SHIPS - Shore Independent Precise Squat Observation
It is a well known fact that vessels additionally immerse in restricted waterways due to hydrodynamic effects.
Several approaches have been made in the past to study this phenomenon by measuring the variation of the height of a vessel compared to a fixed survey station on shore. However, photogrammetric or nivellitic methods are limited in their applicability to either small sections of a waterway, good visibility or both. Even by using DGPS the accuracy of these observations is limited by the dependence on data from tide gauges to obtain the local water level at the vessel. The poorer accuracy arises from the fact that tide gauges measure the water level in some distance to the vessel. Hence, the data have to be interpolated in time and space for the vessel's actual position. Such a procedure may be unreliable when local wind or current influences are neglected. Furthermore, each tide gauge has a system error up to 2 cm in relation to its measurement range.
Concept for a new Method
The Nautical and the Survey Department of the University of Applied Sciences in Oldenburg (Germany) are developing a new method designed to avoid the influences specified above. The main goal of the project is to develop a powerful product to simplify the determination of the exact squat of a vessel.
The GPS-based method is named SHIPS (Shore Independent Precise Squat Observation) and has been tested successfully in several experiments on German waterways.
The novelty of this method is the application of GPS carrier phase observations on a small escort craft to represent the local water level at the vessel and avoiding the use of tide gauges. There are three high quality GPS-units on the observed vessel and one on the escort craft to determine the squat (see Fig.1). The GPS-antennas onboard the observed vessel have to be installed as far apart as possible. For an effective measurement it turned out that two GPS-antennas are well located at both wings of the wheel house and the third GPS-antenna on the forecastle. The motions of the vessel around its longitudinal, transverse and vertical axes can be determined by the evaluation of the height differences of the three onboard GPS-antennas. It is assumed that for small, quasi-static changes in trim and list the rotational axes pass through the longitudinal centre of floatation (LCF) of the vessel. The information about its position can be obtained from the ship's hydrostatic particulars. The change in height of the vessel's LCF can be determined by measuring the height difference between the three GPS-antennas on the vessel and the GPS-antenna on the escort craft.
Fig.1: Schematic presentation of SHIPS for static and dynamic conditions.
The SHIPS-method uses kinematic GPS carrier phase measurements in differential mode with a moving reference-station, which is, as suggested above, onboard the escort craft. The distance between the vessel and the escort craft usually is about 200-500 metres. Due to the short baseline the influence of tropospherical effects is strongly reduced. The ionospherical effects are mostly eliminated by using two-frequency GPS-receivers. We prefer storing the raw data in the internal memory of the receivers - using sampling rates of up to one second - rather than using real time options of the receivers because this allows to check for outliers and data quality and to correct for different effects.
The data analysis is done later in post-processing mode. The non-commercial software package used for evaluation of the collected data was developed at the University of Applied Sciences and will constantly be extended and optimised. Tests have shown that the quality of the derived height differences using this software is approximately 1-2 cm. Considering all error sources, the expected accuracy of the resulting squat will be better than 3-4 cm.
Fig.2: Test measurement on the lower Weser on May-01-2002.