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Abstract

Time-delay interferometry (TDI) is a data processing technique for LISA
designed to suppress the otherwise overwhelming laser noise by several orders
of magnitude. It is widely believed that TDI can only be applied once all phase
or frequency measurements from each spacecraft have been synchronized to a
common time frame. We demonstrate analytically, using as an example the
commonly-used Michelson combination X, that TDI can be computed using the raw,
unsynchronized data, thereby avoiding the need for an initial synchronization
processing step and significantly simplifying the initial noise reduction
pipeline. Furthermore, the raw data is free of any potential artifacts
introduced by clock synchronization and reference frame transformation
algorithms, which allows to operate directly on the MHz beatnotes. As a
consequence, in-band clock noise is directly suppressed as part of TDI, in
contrast to the approach previously proposed in the literature (in which large
trends in the beatnotes are removed before the main laser-noise reduction step,
and clock noise is suppressed in an extra processing step). We validate our
algorithm with full-scale numerical simulations that use LISA Instrument and
PyTDI and show that we reach the same performance levels as the previously
proposed methods, ultimately limited by the clock sideband stability.