Microwave-Like Instruments

**Figure: Limb Viewing Geometry of MLS.**

One of the most successful instruments which makes use of the THz spectral region for atmospheric remote sensing is the Microwave Limb Sounder (MLS). The First version of MLS was launched as a part of the Upper Atmospheric Research Satellite (UARS) in 1991 and operated successfully for more than a decade. It measured thermal emission from the limb of the Earth's atmosphere to retrieve vertical profiles of selected atmospheric gases, as well as temperature and pressure. A second generation of MLS, Microwave Limb Sounder Home designed to observe a broader range of molecular species launched on July 15, 2004 as a part of the AURA mission of NASA's Earth Observing System (EOS).

Figure: Block Diagram of the MLS.

Figure: Block Diagram of the Optical Multiplexer.

**Figure: Spectral Regions.** Spectral regions
down converted to the IF.

Figure: Limb Viewing Geometry of MLS shows the limb viewing geometry of MLS. A significant feature of MLS is that because it operates in the Submillimeter/Terahertz region, the natural thermal emission of the atmosphere provides the source of the observed radiation, whereas at infrared and shorter wavelength molecular species are ordinarily observed as absorptions in the solar radiation. This latter viewing geometry restricts observations to sun rise and sun set.

Figure: Block Diagram of the MLS.

. The mixers down-convert the spectral information to an ~ 2 - 20 GHz intermediate frequency (IF) where they are amplified and sent to multichannel filter banks.

Figure: Block Diagram of the Optical Multiplexer.

a simulated spectrum for the 640 GHz channel. To increase S/N a smaller antenna is used to feed separately the 2.5 THz radiometer from a smaller 0.25 meter antenna.

**Figure: Simulated Spectra.** Simulated spectra in the 640 GHz channel at three pressure alt-
itude/pressure points (upper 100 mb tangent point, middle 30 mb tangent point, and lower
10 mb tangent point).

Finally, in

Figure: Geophysical Measurement Capabilities. Geophysical measurement
capabilities by radiometer. The solid lines indicates profiles which can be obtained
from single observations or daily averages, the dotted lines represent measurements
which require zonal or other averages and circles goals for difficult measurements.
The dashed line for ClO indicates enhancements associated with polar winter vortices.

and by continuum contributions. At high altitudes the decreasing number density of the atmosphere with altitude finally dominates the advantages that come with narrower line-widths, and signal levels drop below the noise. Thus, the initial splitting of the photon stream in the optical multiplexer, subsequent mixing followed by amplification at intermediate frequencies, and the availability of multichannel filter banks, at the IF to simultaneously measure and record the atmospheric spectra result in an instrument of high resolution and large multiplex advantage. Because broadly tunable instruments in this spectral region are not yet possible, an important design consideration for these instruments has been a careful analysis of the spectral characteristics of all known atmospheric molecules. This analysis makes possible the selection of center frequencies for each channel which maximize the number of species that can be observed within the available IF and filter bank width. While many of the strong lines used for retrievals have been known to spectroscopists for some time, the elimination of the possibility of spectral coincidences from unknown lines, especially the weaker lines (excited vibrational states, isotopic species, high lying rotational states, etc.) of species abundant in the atmosphere (e.g. ozone or nitric acid) has been a significant spectroscopic challenge.