The scene is a politically unstable, strategically important country where terrorists are hiding in a large cave system. In addition to using traditional guns and bombs, both the terrorists and the U.S. Air Force will face off in a whole new military arena – navigation warfare.

Navigation warfare centers on the use of the Air Force Global Positioning System (GPS) satellite navigation system by the U.S. military and its allies, and attempts to disrupt the system by adversaries.

In this scenario, an informant in the terrorist ranks has given the United States the precise location of the terrorists. The United States must attack the location to seal in the operatives, but strike with great precision to avoid civilian casualties in the same area. Strike planners know they can do that by using GPS-equipped Mk. 82 500-pound Joint Direct Attack Munitions (JDAMs).

The underground enclave has 40 separate entrances, some several miles apart, giving the terrorists ample escape routes. That means the attack must simultaneously hit 40 separate locations, putting two 500-pound bombs on each target within seconds of each other.

One consideration is that with so many targets and bombs, some misses are possible. But planners have an extra element on their side. They know the batting average of every GPS player in the game. Among the two dozen primary GPS satellites in the GPS constellation, all have subtle personalities and very slight signal strength differences that can affect precision. To ensure the most accurate attack, planners consult Air Force Space Command data showing when “sweet spots” in the constantly moving GPS constellation will place the best combination of four to six satellites over the target area.

The GPS graphical data show spikes at specific times when lower capability combinations of GPS satellites will move over those same areas. Wherever there is a spike in the graphs, somewhat less GPS accuracy may occur. With that in mind, U.S. commanders follow a GPS program axiom – “don’t strike when there is a spike” – in determining the precise time to schedule a GPS-guided attack. That way the most accurate combination of GPS spacecraft will be in position to support the attack.

The 40 target points are within 5 miles of each other and are under continuous surveillance by a single Global Hawk remotely piloted vehicle loitering invisibly 50,000 feet above the target area.

But just after takeoff of the strike aircraft, the informant is captured by the terrorists and tortured into revealing that an attack is on the way. What the terrorists do not know, however, is that there is another informant in their midst – an electronic one they mistakenly think will work to their advantage.

The terrorists know if they move during daylight they will be caught in the open and attacked. But given past experience, they believe by hunkering down within the cave system they can survive an attack. They would then depart at night, believing some cave exits would surely be unblocked. They have plotted where each entrance is with their own GPS receivers.

The bad guys are also equipped with GPS signal jammers developed by a company in Russia. They plan to jam the GPS systems of the strike aircraft and the individual GPS-guided bombs. Being well-funded terrorists, they are also equipped with the latest model of GPS receivers, which they turn on briefly to check if the jammers are working. That is a big mistake. They have just awakened that other impostor in their midst.

Within 5 seconds, a U.S. Air Force officer monitoring the Global Hawk surveillance data exclaims “Gotcha!” The large drone is equipped with the latest GPS Jammer Location (JLOC) software. The software in the Global Hawk allows it to communicate electronically with the main JLOC processing center in the United States that recognizes the jammer as a model built by a particular Russian company. It is a jammer version U.S. commanders have seen before and know how to disrupt.

The JLOC software is built into the terrorists’ GPS receivers. When the terrorists turned on their GPS receivers to test their jammers, they unwittingly broadcast a digital message to the Global Hawk, saying “GPS jammers present.” The receivers also provided the exact location where those jamming transmitters are located.

Due to the electronic GPS spy in their ranks, the United States now has even better location data as well as information that specific GPS jammers are present. This gives the United States time to add extra assets to the attack to defeat the jammers.

At the appointed time, 80 bombs begin falling on the al Qaeda target area. The terrorists are immediately confused. There is only one aircraft visible high in the sky, yet dozens of bombs are hitting the cave entrances.

The bad guys are being hammered by another star player on the American military team – the weapons capability of the B-2 stealth bomber. All 80 bombs are coming from a single B-2, carrying a bomb load comparable to six World War II B-24s. The 80 bombs are dropped in a single 22-second bomb run and each bomb is precisely steered by GPS signals coming from 12,000 miles away in space.

On the receiving end, the terrorists have turned on their jammers, but they die wondering why they are not working. The GPS signal a senior al Qaeda leader unwittingly broadcast has enabled two of the bombs to home in specifically on his personal coordinates.

