Waltham-based defense contractor Raytheon Corp. has received a $20.7 million contract to build 862 decoy units for Air Force warplanes that throw off the sensors in enemy attackers’ missiles. The units are towed behind F-16 and F/A-18 fighter-jets, and B-1B bombers. Work on decoys will be carried out at the company’s Electronic Warfare Systems organization, located in Goleta, CA. Raytheon has a significant presence in Marlboro.