team is trying to conserve the spacecraft’s remaining laser lifetime in order
to stretch out the time-series of its ice-sheet measurements as far as possible.
As far as engineers on Earth can tell,
the GLAS laser troubles were caused by
diode pump modules that failed due to
contamination problems. In the cold
vacuum of space, indium solder in the
device grew “whiskers” that in turn led
to electrical shorting.
The GLAS instrument was also the
first spaceborne lidar to have a large,
fixed-focus, meter-class beryllium
telescope coupled to a complex detector
package. Initial results, before the laser
problems revealed themselves, demonstrated that photon-counting detection
at 532 nm could be an effective way to
study the atmosphere from space.
ICESat was the first mission that
was specifically designed for measuring ice sheets, said Robert H. Thomas,
a glaciologist at NASA’s Wallops Flight
Facility (Wallops Island, Va., U.S.A.).
With ICESat, scientists have been able
to measure the inland retreat of the so-
called grounding line that marks where
an ice sheet slides off land and flows
horizontally over the ocean.
Greenland’s ice sheet is thinning
quite rapidly in places, but ICESat
misses some of those basins. “So if you
only have ICESat data, you don’t know
those areas are thinning,” Thomas said.
Aircraft have the ability to fly at specific
times and altitudes and can measure ice-sheet thickness as well as elevation. Some
of these glaciers are thinning fast—by
several tens of meters per year—and
glaciologists want to understand why.
Thomas uses both aircraft and spacecraft
data in his research.
In 2007, the National Research
Council’s survey of the Earth-science
community ranked a follow-on ICESat-II mission as one of its highest priorities
for the next decade. NASA’s Web site for
science missions says that the project is
expected to launch in 2015. According
to Thomas, the future lidar would have
a multi-beam laser that would make
more accurate measurements of sloping
surfaces. The GLAS beam footprint—
(Top) The flight model of the Phoenix Mars
Lander’s lidar just before it was placed on
the spacecraft during assembly operations.
(Bottom) The lidar experiment for the Lunar
Reconnaissance Orbiter (LRO).
70 to 100 m across—caused larger than
desired errors in measurements of slopes
of more than a few degrees.
In April 2006, NASA launched the
Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO)
spacecraft. CALIPSO is a joint mission
of NASA and the French space agency,
Centre National d’Études Spatiales
(CNES), but NASA Langley built the
laser for its lidar experiment. The lidar
system provides high-resolution vertical
profiles of aerosols and clouds.
Currently CALIPSO has the highest-power laser in space, said Upendra N.
Singh, chief technologist of the systems
engineering directorate at NASA Langley Research Center (Hampton, Va.,
U.S.A.). It performed well until March,
when ground crews detected a small
pressure leak in the container holding
the instrument’s primary laser. The lidar
was shut down while the crew switched
the instrument over to its backup laser,
but it was expected to operate again by
Wind measurements from space
The next frontier of lidar is getting a
better picture of terrestrial wind patterns,
especially in places where scientists can’t
easily place an anemometer or wind-speed
gauge, like the middle of the ocean.
“Wind is one of the most critical
things that hasn’t been measured from
space,” Singh said. Today’s meteorologists can predict two or three days out
reasonably well, but forecasts of five
to 10 days in advance are less certain.
Weather scientists would like to improve
the accuracy of their five- to 10-day
forecasts to the current level of the two-to three-day forecasts, but one of the
critical missing components is a three-dimensional profile of wind.
Even across the United States, government meteorologists have numerous
weather stations but sparse coverage
of the vertical wind field and thus
wind gradients with altitude. Over the
Pacific Ocean, in the polar regions and
throughout the southern hemisphere,
huge regions of the Earth’s atmosphere
have no wind-field coverage. And, even
though climate models have a longer-term focus than the “operational”
weather forecasts, they too depend on
understanding wind-field patterns at
high horizontal and vertical resolutions.
Another key issue with wind monitoring is timeliness. Wind fields are quite
ephemeral, and scientists need to capture
them globally quite often. A satellite in
low Earth orbit will complete 15 circles
of the planet every day.
According to Singh, there are two
ways to measure wind speeds: with
aerosols and with air molecules. Sensing backscatter from aerosols works in
Earth’s atmosphere below altitudes of
8 or 9 km. Above that altitude, one can
use shorter wavelengths to scatter laser
light off the actual molecules in the air
to get the wind speed. In meteorological circles, this is called direct detection,