Multibeam Sonar Theory of Operation

Introduction to Multibeam Sonar:
Projector and Hydrophone Systems Multibeam Sonar Theory of Operation
where:
or equivalently:
S(t) = A(t) cos (φ(t)) (3.10)
φ(t) = 2πft (3.11)
The phase φ(t) is simply a measure of where a signal is in its oscillations. An instantaneous
measure of a signal will yield S(t), as described above, but what you are really interested in is
A(t), the amplitude. If measurements of S(t) and φ(t) are made simultaneously, A(t) can be
extracted using Equation 3.12.
The SEA BEAM 2100 measures both signal strength S(t) and phase φ(t). Both pieces of
information are converted from continuous analog signals to a series of discrete digital
measurements through the analog-to-digital converters. These measurements are made at the
sampling rate, which is between 1 and 3 milliseconds. All of the signal and phase data for all
hydrophones from one of these samples is called a time slice.
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(3.12)
To form a steered beam, the signals from the hydrophones S1(t) through SN(t) must be combined
with the correct adjustments for the path differences between different elements. Recall from
Equation 3.7 that the path difference of a signal from steered beam angle θ between two
elements of an array spaced d apart is given by:
d sin θ (3.13)
If the N hydrophones are numbered i = 0 through i = N 1, the path differences required to
properly combine the signals of each are given by:
id sin θ (3.14)
For computational purposes, it is easier to speak in terms of the phase difference between
hydrophone elements rather than the path difference. The phase difference is a measure of the
fractional number of wave oscillations that occur in the path difference. The phase difference in
one complete oscillation or cycle is 2π. The fractional difference in phase caused by the path
difference id sin θ is given by:
2π
id sin θ
(3.15)
λ
The in-phase signal received from an angle θ by each hydrophone is the measured signal adjusted
by the appropriate phase delay. Using Equations 3.9 and 3.15, you can find the signal of
hydrophone element i with the appropriate phase delay required to form a steered beam at
angle θ, called Bi(θ), in terms of the instantaneous measurements Si and φi within a time slice:
(3.16)
(3.17)
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