| the propagation time between two points on a ray path defined in terms of the group velocity vector by the line integral taken along the ray path:
where vg is the algebraic value of the projection of the group velocity vector onto the tangent to the ray path, and ds is the corresponding curve element
NOTE – In practice, if the medium is not too absorbent, nor too anisotropic, nor too dispersive, the group delay is equal to the propagation time of the signal element used to define the group velocity.
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