- Microlasers Developed for Altimetry
Brent D. Johnson
When Lewis and Clark set out from the head of the St. Louis River in 1804 to chart the unexplored territory west of the Mississippi, each mile surrendered by the wilderness was measured in the plodding steps of an expedition that took three years to survey 8000 miles. Today's explorers, who have their eyes fixed on the planets, measure the martian wilderness with the steady pulse of a laser, covering hundreds of thousands of miles in a single day.
A test flight shows (from top to bottom) an aerial photograph taken by the onboard digital camera, a ground photograph of a single-story home and the profile as seen by the laser altimeter. The color lidar points correspond to individual photons detected by different color quadrants and have not been corrected for small differential channel delays.
As part of NASA's Mars Global Surveyor Mission, the Mars orbiter laser altimeter instrument, equipped with a 50-cm telescope and an actively Q-switched Nd:YAG laser producing 50 mJ at 10 Hz, made almost a billion range measurements of the planet's surface. These measurements are spaced approximately every 300 meters along the satellite ground track and provide a somewhat porous but high-vertical-resolution topographic map.
John Degnan, head of the geoscience technology office at NASA's Goddard Space Flight Center, has been developing an advanced photon-counting laser altimeter that he hopes will produce more-detailed maps from space while consuming as little energy as possible. The concept combines low-energy (a few millijoules), high-repetition-rate (~1 kHz) lasers with photon-counting detector arrays connected to multichannel timers so that each laser pulse produces a three-dimensional image
of the topography within the laser footprint.
The transmitter and receiver fields of view are overlaid at the surface using a counterrotating optical wedge scanner that provides a fairly uniform and contiguous linear scan at 45° to the spacecraft's direction of flight and that also compensates for transmitter point-ahead during high-speed scans of the surface. With this system, Degnan hopes to provide globally contiguous 3-D maps of a planetary surface with horizontal resolution of a few meters and decimeter vertical resolution.
In aircraft tests of the photon-counting concept, a microlaser altimeter, or "microaltimeter," has been flown several times near midday on the NASA P3B aircraft at altitudes up to 22,500 feet. The passively Q-switched Nd:YAG microchip laser transmitter from Northrop-Grumman Poly-Scientific (formerly Litton Airtron Synoptics) has a volume of 8 mm3 and produces a few microjoules of energy in a subnanosecond pulse at rates up to 7 kHz.
It is powered by a single 1.2-W continuous-wave laser diode and is frequency-doubled by a passive LBO nonlinear crystal.
Although the expected mean signal strength per laser fire was typically on the order of one photoelectron, the instrument successfully recorded -- and extracted from the solar background -- highly contiguous returns from buildings, tree canopies and water surfaces and even performed shallow water bathymetry to depths of a few meters. The residuals off modestly sloped rooftops suggested a range precision near the instrument limit of 5 cm.
Flights over Ocean City, Md., and Assateague Island, Va., are planned to check the absolute accuracy of the instrument against available digital elevation models in the area.
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