  We study the time evolution of the impact probability for synthetic but
realistic impacting and close approaching asteroids detected in a simulated
all-sky survey. We use the impact probability to calculate the impact warning
time as the time interval between when an object reaches a Palermo Scale value
of -2 and when it impacts Earth. A simple argument shows that warning time ~
D^x with the exponent in the range [1.0,1.5] and our derived value was
$x=1.3\pm0.1$. The low-precision astrometry from the single simulated all-sky
survey could require many days or weeks to establish an imminent impact for
asteroids >100m diameter that are discovered far from Earth. Most close
approaching asteroids are quickly identified as not being impactors but a
size-dependent percentage, even for those >50m diameter, have a persistent
impact probability of >10^{-6} on the day of closest approach. Thus, a single
all-sky survey can be of tremendous value in identifying Earth impacting and
close approaching asteroids in advance of their closest approach but it can not
solve the problem on its own: high-precision astrometry from other optical or
radar systems is necessary to rapidly establish an object as an impactor or
close approacher. We show that the parallax afforded by surveying the sky from
two sites is only of benefit for a small fraction of the smallest objects
detected within a couple days before impact: probably not enough to justify the
increased operating costs of a 2-site survey. Finally, the survey cadence
within a fixed time span is relatively unimportant to the impact probability
calculation. We tested three different reasonable cadences and found that one
provided ~10x higher (better) value for the impact probability on the discovery
night for the smallest (10m diameter) objects but the consequences on the
overall impact probability calculation is negligible.
