  The details are presented of a new evolution algorithm for the characteristic
initial-boundary value problem based upon an affine parameter rather than the
areal radial coordinate used in the Bondi-Sachs formulation. The advantages
over the Bondi-Sachs version are discussed, with particular emphasis on the
application to the characteristic extraction of the gravitational waveform from
Cauchy simulations of general relativistic astrophysical systems.
