  The LISA mission is the future space-based gravitational wave (GW)
observatory of the European Space Agency. It is formed by 3 spacecraft
exchanging laser beams in order to form multiple real and virtual
interferometers. The data streams to be used in order to extract the large
number and variety of GW sources are Time-Delay Interferometry (TDI) data. One
important processing to produce these data is the TDI on-ground processing
which recombines multiple interferometric on-board measurements to remove
certain noise sources from the data such as laser frequency noise or spacecraft
jitter. The LISA noise budget is therefore expressed at the TDI level in order
to account for the different TDI transfer functions applied for each noise
source and thus estimate their real weight on mission performance. In order to
derive a usable form of these transfer functions, a model of the beams, the
measurements, and TDI have been developed, and several approximation have been
made. A methodology for such a derivation has been established, as well as
verification procedures. It results in a set of transfer functions, which are
now used by the LISA project, in particular in its performance model. Using
these transfer functions, realistic noise curves for various instrumental
configurations are provided to data analysis algorithms and used for instrument
design.
