Pre-Phase A Report - Lisa - Nasa

4.1 Sensitivity 81
averaged transfer function
1
10 −1
10 −2
10 −3
ϑ=0 envelope
10 −2
10 −1
frequency [Hz]
declination δ = 0°
δ = 30°
δ = 60°
δ = 90°
Figure 4.2 Magnitude of the normalised LISA transfer function in dependence
upon frequency after averaging over the orbit and all possible source azimuths
and polarisations, shown for source declinations of 0 ◦ (red), 30 ◦ (green),
60 ◦ (blue) and 90 ◦ (yellow). For comparison, also shown are the envelope for
normal incidence (black), the same line reduced by √ 5 (black, broken line)
and the transfer function proper for such a case (white).
optimal directions and polarizations. The measurement time is generally assumed to be
1 year, even though the lifetime of LISA would make longer measurement times possible.
It is with these assumptions that the sensitivity curves in the figures of Section 1.2 have
been drawn.
4.1.3 The noise types.
The sensitivity of LISA is determined by a wide variety of noise sources, and by the
degree to which their effects can be kept small. There are two main categories of such
sensitivity-limiting noise effects:
• A first one that fakes fluctuations in the lengths of the optical paths, which we
will call optical-path noise. This category of disturbances includes different types of
noise discussed earlier, such as shot noise and beam pointing instabilities. These
contributions will, in general, be uncorrelated, and therefore adding quadratically
the four contributions from the four passes gives the final (apparent) fluctuation in
optical path difference.
• The second category is due to forces (or accelerations) acting on the proof masses.
The drag-free environment will effectively shield the proof masses from outside influences,
but some residual acceleration noise will remain. These accelerations will
lead to displacement errors δx of the proof masses, and for each pass these errors
have to be counted twice to arrive at the (real) fluctuation in optical path difference.
Corrected version 2.08 3-3-1999 9:33
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