IPF 2010: Probing planets with lidar

Lidar, an abbreviation for light detection and ranging, is changing the remote-sensing landscape.

Radar, lidar’s better-known predecessor, works by transmitting radio waves and collecting their reflections to detect, locate, and track aircraft, missiles, raindrops, and other objects. Lidar can do more.

Thanks to their shorter wavelengths and the use of powerful pulsed lasers to generate them, lidar beams are much narrower than radar beams. Lidar can not only detect and track objects; by sweeping a beam across them, it can also image objects—with centimeter resolution from several hundred kilometers away. Moreover, unlike radar’s radio waves, which reflect strongly only from large metallic objects, lidar’s IR, visible, or UV waves reflect or backscatter strongly from objects that are small, insulating, or both.

It’s no surprise that space agencies such as NASA are major lidar users. At this year’s Industrial Physics Forum (IPF) in Rochester, New York, presenter James Abshire of NASA’s Solar System Exploration Division traced the surprisingly long history of lidar applications.

The first space-borne lidar, the Apollo Laser Altimeter, took to the skies in 1971 aboard Apollo 15, and subsequently aboard Apollo 16 and Apollo 17. Firing off a 694-nm ruby laser at a rate of 0.05 Hz, the Apollo Laser Altimeter determined the elevation profile of the Moon’s surface.

In 1997, the Mars Observer Laser Altimeter aboard the Mars Global Surveyor began its mission to map the red planet. Equipped with a more powerful and shorter-wavelength laser—a 1064-nm Nd:YAG laser with a pulse frequency of 10 Hz—MOLA revealed the topography of Mars in stunning detail, as demonstrated by the image here of Olympus Mons, the tallest mountain and volcano in the solar system.

Space missions to planets, moons, and asteroids continue to rely on lidar to probe topography. Launched last year, NASA’s Lunar Reconnaissance Orbiter is in the process of generating a complete three-dimensional map of the Moon’s surface. The spacecraft’s Lunar Orbiter Laser Altimeter is the first multibeam space lidar. Diffractive elements split LOLA’s primary beam into five secondary beams that can determine a planetary surface’s range, slope, and direction simultaneously.

Down to Earth

Lidar users have not abandoned their home planet. Whether on spacecraft or airplanes, lidar is a major remote-sensing tool in the Earth sciences. In the latest National Research Council decadal survey , 6 of the 10 new Earth science missions will carry lidar instruments.

Among the more recent lidar-toting Earth-measuring satellites was NASA’s Ice, Cloud, and Land Elevation Satellite , whose successful seven-year mission ended in February. Designed by Ball Aerospace, ICESat‘s lidar system was composed of a dual-wavelength (532/1064-nm) beam, which measured the location and composition of clouds as well as the topography of forests. The system also gleaned data on the acceleration of ice-sheet melting at the North and South Poles.

Ball Aerospace engineer Carl Weimer, who also presented at this year’s IPF, pointed out that while greenhouse gases are warming the planet, atmospheric aerosols are cooling it. To profile clouds, aerosols, and their interactions, Ball Aerospace, NASA, and CNES, (France’s space agency), teamed up to design and launch the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation, or CALIPSO. Launched in 2006, CALIPSO has joined the A-Train , a collection of five Earth-monitoring satellites flying in close formation.

CALIPSO carries the first polarization-sensitive, dual-wavelength (532/1064 nm) lidar system, a visible-wavelength wide-field camera, and a passive IR radiometer. The lidar system captures data from the ground up to an altitude of 40 km.

Because it is sensitive to aerosols and similarly sized particulates, CALIPSO‘s lidar has also been considered for a new approach to hurricane forecasting. Dust suppresses cloud formation and subsequently inhibits convection, which powers hurricanes. Monitoring the storms that carry dust from the Sahara across the Atlantic Ocean could help identify which tropical disturbances will become hurricanes.

Jermey N. A. Matthews

All the talks at IPF 2010 were recorded and are now available on video .