Anyone can stick their finger in the air and tell which way and roughly how fast the wind is blowing. But you can't do it everywhere, all the time.
One of the challenges faced by modern weather and climate scientists has been figuring out what the winds are doing across the planet. It's easy enough to measure them by putting an anemometer atop a building or on a balloon, or by clocking the speed of clouds as they move on the Earth below a satellite.
Direct measurements of the tropospheric winds have been cited as a major missing element in NASA's Mission to Planet Earth program. More accurate and precise measurements would provide a greater impact on numerical weather prediction models than any other space-based observation.
To measure the winds in clear air, NASA's Marshall Space Flight Center and the Global Hydrology and Climate Center (GHCC) have been selected to develop the Space Readiness Coherent Lidar Experiment (SPARCLE), a test model of a large-scale laser that could measure winds across the planet where there are no instruments.
Dr. G.D. Emmitt of Simpson Weather Associates in Charlottesville, Va., is the co-principal investigator for SPARCLE science, and Dr. Michael J. Kavaya of NASA/Marshall is co-principal investigator for technology. NASA/Marshall is the lead center and will work closely with Langley Research Center and the Jet Propulsion Laboratory in developing SPARCLE.
"This is an exciting time for a number of people who have, until now, had only paper studies to convince their colleagues that winds could be measured accurately from space with lasers," Emmitt said. "The global community of atmospheric researchers, weather forecasters, and weather-sensitive industries will be watching as NASA explores this new frontier of space-based laser sensing of the Earth's winds."
The "readiness" part means that SPARCLE will be limited in capabilities and time; only about 50 hours of observations on its Shuttle mission. However, scientists expect that it will light the way for more ambitious instruments aboard large satellites dedicated to monitoring the environment.
SPARCLE was selected for development under NASA's New Millennium Program which is designed to develop advanced new technologies for 21st century space missions.
For all its ambition, SPARCLE will start with a modest size; two Getaway Specials, cans the size of a large wastepaper basket. Using a "GAS can" will allow scientists to put their optical and electronic systems in a pressurized environment and not have to ruggedize them for space.
SPARCLE will aim pulses of eye-safe laser light into the atmosphere and measure the light which is reflected back to it by dust and aerosols in the atmosphere. The time between pulse and echo will (like radar) determine the distance to an object. The shift in the color of the light will tell how fast the particles are moving along the laser's line of sight.
By analyzing a series of pulse echoes, scientists using SPARCLE will be able to build a model that shows the movement of clear air.
The heart of SPARCLE will be a compact optical system in a GAS can with an opening lid (the second GAS can will hold the electronics). A transmitter laser will send a pulse of low-intensity infrared light through a telescope to the Earth below. A rotating glass wedge will allow some pointing control since the GAS cans are firmly bolted to the sill of the Shuttle payload bay.
The return signal will be combined with another laser beam so the two interfere slightly to yield a more easily measured radio wave in place of the two light waves. The technique is called heterodyning. The frequency of the radio wave will match the difference between the outgoing and incoming signals (adjusted so the system is not acting as a speedometer for the Shuttle).
SPARCLE is a low-cost, quick-development project. The price tag is about $15 million, and the first flight is scheduled for 2001.
If successful, the program could lead to a larger laser facility, with a significantly larger aperture aboard operational meteorological satellites early in the 21st century.