A method and a system of determining a surface location defining an interface between an object to image and an antenna is provided. The method uses geometric principles and the fact that an impedance mismatch at the interface results in significant reflection. A propagation time from the surface to each antenna of a plurality of antennas is estimated. The propagation time locates the surface on a circle centered at each antenna and having a radius calculated using the propagation time. A tangent line connects the intersection of adjacent circles with the surface. The surface is estimated to be located at the tangent point where the circle and the shared tangent line between adjacent antennas intersect. Multiple tangent points may be averaged for each interior antenna. A curve is fit to the set of tangent points to provide an estimate of the surface location.