The U.S. Department of Defense is spending more research dollars on developing a jam resistant replacement, or backup (depending on who you talk to), for GPS. The best candidate is an improved INS (Internal Navigation System). These have existed for nearly a century but have gotten smaller, cheaper, and more reliable as electronic components did the same over the last half century. Basically INS uses three gyroscopes and three accelerometers to constantly measure changes in direction and changes in velocity. With that the INS will always know where it is in relation to its initial starting point (which can be obtained initially via unjammed GPS or older means). Miniature INS devices have long served as a backup for GPS guided weapons. But while GPS guidance can land a bomb or missile within 10 meters (32 feet) of a target, INS can only achieve 30 meter accuracy. GPS also has the advantage of not needing to have its exact position entered after the INS is turned on. On the upside, that means INS cannot be jammed or spoofed.
But now American researchers believe they have new concepts and technology that will greatly improve current INS accuracy and cost. Prototypes have been nearly as accurate as GPS and almost as small. Cost is still a factor, with the new INS still costing more than 10 times what GPS does. But this is all a big improvement over what has been available before. The new INS can now be used to monitor GPS and alert the operator that their GPS has either developed a problem or is being jammed. The new INS is also useful for some fast missiles that often lose their GPS signal as they maneuver. Another urgent chore for INS is to alert users that their GPS is being spoofed (sent a false signal that is luring the user away). Thus, even with the ability of anti-jamming tech to keep up with jammer technology, there is still a demand for a new INS.
Many Department of Defense navigation and electronics experts believe current anti-jamming efforts are sufficient to keep GPS viable, but the new INS technology has attracted a lot of attention because in the military a backup is something that is always appreciated because when equipment fails in combat it’s literally a matter of life or death. Meanwhile, the U.S. is building and testing more compact GPS anti-jamming systems for smaller (as small as 200 kg/440 pounds) UAVs. This is part of a program to equip all American UAVs, even the smallest ones, with more secure GPS. While all UAVs can be “flown” by the operator, the GPS makes it a lot easier for the operator to keep track of exactly where his UAV is at all times and sometimes the UAV is programmed to simply patrol between a series of GPS coordinates. If the GPS jams or fails the operator can usually use the video feed to find landmarks on the ground and bring the UAV back to where it can be seen and landed.
While American troops have not yet encountered much (if any) battlefield GPS jamming, the threat exists. The most tangible evidence of this comes from North Korea, which has long made, sold, and itself used GPS jammers. In 2012 North Korea attacked South Korea with a massive GPS jamming campaign. The jamming began in late April and continued for over two weeks. It took less than a day to confirm that the signal was coming from North Korea and was mainly aimed at the South Korean capital (Seoul). The jamming had little impact inside the city itself (the ground based jamming signal was blocked by buildings and hills) and was only noted by several hundred aircraft landing or taking off from local airports and over a hundred ships operating off the coast. In all these cases the ships and aircraft had backup navigation systems, which were switched on when GPS became unreliable. This is how navigation systems, especially those that rely on an external (satellite) signal are designed.
This was the third time North Korea has used GPS jamming against South Korea. For most of March, 2011, North Korea directed a GPS jamming signal across the border towards Seoul. A separate jammer has been directed at cell phone traffic. The GPS jamming signal could be detected up to a hundred kilometers south of the DMZ.
The usual response to GPS jamming is to bomb the jammers, which are easy to find (jamming is nothing more than broadcasting a more powerful version of the frequency you want to interfere with). But such a response could lead to more fighting in Korea, so the south protested and refrained from responding with force. The jamming was a nuisance more than a threat and most military equipment is equipped with electronics and other enhancements to defeat it. The North Korean jamming confirmed what was already suspected. So now South Korean and American electronic warfare experts have an opportunity to study the effects of jamming on a large metropolitan area. It caused intermittent problems for users of GPS devices and many more cell phone connectivity problems. There were briefer and less powerful jamming incidents in August and December of 2010.
Meanwhile, this is old news for the U.S. Department of Defense, which has spent most of the last two decades developing anti-GPS jamming technology. For years military aircraft have been equipped with complex and expensive GPS receivers that will usually continue to work even if they are being jammed. There are several ways you can defeat attempts to jam GPS signals. While some of the methods are well known, others are classified. No one has successfully used GPS jammers in combat yet but the potential is there. Now the North Koreans are giving large scale demonstration of GPS jamming. Anti-jamming technology is increasingly complex. None of the major players (the U.S., Russia, China, Israel, and several other industrialized countries) are talking, and for good reason. If you don't know what techniques the other guys are using you can't deal with them.
China and Russia are both selling GPS jammers. In 2007, China brought to market a powerful truck mounted GPS jamming system. These "GPS jamming vans" are meant to create a protective "bubble" over an area the van is in the middle of. Sales have been slow. A year before the 2003 invasion of Iraq it was believed that Saddam had bought many GPS jammers, to deal with U.S. JDAM GPS smart bombs. The JDAM has a backup INS, which was no secret, and the Iraqi GPS jamming efforts had no significant effect on the 2003 campaign.
There are several approaches to defeating GPS jamming, and knowing which one each American GPS guided weapon uses makes it easy to develop a way to jam the "jam-proof" GPS. So the U.S. Air Force is understandably reluctant to discuss what they are doing. Given the cost of jam proofing all existing GPS weapons, it's more likely that jam-proof GPS weapons will only be used against targets where the GPS accuracy is vital. Against most targets the accuracy provided by the inertial guidance system will do. Also note that you can bomb GPS jammers with a bomb equipped with a guidance system that homes in on a GPS jamming signal. For that reason it's thought that any use of GPS jammers will involve dozens of jammers in each area so protected. The GPS jamming has no effect on the even more accurate laser guided bombs, and some countries buy smart bombs with both laser and GPS/INS systems. Most countries are working on anti-jamming tech in anticipation of encountering more jamming if war comes.