Online Journal of Space Communication


Relative navigation of an aircraft (fixed wing or helicopter) close to ships at sea is a unique navigation problem. Shipboard helicopter operations provide a difficult operational environment. Wind over deck and wake turbulence shed by ship super structure offer challenging and unpredictable conditions during takeoff and landing. This is especially true in the operational environment that includes sea-state six, with its associated twenty-foot waves and thirty-three knot winds. Anything other than calm seas can create pitch, roll, yaw, and heave of the landing platform. Different sea going vessels behave in a variety of ways due to their size, hull design, stabilization systems, etc. Of particular concern in this environment is the performance consistency during takeoff, landing and sling load re-supply operations. A helicopter pilot operating off such a platform must observe the heave, pitch, and roll motion of the landing platform and determine the landing contact time based on human reaction time as well as aircraft performance. In an attempt to automate this difficult task, a relative navigation system prototype has been jointly developed by Novatel and Boeing.

This paper describes such a system. The relative navigation system consists of a pair of integrated Inertial Differential Global Positioning System (IDGPS) systems communicating with standard RTCA messages. A fixed integer carrier based solution enables the relative system to reduce the uncorrelated low latency position error between the two systems to less than 50 cm. The shipbased inertial unit provides its position, attitude, pseudorange and carrier measurements, as well as the position of an eccentric point (the landing mark) to the helicopter-based unit. The helicopter generates a precise carrier-based vector between the vessel and its antenna and uses this to compute a GPS position that has a high relative accuracy to the ship-based unit. This in turn is used to update the helicopter inertial unit so a low latency position can be generated there. From this, a high accuracy, low latency relative position is generated at the helicopter, along with the relative motion and attitude data required for safe and consistent landing or slinging operations.

The system requirements and design are detailed, and an attempt is made to provide insight into the implementation difficulties and solutions. Test setup details and results are provided.

Reprinted with permission from The Institute of Navigation (http://ion.org/) and The Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation, (pp. 979-988). Fairfax, VA: The Institute of Navigation.



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