Cramer-Rao bound investigation of swath bathymetry accuracy

The Cramer-Rao bound is employed to establish a lower bound on the estimation accuracy of bottom detection and angle estimation methods for swath bathymetry. The angle estimation bound is converted to a distance along an arc at the given range and then compared to the range estimation accuracy obtained for bottom detection methods. It is determined that for bottom detection methods the accuracy decreases as the angle for which the estimation is made moves from vertical to horizontal. Also, if the backscatter decorrelates across the beam (the usual situation in practice), the estimation accuracy plateaus and increasing the signal-to-noise ratio no longer increases the accuracy. Angle estimation methods tend to be more accurate away from the nadir angle (vertical for a flat bottom), complementing bottom detection methods, and the accuracy does not plateau as quickly with increasing signal-to-noise ratio. Therefore, with regard to performance with increasing signal-to-noise ratio, angle estimation has an advantage over bottom detection. It is also shown that the accuracy of angle estimation techniques is affected detrimentally by acoustic pulse spread on the bottom and in a relative sense the effect is more severe for large arrays. Therefore, there is a potential for obtaining good swath bathymetry with small arrays using angle estimation techniques