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.