Improved Stratospheric And Mesospheric Sounder via WWW

The ISAMS home page on the WWW is "http://www.atm.ox.ac.uk/main/". The UARS satellite is in an orbit inclined at 57 degrees (?TBD) to the equator, and which precesses 5 degrees longitude per day relative to the sun. Thus, for one day's data, local solar time around each orbit is an approximately constant function of latitude (differing by 12 hours for ascending and descending nodes) but over a period of 72 days the entire diurnal cycle is sampled at each latitude. Thermal stability requires that one side of the satellite (+Y) is never illuminated by the sun. To maintain this situation in an orbit that precesses relative to the sun (i.e. not sun-synchronous), the satellite has to be yawed through 180 degrees approximately every 36 days. ISAMS is a limb sounder, which, at the satellite altitude of 600km (?TBD), places the tangent point at 23 degrees from the orbital track. When the satellite flies in the +X direction (defined as forwards), viewing to the (usual) anti-sun side (+Y) gives coverage from 80N to 34S. Conversely, when the satellite yaws to fly in the -X direction (backwards), the coverage viewing to +Y is from 34N to 80S. ISAMS also has the ability, when the sun-satellite geometry is favourable, to view to the -Y side for parts of orbits, increasing the potential coverage to 80N-80S for either satellite flight direction. Spectral selection is achieved by pressure-modulator cells (PMCs) (Taylor, 1983) whose pressures, therefore transmission characteristics, may be altered in flight. The various observing geometries, plus the ability to program the scan pattern and PMC pressures, lead to a large number of operating modes for ISAMS, which may be characterised by some combination of: (1) PMC Cell Pressure in each of the 8 Cells (2) Anti-Sun/Sun side viewing (+/- Y side of spacecraft) (3) Satellite Direction (+/- X direction of motion) (4) Day/Night at tangent point (5) Ascending/Descending node (6) Limb Scan program Whether the tangent point is illuminated or not does not affect the operation of the ISAMS instrument, but it is convenient to add it to this list.