m CO 
band. These data
are used to maintain nadir pointing for the remote sensing instrument
suite. The set of 5.5 
tall triangular fields of view normally
straddle the limb, and cover quadrants 90 apart around the limb.
Horizon Science Experiment for Mars Global Surveyor
T.Z. Martin (JPL/Caltech)
The Mars Horizon Sensor Assembly on the MGS orbiter monitors the
orientation of the spacecraft relative to the limb by sensing
atmospheric emission in the 15 m CO band. These data
are used to maintain nadir pointing for the remote sensing instrument
suite. The set of 5.5 tall triangular fields of view normally
straddle the limb, and cover quadrants 90 apart around the limb.
As an engineering device, the MHSA benefits from Mars' atmosphere
being spatially bland at 15 m. However, these data will carry
information about the thermal state of the atmosphere, which is
subject to diurnal, seasonal, latitudinal, and dust-storm related
variations, as well as possible wave effects. The Mariner 7 IRS,
Mariner 9 IRIS, and Viking IRTM all demonstrated such variability.
The Horizon Science Experiment (HORSE) is intended to glean new
insight into atmospheric variation from the MGS horizon sensors,
with continuous data flow to the Earth in the engineering stream,
and a rapid buildup of spatial coverage. MHSA data will also be
used to monitor atmospheric thermal behavior during the aerobraking
of MGS in late 1997.