As the several GPS satellites involved in the strike move away from the target area at 17,000 mph, so do four other much slower but secret participants added specifically to counter GPS jamming. These are four small Predator drones flying around the target area at about 25,000 feet. Known for their reconnaissance and Hellfire missile capability, these Predators have a less well-known mission. On the tail of each there is a sticker that reads “NAVWAR” – short for Navigation Warfare, the GPS program’s development division set up specifically to counter jamming.

Equipped with small electronics boxes to boost GPS signals, they were sent to orbit the strike area as “pseudolites” – pseudo satellites – essentially very low-altitude versions of the GPS satellites. Each Predator pseudolite was used to boost the signal strength of the GPS timing signals from space, overwhelming the enemy’s GPS jammers.

Every element of this imaginary attack scenario has already actually occurred in Iraq and other locations. It illustrates the importance adversaries place on jamming GPS and the major efforts under way to counter that jamming.

The GPS navigation satellite system has become an indispensable part of warfare and a key element of civilian infrastructure.

Hundreds of years ago precision navigation was developed and practiced only by sea captains. In the early 20th century, it was also practiced at the macro scale by Army commanders and the micro scale by aircraft pilots as well as seafarers.

The operational status of GPS in the 1980s began a rapid proliferation of personal navigation systems for all manner of military operations, and eventually civil operations. It was first used by rental car, bus and rail fleets, and finally now by private cars, boaters, and individuals. In the late 1990s, about 100,000 civilians used formal GPS navigation services; now the figure exceeds 15 million, according to the financial analysis firm Frost & Sullivan. Several million more people receive less-scripted GPS data on their cell phones.

The GPS’ value to military operations and the civilian economy has increased efforts by adversaries to develop jamming techniques. At the present time, this involves only electronic warfare, not physical attack. But the Chinese test of an anti-satellite weapon (ASAT) in 2007 has heightened concern that someday the GPS satellites could be threatened by space mines or ground-to-space missiles. There is little the GPS satellites can do to avoid a high-speed ASAT. But that risk has spawned Pentagon interest in smaller, more quickly launched navigation spacecraft to quickly replace any spacecraft attacked by an ASAT or hostile ground-based laser.

Still only in concept form, such a capability must wait on the development of a more rapid space-launch capability. Until then the emphasis will be on protecting the GPS system in an electronic warfare jamming environment, and maintaining several backup GPS spacecraft in orbit that can be quickly moved into an operational slot if necessary.

“GPS satellites produce low-power signals that must travel great distances to reach the receiver. A jammer, on the other hand, can produce a stronger signal much closer to the receiver. Since signal power diminishes as the square of the distance traveled, the jammer has a distinct advantage,” said Anthony Abbott, a senior GPS manager at the Aerospace Corp., Los Angeles, Calif.

“This vulnerability has been identified as a high priority within the Department of Defense, and numerous programs have been established to develop near-term solutions for today’s potential threats and more extensive long-term solutions for projected future threats,” said Abbott in an Aerospace Corp. assessment. The company is the primary think tank used by the Air Force. It is heavily involved in countering threats to the GPS constellation.

Some elements of these efforts to counter GPS jamming involve:

NAVWAR: The DoD GPS Joint Program Office has established the Navigation Warfare (NAVWAR) program to address the electronic warfare threat to the GPS constellation. The NAVWAR program is tasked with protecting Pentagon and allied use of GPS during times of conflict, preventing its use by adversaries while maintaining normal availability to the civil user outside the area of conflict.

The U.S. Air Force is planning the deployment of a new, more powerful, x100 jam-resistant GPS Block III satellite constellation, but has been subject to funding delays and will not be deployed for several more years. Another countermeasure aspect is the deployment of anti-jam equipment with existing and new receiver systems. The NAVWAR office leads a range of GPS counter-jamming programs that include new antenna designs as well as operational strategies that can negate jammers.

Pseudolites: Low-altitude signal boost systems are an example of one GPS anti-jamming capability that is largely classified, but likely to have been employed recently in Iraq.

