$5-10$ A pulsed radar source is at rest at the point $\backslash ($ x$=0\backslash ).$ A large meteorite moves with constant velocity $\backslash ($ v $\backslash )$ toward the source; it is at the point $\backslash ($ x$=-$l $\backslash )$ at $\backslash ($ t$=0\backslash ).$ A first radar pulse is emitted by the source at $\backslash ($ t$=0\backslash ),$ and a second pulse at $\backslash ($ t$=$t$\_\{0\}\backslash$left$($t$\_\{0\}$