$d\left[\mathrm{cm}\right]$	layer thickness
$c\left[\mathrm{cm}/\mathrm{ns}\right]$	speed of light propagation in a vacuum, $c=30\mathrm{cm}/\mathrm{ns}$
$t\left[\mathrm{ns}\right]$	two-way travel time between reflections from boundary surfaces
${\epsilon }_r[-]$	dielectric constant of the layer.