Use of lightweight on-line GPS drifters for surface current and ice drift observations

Abstract

A family of lightweight (less than 15 kg), long-life (up to 3–4 months) and low-cost drifters are developed in the Tallinn University of Technology and local engineering companies. Such buoys have a rugged and simple housing construction with the diameter of 11 cm and length of 1 m. The buoys are equipped with GPRS based two-way communication and they are programmable during the mission. The data are transferred in real time into one or several FTPs for the operational use in circulation and ice models for data assimilation and/or model validations. The buoys are used also for the estimation of oil drift and ice dynamics properties, e.g. ice drift, compacting, ridging, etc. Real-time (on-line) data transfer is performed via the GPRS protocol with a typical time interval of 15 minutes, but it can be programmed otherwise. The data received in FTP are ready to be used for model assimilation and in different operational applications and services like the drift of surface objects, ice and oil drift. It is possible to view the buoy data in real time via a special web solution. Two major measurement campaigns have been performed using light drifters. Firstly, the validation of High Resolution Operational Model for the Baltic Sea (HIROMB) surface currents in Estonian coastal waters in 2007 and secondly, the study of dynamic ice drift. In both cases the light on-line drifters showed good performance and stability for long-life operation. The percentage of fault positioning was higher in water, reaching up to 20%, and was mainly caused by the reflections of GPS signal from the waves, since a very small part of the buoy remains above the sea surface. In ice the fault positioning was lower, less than 10%. In the case of an ice covered sea, the buoys were from time to time pushed under ice for some period during which the positioning systems failed to transfer the signal. In 2007 during the period from August till November 17 experiments were carried out with surface drifters released in small coastal bays and the open sea of the Gulf of Finland. The longest period when a buoy was drifting lasted for 8 days. It moved across the Gulf of Finland covering a little less than 50 miles. In accordance with the drifter experiments the drifter trajectories that imitate oil spill drift were calculated from HIROMB currents and wind. The calculations were carried out with a different wind factor between 0–3%. The results of modeling showed close agreement with drifter data in some cases, but no unique wind factor suitable for the Gulf of Finland could be identified. In March-April 2010, an extensive ice drifter experiment was conducted in the eastern part of the Gulf of Finland. The development of light on-line GPS drifters continues. The platform of data logging and transfer allows adding a number of parameters to be measured by the drifter and transmitted on-line. To begin with, water temperature and salinity sensors will be added.