom 25.

Dish-shaped feet 1 7 prevent the frame from sinking in layers of mud and ensure its stability.

The carrier 12 and the arm 13 can be surrounded by a cover 1 8 shown in dash lines.

The transducer assembly 10 has a small aperture and therefore a large aperture angle 6B (Fig. 3), which may for example be 1200 and can be as much as 1500. Owing to the rotation of the arm the transducer assembly 10 moves with an angular velocity co in a defined spatially fixed circular path 1 9 about a fixed centre 20. The path 1 9 in a horizontal plane therefore constitutes a prescribed invariable track which can be traversed repeatedly with absolute constancy. The transducer assembly 10 operates alternately as a transmitter and as a receiver and, in the transmission plane, emits short CW pulses of constant frequency or FM sweeps of linearly increasing frequency. The pulses are transmitted in a fixed phase pattern based on a constant reference signal.

The sound field produced by reflection of the sound energy is scanned at successive positions in the path 19, the carrier having moved in the interval between transmission of a pulse and reception of a resulting echo only so far that the difference in distance between the target and the transducer assembly 10 is at most A/4, where A is the wave length of the sound energy.

The carrier 12 is shown in Fig. 3 in three different scanning positions I, II and Ill. In position I of the carrier 12 a point target 26 has just been picked up by the transducer array 10 of opening angle SB. This target 26 is in view at each scanning position of the carrier 12 between the positions I and Ill. At position Ill the target 26 leaves the observation zone of the transducer array 10. The synthetic aperture Ls of the underwater antenna with respect to the target builds up over the angular range , which is derived from the zone in which the target remains under observation within the aperture angle OB of the transducer assembly 10, since all values scanned within the angular range XS, at the various scanning positions can be coherently evaluated.To this end the received echoes, as in the case of a synthetic aperture radar, are detected in a signal processing unit 22 (Fig. 1) in terms of phase and amplitude with reference to a reference signal by a quadrature detector and stored. To effect azimuthal signal compression the stored values associated with a resolution cell, after phase correction if necessary, are processed for the purpose of focussing the antenna on an image element by Fourier transformation followed by matched filter operation or by means of a filter bank. The construction of the signal processing unit 22 and the signal evaluation therein are as described in detail in reference [2]. As the result there appears for each reflecting target at the output of the unit 22 a sharp pulse, which is portrayed correctly in terms of bearing and distance on a display screen.

As shown schematically in Fig. 1, the unit 22 is disposed above the water surface 21 at a location remote from the rotating transducer array 10, e.g.

on a ship, and connected to the transducer array by a coaxial cable 23 for transmission of information and supply of the transmission energy. The energy supply for the motor 15 can be through a cable or from a battery 24 mounted on the framework 16.

The time T per revolution of the carrier 12 and the transducer 10 in the path 19 of radius r (Fig. 3) is, in consideration of the above-mentioned A/4 condition, for a maximum range Rmax of the antenna given by the expression: 1 67r RmaX r 56s T= sin 2 in which, to a good approximation, S can be replaced by OB. For an aperture angle OB of 1200 this reduces to:

where c is the propagation speed of sound in water. The azimuthal resolution is:

and radial resolution, as in all sonar antennae: c.T ar= 2 in the case of CW pulses of duration T and c ar= 2.B in the case of an FM-sweep of band width B.

In a typical example the following data are obtained: Aperture angle of the transducer array 10: 0B=1200 Radius of the path 19: r=1 m CW-transmission pulses of pulse length: T=0,3 ms and frequency: f=242 kHz Time per revolution of the carrier 12: T=677 s/revolution Maximum range: Rmax=140 m Azimuthal and radial resolution: Saz=Sr=0,25 m.

When the panorama obtained by the underwater antenna is displayed on a screen F in accordance with present day technology, 106 image points are available so that the display represents a panorama having a surface area of 6.25 ha.

Fig. 2 shows another embodiment of synthetic aperture underwater antenna. In this case there is an electro acoustic transducer 30 for each scanning position along the path 1 9. All the transducers 30 are mounted on the perimeter of a disc-shaped carrier 31. The carrier 31 and the transducers 30 are surrounded by a cover 32. The carrier 31 is supported in a horizontal position on a framework 33. The latter is again supported on the sea bottom 35 by dish-shaped feet 34. As before a signal processing unit 36 is provided above the water surface 37 and connected to the transducers 30 by a coaxial cable 38 for transmission of information and supply of transmission energy.

By the provision of associated time delay members groups of, for instance three, adjoining transducers 30 are connected electronicaily to form transducer assemblies of large aperture angle 0B By switching adjoining transducers 30 on and off rotation of the transducer assembly is achieved, as an alternative to the mechanical rotation of the transducer assembly 10 in Fig. 1.

The invention is not limited to the above described embodiment. Thus instead of a single carrier 1 2 with its supported transducer assembly 10 it is possible to use several such assembly carrying arms which rotate simultaneously along the path at a constant distance from one another.

Also the closed path of rotation can take another form, e.g. elliptical. Also the closed rotational path can be replaced by a rectilinear path traversed repeatedly by return movements of the carrier 1 2.

However, rotation of the carrier 12 in a circular path has great advantages from the viewpoint of cost both mechanically and in electronic signals evaluation.

The numerals in brackets in the above text which refer to the state of the art relate to the following literature: [1] Skolnik, M. I.: Introduction to RADAR System, McGraw Hill, 2.Auflage 1980, [2] Söndergaard, F.: A Dual mode digital processor for medium resolution synthetic aperture radars, RADAR 77, IEEE publication, No. 155, [3] Cutrona, L. J.: Comparison of Sonar system performance achievable using synthetic aperture techniques with the performance achievable by more conventional means, JASA, Vol. 58, No. 2, pp. 336.

Claims

1. An acoustic underwater antenna having a synthetic aperture produced by successive scanning of a sound field at spaced positions along a path and coherent processing of the scanned values, characterised in that the path in which the scanning positions lie is spatially fixed and constitutes a prescribed invariable track which is traversible repeatedly for repeated scanning of the sound field.

2. An antenna according to claim 1, characterised in that the path lies in a substantially horizontal plane.

3. An antenna according to claim 1 or claim 2, characterised in that the path is a closed path of revolution, preferably a circle about a centre fixed with respect to the bottom of the water.

4. An antenna according to one of claims 1 to 3, characterised by the provision of at least one carrier which moves, preferably at constant speed, along the path and carries an assembly of electro-acoustic transducers having an aperture angle of less than 1800.

5. An antenna according to claims 3 and 4, characterised in that the carrier rotates at constant angular velocity in a circular path.

6. An antenna according to claim 5, characterised in that the carrier is fixed to an arm attached to a vertical drive shaft.

7. An antenna according to claim 6, characterised in that the arm, the carrier, and the transducer array at the path are surrounded by a cover.

8. An antenna according to one of claims 1 to 3, characterised in that each scanning position is occupied by an electro-acoustic transducer and groups of successive transducers are connected to form transducer assemblies having a small aperture angle, preferably with identical time delay.

9. An antenna according to one of claims 1 to 3, characterised in that an assembly of electroacoustic transducers having an aperture angle less than 1800 is provided at each scanning position and the transducer assemblies are actuable in time succession, preferably with identical time intervals.

10. An antenna assembly according to one of claims 4 to 9, characterised in that the aperture angle of the or each transducer assembly is not much less than 1800, e.g. 1200.

11. An antenna according to one of claims 1 to 10, characterised in that the portions of the antenna receiving the scanned values is disposed in water at a location below and spaced from the portion processing the scanned values and preferably on the bottom of the water.