Laser detection system on space satellite should yield wealth of data

By Neal Singer

The first step in deploying lasers on space satellites will take place
Sept. 9, when NASA sends a lidar system into orbit on the space shuttle
Discovery, says the UI researcher who helped design the major experiments
for the mission. 

The lidar system, dubbed NASA LITE, is expected to be aboard the shuttle
when it is launched at 4:30 p.m. EDT from Cape Canaveral, Fla., to monitor
Earth's atmosphere. The shuttle is scheduled to return Sept. 18.

Lidar, an acronym derived from light detection and ranging, emits laser
beams rather than radio waves to obtain information. LITE derives from
Lidar in Space Technology Experiment. 

"Through use of a pulsed laser beam as an active sensor, we can measure
light back-scattered from air molecules, clouds and aerosols, or
reflections from Earth's surface, and infer information about
characteristics of the atmosphere," says UI electrical and computer
engineer Chester S. Gardner. The system will be firing 10 times a second in
the green, ultraviolet and infrared ranges.

The returning light, captured by a telescope adjacent to the laser on the
satellite, should yield the best information on concentrations of aerosols,
movements of storm systems and heights of clouds. Later lidar systems will
study atmospheric ozone depletion, the accumulation of carbon dioxide and
water vapor in the atmosphere, and the expansion or shrinkage of polar ice
caps.

The information is expected to be much more detailed than that obtained by
current methods. Satellites now use a horizon-scanning technique that
measures the thinning of sunlight as it passes through the edge of Earth's
atmosphere. The technique can identify chemical constituents of the
atmosphere, but its resolution is relatively poor, he said.

A major challenge for LITE will be to measure water reflectivity at
different amounts of surface turbulence. In an attempt to accomplish this,
the shuttle will pitch forward 30 degrees as it passes over specific sites
and will rotate at a speed that allows it to keep the laser beam focused on
specific sites. The process is expected to help calibrate water reflections
of different size waves for future generations of orbiting lidar systems.

Calibration stations around the globe will beam lidars upward to compare
their readings with the shuttle's lidar as it passes above them. The
largest such check station in terms of telescope size - 3.5 meters - will
be at the Starfire Optical Range in Albuquerque, N.M., and will be staffed
by UI electrical and computer engineer George Papen and several UI graduate
students.

Gardner, who for eight years has been part of the 12-member LITE advisory
committee, will lead the day shift at the Project Science control room at
the Johnson Space Center in Houston during the 45 hours of intermittent
observation expected from the lidar system. He and his colleagues will
advise mission controllers on science problems that may arise during the
tests.