Experimental test of the three subaperture method for estimating target elevation angle

Experimental radar measurements were taken on a target aircraft flying low over a smooth reflecting sea for the purpose of testing the three subaperture method, which is a non-linear technique for estimating the elevation angle of a a target in the presence of multipath. The experimental array employed three vertically mounted dishes and three receivers, and the system stability proved to be more than adequate to permit interchannel gain and phase calibrations to be made by an appropriate numerical processing of the data. The standard deviations of the angle estimates were found to conform to theoretical expectations, but the biases were often large and exhibited a certain peculiar degeneracy in the vicinity of multipath nulls; viz, a sharp and rapid increase as the target approached a null followed by an abrupt change in sign as the target emerged from the null. Computer simulations were used to determine whether this peculiar angle flip-flop phenomenon could have been caused by: (1) refractive ray bending, (2) uncompensated array tilt, (3) diffuse scattering from the sea surface, or by (4) diffraction of the echo signal round obstacles located in the near vicinity of the radar. It was found that although the first two mechanisms can produce large biases, such biases are symmetric about the multipath nulls, whereas the latter two mechanisms can produce biases which are both large and skew-symmetric about the nulls. It is therefore concluded that one of the latter two mechanisms was responsible for the observed large biases.