  An inverse obstacle scattering problem for the wave governed by the Maxwell
system in the time domain, in particular, over a finite time interval is
considered. It is assumed that the electric field $\mbox{\boldmath $E$}$ and
magnetic field $\mbox{\boldmath $H$}$ which are solutions of the Maxwell system
are generated only by a current density at the initial time located not far a
way from an unknown obstacle. The obstacle is embedded in a medium like air
which has constant electric permittivity $\epsilon$ and magnetic permeability
$\mu$. It is assumed that the fields on the surface of the obstacle satisfy the
impedance-or the Leontovich boundary condition $\mbox{\boldmath
$\nu$}\times\mbox{\boldmath $H$} -\lambda\,\mbox{\boldmath
$\nu$}\times(\mbox{\boldmath $E$}\times\mbox{\boldmath $\nu$})=\mbox{\boldmath
$0$}$ with $\lambda$ an unknown positive function and $\mbox{\boldmath $\nu$}$
the unit outward normal. The observation data are given by the electric field
observed at the same place as the support of the current density over a finite
time interval. It is shown that an indicator function computed from the
electric fields corresponding two current densities enables us to know: the
distance of the center of the common spherical support of the current densities
to the obstacle; whether the value of the impedance $\lambda$ is greater or
less than the special value $\sqrt{\epsilon/\mu}$.
