The LISA Low-Frequency Detector

8 The LISA Low-Frequency Detector

Update Current Doppler tracking observations piggy-back on spacecraft mainly serving the planetary science community. Future low-frequency detectors could be dedicated GW missions – fully space-based – involving separated drag-free test masses [24

, 69 , 94 ]. The LISA (Laser Interferometer Space Antenna) mission is currently (2008) in the design and technology development stage. The three LISA sciencecraft will form an approximately equilateral triangle with nominal 5 $×$ 10⁹ m armlengths (time-variable by $∼$ 1% due to celestial mechanics). Six one-way laser-driven optical links between spacecraft pairs will monitor Doppler (or phase) fluctuations as the test masses respond to incident GWs¹¹ . The principal advantages to moving all the apparatus to space are that the environment is very stable and drag-free technology can be employed. The final noise level can then in principle be set by (very small) optical-path and proof mass noises [24 ]. LISA’s anticipated sensitivity is excellent: $∼$ 10^–23 for sinusoidal signals in a one year integration.

To reach the levels of the secondary optical-path and proof-mass noises, however, LISA must first cancel laser phase noise (which is otherwise overwhelming, $≃$ 160 dB larger than the secondary noises). Since LISA’s armlengths cannot be made equal and constant, conventional laser noise cancelling methods, e.g., Michelson interferometry, will not work. LISA will use a technique based on the transfer functions of signals and noises to the inter- and intra-spacecraft Doppler data called “Time-Delay Interferometry” (TDI; see, e.g., [109 ]), to cancel the laser phase noises¹² . TDI had its genesis in Doppler tracking where, as with LISA, time-of-flight of GWs and electromagnetic waves must be treated explicitly in the analysis.

Other ideas from spacecraft Doppler tracking may also be useful for LISA. As with spacecraft tracking, noises enter LISA’s TDI time series with well-defined transfer functions. The time-domain transfer function approach used in the analysis of spacecraft tracking is also used in the Synthetic LISA simulation package [115 ]. At least over the Fourier band where noise excitations are well-resolved (f $>$ 1/T₁, where T₁ is the one-way light time for a LISA arm $≃$ 16.7 s), time domain signatures should be useful in characterizing and isolating specific noise sources (analogy with Section 4) in the LISA data. Use of signal transfer functions for burst waves to classify data intervals as candidate signal-like or not (analogy of Section 5) may also be useful.