ABC and NASC
ABC and NASC are measures of area scattering rather than volume scattering. They are calculated when integrating a region, cell or selection and available for export as analysis variables when integrating by regions, or by cells. NASC or nautical area scattering coefficient is identical to sA as used by Simrad and is the area backscattering coefficient (ABC) scaled by 4ϖ and 1 nautical mile squared. This terminology follows MacLennan et al (2002).
ABC (Area Backscattering Coefficient - m2m-2)
ABC is also known as sa.
ABC is calculated as:
Where
Sv = mean volume backscattering strength of the domain being integrated (dB re 1 m2m-3).
T = mean thickness of the domain being integrated (m).
NASC (Nautical Area Scattering Coefficient - m2nmi-2)
NASC is also known as sA.
NASC is calculated as:
Where
4ϖ = Steradians in a sphere - converting "backscattering" cross-section to "scattering" cross-section.
1852 = meters per nautical mile (m/nmi)
Sv = mean volume backscattering strength of the domain being integrated (dB re 1 m2m-3).
T = mean thickness of the domain being integrated.
ABC = Area Backscattering Coefficient (m2m-2)
Notes:
- Changes to the Exclusions and Analysis settings on the Analysis page of the Variable Properties dialog box can reduce the Thickness_mean and hence the volume that NASC is valid over. Changes to the Threshold settings of the Data page of the Variable Properties dialog box may change data values in the analysis domain. Such changes can affect the Echoview values for Sv_mean and/or Thickness_mean and the final NASC value. For more information refer to About samples and echo integration.
- Care is required when using Exclusions settings, stray bottom signal (associated with an Exclude below line) can introduce large errors to the calculated NASC.
See also
Area backscattering strength
Mean_height
Sv mean
Thickness mean and Height mean