Abstract: A method is provided for removing gimbal periodic reorientation (indexing) readout errors in a navigation system having multiple IMUs mounted to a platform. Each IMU has multiple gyroscopes providing attitude outputs. Attitude readout errors bias due to periodic gimbal motions is determined in each IMU along each gyroscope attitude axis. Attitude outputs of the gyroscopes are time-aligned, as necessary. Onset times of indexing of each gyroscope is determined. A difference is formed between gyroscope attitude outputs. Steps in this difference of attitude outputs are assigned to the respective gyroscope causing the step in attitude. Cumulative sums of the steps associated with the respective gyroscopes are formed. The mean and linear trend in the respective cumulative sums are removed from the respective cumulative sums to form the final error correction, corresponding to the time interval associated with the steps.

Abstract: Local ES refractivity for a geographical position on a given date is determined by locating the geographical position within one of the Delaunay triangles in a grid of Delaunay triangles defined by a plurality of data points in a database containing temporal (e.g. monthly) average refractivity values at various locations around the globe. Each of the data points in the database form the vertices of the Delaunay triangles and by interpolating the temporal averaged refractivity of the vertices of the triangle enclosing the geographical position to the given date and then interpolating the refractivity of the vertices to the geographical position within the triangle, the local ES refractivity of the geographical position is estimated.