SHIPS: A New Method for Efficient Full-Scale Ship Squat Determination

摘要

Recent full-scale experiments on measuring ship squat are presently based on differential GPS in kinematic mode where the changes in ellipsoidal height between receivers on the vessel and at shore are observed. In most of these measurements it has been necessary to correct for the change in water level. For this purpose all available tide gauges must be read out and the tide wave must be modelled such that an interpolation in space and time can obtain the water level at the ship. Some authors quote that a precision of about 6 cm can only be achieved if additional tide gauges are deployed for the campaign. Still it must be doubtful whether it is justified to neglect water level topography and wind influence. The authors suggest the SHIPS method (SHore Independent Precise Squat observation), in which the disadvantages of the laborious and error-prone use of tide gauges are avoided. Besides this the SHIPS method gives the possibility to observe the squat independently of existing facilities at shore. The method is also based on GPS and one receiver is placed on a small escort craft. Three GPS receivers onboard the ship give the heights of the centre of floatation relative to the antenna on the escort craft. During data taking the escort craft is traveling 200-500 m ahead of the vessel under investigation. This distance is small enough that the difference in water levels can be modelled with negligible errors. The squat effect of the escort craft, though small, is supposed to be know from a separate calibration experiment. Consequently, when taking the difference between heights of the centre of floatation of the large ship and antenna heights of the small boat, the unknown water level is eliminated. The method has been tested in several experiments on German waterways. The results and experiences of these tests will be presented and discussed.