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Introduction to the new FUSE target visibility tool: VIZ
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Updated 1-Aug-2002 (B-G)
Updated 15-Jul-2003 (B-G)
Following two reaction wheel failures in late 2001, the FUSE attitude control
system was significantly modified to permit the continuation of the mission.
The new hybrid pointing system uses the two remaining reaction wheels plus the
three-axis magnetic torquer bars (MTB) to control the satellite's slewing and
pointing. Because the MTBs are not strong enough to allow full torque authority
for all combinations of time and target position, the instantaneously
available sky is more restricted than in the nominal attitude control
configuration. The FUSE project at JHU has developed a software tool ("VIZ")
which allows users to evaluate the availability of any target for stable pointing
control.
Due to ongoing development of the new attitude control system, the mission
capabilities in this area are dynamic and further
improvements are expected. Because of this, several of the
spacecraft and scheduling parameters in VIZ may be modified by the user.
The defaults in VIZ reflect our best estimate of what the
attitude control and scheduling parameters will be at the beginning of cycle 4.
Cycle 4 proposers who find that their target has less availability than the desired
exposure time requires, may want to explore whether a more
aggressive parameter set might provide sufficient availability to accomplish
the desired observations. If non-default parameters are used to determine
target visibility, these parameters must be cited and justified in the proposal.
When using the VIZ tool, note that:
- The possible exposure time is only a fraction of the visibility period
(~30%) since slews to and from the target, Earth occultations and SAA passes have to be
taken into account.
- The channel alignment for short observations at low declination may be problematic,
and that, therefore, the short wavelength (SiC) throughput may be impaired.
The main operational and scheduling parameters of the spacecraft are the anti-Sun
angle (beta), RAM zone avoidance, Moon avoidance, ROLL angle, torque authority and
reaction wheel angular momentum management. Of these, the imposed constraints of
the Moon avoidance have not changed from previous cycles. The nominal limits on
beta angle also remain the same (30°-95°). However, for targets with limited availability, lower
than nominal beta angles may be needed and may be possible to implement. If such
observations can be performed, it is likely that only LiF1 coverage can be guaranteed.
Parameters, still under study, are:
RAM avoidance angle: Default in VIZ: 10 degrees
At present (August 1, 2002), the FUSE project is implementing a decrease of the
RAM avoidance angle from 20° to 15°. We expect that by the start of
cycle 4, with the additional time since Solar maximum, the RAM avoidance will be reduced to
10°. Even at this angle, incoming atmospheric particles will still hit the
baffles. However, at lower angles, some particles may strike the mirrors directly.
As we evaluate the atmospheric density at the FUSE altitude with time and the
possible impact on the optics, further reductions in the RAM avoidance angle may
become possible.
Improved momentum unloading algorithm: Default in VIZ: Optimized unloading
The MTBs are used both in direct attitude control and to spin down the remaining
reaction wheels (unloading angular momentum). At the present time, the onboard software
is not optimized to perform these two functions concurrently. An improved algorithm is under
development which is expected to be ready for uplink to the spacecraft in October, 2002.
We expect to have this improvement fully operational before the start of cycle 4.
Spacecraft ROLL offsets: Default in VIZ: Nominal ROLL
As the torque authority is a vector entity, the ROLL of the spacecraft can influence the
instantaneous sky availability. However, non-nominal ROLL angles impact the thermal
environment of the instrument and in particular for the Fine Error Sensor (FES) camera. Studies are under
way to quantify the extent to which non-nominal ROLL angles can be used. It is likely
that the limits on such offset ROLLs will be within -10° to +20°. Offset ROLL angles, in general, have
a modest impact on target availability.
Partially stable orbits: Default in VIZ: Not implemented
It is possible that orbits where torque authority is only lost while the target
is occulted by the Earth may become usable for observations. This will be studied
by the FUSE project in the coming months. At the present time we do not have adequate
information to include this , but we do not expect to have implemented such
a possible modification by the beginning of cycle 4. This is also not supported
by VIZ at the present time.
For questions or comments, please contact FUSE User Support at
fuse_support@pha.jhu.edu.
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