" When the plane approached the field, three team members were to position themselves on it in the shape of an inverted L. The pilot would be flying at a low altitude—usually no more than 1,500 feet—to enable him to make out the distinguishing features of the surrounding countryside. As he came near, he would flash a prearranged Morse code signal with his craft’s signal light. If the head of the reception committee, using a flashlight, responded with a prearranged signal of his own, the pilot would prepare to land; if he didn’t see the correct signal, he was under orders to return to base. "

― Lynne Olson , Madame Fourcade's Secret War: The Daring Young Woman Who Led France's Largest Spy Network Against Hitler