Error model for SRGPS ship reference point position coordinate estimation

This paper develops an error model for the position errors of the Ship Reference Point (SRP) in the Shipboard Relative GPS (SRGPS) variant of the Joint Precision Approach and Landing System (JPALS). The SRP position is estimated by fusing ship attitude information with knowledge of the location of the GPS reference antennas. The attitude and antenna installation geometry are used to transfer GPS measurements made by antennas located at various parts of the ship to the SRP. The error model developed quantifies the errors in the SRP location as a function of antenna-SRP relative geometry, ship structural flexibility and attitude estimation quality. Attitude estimation quality is captured in an attitude error covariance matrix determined from a navigation covariance analysis while errors due to ship flexibility are captured in a structural flexure covariance matrix. Using data collected from sea trials conducted on two aircraft carriers it is shown that the statistics of the errors due to ship flexure are non-stationary as they depend on ship velocity, acceleration and sea state. Ship maneuvers also affect the attitude error covariance matrix if a GPS-Aided INS is used to estimate ship attitude. The results of a preliminary error budgeting and allocation shows that if an INS/GPS system is used for attitude estimation, most of the SRP position estimation error will be taken up structural flexure and not INS/GPS sensor errors. However, typical INS installation errors can lead to errors that are larger than the errors due to both flexure and INS/GPS sensor inaccuracy.