Subspace-based algorithms for narrowband direction-of-arrival (DOA) estimation require detailed knowledge of the array response (the array manifold) and assume that the noise covariance matrix is known up to a scaling factor. In practice, these quantities are not known precisely. Estimation accuracy can degrade significantly when the array response or the noise covariance deviate from their nominal values. In this work, we examine the resolution threshold of the MUSIC algorithm when the array response is perturbed from its assumed value and when the noise covariance does not match the assumed model. Analytical expressions for the resolution threshold are derived and verified by computer simulation. We also demonstrate the fact that preprocessing of the array data can improve somewhat the resolution in the presence of model errors. This work makes extensive use of the contributions of various authors.