A method and apparatus are presented for improving accuracy in a navigation system. A method of determining a physical characteristic of a receiving device in a navigation system, includes receiving a plurality of navigation signals from a plurality of navigation signal sources, respectively; determining a plurality of measurement estimates, based on the received navigation signals; and determining a physical characteristic of the receiving device by estimating a first set of one or more parameters in an error model, based on at least the plurality of measurement estimates. The error model includes a first error contribution for modeling noise and a second error contribution for modeling indirect signal propagation of the plurality of navigation signals.