Pre-Phase A Report - Lisa - Nasa

166 Chapter 8 Technology Demonstration in Space
8.3.4 Drag-free control
Verification of the performance of the drag-free control system is a key element of the
ELITE mission concept. Since the spacecraft contains two inertial sensors (as with the
new LISA baseline), it cannot be manoeuvred to centre each proof mass within its housing
electrodes. Instead, there will be a single location (the “drag-free null”) within the
spacecraft where the acceleration is minimised by the drag-free control system. Forces
will have to be applied on the proof masses to compensate for any force gradients relative
to this drag-free null. ELITE will test various control strategies. For example, perhaps
the simplest approach is to have the spacecraft translation controlled to centre, in all
three components, on one of the two proof masses (i.e. locating the drag-free null at the
centre of one proof mass). The attitude of the spacecraft could then be controlled using
the information from the other proof mass. The other proof mass will need to have forces
applied to it to follow the primary proof mass. These forces would be applied electrostatically
by means of its sensing electrodes. The magnitude of the applied force in each
component would be comparable to the expected forces on the primary proof mass, i.e.
corresponding to an acceleration of order 10 −10 ms −2 . A key objective for ELITE will be
to demonstrate that these forces can be applied in such a way that the noise introduced
within the measurement band is acceptable. Also, the orientation of each proof mass
needs to be controlled to match the orientation of its housing at the nanoradian level.
8.4 ELITE Satellite design
The ELITE system will consist primarily of institute-provided payload elements (e.g. inertial
sensors, thrusters, lasers/optical package, flight computer, etc.). The spacecraft,
which comprises the structure, power, communication etc. subsystems will be ‘built
around the payload’ using commercially-available off-the-shelf components as far as possible.
In order to minimise costs, the satellite will be single-string (in terms of failure
modes) with limited functional redundancy and graceful degradation of all items.
8.4.1 Power subsystem
For the six-month duration, the GEO orbit can be chosen to be free of eclipses. Then the
nominal continuous power requirement is ≈ 150 W, including ≈ 25 W for battery charging
(for safe-mode).
8.4.2 Command and Data Handling
A main central processor unit (CPU, e.g. RAD 6000) will be responsible for all command
& data handling, and computation of control laws. A smaller backup processor will be
used for initial set-up and safe-modes. Command and data I/F between the processors and
all sensors and actuators will be via a 1553 bus with maximum throughput of 100 kbps.
RS 422 serial communication may also be an option for some payload items [TBD]. Control
of the 1553 bus can be software-switched between the processors. The main CPU will
3-3-1999 9:33 Corrected version 2.08