Surface currents from AIS ship data — a Hermess proof-of-concept study.
In this document surface currents along main shipping lanes and a cross track in the English Channel are derived from AIS (Automatic Identification System) ship data and compared with operational model data. Three techniques are tested: a tidal analysis of speed variations, assessment of cross currents from the difference between course and heading, and slowly varying currents from filtered speeds of ships sailing in opposite directions. The results show a good agreement between the data sets and indicate that AIS-derived observations may form a valuable data source to improve the reliability of current models worldwide.
Surface currents are important for shipping activities, to improve efficiency and save fuel and also to better estimate arrival times. In shallow coastal waters tidal currents are generally dominant. Surface currents are routinely forecasted by dedicated models. To adjust these models, current measurements are needed, e.g. from buoys or drifters. Such measurements are scarce in general and absent in busy shipping lanes. As the speed over ground of ships is directly affected by surrounding surface currents, an analysis of speed time series can reveal valuable information about these currents. Nowadays, almost every ship transmits frequently (every 10 seconds) its position, speed, heading and course, and these AIS signals are collected by land based stations and satellites.
At HERMESS, for over ten years analysis methods were developed to assess surface currents from AIS data, treating all ships as drifters with a motor. These methods were successfully applied in proof of concept studies to large ocean eddies [1], tidal areas [2] and rivers [3]. The basic assumption is that during voyages ships keep the engine power constant for a longer time, so that modulations of the cruising speed, i.e. the speed relative to the surrounding water, can be attributed to current variations. Deviations from this assumption, sudden acceleration or deceleration, can be detected and these time series are filtered out. As the cruising speed is unknown and has to be assessed for each ship from the time series, the analyses yield variations in the surface currents and no absolute values. For the determination of tidal constants along the route this is not a problem.
In this study AIS data along two busy shipping lanes in the English Channel are analyzed and compared with high-resolution model data from the CMEMS North Sea model [4], as distributed by Copernicus. First the consistency of the two datasets is considered, giving an indication of the model accuracy. Next, it is investigated how modeling of the tidal currents, which are dominant in this area, can be improved.