Advanced Techniques - Kongsberg Simrad EM 300 Operation Manual

Kongsberg Simrad EM 300 / Base version
356
sidescan systems, an approximate measurement of depth has been
achieved by using phase detection through an interferometric
principle. By comparing the phase of the returned echo on two or
more physically separated transducers, the angle of arrival may be
estimated in addition to the range. However, this only works when
the angle of incidence is large or the seabed is relatively flat, as the
phase information may be destroyed when simultaneous echoes
are received from different parts of the seabed. Note that a similar
phase detection principle is used with success in short or
ultra-short baseline acoustic positioning systems, in radars
(usually called monopulse) and in optics (usually called splitbeam,
a term which is also used in connection with ultra-short baseline
positioning systems).

Advanced techniques

Phase detection method
The problem of multiple echoes with the phase detecting sidescan
sonars can to a large extent be overcome by combining phase
detection with the multi-beam principle. For each beam, two
additional so-called halfbeams are formed from different
sub-arrays of the receive transducer, with all three beams having
the same pointing direction. The phase difference between the
halfbeams, which is a measure of the angle of arrival of the
returned echo, is calculated from the complex conjugate product
of the two received signals, usually after some averaging of the
product is done. The degree of averaging needed is determined by
a combination of pulse duration, range and incidence angle, and is
in practice derived from extensive simulations and experimental
verifications. The averaging stabilizes the phase determination,
reducing the effect of glint or random variation in signal strength
due to destructive interference from different parts within the area
producing the echo at any given time.
The distance between the acoustical centre of the two sub-arrays
used to form the halfbeams determines the relationship between
electrical phase and angle of arrival. When the phase is zero, the
returned echo from the bottom comes in the centre of the beam.
The phase as a function of time will theoretically be a second order
function for a flat bottom, modulated by local changes in bottom
curvature and by noise. A curve fit made to this time series of
phase, from which the zero phase crossing can be determined, will
therefore allow a very accurate determination of the range to the
bottom in the centre of the beam, with a much higher accuracy than
any method based only on amplitude information can provide.
160719 / /F