detail and are less expensive to fly. Ultimately, space-based observations that are
global and limited in spatial and temporal resolution need to be calibrated with
and validated against regional observations from the ground or aircraft.
The laser is still the biggest risk in any
spaceborne lidar instrument, said Geary
Schwimmer, a former NASA researcher
now with Science and Engineering
Services Inc. (Columbia, Md., U.S.A.).
“You’re concentrating a lot of energy in
materials that break down and are damaged as a result of that energy,” he said.
When a human technician isn’t around
to remove contamination from the laser
optics, the laser could break down.
Early lidar experiments
NASA’s last three Apollo missions to
the moon each carried laser ranging
experiments—possibly the first lidar
systems to fly into space.
While his colleagues explored the
lunar surface for four days in mid-1971,
command module pilot James Worden
spent four days alone in orbit, running
the Scientific Instrument Module
(SIM) in the Apollo 15 command
module. Among other experiments,
SIM contained a laser altimeter that
used a Q-switched ruby laser giving
off 200-mJ pulses.
Worden collected altimeter data for
approximately four and a half orbits
before the laser, which had been operating intermittently, failed altogether. The
same type of experiment collected data
for seven and a half lunar orbits aboard
Apollo 16 in April 1972 and for 12 orbits
aboard Apollo 17 in December 1972.
Used together with a mapping camera
for calibration, the altimeter measured
the height of the command module
above the surface of the moon to ± 2 m.
Previous studies of the lunar cross-section, based on shadows in Earth-based photographs and ground-based
radar, had much larger errors, especially
near the limb (the apparent visual edge
of the lunar disk). Ultimately, the data
helped lunar scientists to estimate the
relative altitudes of highlands, plains
and craters in unexplored regions of the
(Top) Apollo 15 command module
Endeavour, in a photo taken from the lunar module
Falcon in lunar orbit, shows the instrument
bay containing the lunar mapping camera
and ruby lidar. (Bottom) The Apollo lunar
mapping camera on a lab bench prior to
flight. The lidar occupies the lower left portion of the instrument.
[ Schematic view of a ]
Lidar line-of-sight Backscattered light
European Space Agency
A short laser pulse is emitted towards
the atmosphere where air molecules
and particles reflect a small portion
of the light pulse back to the lidar. A
telescope collects the light and directs
it to the receiver. The signal is recorded
as a function of time to determine the
altitude of the scattering layers.
moon, particularly on the far side, and
to analyze the ellipsoidal shape of the
moon and find its center of gravity.
Lidar experiments on
the space shuttle
One of the first lidar systems to examine
Earth from space was the Lidar In-space
Technology Experiment (LITE), which
orbited on the space shuttle Discovery in
September 1994. The experiment was
designed primarily to prove that modern
lidar technology, with Nd:YAG lasers
and, increasingly, solid-state photodetectors, could work in space. LITE nevertheless provided new views of the cloud
structure of the atmosphere and gathered
much information about tropospheric
and stratospheric aerosols.
The LITE backscatter lidar, under
development since 1988 at NASA
Langley Research Center, included
two flash-lamp-pumped, Q-switched
1,064-nm Nd:YAG lasers, frequency-doubled and -tripled to 532 and 355 nm.
(Two lasers were flown for redundancy;
LITE used only one at a time.) To collect the reflected beams, the instrument
had a 1-m telescope and beamsplitters to
direct the incoming light into the detectors (two photomultiplier tubes and an
LITE operated for more than 220
hours during the space shuttle mission
and gathered the first global measurements of the height of the planetary
boundary layer. LITE also measured
the directional reflectance of the sea
surface and its dependence on the speed
of surface winds, which in turn drive the
A second diode-pumped, Q-switched
Nd:YAG lidar system, the Shuttle Laser
Altimeter (SLA), was a secondary payload
on two space shuttle missions. SLA-I
flew aboard Endeavour in January 1996,
and SLA-II rode Discovery in August
1997. Operating for about 80 hours at
an altitude of 305 km, SLA-I measured
Earth’s topography to a vertical resolution of 0.75 m.
SLA-II’s team at NASA Goddard
Space Flight Center (Greenbelt, Md.,
U.S.A.) assembled the instrument from