- Ducted propagation above the ocean surface can seriously impact shipboard radar and communications. Point-to-point microwave measurements have been proposed as a means of estimating tropospheric refractivity for the purposes of characterizing surface-based ducts. This paper addresses the theoretical performance of refractivity estimates that can be made by combining field measurements at different frequencies with prior statistics of refractivity variation. Parameterizing the refractivity profile using empirical orthogonal functions derived from a historical database, both Cramer-Rao performance bounds and the maximum a posteriori (MAP) estimate are discussed using coherent or incoherent signals. Results obtained using a realistic model of refractivity conditions off Southern California suggest that multifrequency propagation measurements can significantly improve the estimation of refractivity and propagation loss profiles.