Space-based instrumentation

Shuttle laser to measure atmospheric windsYou don`t need an orbiter to know which way the wind blows, unless you are a climate modeler in search of accurate data about the speed, direction, and vertical profile of atmospheric winds. A recently approved space shuttle mission will begin to provide such information, offering the first-ever direct measurement of tropospheric winds from space using an infrared (IR) laser located in the cargo bay of the space shuttle.

Space-based instrumentation

Shuttle laser to measure atmospheric windsYou don`t need an orbiter to know which way the wind blows, unless you are a climate modeler in search of accurate data about the speed, direction, and vertical profile of atmospheric winds. A recently approved space shuttle mission will begin to provide such information, offering the first-ever direct measurement of tropospheric winds from space using an infrared (IR) laser located in the cargo bay of the space shuttle.

Extending technology tested aboard research aircraft, the SPAce-Readiness Coherent Lidar Experiment (SPARCLE) will detect the frequency Doppler shift of IR laser light pulses as they reflect off dust, clouds, and aerosols in the troposphere, the lowest stratum of the atmosphere. Such shifts will provide direct information about the velocity vectors and vertical profile of tropospheric winds, data that are "to¥of the wish list" of weather researchers, according to Dave Emmitt from Simpson Weather Associates (Charlottesville, VA), a co-principal investigator on the project. The information would improve weather and climate forecasts and models and would also be of use to commercial and military aviators for flight planning. Set for a 2001 launch at an estimated cost of $15 million, SPARCLE will be managed by NASA`s Marshall Space Flight Center (Huntsville, AL) as the second Earth-orbiting mission in the agency`s New Millennium Program.

Holmium,thulium:YLF laser

Researchers at the Remote Sensing Technology Branch of NASA Langley Research Center (NASA/LaRC; Hampton, VA) made a breakthrough last July by demonstrating, for the first time, an all-solid-state, room-temperature, diode-pumped holmium,thulium-doped YLF laser that produces an eye-safe, 2.05-µm output of 600 mJ at 10 H¥(see Fig. 1 on p. 30). This is the highest energy ever produced and is at least an order of magnitude greater than so far achieved from a 2-µm diode-pumped laser at room temperature.

"This turned out to be a very timely development as it led NASA/LaRC to design and develo¥a room-temperature, diode-pumped 100-mJ, 6-H¥laser transmitter for SPARCLE--a key technology for space-based coherent wind lidar," said Upendra N. Singh, Project Leader of the 2-µm Laser Team at NASA LaRC. Singh is also Contracting Officer`s Technical Representative (COTR) for the SPARCLE Shuttle Mission.

Wind lidar

The laser, together with directing and receiving optics and a detector, will be flown in two shuttle "hitchhiker" canisters at an altitude of 300 km (see Fig. 2 on p. 33). The 100-mJ laser pulses will be directed toward Earth in a 25-cm-diameter beam at an angle of 30° in order to provide both horizontal and vertical Doppler information. Reflection from atmospheric particles is then collected back at the shuttle, where, after heterodyne mixing by one of two onboard CW lasers, an indium gallium arsenide detector registers the few backscattered photons.

Only four or so photoelectrons at the detector output are enough to provide accurate wind measurements, out of the 1018 photons emitted in each laser pulse. This remarkable photon efficiency arises because the wind information is obtained from a frequency measurement of photons. Today, such wind information is inferred indirectly from measurements of temperature and moisture fields.

NASA/LaRC will transfer the diode-pumped Ho,Tm:YLF laser technology, together with a breadboard laser, to Coherent Technologies Inc. (CTI; Boulder, CO). NASA/LaRC will also provide technical guidance while CTI builds the laser transceiver for the project, extending its experience designing airborne lidar systems to the rigorous shuttle environment. According to Sammy Henderson, CTI`s chief technical officer for SPARCLE, one of the most challenging aspects of the system design is its need for highly accurate pointing. "You`re flying at 7 km/s, and you`re trying to measure to one m/s, so you have to have knowledge on the order of 100 µrad of exactly where you`re pointing," he said.

Michael J. Kavaya, the project`s co-principal investigator handling lidar instrument design and engineering issues from Marshall Space Flight Center, said that developing a more-powerful laser subsystem has been a major challenge. "Just a year ago the state of the art in producing wind-quality, coherent narrow-bandwidth pulses was 20 mJ. NASA`s Langley Research Center has recently demonstrated 600-mJ pulses, so we feel that the 100-mJ operating point for SPARCLE is a reasonable one," he said.

Other partners in the development of SPARCLE include NASA`s Goddard Space Flight Center (Greenbelt, MD), Langley Research Center (Hampton, VA), the Jet Propulsion Laboratory (JPL; Pasadena, CA), the University of Alabama at Huntsville, and several private companies.

"We have a long history of doing ground and airborne wind measurements," said Emmitt, noting that earlier versions of coherent systems were quite large. "By going to 2-µm technology and doing good engineering, we`ve gotten the system down to a very lightweight package that`s ready for trial in space."

Satellite system could follow

If SPARCLE performs in its technological demonstration, a more robust system could be based on a free-flying satellite several years later. SPARCLE is trying to match the schedule of National Oceanic and Atmospheric Ad ministration`s Integrated Program Office for the National Polar-orbiting Operational Environmental Satellite System (NPOESS) satellite, which launches in 2007.

Though it is only a technological demonstration with a limited operational time of about 50 hours, the project also aims to collect the maximum amount of data about the atmosphere in order to provide initial results and to plan future missions in the most optimum way. "Indeed," 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."

David Appell

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