A New Design for Coastal Drifters

TELEFOS: A New Design for Coastal Drifters

Zervakis, Vassilis

Exploiting the Expansion of Cellular Telephony in Measuring Coastal Currents and Dispersion

The use of Lagrangian measurements to monitor the surface circulation of the ocean greatly expanded in the 1980s and 1990s through the exploitation of improvements in satellite technologies and the global positioning system (GPS). The global World Ocean Circulation Experiment (WOCE) took advantage of such progress and adopted the WOCE Lagrangian drifter for studies of the circulation of the surface layer of the global ocean. The drifter is designed to record the current at 15 meters’ depth and can be deployed very easily, as it is pre-programmed and its drogue is self-extendable. In the framework of the WOCE project, over 4,000 such drifters were deployed and tracked by satellite from 1990 to 2002. Today, the WOCE drifter is used worldwide, existing as the basis for open-ocean Lagrangian measurements of the surface circulation.

However, the WOCE-type drifters are designed for open-ocean applications. seeking to reduce the drag-surface ratio in order to minimize windage, the drogue of the drifter is five to seven meters long. Thus, the current measurements represent a vertically averaged current of the 12.5 to 17.5-meter water layer, or even the 11.5 to 18.5-meter layer. Furthermore, the WOCE drifter utilizes satellite communication, which has several advantages and some disadvantages. The most obvious and highly significant advantage is global coverage. Among the disadvantages is the relatively high cost and the indirect, oneway communication with the drifter, which makes the instrument expendable.

The great expansion of global system for mobile (GSM) communication technologies that took place in the late 1990s permitted the exploitation of a low-cost, readily available and widely expanded technology in the design of surface/subsurface drifters for use in coastal and land-locked seas, archipelagos, estuaries and lakes. The idea of a GSM-based drifter was born by the near-full coverage that cellular telephone providers offered over the Aegean Sea by the late 1990s. The Aegean Sea (eastern Mediterranean) is almost land-locked and seeded with a great number of islands, thus an ideal setting for the deployment of GSM antennas providing almost full coverage. The use of GSM devices for data transmission is sufficient for studies throughout the Aegean Sea. The idea of developing a GPS/GSM drifter suitable for coastal studies, but fully satisfying needs for small seas and archipelagos, was thus born in the Hellenic Centre for Marine Research (HCMR), and adopted by MARAC Electronics.

The authors’ requirements, besides the use of two-way GSM communications and a fully programmable drifter throughout the experiment, were that the instrument would be better-suited for coastal studies than the WOCE drifter.

A coastal environment is usually characterized by more stratified conditions, thinner layers of water and higher vertical shears than the open ocean, thus the requirements are for wider and shorter drogues. Several drifter designs seek to satisfy such criteria. However, the industry’s standard when aiming to record surface (zero to one-meter layer) currents has become the surface drifter designed by Russ Davis. That design’s drogue consists of vertical sails attached on horizontal beams. The electronics are enclosed in the spine of the drogue and the drifter stays afloat (and follows the waves’ slope) by four small floats attached on the drogue’s top beams.

The architectural design of the new drifter was based on Davis’s drifter, yet, it was modified so as to permit the deployment of the drogue at deeper depths. The new drifter was named telephonically tracked floats for oceanographie studies (TELEFOS).

The above acronym was picked after the name of a character from Greek mythology. Telefos, king of Mysia and son of Hercules, was wounded by Achilles in battle; his wound would not be healed, and he was informed by an oracle that he could only be cured by the same hand that wounded him. As a result, Telefos became an ancient drifter whose fate was to start on a long wandering journey throughout the ancient world searching for Achilles in order to heal his wound. Today, his technological descendant is a modern-day drifter seeking to reveal coastal currents.

Architectural Design

The drifter’s design addresses the need to perform coastal studies, i.e., to monitor currents with high vertical resolution. Thus, the authors adopted a Davis-type design, but modified it so that the drogue could be easily deployed several tens of meters underwater. The novelty of the design lies in the use of a hollow PVC cylinder as a spine to which the drogue is attached. The surface module, equipped with the electronics (GPS, GSM and ultrahigh-frequency (UHF) modules, microprocessor and batteries) has a cylindrical body of slightly smaller diameter than the drogue spine. In the configuration for measuring surface currents, the surface module is placed inside the drogue spine and the four donutshaped floats are attached on the four top corners of the drogue. In the configuration for measuring sub-surface currents, the two modules are deployed separately, connected with a rope-line, and the four donut-shaped floats are attached on the surface module. The design is quite compact, modular and flexible. The relatively small height of the drogue allows studies in highly stratified environments, with good vertical resolution of the velocity field, unlike the WOCE open-ocean drifter design.

