Title: Thermal stability of a 4 meter primary reflector for the Scanning Microwave Limb Sounder
Primary Author: Cofield, Richard 
Organization: Jet Propulsion Laboratory
Co-Author(s): Eldon P. Kasl, DR Technologies, Inc.

We describe the fabrication and thermal-stability analysis and test of a composite demonstration model of the Scanning Microwave Limb Sounder (SMLS) primary reflector, having full 4m height and 1/3 the width planned for flight. SMLS is a space-borne heterodyne radiometer which will measure pressure, temperature and atmospheric constituents from thermal emission between 180 and 660 GHz. Current MLS instruments in low Earth orbit scan pencil-beam antennas (sized to resolve about one scale height) vertically over the atmospheric limb. SMLS, planned for the Global Atmospheric Composition Mission of the NRC Decadal Survey, adds azimuthal scanning for better horizontal and temporal resolution and coverage than typical orbit spacing provides. SMLS 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 highly astigmatic in the horizontal, having a beam aspect ratio ~1:20. Symmetry about the nadir axis ensures that beam shape is nearly invariant over +/-65 degrees of azimuth. The antenna feeds a low-noise SIS receiver whose FOV is swept over the reflector system by a small scanning mirror. Using finite-element models of antenna reflectors and structure, we evaluate thermal deformations and the resulting optical performance for 4 orbital environments and isothermal soak. We compare deformations with photogrammetric measurements made during wide-range (ambient+[-97,+75]degrees C) thermal soak tests of the primary in a chamber. This range exceeds predicted orbital soak ranges by large factors, implying in-orbit thermal stability of 0.21 micron ms/degree C, which meets SMLS requirements.