A receiving beam is formed using 60 elements arranged in a 90.degree.
sector area of a cylindrical receiving array. Receiving beam 0 is formed
with elements 0-59 divided into two (front and rear) groups. Specifically,
the elements 0-59 are divided into rear group 1 (elements 0-8, 51-59) and
front group 2 (elements 9-50) in the beam direction, and the receiving
beam is formed by using sample data obtained from the transducer elements
0-8, 51-59 of group 1 in a current scanning cycle as well as sample data
obtained from the transducer elements 9-50 of group 2 in a preceding
scanning cycle. This approach makes it possible to form the receiving beam
such that incoming waves of a short-pulse signal would simultaneously hit
the relevant transducer elements of the receiving array and thereby
improve directivity and sensitivity of the receiving beam for the
reception of short-pulse waves.
