The Scanning Microwave Limb Sounder (SMLS) is a space-borne heterodyne radiometer which will measure
pressure, temperature and atmospheric constituents from thermal emission in [180,680] GHz. SMLS, planned for
the NRC Decadal Survey's Global Atmospheric Composition Mission, uses a novel toric Cassegrain antenna to
perform both elevation and azimuth scanning. These will provide better horizontal and temporal resolution and
coverage than were possible with elevation-only scanning in the two previous MLS satellite instruments. SMLS is
diffraction-limited in the vertical plane but highly astigmatic in the horizontal (beam aspect ratio ~1:20). Nadir
symmetry ensures that beam shape is nearly invariant over ±65° azimuth. A low-noise receiver's FOV will be
swept over the reflector system by a small azimuth-scanning mirror. We describe the fabrication and thermalstability
test of a composite demonstration primary reflector, having full 4m height and 1/3 the width planned
for flight. Using finite-element models of reflectors6 and structure, we evaluate thermal deformations and optical
performance for 4 orbital environments and isothermal soak. We compare deformations with photogrammetric
measurements made during soak tests in a chamber. The test temperature range exceeds predicted orbital ranges
by large factors, implying in-orbit thermal stability of 0.21 micron rms/°C; this meets SMLS requirements.
The Microwave Limb Sounder instrument was launched aboard NASA's EOS AURA satellite in July, 2004. The overall scientific objectives for MLS are to measure temperature, pressure, and several important chemical species in the upper troposphere and stratosphere relevant to ozone processes and climate change. MLS consists of a suite of radiometers designed to operate from 118 GHz to 2.5 THz, with two antennas (one for 2.5 THz, the other for the lower frequencies) that scan vertically through the atmospheric limb, and spectrometers with spectral resolution of 6 MHz at spectral line centers. This paper describes the on-orbit commissioning the MLS instrument which includes activation and engineering functional verifications and calibrations.
The Microwave Limb Sounder (MLS) instrument, launched in July of 2004 on NASA's EOS Aura satellite, has been in its nominal science operating mode since August 2004. The objective of EOS MLS is to obtain measurements of atmospheric composition, temperature and pressure through observations of millimeter- and submillimeter-wavelength thermal emission as the instrument field-of-view is scanned through the atmospheric limb. The MLS instrument has completed activation, in-orbit calibrations have been performed leading to adjustments to radiometric calibration (Level 1) algorithms, a software upgrade was implemented for more robust operation of the laser local oscillator, and engineering performance trends have been established. This paper discusses the current status of the MLS instrument which now continuously provides data to produce global maps of targeted chemical species as well as temperature, cloud ice, and gravity wave activity. Performance trends are assessed with respect to characterization during initial on-orbit activation of the instrument, and with data from ground test verification prior to launch.
An advanced Microwave Limb Sounder (MLS), now in concept development for a potential future mission, is a space-borne heterodyne instrument to measure pressure, temperature, and atmospheric constituents from thermal emission between 120 and 2400 GHz. Previous MLS instruments used pencil-beam antennas sized to resolve ~1 vertical scale height. Current atmospheric models need better horizontal resolution than orbit spacing provides. To meet these needs, a new antenna concept combines the wide scan range of the parabolic torus with unblocked offset Cassegrain optics. The resulting system is diffraction-limited in the vertical plane but extremely astigmatic, with beamwidths 0.13×2.5°. Nadir axis symmetry ensures that this Beam Aspect Ratio (BAR) is invariant over ±33 degrees of azimuth. The antenna can feed either an array of receivers or multiplexed low-noise receivers whose FOVs are swept by a small scanning mirror. We describe 3 stages of antenna design: First, using a paraxial-optics method, we choose conic profiles given vertical resolution orbit geometry, then develop the surfaces by nadir axis rotation, matching axisymmetric feeds to the BAR. A ray-trace program validates the design and generates alignment and deformation tolerances. Finally, a physical optics analysis verifies reflector surface currents and radiation patterns.
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