Boeing Co. said the Air Force has awarded the Chicago-based defense contractor a $138 million option to build three additional Global Positioning System, or GPS, satellites.

The satellites will be built at Boeing’s Satellite Development Center in El Segundo. The exercised option is the final option for three GPS satellites the company was already under contract to build an additional nine. The satellites will be part of the military’s NAVSTAR GPS joint program office. The first satellite in the constellation is supposed to be launched early next year, Boeing’s spokeswoman Diana Ball said.

“As the requirements emerge and as satellites age, new demand will emerge as is evident by the Air Force taking this option,” she said.

Boeing now constructs nearly all its unmanned spacecraft, which are 70 percent military and 30 percent civilian according to Ball, at the El Segundo plant. Boeing has made 43 GPS satellites since 1974. The El Segundo facility employees about 6,000 people.