Communications – electrical: acoustic wave systems and devices – Echo systems – Side scanning or contour mapping sonar systems
Reexamination Certificate
2002-01-16
2003-07-15
Lobo, Ian J. (Department: 3662)
Communications, electrical: acoustic wave systems and devices
Echo systems
Side scanning or contour mapping sonar systems
Reexamination Certificate
active
06594200
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a synthetic aperture sonar and a synthetic aperture processing method, and in particular, to a synthetic aperture sonar using an actual aperture array division and transmission and reception multiplexing method, and a synthetic aperture processing method for accelerating a traveling speed.
2. Description of the Related Art
Synthetic aperture radars (SAR) are used in artificial satellites and aircrafts as radar systems with high spatial resolution. What synthetic aperture processing is applied to a sonar on the basis of the same principle as this radar is a synthetic aperture sonar (SAS).
FIG. 8
is an explanatory diagram of the basic principle of a Strip-Map Type synthetic aperture sonar. With referring to this figure, it is assumed that an actual aperture array
101
travels in the direction of the upper part (an azimuth direction) from a lower part of this figure. First, a first transmission and reception of a signal is performed in the direction E of a detection target (a range direction) at a position A, and next, a next transmission and reception of a signal is performed at a point where the sonar travels a half of actual aperture array length. This transmission and reception of a signal is totally performed N times continuously until the sonar reaches a position C through a position B. At this time, a detection target
102
is always included in an emission range of each beam in N-time transmission and reception. Then, by collecting this N-time received data, and performing synthetic aperture processing (convolution processing), it is possible to obtain the same resolution as the resolution obtained when one-time transmission and reception is performed in an aperture length (synthetic aperture length) H longer than that of the actual aperture array
101
. This is the basic principle of a synthetic aperture sonar.
Although there are several methods also in synthetic aperture processing, here, an object is a Strip-Map type synthetic aperture sonar that is most common.
The Strip-Map type synthesis aperture sonar can be recognized as means for improving the azimuth resolution of a side-scan sonar (SSS) that has been used until now. The side-scan sonar generates a two-dimensional map of sound reflective intensity of a submarine surface by traveling with continuously performing sound transmission and reception. The resolution in a range direction (range resolution) of this submarine surface map is proportional to pulse length when PCW (Pulse Continuous Wave) is used for transmission and reception signals, or to frequency bandwidth when a wide band signal like LFM (Linear Frequency Modulation) is used. Moreover, angular resolution is determined by the width of a beam that an echo sounder transducer array forms. That is, angular resolution is proportional to an actual aperture length or a center frequency of the sonar.
On the other hand, a synthetic aperture sonar is a method for obtaining angular resolution (or an azimuth resolution) higher than a usual SSS by generating a long virtual array (synthetic aperture array) with using a plurality of continuous transmission and reception signals as described above.
Usually, the synthetic aperture sonar performs the processing of changing the length of a synthetic aperture array in proportion to a range. Consequently, on the submarine map obtained, spatial resolution in the azimuth direction (azimuth resolution) becomes constant regardless of the range. This azimuth resolution is restricted to D/2, that is, a half of actual aperture array length D from grating lobe suppression conditions. This just means that a space-sampling period becomes D/2, and the spatial resolution obtained as the result of performing synthetic aperture processing is also restricted to D/2 or less.
Moreover, letting the maximum range be R, the traveling speed V is restricted as follows:
i V≦(
D/
2) (
c/
2
R
) (1)
Here, item c denotes a underwater acoustic velocity. Although this trade-off condition of V and R can be also applied to a radar, the traveling speed, range, and resolution are restricted severely in comparison with the radar in a sonar due to the slowness of the underwater acoustic velocity c.
Several methods that improve implementation efficiencies of sonars with exceeding this trade-off restrictions are proposed. For example, there are a vernier division method (M. A. Lawlor et al., “Further results from the SAMI synthetic aperture sonar”, IEEE OCEANS'96, 1996, Vol. 2, pp 545-550), a Japanese Patent Laid-Open No. 10-142333, etc. Each of these methods performs a plurality of space sampling by one-time transmission and reception, and uses an actual aperture array by dividing the actual aperture array. The azimuth resolution improves in proportion to this number of divisions, and it is not necessary to make the traveling speed slow.
Here, an example of the vernier division method will be described with referring to
FIGS. 9 and 10
. Although the method in
FIG. 9
is a usual synthetic aperture method, the method in
FIG. 10
is a two-vernier division method where the actual aperture array is divided into two pieces. First, the usual synthetic aperture method will be described.
With referring to
FIG. 9
, in ping
1
, a first transmission of a signal is first performed from an array
110
with a length D to the detection target
102
at an azimuth position S
1
, and receives a reflected (echo) signal from the detection target
102
. Next, in ping
2
, a second transmission of a signal is performed from the vernier
110
to the detection target
102
at a position (azimuth position S
2
) where the sonar travels D/2 from the ping
1
, and receives a reflected signal from the detection target
102
. Next, in ping
3
, a third transmission of a signal is performed from the vernier
110
to the detection target
102
at a position (azimuth position S
3
) where the sonar travels D/2 from the ping
2
, and receives a reflected signal from the detection target
102
. This transmission and reception is repeated N times continuously, and synthetic aperture processing is performed on the basis of N-time input signals. In this manner, since transmission and reception is performed in one ping with the sonar traveling D/2, azimuth resolution RES becomes D/2 and a speed V of the array
110
becomes PRF·D/2. Moreover, since transmission and reception is performed with using a whole array, it is not possible to define a phase-equivalent overlap point, and hence it is difficult to correct fluctuation with the overlap method.
Next, the two-vernier division method where an actual aperture array is divided into two pieces will be described. With referring to
FIG. 10
, the array
110
in
FIG. 9
is divided into two pieces in the two-vernier division method in
FIG. 10
, and hence, an array consists of a rear vernier
111
and a front vernier
112
. Then, the rear vernier
111
(a sub array with a length of D/2) transmits a signal, and both the rear vernier
111
and the front vernier
112
receive a reflected signal from the detection target
102
. At this time, an input signal of the rear vernier
111
is equal to a transmission and reception signal at an azimuth position S
1
,
1
(a position where the sonar travels D/4 forward from a backmost portion of the rear vernier
111
). In addition, an input signal of the front vernier
112
is equal to a transmission and reception signal at an azimuth position S
1
,
2
(a position where the sonar travels D/4 forward from the azimuth position S
1
,
1
). This relation is called a Displaced Phase Center. Since the azimuth resolution RES becomes twice as many as a usual method, that is, D/4 by performing this transmission and reception whenever the sonar travels D/2, the speed V of the array
110
becomes PRF·D/2 similarly to the usual method. Moreover, it is not possible to define a phase-equivalent overlap point, and hence it is also difficult to correct fluctuation with the overlap method. In addition, an example of the metho
Foley & Lardner
Lobo Ian J.
NEC Corporation
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