The Microwave Limb Sounder (MLS), one of the key instruments on the Upper Atmospheric Research Satellite, (UARS) is currently playing an important role in understanding the chemistry of the earth's upper atmosphere and, in particular, monitoring of the critical ozone layer that protects the earth's surface from damaging effects of ultra-violet radiation from the sun. A follow-on instrument, EOS-MLS, will fly as part of the NASA Office of Mission to Planet Earth's (MTPE's) Earth Observing System (EOS) program and will provide continued monitoring of the ozone layer and other key chemical species in order to enable understanding of the long term dynamics of the earth's upper atmosphere.
As a cost saving measure, EOS-MLS will be similar in may ways to its predecessor UARS-MLS. Many of the key systems will be rebuilds of the previous hardware designs. There are, however, two key differences. Advances in microwave detector technology, developed through research supported as part of the NASA Office of Advanced Concepts and Technology (OACT) Submillimeter Sensors Program will enable EOS-MLS to carry more and higher frequency channels that will enable improved monitoring of upper atmospheric chemistry. The inclusion of the higher frequency channels and a more dynamic thermal environment on orbit will place more demands on the precision of the primary reflector.
The EOS-MLS will have a 1 square meter, elliptical shaped (1.6m x 0.8m), vertically scanned primary reflector for collecting the atmospheric signals. UARS-MLS used an aluminum reflector for this purpose. The requirements for higher surface accuracy, greater thermal stability and rapid scanning are major cost and weight drivers for the EOS-MLS instrument. Advances over the last several years in large (1 meter class) precision composite mirrors as part of the NASA OACT Precision Segmented Reflector (PSR) Program have enabled the EOS-MLS team to replace the aluminum reflector with a composite version in their baseline configuration. Replacing the 20 kilogram aluminum reflector used on UARS-MLS with a less than 10 kilogram composite version will result in substantial savings in weight, power and mechanical complexity of the scan system. In addition, replacing the aluminum with a composite material that has similar thermal conductivity and heat capacity but a factor of about 100 lower coefficient of thermal expansion will result in significantly less figure and focus change as the reflector moves into the sunlight and warms up by about 20 degrees C. This change in materials is necessary to meet the 8.5 micrometer (root-mean-square) surface accuracy requirement for the primary reflector. The implementation of a precision composite reflector in the EOS-MLS package represents the first application of this important technology developed by the PSR program. In the future, this technology will have wide applications and long range benefits for instruments from the microwave to the infrared wavelengths.
The NASA Office of Mission to Planet Earth, in collaboration with the Office of Advanced Concepts and Technology, hopes to develop a demonstration antenna test-bed for EOS-MLS including the primary reflector, secondary mirror, and appropriate backup structure meeting the design requirements for the mission. A series of functional tests on the test-bed would validate antenna performance including beam pattern, stability, and thermal cycling. This test-bed would then be available for system level testing of the front-end components and might serve as the basis for an engineering model of the flight antenna.