Triton Isis - Manual (Page 201)

June 2004 Isis® Sonar User's Manual, Volume1 187
9.8.1 Overview
After sonar imagery has been corrected for slant-range distortion and the
water column has been removed, prominent across-track variations in signal
intensity generally remain in the imagery. These variations result in track-
parallel stripes of high and low intensity, commonly referred to as beam
pattern. You can use Isis to eliminate, to a large degree, the unwanted
beam pattern that sometimes appears in your imagery.
The actual causes of these variations are the transmit and receive
characteristics of the transducers, increased attenuation of the acoustic
signal at far range, and the grazing angle between the acoustic ray path and
the seafloor. Most sonars include time-varied gain (TVG) amplifiers in the
towfish that accurately correct for signal attenuation. These corrections are
helpful in reducing the dynamic range of signals received by the topside
equipment.
The Isis beam angle and grazing angle compensation feature (BAC and
GAC) goes further by allowing independent compensation for beam pattern
and grazing angle distortions in record and playback modes. Isis can apply
either type of compensation to as many as four channels in a single
Waterfall window.
These compensations can only be applied to slant-range-corrected data.
Both compensations require reliable towfish altitude information and are
much more accurate when a reasonable bottom slope is entered (i.e., from
a regional chart or detailed bathymetry data).
Beam angle and grazing angle compensations act independently for each
waterfall window. Therefore, it is important to select the desired waterfall
window first before selecting Beam Angle, Grazing Angle.
Select the desired waterfall window before choosing
the Beam Angle, Grazing Angle command.
9.8.2 Beam Angle Compensation
Each transducer array has a distinct beam pattern whose intensity varies
with the angle off the acoustic axis, or boresite. If far-field transducer
calibration patterns are available for a sonar’s transducers, Isis can use
these to derive a beam angle compensation (BAC) from them. BACs are
Chapter 9: Using the Tools Menu