Pre-Phase A Report - Lisa - Nasa

8.1 ELITE – European LISA Technology Demonstration Satellite 163
within the measurement bandwidth (MBW) of10 −4 to 10 −1 Hz. The requirements
along the other axes are relaxed by approximately two orders of magnitude.
2. In order to suppress the effects of motion-modulation of local (spacecraft-induced)
disturbance fields (magnetic, electrostatic, gravitational, etc.) and to suppress the
effects of optical-path fluctuations, the relative displacement between each proof
mass and the spacecraft, along the intereferometer axis, must be less than
5×10 −9 m/ √ Hz
within the MBW. This corresponds to a relative acceleration of
≈ 10 −13 ms −2 / √ Hz (at 10 −3 Hz) ,
which is seen to be two orders of magnitude above the requirement on the inertial
acceleration of the proof mass (item 1). This means that the spacecraft drag-free
control requirement (i.e. relative to the proof mass) is relaxed compared with the
requirement on the inertial isolation of the proof mass. This relaxation is made
possible by the optical arrangement whereby the light is effectively referenced to
opposing surfaces on each proof mass.
The corresponding relative displacement requirements along the non-sensitive axes
arise only from the need to suppress nonlinearities and to avoid sensor saturation
(e.g. of the op-amps in the capacitive circuits) which suggests that the displacements
must be less than
5×10 −6 m/ √ Hz
within the MBW. However, since each spacecraft contains two proof masses, and
since the interferometer axes for each arm are not parallel, the more rigid requirements
will prevail in two planar directions, and only the out-of-plane direction will
be relaxed.
3. The spacecraft attitude must be precisely controlled for two reasons: (i) to ensure
that the receiving spacecraft remains locked-on to the same portion of the incoming,
non-perfectly-spherical optical wavefront; and (ii) to ensure that the attitude motion
does not yield excessive translational motion at the location of each proof mass
(which are spatially separated by tens of centimetres). For nominal parameters,
(i) imposes that the pointing must be controlled to within
θdc δθ ≤ 3×10 −16 rad 2 / √ Hz ,
where θdc represents the dc pointing error, and δθ represents the pointing jitter
across the MBW. Assuming that θdc canbecontrolledto≈ 3×10 −8 rad (depending
on photodiode drift, etc.), this imposes a jitter requirement of
δθ ≤ 10 −8 rad/ √ Hz
within the MBW. Depending on the actual geometrical layout, condition (i) will be
more or less demanding than condition (ii). For example, assuming that the proof
masses are spatially separated by 0.2 m, and that the drag-free null passes through
Corrected version 2.08 3-3-1999 9:33