Monitoring environment with GPS

ENGINEERS AT at Purdue University and NASA have developed a new technique for monitoring the environment by using routine signals that already are being beamed to Earth by global positioning systems. Conventional techniques for gathering information about global climate change and other environmental data require systems that have their own transmitters and receivers on satellites or aircraft.

The equipment, though, is relatively heavy and consumes large amounts of power. The new technique could be used for alternative or complementary systems on aircraft and satellites. The method does not require a transmitter because it uses the signals already being transmitted by GPS satellites. That makes it possible to design a system that only requires a receiver, other hardware and special software that is one-tenth the weight and uses about one-tenth the energy of conventional systems, said James Garrison, an assistant professor of aeronautics and astronautics at Purdue University. Garrison presented a paper on this technique at a special session during the International Geoscience and Remote Sensing Symposium, in Sydney, Australia. Satellite.

GPS systems are constantly broadcasting microwave signals to Earth. The signals are used for everything from military maneuvers to directions for lost motorists, and ship navigation to measurements for agriculture. The systems work by timing how long it takes for signals to get from the satellite to a receiver on the ground.

Because the speed of the signal is known, the difference between when the signal is sent and when it is received reveals the distance of the receiver. Signals from several satellites are needed to provide a three-dimensional fix - a precise location of the receiver on the Earth's surface. Those same microwave signals, however, also bounce off of the Earth's surface and oceans and can be picked up by receivers on aircraft or satellites, recorded and used to study conditions on the planet.

The new system, invented by Garrison and NASA engineer Stephen Katzberg, would use GPS signals to collect environmental data for studies aimed at understanding global climate change. "The small size and low cost of the instrumentation would make operational deployment on balloons or small aircraft very affordable," Garrison said.

Because water is an especially good reflector of GPS signals, the new technique is promising for studying how ocean-related conditions affect global circulation and world climate. For example, scientists monitor wind patterns near the surfaces of the oceans to track global circulation, which influences climate. As each GPS signal bounces off of the water, it reflects from numerous facets on the rough, undulating surface.

The signal is then picked up by a receiver, located on an aircraft or a satellite, and then software interprets the data, providing detailed measurements of the changing ocean surface. "The roughness of the surface is directly related to near-surface meteorological conditions," said Garrison, a former NASA engineer who began the research while working for the space agency.

Because the system is lighter and uses less energy than conventional systems, it would be ideal for installation on small remotely piloted drones called unmanned aerial vehicles. It also would have benefits for satellite systems, he said.

The new technique also has other advantages. It works well when there is heavy moisture in the atmosphere during storms, unlike some of the conventional remote-sensing systems. It also is easier to use because it does not require complicated calibrations now required for conventional systems.

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Section  : Science & Tech
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