The GPS system is thought to have at least one backup units associated with each constellation thus orbit changes can make a fairly quick fix to the constellation should a satellite go down. Control information is classified, however, since the signals are also publicly receivable and decoded, it is not that difficult to determine where each constellation resides for a corresponding region on the planet's surface, making the system an obvious target for anti-satellite warfare during wartime.
For this reason, it is also thought that the military can real time control the system so as to operate for brief periods of time, and it is also thought that the constellations can be moved if necessary (however this is a big stretch because there is a finite amount of propellant aboard the satellites ...if any at all).
A GPS receiver (public models avaialable at $150 street price today) is needed to make use of the system, thus during the Gulf War, many vehicles were quickly retrofitted to accept either a military quality system or a public system (the military system is thought to carry the navigation info in encrypted form).
Today, aircraft receivers are becoming widely used in both commercial and private flight opertions, adding both safety and tracking capabilities never before experienced in the flying world.
Several small companies have made their fortunes on the military sponsered programs (Trimble Navigation is a good example), demonstrating the profitability of a good idea especially when it is allowed to migrate from the military to commercial usage (like many of the navigational systems for aircraft and sea going vessels today!).
According to Air Force Magazine 1, the NAVSTAR satellites are 5 ft in width and 17 ft, 6in in length with solar panels unfurled; and weigh in at about 1,860 lbs once reaching orbit.
The same entry in the magazine cites the performance as "...time can be figured to within one-millionth of a second, velocity within a fraction of a mile an hour, and location to within a few feet. The constellations orbit the earth every 12 hours. The last of the required constellations for worldwide coverage was completd in July of 1995 and consists of 24 satellites (26 have been launched).