All light, whether X-rays, visible light, or radio waves, is part of the electromagnetic spectrum. The difference is in the wavelength (or frequency or energy) of the light. Our society knows how to measure visible light and radio waves, but in between are the millimeter and sub-millimeter regions, where the wavelengths are too short for radio and too long for visible light techniques. Many molecules, including ozone and many chemicals that destroy ozone, emit light at unique frequencies in this part of the spectrum. Using these emissions, we can measure the amount of these chemicals in the earth's protective ozone layer.
Since 1974, The NASA Office of Space Access and Technology (OSAT, formerly know as the Office of Advanced Concepts and Technology, and before that the Office of Aeronautics and Space Technology) and the NASA Office of Mission to Planet Earth (MtPE) have jointly developed the Microwave Limb Sounder (MLS) instrument. OSAT has developed critical technology elements including the antenna, mixer and electronic components while MtPE was responsible for the instrument development. On several occasions, scientists have flown a balloon version of the instrument, obtaining science data and demonstrating the technology.
In 1991, NASA launched the MLS instrument on the Upper Atmosphere Research Satellite (UARS). The UARS MLS has observed atmospheric thermal emissions from chlorine monoxide (ClO), ozone (O3), water vapor (H2O), sulfur dioxide (SO2), and molecular oxygen (O2), at frequencies of 63, 183 and 205 GHz. Continuous day and night measurements have given global maps of the vertical distribution of these molecules. The vertical resolution is approximately 3 km. The instrument has demonstrated ozone measurements with one percent accuracy.
The UARS MLS uses high spectral resolution heterodyne radiometers. The instrument combines the signal from the atmosphere with reference frequencies from on-board local oscillators. This creates beat or difference signals that are at much lower frequencies. The instrument can handle these beat frequency signals with conventional electronics. The specific, OSAT supported technologies involved in the UARS MLS include the local oscillator injector, the dual mode feed-horn, quasi-optical filter technology, and gallium arsenide (GaAs) Schottky diode development.
OSAT continues to develop technology for the next generation MLS for the Earth Observing System (EOS), pushing the upper limit of the frequency to 600 GHz and beyond. A key technology challenge is the measurement of the hydroxyl radical (OH), which can only be measured at frequencies of 2.5 THz (2,500 GHz) or higher. This is more than ten times higher in frequency (shorter in wavelength) than the highest frequency of the UARS MLS.
In a related development, advances over the last several years in large (1 meter class) precision composite mirrors as part of the NASA OSAT Precision Segmented Reflector (PSR) Program have enabled the EOS-MLS team to replace the aluminum reflector with a composite reflector in their baseline configuration.
For more information contact Gordon Johnston.
Gordon.Johnston@hq.nasa.gov