Backprojection algorithm for subsurface radar imaging: Computing the round-trip time delay

We have shown that for the equations arising in subsurface SAR imaging, the Newton´s method is robust, provided that the initial approximation has the same sign and is greater in absolute value than the solution of the nonlinear equation. We also give an explicit expression for the initial approximation that satisfies these conditions, and show how we can modify it in order to take advantage of previous computations.