Quote from : Star Wars: Episode VI - Return of the Jedi Movie

The Emperor: Everything that has transpired has done so according to my design. Your friends, up there on the sanctuary moon, are walking into a trap, as is your Rebel fleet. It was *I* who allowed the Alliance to know the location of the shield generator. It is quite safe from your pitiful little band. An entire legion of my best troops awaits them. Oh, I'm afraid the deflector shield will be quite operational when your friends arrive.