A new use for global positioning systems

Engineers at Purdue University and NASA have developed a new technique for monitoring the environment by using routine signals that are beamed to Earth by global positioning systems.

Engineers 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 (GPS) 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 because it does not require a transmitter. This is 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.

Satellite GPS 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.

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. A receiver, located on an aircraft or a satellite, then picks up the signal, and then software interprets the data, providing detailed measurements of the changing ocean surface.

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

‘In the design of satellites, power is a big concern because systems that use a lot of power require larger solar arrays,’ explained Garrison. ‘The larger solar arrays require a larger satellite structure, and large solar arrays also put more demand on attitude control.’

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.

Tests using aircraft equipped with the system are said to have yielded significant data, which will be used to improve the technology so that it might be incorporated into satellites, concluded Garrison.