Tucson-based Raytheon Missile Systems has won a contract option worth $523 million to build 47 Standard Missile Block IB ballistic-missile interceptors.

The option awarded by the U.S. Missile Defense Agency is part of a $2.3 billion contract awarded to Raytheon in 2015 for the production of 52 interceptors, including production options for 2016, 2017 and 2018 for Raytheon to build up to 52 interceptors each year, Raytheon said.

The SM-3 is used aboard U.S. Navy ships as part of Aegis Ballistic Missile Defense system, which is designed to counter short- to intermediate-range ballistic missile threats.

The latest, IB version, which adds advanced targeting, propulsion and maneuvering capabilities, is deployed aboard some Aegis-equipped ships and is planned for a land-based system under construction in Romania.