  Time-delay interferometry (TDI) is a data processing technique for
space-based gravitational-wave detectors to create laser-noise-free
equal-optical-path-length interferometers virtually on the ground. It relies on
the interspacecraft signal propagation delays, which are delivered by
intersatellite ranging monitors. Also delays due to onboard signal propagation
and processing have a nonnegligible impact on the TDI combinations. However,
these onboard delays were only partially considered in previous TDI-related
research; onboard optical path lengths have been neglected so far. In this
paper, we study onboard optical path lengths in TDI. We derive analytical
models for their coupling to the second-generation TDI Michelson combinations
and verify these models numerically. Furthermore, we derive a compensation
scheme for onboard optical path lengths in TDI and validate its performance via
numerical simulations.
