Loser: Flying Away

Forecast bad for tiny airborne probes

4 min read

There's a well-known joke among meteorologists: the weather service once came up with a computer model that could make perfect one-day forecasts, but they took two days to compute.

Whether we will have perfect forecasts in the future is a matter of debate among meteorologists, but they agree that what we need most to improve weather predictions--besides a deeper understanding of atmospheric physics--is better data to feed into numerical models. To get that data, suppose we built microscopic probes that we could disperse by the billions into the atmosphere to form a monitoring network that would engulf the planet, taking in situ measurements and providing a meteorologist's dream of input information.

That is the idea behind GEMS, or Global Environmental MEMS Sensors. These dust particle-sized airborne probes with embedded microelectromechanical sensors would measure variables like temperature, pressure, humidity, and wind velocity and would spontaneously form a network that would report the measurements to remote receiving stations.

Dust in the Wind: A speck-sized probe in this rendering from ENSCO Inc. is supposed to float in the air and monitor temperature, pressure, and other weather variables.

The concept was proposed by ENSCO Inc., a science and engineering R and D company headquartered in Springfield, Va., which received a US $500 000 grant in September 2003 from the NASA Institute for Advanced Concepts to study the idea's feasibility.

Proponents say GEMS could revolutionize weather forecasting because its probes would produce data with unprecedented spatial and temporal resolution. The problem is that the tiny sensors would not stay aloft indefinitely. ENSCO estimates that probes with dimensions of about 100 micrometers would stay aloft for hours to several days and that a global network with one probe in each cubic kilometer of the lower atmosphere--where most weather happens--calls for some 10 billion probes. It would be necessary to continuously replace the probes that fall to the ground or that are washed out by rain. And then, even if each probe costs just a few cents, producing and deploying billions of them would make GEMS quite an expensive enterprise.

ENSCO says the number of probes needed and the cost per probe are malleable numbers. But even as a mere paper study, GEMS has accomplished the seemingly impossible: making satellites--known to cost a lot of money--look cheap. "If you need global monitoring on a continuous basis, the only way that I see to do it is from satellites," says Benjamin Herman, a professor of atmospheric sciences at the University of Arizona, in Tucson. With large amounts of people and resources devoted to satellite research, space-based sensing systems and other remote-sensing technologies are advancing quickly.

"The basic [satellite] technology is out there," says Robert Boldi, a researcher in the weather-sensing group at the Massachusetts Institute of Technology's Lincoln Laboratory, in Lexington. He says satellites equipped with ever more powerful lidars, devices that use laser pulses to determine wind velocity, and next-generation microwave sounders, which rely on RF waves to see through clouds and determine variables like temperature and humidity, would eventually provide the data to improve forecasts--maybe not with the same resolution promised by GEMS, but enough for what most meteorologists need.

Developing the dust-sized probes also poses fiendish technological challenges. Packaging a menagerie of nanoscale sensors might not be an obstacle, but powering the probes and getting them networked are problems still to be resolved, says Michael Simpson, a principal investigator with the Molecular-Scale Engineering and Nanoscale Technologies Research Group at the Oak Ridge National Laboratory in Tennessee. "To me, it all comes down to the kind of power and communications [used in GEMS]," he says.

GEMS (Global Environmental MEMS Sensors)

Goal: Deploy a global network of dust-sized airborne probes to monitor the atmosphere and improve weather forecasting

Why it's a Loser: Satellite and remote-sensing technologies offer more cost-effective ways to monitor the environment on a global scale. Deploying microscopic probes globally would be politically difficult and would raise environmental concerns

Company: ENSCO Inc.

Center of Activity: ENSCO facility in Cocoa Beach, Fla.

Number of People on the Project: 3

Budget: US $500 000 (feasibility study and prototype development)

Storing a lot of energy in such small objects would be hard, Simpson notes, and RF communications require a lot of power, not to mention a sizable antenna. A probe transmitting in the gigahertz range, for instance, would need a centimeter-sized antenna, an enormous flagellum for such a tiny probe. An alternative might be employing optical technology, but that would require a type of line-of-sight communications, making things much more complicated.

Then, too, disseminating invisible probes all over the seven continents would certainly raise a number of political, environmental, health, and privacy-related concerns. Would other countries accept U.S.-invented invisible and intrusive devices falling on their territories? Would the probes be harmful to the environment or to anyone unfortunate enough to breathe them in? Would society accept these tiny sensors floating around monitoring weather variables--and some would ask, who knows what else?

Nevertheless, the idea of having a self-organizing network of tiny sensors that can monitor their surroundings--"smart dust," as some people call them--has a number of interesting applications. Examples include monitoring chemicals in a factory or troops on a battlefield.

In the same way, GEMS might prove invaluable for tracking other localized events, like a hurricane approaching the Florida coast. GEMS data could then complement the information collected with weather balloons, satellites, airplanes, ground-based radars, and other equipment.

A technology like GEMS "enters into the complex composite of observing systems that we have [and] adds a capability, if we can make it work well," says Alexander MacDonald, director of the Forecast Systems Laboratory, the leading meteorological research and development lab of the National Oceanic and Atmospheric Administration, in Washington, D.C.

In MacDonald's view, researchers should focus on larger devices that they could actually build and release in a thunderstorm or in areas where there are few observations being made, like over the oceans.

That is indeed how the project seems to be evolving. "The near-term objective," says John Manobianco, who is leading the study at ENSCO, "would be, say, in the next five years, to design prototypes and actually test them and release them over limited areas."

For monitoring things like hurricanes and thunderstorms, GEMS might become a very valuable technology. But the leap to a global scale poses huge technical, political, and cost barriers, making the idea of a planetary network of speck-sized flying probes more likely to disappear in the air--like dust.

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