So here's your chance to contribute to science. The object is to design an inexpensive instrument for monitoring the ocean close to shore-something that perhaps could be built for only a few hundred dollars. To get you drifting in the right direction, this column lays out the requirements and discusses some hardware that may help. But don't limit yourself to the ideas presented here. I'll forward the best solutions to the federal agencies involved and post them on the World Wide Web site of the Society for Amateur Scientists. The winning designs in this informal competition will also earn laurels by appearing in a forthcoming column.

The drifter should be able to follow coastal currents for a full month, reporting its position and the condition of the sea around it. But it should be neither a danger to navigation nor a threat to marine life. The device should thus be free of items that might ensnare, choke, poison or otherwise injure marine organisms. The ideal drifter would be largely biodegradable. Or it could be made so that whatever parts don't break down quickly are encapsulated and sink promptly to the bottom, where they cannot do any harm.

Indeed, part of the challenge is first to decide just what poses an environmental danger. Only then can you design a drifter to be as benign as possible. Keep in mind also that the instrument must be small enough not to create a hazard to shipping and yet should be visible to swimmers, surfers and boaters so that they can steer clear. Finally, it should be able to survive storms or choppy seas, and it must move with the currents, not with the winds.

Finding suitably biodegradable parts will require some experimentation. Cardboard supports, for example, might rapidly become waterlogged. And floats made from steel cans may corrode too quickly. To slow these processes, you might coat such components with a layer of shellac, wax or some other natural substance.

My own conception is illustrated at the left. A large keel and a low profile above the water prevent the drifter from tipping and ensure that it moves with the currents. To warn off boaters, a colorful cotton flag attaches to the cardboard tube that protects a radio antenna.

But my proposal merely shows one sample design. There are probably myriad ways to configure a drifter with the necessary attributes, and I look forward to judging your detailed plans. Because the drifter must be able to sense and report its position and the state of the sea around it, I will also select a separate winner for excellence in designing an electronics package that reliably accomplishes all these tasks for the least amount of money.

Though difficult, that challenge is easier than it might at first seem. Satellites of the Global Positioning System (GPS) make it child's play to determine location. Garmin, for instance, sells tiny GPS receivers for less than $200 that could locate a drifter anywhere on the earth to within 100 meters or so. (See here or call 913-397-8200.)

Telemetry is, perhaps, trickier. Drifters deployed in the open ocean use the ARGOS satellites to transmit their data home. But at least for the moment, this service is quite pricey. Fortunately, for coastal studies, "packet radio" provides a low-cost alternative. This technique, pioneered by radio amateurs two decades ago, lets two computers share data without a phone or Internet link. The scheme is to break a data file into smaller chunks called packets, add routing information and send them over the airwaves. Although the transmitters used for this form of communication typically broadcast less than one watt of power, packet radio signals can be received almost anywhere in the U.S. through a nationwide network of amateur-operated stations (called repeaters) that automatically receive, boost and retransmit the information onward to more distant sites.

Packet radio has another advantage. Thanks to the visionary work of Bob Bruninga of the U.S. Naval Academy Ground Satellite Station in Annapolis, Md., generating a map on a personal computer that would locate all such drifters in real time would be a straightforward task. Bruninga created the Automated Position Reporting System (APRS), a software package that plots the data received on a digitized map and shows the precise location of the originating packet radio transmitter, be it on a plane in the air, on a ship at sea or even on a satellite orbiting in space. To find out more, consult http://www.aprs.org.

To use packet radio, the electronics assembly will have to include something called a terminal node controller (or, in the lingo of ham radio, a TNC), along with a GPS unit and an inexpensive transmitter. And at least one sophisticated TNC, the KPC-3 Plus from Kantronics (785-842-7745; $149.95), can simultaneously monitor two different analog sensors, convert the measurements into packets and transmit them. With minor modification, additional analog channels become available. (You'll have to be clever: Kantronics doesn't document or encourage making these changes to the circuitry.) So the drifter could, for example, measure water and air temperature, salinity, intensity of sunlight, and battery voltage.

For a transmitter, check out your local ham radio outlet-but be prepared to be dazzled. Most offer a smorgasbord of gear with a dizzying array of features. Remember, if everything goes well, in a month your transmitter will be lying in Davy Jones's locker, so stick with the simplest model that can do the job. A dedicated data transceiver (a radio that both receives and transmits) is probably the cheapest way to go. MFJ Enterprises in Starkville, Miss. (800-647-1800), for example, sells its model APRS MFJ-8621X2 for about $140. This unit transmits digital data on a standard APRS frequency. Or you may want to consider the Radio Shack HTX-202. It is more flexible and sells for only $199.95. Either unit should be detectable about 10 kilometers out to sea.

Remember, your drifter will need electricity for a month. But don't be seduced by the siren song of solar power: photovoltaic panels, rechargeable batteries and the electronics to connect them together will add complexity and at least $150 to the cost. Instead consider using ordinary alkaline D cells, which can last five times longer than rechargeables. By powering up the sensors and broadcasting its position only every half an hour or so, the drifter should be able to run for several weeks on six D batteries.

I expect that many of the clever people who read this column will provide some novel ideas for oceanographers to consider. The government agencies involved certainly want to enlist science enthusiasts: right now they offer a way for anyone to track some of their drifters over the Internet. Perhaps, with the right design, amateur experimenters and high school students could assist the professionals by building coastal drifters and following their progress. Such activity would help forge a valuable alliance, enriching all parties while revealing some secrets of the sea.

I thank Ellen Prager, Bob Bruninga and Joe Valencic for their help and advice. For more about this and other projects, visit the forum hosted by the Society for Amateur Scientists. You may also write the society at 5600, Post Road, #114-341, East Greenwich, RI 02818, or call 1-877-527-0382.

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.