Communications and Software

The philosophy of the TELEFOS design is to minimize both the cost of the drifter itself and its expendables. The power source is comprised of four alkaline D cell batteries. There are no cables and connectors for programming and data transfer. MARAC Electronics has provided the drifter with a GSM and a short-range UHF communication module. As soon as the drifter is turned on, it calls the base station (using both modules) and receives the program of its new mission. The GSM sampling rate, as well as the frequency of data transmission to the base station, is fully controlled by the user-friendly software at the base station provided with the drifters. All communication, programming and data exchange are performed through either the GSM or the UHF modules. The latter technology provides a no-cost alternative to GSM for studies in small water reservoirs, offering a range of about one kilometer. The GSM technology offers the capability to monitor the drifter fleet anywhere, provided there is a GSM signal both at the experiment site and at the base station site. The base station can also be located on a vessel following the drifters. The software can foresee if the condition of the GSM signal is too low for communication. In that case, the drifter position values are stored in the drifter’s memory and are transmitted to the base station when the GSM or UHF signal allow such communication. Thus, there is no loss of data regardless of the GSM coverage.

The base station software provides the ability not only to program and monitor the drifter fleet, but also to analyze the data and facilitate the following and recovering of the drifters. A special module of the software enables the connection to a GPS onboard the vessel for the recovery of the floats. As soon as a drifter is selected for recovery, the software provides the necessary information (heading and distance) for finding and recovering the instrument. This module, along with the two-way communication, changes the nature of the drifter from an expendable instrument to a recoverable one, which can be reused. This change in the use of the instrument significantly lowers the actual purchase cost due to the added value through its repeated use.

Another software module enables monitoring of the status of each drifter (power of each battery, memory capacity, level of GSM and UHF signal) before and during the experiment. If the energy levels of certain drifters are running dangerously low, it is possible through the two-way communication capability to reprogram the sampling and reporting strategy ofthat particulate drifter in order to continue the experiment uninterrupted.

The TELEFOS drifters are aimed not only at the oceanographers’ market, but also at scientists dealing with coastal constructions, pollution, coastal managements, etc. Thus, the software provides non-expert users the ability to perform Lagrangian data analysis. Some of the software’s capabilities in terms of data analysis are the estimation of mean Eulerian velocity fields (along with variance ellipses), mean and eddy kinetic energy fields, spectral analysis and the estimation of horizontal eddy diffusivities.

Applications

The main application of the TELEFOS drifters is real-time monitoring and analyzing coastal (as well as lake and reservoir) advection and dispersion.

The spatial scale of the phenomena is limited only by the GPS position error on the one hand (a few meters) and the scale of GSM coverage (of the order of 30 kilometers from the coast) on the other. An example of use in a small aquatic basin is their deployment in the rowing facilities for the 2004 Olympic Games, a reservoir about 1,500 by 120 meters.

However, since the introduction of the first nine TELEFOS prototypes to the HCMR’s instrumentation, the authors have used the TELEFOS modules not just as drifters but also as position transponders for different experiments (i.e., for tracking a freedrifting sediment trap mooring in the framework of the Greek-United States collaborative project FACTS, a subsurface array for a primary production experiment).

In all cases, the TELEFOS drifters were selected as the tracking instrument of choice due to the ease of application, two-way communication, flexibility and availability of transponders, and the direct availability of positions on the computer screen at the office or tracking vessel, /st/

For more information, e-mail oceanbiz@sea-technology.com.

By Vassilis Zervakis

Assistant Professor

University of the Aegean

Mytilene, Greece

Michael Ktistakis

Electrical Engineer

MARAC Electronics SA

Athens, Greece :

and

Dimitris Georgopoulos

Research Director

Hellenic Centre for Marine Research

Anavyssos, Greece

Vassilis Zervakis is an assistant professor of physical oceanography at the University of the Aegean. He received his B.S. in physics at the University of Athens, his Ph.D. in physical oceanography at the College of Oceanic and Atmospheric Sciences of Oregon State University and worked as an associate researcher at the HCMR for eight years.

Michael Ktistakis received his B.S. in electrical engineering at the Polytechnic School of University of Patras, Greece. He joined MARAC Electronics in 1987 and works as a senior engineer in the research and development department of the company.

Dimitris Georgopoulos is a research director at the HCMR. He obtained his B.S. in physics at the University of Athens, his DEA (M.S.) in physical oceanography at Paris VI-France and his Ph.D. at the University of Patras, Greece.

Copyright Compass Publications, Inc. Feb 2005

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