1. Apparatus for determining the position and velocity of a moving platform comprising: a radio navigation aid having at least three portions, two of which are distant from said platform and having a known position for providing to said platform a plurality of samples indicative of the position of said platform at respective times, a position estimator for receiving said plurality of samples for generating an estimate of position and velocity both with an unbiased estimator wherein the mean square error of the estimate is the variance and with a biased estimator and selecting said estimate of position and velocity from said biased estimator at times said geometry of said moving platform is substantially co-linear with said two distant portions of said radio navigation aid.