TELEFOS:

A new surface drifter for coastal and lake studies

V. Zervakis1, M. Ktistakis2, D. Georgopoulos1 and A. Kantidakis2

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TELEFOS: the conceptThe idea of designing and building TELEFOS was generated at the Hellenic National Centre for Marine Research, when it was realized that almost the entirety of the Aegean Sea is by now fully covered by cellular telephony:

COSMOTE

TELESTET

VODAFON

Coverage by the three main companies:

The initial idea

NCMR researchers realized that cellular telephony (GSM) could be significantly more advantageous than satellite or UHF tracking of the floats, in terms of:

• Communication cost

• Real-time access to the information

• Flexibility to monitor the floats from anywhere

• Flexibility to reprogram the floats for easier retrieval.

Based on that idea, we started looking for partners to build a surface drifter specially designed for the Greek Seas.

The Consortium

A manufacturing company with great experience in manufacturing navigational equipment, MARAC Electronics, was interested, and a common project started under the auspices (and 50% funded) by the General Secretariat for Research and Technology of the Hellenic Republic.

The new drifter

Through our interactive work, we believe that we have come up with a new drifter that is characterized by the following novelties:

• GSM communication (with option for ARGOS)

• Short-range communication (and programming) through UHF (no hussling with connectors…)

• Novel design, allowing the very easy conversion from measuring surface currents (“Davis type”) to subsurface currents (kite down to 50 m depth).

• Low-cost accessories (alkaline batteries)

• A user-friendly, highly capable software

As the real-time data acquisition will permit the easy tracking and retrieval of the drifters, the instruments will acquire a semi-expendable character.

This lowers the real cost of the initial investment.

The Hardware

•Hull constructed of PVC•Weight < 5Kgs (float only)•Hull 90cm X 15cm•Sail area ~1m2

Characteristics

TELEFOS is designed to measure both surface and subsurface currents

In “surface mode”, the design is based on the Davis type.

In “subsurface mode”, the inner body of the drifter is turned into the surface buoy, and the hollow outer body forms the spine of the kite.

One of TELEFOS novelties lies in the design:

The modification of a drifter from the “surface mode” to “subsurface mode” takes place in a couple of minutes:

Surface mode Simply pulling the float off the kite

Subsurface mode

The Software

As “Telefos” is targeted to a wider market than marine scientists, the software is designed from the start to be as user-friendly and comprehensive as possible.

Selecting the geographic area is a click away….

The software is comprised of

• the project scheduling/drifter programming mode

• The real-time drifter monitoring mode, and

• The data analysis mode.

project scheduling/drifter programming mode

the user sets:

• Region and length of the experiment

• Number and identification of drifters to be used

• Sampling rate (GPS interrogation)

• Reporting rate (GSM calling home)

real-time drifter monitoring mode the user can:

• Observe his/her drifters in real time (and know their positions)

• Change the reporting rate to facilitate drifter chase and recovery

The data analysis mode

A more extensive presentation of the data analysis mode follows. We should just point out that the analysis can be performed not just on TELEFOS data but also on any Lagrangian drifter data, surface or subsurface.The graphical interface provides a user-friendly

way to filter out bad data.

The following computations are possible with the data analysis module:

• Lagrangian velocities, accelerations and Rossby numbers (along the path of the drifters)

• Eulerian statistics (Velocity fields)

• Lagrangian statistics (integral time scales, eddy diffusivities)

• Semi-Lagrangian Spectra

Example with borrowed data-set from WOCE drifters in the Aegean Sea:

Each drifter can be identified by a different color, determined interactively.

Simply moving and letting the mouse over a drifter position, a box appearsshowing •the drifter’s code #•date and time•Position•estimated speed•estimated direction•Rossby number •surface temperature (for the WOCE drifters; TELEFOS are not yet equipped with thermistors)

Using the “zoom” button, we can focus in an area to view details of the tracks

Removing “bad” data, either by deleting single points or by selecting a group of points, is done easily with the mouse.

The details of single points to be erased are shown in inset boxes.

The Lagrangian velocity is estimated just by clicking to the folder “velocity”and leaving the option“grid” unchecked

Checking the option “grid”, we obtain Eulerian means over a grid laid on the region of interest

Moving the mouse over the center of a grid point, we see its statistics.

The size of the grid is controlled by a slide-ruler

The variance ellipse is displayed by clicking to the folder “confidence”

A slide-ruler controlsthe desired confidence interval

There is also a folder option providing the mean kinetic energy

…and one for the “eddy”, or fluctuating kinetic energy

These autocorrelation functions are the first step in estimating integral time scales and horizontal diffusivities.

The work is under progress.

Conclusions

• Telefos will constitute a low-cost, very versatile instrument.

• Telefos novelties improve its cost-efficiency, while facilitating its use.

• We seek to expand the market to non-marine scientists by offering ease of use and user-friendly data analysis.

• The first prototypes have been built; the first four instruments will be delivered to NCMR for sea trials in January 2003, and six more (production-type) will be ready until the summer 2003.

Additional Information: Vassilis Zervakis (NCMR), at zervakis@ncmr.gr Antonis Kantidakis (MARAC) at

kantidakis@marac.gr

What is TELEFOS?

TELEFOS is a state of the art system for measuring currents and calculating dispersion (e.g. pollutants) in the near surface waters. Its concept is based on a new design on an old idea, i.e. of using drifters to monitor the circulation in aquatic bodies.

The system consists of a monitoring station (which may be placed either on land or aboard the deployment/retrieval vessel), and the drifter fleet. The drifters’ positioning exploits the highly accurate GPS system.

The main innovations that TELEFOS brings are:Use of GSM and RF technology for communication.

A modular design allowing two configurations, thus enabling the monitoring of surface and subsurface currents (down to 50 m).A potent, user-friendly software offering real-time monitoring and interactive programming during the mission, a vessel-guiding module for the retrieval of the drifters, and extensive data analysis features (assessment of mean and eddy velocity and kinetic energy fields, lagrangian statistics, horizontal diffusion coefficients, spectral analysis).The software allows the import and analysis of any drifter data.

MARAC ELECTRONICS and the HELLENIC CENTRE FOR MARINE RESEARCH present a new surface drifter (Telephonic monitored drifters for ecological studies - TELEFOS), designed for use in semi-enclosed seas, coastal areas and lakes.

Monitoring surface currents

APPLICATIONS

• Environmental impact studies

Port construction Sewage dispersion from

point sources Aquacultures Experimental validation of

numerical simulations of circulation

Calibration of coastal HF current/wave radars

Area selection tool

Monitoring sub-surface currents

POSITIONING POSITIONING

GPS

COMMUNICATIONCOMMUNICATION

ISM433GSM

DIMENSIONSDIMENSIONS

Weight < 5KgsHull 90cm X 15cmDrogue surface ~ 1m2

OPERATIONAL LIMITSOPERATIONAL LIMITS

-5oC to 50oC Air Temperature-2oC to 45oC Water Temperature

OTHER DETAILSOTHER DETAILS

• Operation depth 0-50m• Weeks or months of operation (depending on the

sampling and communication strategy)• Low cost operation due to:• High drifter retrievability due to the monitoring and boat-

guidance modules.• Use of data compression algorithms. • Use of alkaline “D”-cell batteries for low cost and high

flexibility.• Low cost materials and accessories

Vessel-guidance module

Lagrangian Statistics – decorrelation scales

www.marac.gr - www.hcmr.gr

Real- time drifter- monitoring screen