Generalised algorithm for DOA estimation in a passive sonar

In a recent paper (see ibid., vol.140, no.1, p.12-30, 1993) Rajagopal and Rao stated that the differential covariance matrix technique could be used to resolve P coherent sources, provided that the array contained at least (P + 1) sensor elements. Further investigation shows that the method is only capable of resolving two coherent sources. The authors believe the original premise in the paper was inaccurate and it invalidates the subsequent analysis. The mathematical reason for their new conclusion is given