  We present the selection algorithm and anticipated results for the Time
Domain Spectroscopic Survey (TDSS). TDSS is an SDSS-IV eBOSS subproject that
will provide initial identification spectra of approximately 220,000
luminosity-variable objects (variable stars and AGN) across 7,500 square
degrees selected from a combination of SDSS and multi-epoch Pan-STARRS1
photometry. TDSS will be the largest spectroscopic survey to explicitly target
variable objects, avoiding pre-selection on the basis of colors or detailed
modeling of specific variability characteristics. Kernel Density Estimate (KDE)
analysis of our target population performed on SDSS Stripe 82 data suggests our
target sample will be 95% pure (meaning 95% of objects we select have genuine
luminosity variability of a few magnitudes or more). Our final spectroscopic
sample will contain roughly 135,000 quasars and 85,000 stellar variables,
approximately 4,000 of which will be RR Lyrae stars which may be used as outer
Milky Way probes. The variability-selected quasar population has a smoother
redshift distribution than a color-selected sample, and variability
measurements similar to those we develop here may be used to make more uniform
quasar samples in large surveys. The stellar variable targets are distributed
fairly uniformly across color space, indicating that TDSS will obtain spectra
for a wide variety of stellar variables including pulsating variables, stars
with significant chromospheric activity, cataclysmic variables and eclipsing
binaries. TDSS will serve as a pathfinder mission to identify and characterize
the multitude of variable objects that will be detected photometrically in even
larger variability surveys such as LSST.
