  Realistic models of the Galactic double white dwarf (DWD) population are
crucial for testing and quantitatively defining the science objectives of the
Laser Interferometer Space Antenna (LISA), a future European Space Agency's
gravitational-wave observatory. In addition to numerous individually detectable
DWDs, LISA will also detect an unresolved confusion foreground produced by the
underlying Galactic population, which will affect the detectability of all LISA
sources at frequencies below a few mHz. So far, the modelling of the DWD
population for LISA has been based on binary population synthesis (BPS)
techniques. The aim of this study is to construct an observationally driven
population. To achieve this, we employ a model developed by Maoz, Hallakoun and
Badenes (2018) for the statistical analysis of the local DWD population using
two complementary large, multi-epoch, spectroscopic samples: the Sloan Digital
Sky Survey (SDSS), and the Supernova Ia Progenitor surveY (SPY). We calculate
the number of LISA-detectable DWDs and the Galactic confusion foreground, based
on their assumptions and results. We find that the observationally driven
estimates yield 1) 2 - 5 times more individually detectable DWDs than various
BPS forecasts, and 2) a significantly different shape of the DWD confusion
foreground. Both results have important implications for the LISA mission. A
comparison between several variations to our underlying assumptions shows that
our observationally driven model is robust, and that the uncertainty on the
total number of LISA-detectable DWDs is in the order of 20 per cent.
