What is the Air Force's Space Fence? And why will it cost $2B for an upgrade?

In 1961 the Air Force created a Space Fence that tracks objects in space and is used to protect spaceships and satellites from being struck by "high-speed orbital debris". The Space Fence — also known by its formal name, the Air Force Space Surveillance System — consists of three transmitters and six receivers that stretch across the southern U.S., using radio waves to paint a picture of a slice of space. The items it detected ranged from satellites and debris to meteors. As the article explains, this debris could cripple satellites and cause damage as much as what was reflected in last year's movie Gravity. Since then the only time the space fence was shut down was in September 2013 during the budget cuts. The space fence is 40% of the network's tracking system, and has not come back online since September. A C-band radar was relocated to keep track of the area the space fence covered, but that is only a band-aid. "The U.S. military also uses the data to offer a close approach warning service for owner-operators of the more than 1,000 active satellites in orbit," Brian Weeden of the Secure World Foundation writes. "In 2012, satellite operators conducted 75 maneuvers to avoid potential collisions as a result of those warnings." This year the Air Force budget includes a new and improved space fence for approximately $2B. Why so much? The current space fence could track approximately 23,000 items in space. The new system has expanded its S-Band radar to track 480,000 items in space. The new space fence is expected to be operational in 2017, but due to the delays in award, it may not be operational until 2018.