Triton Isis - Manual (Page 202)

June 2004 Isis® Sonar User's Manual, Volume1 188
stored and read by Isis as beam patterns from which compensations are
created while Isis is running. Refer to Appendix B for a description of the
BAC file format.
If no calibration patterns are available, Isis can derive an empirical BAC
from statistics collected over at least several hundred pings. This is
generally the approach taken since transducer beam patterns are rarely
available.
If the empirical approach is to be taken, select a featureless piece of
seafloor within the survey area with a well-known slope where at least 200
pings can be or have been collected. The greater the number of pings used,
the more successful the compensation will be.
When the statistics are being collected, the towfish altitude should be varied
over the entire range of altitudes expected for the survey. This will help Isis
to eliminate the effect of grazing angle intensity variations from the beam
angle compensation statistics. It is also helpful to collect statistics at
altitudes lower than the expected minimum for the survey in order to
increase the information available at high beam angles. It is very important
that the towfish altitude is well known during the collection of statistics and
during the entire survey for which beam angle compensation is applied.
9.8.3 Grazing Angle Compensation
At far range, the angle at which the acoustic energy strikes the seafloor
shrinks toward zero. As this grazing angle gets smaller, less energy is
scattered back in the direction of the sonar, thereby creating another across-
track intensity variation. By collecting statistics over well-known and well-
behaved seafloor, Isis can derive a grazing angle compensation for each
bottom type and general track heading in a given survey. You can then load
the GAC from a file or collect new statistics for each run.
When calculating a GAC from collected statistics, select a featureless piece
of seafloor of known slope where at least 200 pings can be collected. As
with statistics collection for BAC, the towfish altitude should be varied
between the estimated upper and lower altitude limits for the survey. This
will allow the most thorough sampling of grazing angle independent of beam
angle.
It is also helpful to collect statistics at altitudes lower than the expected
minimum for the survey in order to increase the information available at low
grazing angles. It is very important that the towfish altitude is well-known
Chapter 9: Using the Tools Menu