Maximum approximation errors (top) on axis and (bottom) at 15 degrees for the forward-difference forms of the paraxial and moments approximations over a 30 x 30 mm region of interest. The paraxial approximation error was calculated with the division by range in the linear term of the binomial expansion of the time of flight replaced by a scale factor. A second-degree polynomial in azimuth and range was used to fit the time-of-flight profile on a least-squares-error basis for the moments approximation.