UARS logo
UARS-launched from the space shuttle in 1991-is the first stage of a long-term, international program of space research into global atmospheric change. The UARS mission is to investigate the global photochemistry, energy balance, and dynamics of the earth's upper atmosphere. To accomplish its goal, UARS carries ten complementary experiments, each experiment providing measurements critical to our understanding of processes in the upper atmosphere.
The Cryogenic Limb Array Etalon Spectrometer (CLAES) experiment measures temperature profiles, and concentrations of ozone, methane, water vapor, nitrogen oxides, and other important species, including CFCs, in the stratosphere. CLAES also maps the horizontal and vertical distributions of aerosols in the stratosphere. These measurements are analyzed to better understand the photochemical, radiative, and dynamical processes taking place in the ozone layer.
Ozone-Destructive Species
* Nitric oxide (NO)
* Nitrogen dioxide (NO2)
Reservoir and Sink Species
* Nitric acid (HNO3)
* Chlorine nitrate (ClONO2)
* Hydrogen chloride (HCl)
* Dinitrogen pentoxide
(N2O5)
Carbon Dioxide (CO2) emissions are measured and used to deduce temperature and pressure. Aerosol absorption coefficients are also derived.
CLAES infers the amounts of gases in the stratosphere from the
measurement of the unique infrared emission features associated with
each gas. To separate the often very weak signatures of "trace" gases
such as CFCs from the intense atmospheric background radiation,
requires both high spectral resolution and high sensitivity. This was
done by combining a telescope with an infrared spectrometer and solid
state detectors, and cryogenically cooling the whole instrument to
prevent its own thermal infrared emissions from interfering with the
measurement of weak atmospheric signals. The spectrometer had a
resolving power of about 4000 and operated over the wavelength range
3.5 to 12.9 microns.
The major features of the instrument are shown in the following image:
Illustration of the CLAES Instrument
The instrument comprises the front-end sensor, made up of the
telescope, spectrometer, and detectors, and the back-end cryogenic
cooler, made up of an inner tank of solid neon (at -257 C), and a
surrounding tank of solid carbon dioxide ( at -150 C). The entire
instrument was kept under vacuum during ground test and launch, and
then exposed to the vacuum of space when the telescope door was opened
on-orbit. The cryogens which slowly evaporated as they cooled the
instrument were designed to last about 19 months on orbit, allowing
CLAES to make scientific measurements from October 1, 1991 through May
5, 1993, when the cryogens finally evaporated and the instrument
warmed up. CLAES weighs approximately 2600 lbs, is approximately 9 ft
long and 4 ft in diameter, and used an average of about 25 watts of
power on orbit.
The next images show CLAES and the various other instruments
integrated with the UARS spacecraft and undergoing prelaunch tests,
and launch of the UARS on space shuttle Discovery.
UARS Instruments undergoing pre-launch tests
UARS Launch by the shuttle Discovery
Space Shuttle Discovery carrying UARS was launched on September 12, 1991 from Kennedy Space Flight Center. UARS was released to orbit on September 15, 1991, and CLAES began scientific observations of the earth's upper atmosphere October 1, 1991.
Observing geometry and global coverage
CLAES looked out from one side of the UARS at a fixed 90 deg angle to the velocity vector and was mostly pointed above the earth horizon (or "limb") to observe the upper atmosphere between about 10 and 60 km above the surface. This is the region encompassing the Stratosphere and the lower Mesosphere, as shown in the next drawing.
CLAES viewing geometry in orbit
The instrument used an array of detectors providing 20 "footprints" at
the earth limb between the 10 and 60 km altitude levels, each one
separated by 2.5 km to provide vertical profiles of each gas with this
vertical resolution. The UARS orbit has a 57 deg inclination, and this
allowed CLAES to view to 80 degrees latitude in one hemisphere, and to
34 degrees in the other. This orbit precesses 180 degrees in about 36
days relative to the Sun. Because of this, UARS is rotated 180 degrees
in yaw approximately every 36 days such that one side of the spacecraft
is kept facing away from the sun and instruments on this side, including
CLAES, stay cool. This means that CLAES alternately views from 34N to
to 80S or 34S to 80N in 36 day periods.
The next diagram shows the latitude/longitude tracks of the center of
the CLAES field of view at the earth limb for one day for a
north-looking case. A south-looking case would look essentially the
same, with the latitude limits changed to 34N and 80S.
CLAES Measurement tracks in one day
There are 15 orbits per day, and since CLAES takes about 65 seconds for a complete measurement cycle it acquires about 1300 measurement sets per day. As the figure shows, measurement tracks are particularly dense near the polar regions, allowing for good coverage of ozone-related chemistry and dynamics over the Arctic and Antarctic.
CLAES produced a 19-month global database showing the vertical distributions of important ozone-layer gases in the stratosphere and their variation with time of day, season, latitude, and longitude. With the other UARS instruments, these data are contributing to a better understanding of the processes that control ozone depletion in the middle and northern latitudes, including volcanic effects, as well as the seasonal development and breakup of the Antarctic ozone hole.
