Ship attitude accuracy trade study for aircraft approach and landing operations

Joint Precision Approach and Landing System (JPALS) and other ship-based approach and landing systems implemented on ship require accurate attitude determination to move sensor measurements to the touchdown point, determine an accurate path for the aircraft to approach and land, and provide attitude information for the departing aircraft to initialize their navigation systems. GPS-aided inertial units (EGIs) can provide attitude information. However they are subject to attitude errors caused by inherent gyro and GPS errors. The error that can cause the biggest problem with the EGI attitude is the bore sight error; or aligning the inertial system to the body of the aircraft carrier. The attitude error that is provided by the EGI can be mitigated by implementing GPS carrier phase measurements from GPS receivers located in a good relative geometry around the ship. The attitude accuracy produced by the GPS relative carrier phase measurements is determined mostly by a good relative geometry of the sensors. Placing the GPS sensors in relative locations so that all three rotation axes have observability and that they are placed far apart can greatly increase the accuracy. However, as with the bore sight error of the EGI, errors in the knowledge of the relative position between antennas will decrease the accuracy of the attitude determination. Adding to this problem is the fact that the ship will expand and contract with changes in temperature. Results show that it is difficult to distinguish between the attitude biases caused by bore sight errors and relative survey errors of the GPS antennas. In order to resolve the error in both survey and bore sight error, survey error needs to be known with significant accuracy independent of the real time GPS and inertial measurements.