The U.S. Air Force recently discovered that, because of a flawed (and untested) software update, 86 military systems that use a GPS jamming feature, had their GPS service greatly degraded. A software patch was quickly developed and distributed. The incident is not unique. Upgrades, even though thoroughly tested, sometimes later prove to be flawed. This sort of thing isn't new. In the early days of World War II, most American submarine torpedoes proved to be flawed, and it took months of increasingly vociferous complaints from sub captains to even get the navy to investigate the possibility of a problem (which was very real, and took a while to fix). Two decades later, a design flaw in an American SLBM (sea launched ballistic missile) warhead, rendered most of those weapons useless until fixed. There are many similar examples, all over the world.
But GPS is different. GPS has become a vital component for U.S. combat forces, and several nations have developed and sell GPS jammers. The U.S. Air Force has equipped its GPS weapons with electronic and software features that help overcome this jamming. DARPA (Defense Advanced Research Projects Agency) is developing an even more powerful anti-jamming system that also solves a more common problem; weak GPS signals. When a GPS guided weapon goes after a target in a canyon, the GPS signals are often so weak that the guidance system must revert to the backup, and several times less accurate, inertial guidance system.
RSN (The Robust Surface Navigation) system uses math, statistics and modified antennae to take signals (GPS and non-GPS) and create accurate estimates to make up for the lost GPS data. If this substitute is judged, by the software, as more accurate than the backup system, it uses the RSN estimate, to maximize accuracy. Other signals can come from nearby aircraft, or navigation beacons set up on the ground. The RSN system would enable the U.S. to jam GPS signals in a combat zone where the enemy was using GPS guided weapons, but does not have RSN.
The lesson being learned is that you can't have too many backups, and protection from unexpected failures. It's Murphy's Law. That things fail when you least expect it, and in the worst possible way.