8 The LISA Low-Frequency Detector

UpdateUpdate 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*, 105, 90, 115]. The LISA/eLISA (Laser Interferometer Space Antenna) mission is currently (2015) in the design and development stage, with a technology demonstration mission, LISA Pathfinder, launched in December 2015. 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 GWs.¹³ 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., [116*]), to cancel the laser phase noises.¹⁴ 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.