Pre-Phase A Report - Lisa - Nasa

2.8 The LISA concept 47
telescopes and 11 ◦ inclination of the orbits to the ecliptic, the optical axis of one of the
telescopes will pass within 15 ◦ of the sun roughly 40 % of the time. Earlier theoretical
studies of this problem indicated that a combination of multilayer UV and IR reflecting
filters plus a narrowband optical transmitting filter could reduce incident sunlight by a
sufficient amount, but such filters have not been designed in detail or constructed. The
problem of constructing such filters appears to be made even more difficult if they need
to be 30 cm in diameter, rather than the 15 cm diameter assumed in the earlier studies.
A third technological issue concerns the need for generating an extremely stable clock
frequency for use in cancelling out the Doppler shifts in the observed signals. For the
geocentric mission, the Doppler shifts vary with about 27 day period between plus and
minus 300 MHz. This is more than two orders of magnitude larger than the difference in
Doppler shifts for the two preferred arms of the LISA interferometer, for which the initial
orbit conditions are chosen to keep the Doppler shifts low, and a factor 20 higher than
for the third arm in LISA. Thus, while LISA can determine the phase noise in its Ultra
Stable Oscillators (USOs) to sufficient accuracy by fairly simple means, as discussed later,
this task is considerably more difficult for the geocentric mission.
For LISA, roughly 200 MHz sidebands generated from the USO are modulated onto the
laser beams, with roughly 10 % of the power in the sidebands. Measurements of phase
jitter in the beats between the sidetones and the carrier after transmission over an interferometer
arm determine the phase noise in the USO. However, for the geocentric mission,
two separate lasers with a difference frequency of perhaps 5 GHz probably would need to
be used. Thus, the number of lasers that must survive in at least four of the spacecraft
is doubled, since the accuracy of the results would be very strongly degraded if accurate
corrections for the Doppler shifts were not available. If optical modulators with 5 GHz
or higher frequencies and substantial sideband power are used instead of two separate
lasers, the efficiency and long term reliability of the modulators are much more significant
technological challenges than for the roughly 200 MHz modulators needed for LISA.
In view of the three important technological issues discussed above and the loss in sensitivity
for a geocentric mission if tighter requirements are not imposed on the inertial
sensors, we believe that the LISA approach of using heliocentric orbits should be preferred.
Cost is clearly a very important issue, but we expect that the main cost drivers
for a gravitational-wave mission will continue to be the design and construction of the
individual spacecraft and payloads, and in insuring the reliability of all of the systems
that have to work simultaneously in at least four of the six optical assemblies.
2.8 The LISA concept
2.8.1 Overview
Conceptually, the idea of implementing an interferometer in space is straightforward,
but the practical realisation requires an intricate blend of optical technology, spacecraft
engineering and control. For a start, the interferometer mirrors can not simply float freely
in space — they must be contained inside spacecraft. Nonetheless, they can be arranged
to be floating almost freely inside the spacecraft, protected from external disturbances by
Corrected version 2.08 3-3-1999 9:33