Carrier Phase Ambiguity Resolution for Ship Attitude Determination and Dynamic Draught

SUMMARY A newly developed GNSS (Global Navigation Satellite System) carrier phase ambiguity resolution method to determine the full attitude and dynamic draught of ships is analysed. GNSS receivers placed onboard a ship can provide both absolute vertical motions relative to a fixed vertical reference on earth, to be used (in combination with nautical chart datum) to estimate the UKC (Under-Keel Clearance), and relative baseline measurements which are employed to estimate the attitude of the ship. The advantage of the GNSS-RTK (Real Time Kinematic) solution compared to other techniques (e.g. Inertial sensors) is that it is driftless and, if carrier phase observations are used, still of high accuracy. The difficulties of using such method lie mostly in the ambiguity nature of the phase observations: in order to fully exploit their higher precision compared to the code measurements, the ambiguities must be solved. To apply the RTK technique for the application of this study, it is necessary to solve for the ambiguities in the shortest time possible, ideally on a single-epoch base; therefore one needs a reliable ambiguity resolution algorithm. The algorithm we make use of in this contribution is an extension of the well-known LAMBDA (Least-squares AMBiguity Decorrelation Adjustment) method, that is currently the standard method for solving unconstrained GNSS ambiguity resolution problems. Its modification, the so-called Constrained LAMBDA method, rigorously incorporates into the integer estimation process the nonlinear constraints as given by the known GNSS antennas geometry configuration. The main advantage of the method is that it avoids the use of multiple high grade antennas/receivers to be placed onboard the ship to estimate its attitude and UKC, providing a reliable baseline solution on an epoch by epoch base. The method is extensively tested on data collected onboard large ships sailing into the harbour of Hong Kong, and a performance comparison between the classical RTK approach and the new method is given.