A sonar example with other than circular and rectangular apertures
Ultrasim supports rectangular and curved arrays and annular arrays directly from its menus. But sometimes it is of interest to simulate unusual combinations of elements such as the four circular apertures shown here.Richard Nilsson, Uppsala University, Sweden, made me aware of
the Thinned
Array Sidescan Sonar page. It simulates
four 50 kHz circular fishfinder transducers, each with diameter 44 mm and with minimum 60 mm between
centers and various ways of combining them into a poor man's sidescan sonar.
This has been used as an example here.
This can be done by starting with a 2D array which is larger
than the area covered by the four apertures. The 2D array in this example is set up
with one point per mm.
Information about each
point in the aperture is stored in the array elem_pts, see the manual
for details. An example of an
element in this array is
elem_pts(:,10)' = [ -0.1295,
-0.0205,
0, 1.0000, 10.0000].
These elements are the x, y, and z coordinates of the element followed
by the azimuth and elevation element numbers. Ultrasim has a
feature which makes it possible for all processing to ignore an element
and this is done by setting the azimuth number to a negative value,
i.e.
elem_pts(4,10) = [ -1.0000].
This can be done in a script which is run from the command line. It will thin the full 2D array
down to the four circular subapertures above. The code can be found in thin4.m. The
resulting beam pattern is shown below. The result is similar to Figs 2a and 2b of
Simulation
of Sidescan Transducer Arrays.
An irregular distribution of the four circular aperture has
been proposed in Simulation
of Sidescan Transducer Arrays, the second set of Figs 3a and 3b.
The code of thin4b.m
does that. The aperture is shown below.
The beam pattern is shown here.
Note that this beam pattern has a narrower mainlobe,
but higher side/grating-lobes than before. You gain some and
you lose some, unlike in the simulation of Simulation
of Sidescan Transducer Arrays
which shows the unlikely result of
both an improved mainlobe and improved sidelobes.
10.10.2006.