Lockheed Martin, based in Littleton, has received a $509.8 million modification (P00181) to a previously awarded contract (FA8807‐18‐C‐0009) for Global Positioning System III Follow-On Space Vehicles 21 and 22, according to the U.S. Department of Defense. This modification raises the total cumulative value of the contract to $4.1 billion. Work under this contract will take…