1. A navigation system that operates in the presence of multipath, comprising: a navigation processor that receives data from inertial sensors and computes a navigation solution; and a Kalman filter that includes a multipath error model, wherein the Kalman filter receives the navigation solution and data from a global positioning satellite (GPS) receiver, and uses the navigation solution, the data from the GPS receiver, and the multipath error model to calculate an estimate that is provided to the navigation processor.