Another GPS satellite (a second generation model) was launched last week, bringing the number of GPS birds up to full strength (24 active and four spares.) Five GPS satellites have been "retired" (allowed to burn up in the atmosphere after breaking down, and one was lost in a failed launch.) The cost of building a GPS satellite and putting into orbit is about $100 million.
The GPS system was partially in service during the 1991 Gulf War, and that was enough to make life a lot easier for U.S. troops moving through the deserts of Arabia and Iraq. The system became fully operational in 1993, and a full 28 satellites were in place in 1995. The first generation of satellites have an expected life of 7.5 years. The second generation birds will last ten years and weight 2370 pounds each. The third generation satellites will weigh 3758 pounds and last 15 years. The U.S. Air Force Space Command runs the GPS satellite network.