In response to the anomaly experienced earlier this year (see
INTEGRAL POM June)
a scheduling strategy that minimises the accumulation
of angular momentum has been implemented. The goal is making operations
entirely free from the need for Reaction Wheel Biases (RWBs), and thus
thruster usage.
This new strategy involves spending approximately half the observing
time pointing to opposing directions in the sky in order to cancel out
as much as possible the accumulation of angular momentum caused by the
radiation pressure on the Solar panels. The strategy is enhanced during
the time that the spacecraft is within the Earth's radiation belts by
selecting a perigee passage attitude which makes use of the gravity
gradient torque close to Earth. This new approach is referred to as the
`Z-flip' strategy, in reference to the reversal of the Sun pointing axis.
The graph in the image shows the first in-orbit demonstration of the
Z-flip control of INTEGRAL's angular momentum, during Revolution 2243
(23-25 June). Solar photon pressure, acting most strongly on the Solar
panels, imposes an external torque on the spacecraft, anticlockwise in
the inertial frame, increasing the total angular momentum and the absolute
speed of the counteracting reaction Wheels 2 and 4 (left half of the graph).
The slew manoeuvre between two targets, about 155 degrees apart,
consisted of two slews separated by roughly 2 hours.
The angular momentum of Wheels 2 and 4 are effectively swapped by the
155 degrees slew, implying also a sign change of their momentum vector.
The external torques then drive the wheel speeds back towards zero from
the other side of the X-axis compared to before the slew (right half of
the graph). Wheel 3 drifts only slowly as there is no Solar radiation
pressure torque component; its momentum can be effectively controlled in
perigee using the gravity gradient torque.
At a typical wheel speed drift rate of 20 RPM/hour the Z-flip strategy
compensated for about 55 hours of Solar radiation torque induced drift
on Wheel 2 and for about 64 hours of Solar radiation torque induced
drift on Wheel 4.
To keep flexibility in target selection and observation duration
necessary for the execution of the observing programme, mission planning
is being changed from the per Revolution basis to the Z-flip approach
that considers several (3, or possibly more) Revolutions at a time. This
allows to balance the spacecraft angular momentum over a longer time
period and gives greater flexibility to the planning process.
