  We discuss a Galaxy-wide coordinated signaling scheme with which a SETI
observer needs to examine a tiny fraction of the sky. The target sky direction
is determined as a function of time, based on high-precision measurements of a
progenitor of a conspicuous astronomical event such as a coalescence of a
double neutron star binary. In various respects, such a coordinated scheme
would be advantageous for both transmitters and receivers, and might be widely
prevailing as a tacit adjustment. For this scheme, the planned space
gravitational-wave detector LISA and its follow-on missions have a potential to
narrow down the target sky area by a factor of $10^{3\textit{-}4}$, and could
have a large impact on future SETI experiments.
