BLEU knew that an ideal system would require components based on the ground and in aircraft. The former system would have to consist of a signal without the land use problems of the cable-based system or the accuracy issues of ILS. All incoming aircraft would need to be equipped with a sensor to receive the signal, a super-precise altimeter, and a reliable autopilot.[1] The barometric altimeters in use at the time were only capable of providing a rough estimate of altitude, so BLEU developed a plane-based radar altimeter to complement its beam so planes knew when to flare up for landing.[14]