Antenna design and signal radiation: A primary GPS anti-jamming technique is “the controlled reception pattern antenna,” Abbott said. This device consists of an array of six antenna elements arranged in a hexagon around a central reference element. The elements are all connected to an electronics box that controls the phase and gain of each element’s output and combines the seven elements into a single output.

This signal processing produces an adaptive gain pattern that can be manipulated to place a null in the direction of an undesired signal source. “The underlying principle is fairly straightforward,” he said. “Received GPS signals are rather weak and cannot be detected or measured without a signal-correlation process; therefore, the processing algorithm assumes that any measurable energy above the ambient noise must be a jamming signal, and so it computes the necessary electronic actions to null out that source.”

Selective Availability Anti-Spoofing Module (SAASM) software upgrade: Developed by the Microsemi Company of Irvine, Calif., this upgrade to GPS ground station software employs time/frequency generation, synchronization systems, crypto key management, and other classified operational capabilities. Placing upgraded SAASM software in GPS ground stations has had the desired anti-tamper effect needed to secure not only military positioning and timekeeping systems, but civilian systems as well.

Originally there were two major drawbacks to the low-level frequency and worldwide proliferation of GPS systems. The first was the ease with which these frequencies can be jammed. The second drawback is the potential for spoofing, or deliberate attempt by outside sources to mimic a legitimate signal to transmit false information and locations. Preventing spoofing with SAASM software is crucial for mission critical situations where misdirecting instructions and locations of the enemy could spell tragedy for troops on the ground.

Advanced Concepts: “With the recent advances in tiny micro-electromechanical systems, new architecture concepts have now come within reach,” said Abbott. “One such technology, the micro-electromechanical inertial measurement unit [IMU], will have a significant impact on the future design of user navigation sets,” he said. Under this concept, extremely small IMUs that precisely measure the direction of motion and velocity are being integrated with GPS units, at least in a developmental phase. These IMUs cannot be jammed.

Each of 24 primary GPS satellites orbit in one of six orbital planes at a 12,544-mile altitude. The first satellite was launched in 1978, and the system began partially operating with 21 satellites in 1993. The program is in the midst of an $8 billion upgrade. The first of 12 new Boeing 2F spacecraft was launched from Cape Canaveral May 27, on board an Air Force/United Launch Alliance Delta IV rocket. These Boeing 2F models will be followed by 18 even more advanced Lockheed Martin spacecraft later this decade.

Each GPS satellite carries an extremely accurate atomic clock. The new spacecraft will be accurate to a billionth of a second.

The satellites know their own position based on tracking data sent to them from several Earth-based GPS update sites. Each satellite then continuously rebroadcasts that continuously updated position data along with its time signature.

The signals travel to a ground receiver, delayed slightly by the amount of time it takes the signal to travel from the satellite to the receiver.

The receiver calculates the range to the satellite using that delay and the timing data, and then calculates its own position for any given moment. The Achilles heel of GPS is its susceptibility to jamming, because the signals from each satellite are extremely weak. The Iraqi army, for example, tried unsuccessfully to jam GPS receivers.

Now both China and North Korea have major GPS jamming programs under way. Both countries use the civilian band for their own military operations, and the North Korean submarine that recently torpedoed a South Korean patrol boat was in all likelihood using a civilian GPS system to navigate. Both China and North Korea have numerous efforts in development to jam GPS when military conflict threatens.

In addition to deploying Russian-built GPS jammers to its own forces, the North Koreans have been caught trying to export Russian-built GPS jammers to Iran and Syria as well as smaller Middle East-based political groups hostile to the United States. When the Russians halted that trade, the North Koreans began making their own jammers based on the design of the Russian device. The Russian system was used by Iraqi forces early in the 1991 Gulf War, but was rapidly countered by U.S. tactics.

According to the Web site GPS World, “South Korean military authorities are deeply concerned that the device could create havoc at home if war broke out along the border.” The South Korean army will have accumulated 900 GPS-guided JDAM bombs in its arsenal by 2012, noted GPS World.

Intelligence reports on the Chinese also paint a worrying picture. The reports indicate that during battle exercises, China has deployed vans that are apparent GPS jamming systems. The Pentagon is concerned about Chinese jamming capability, especially if China tries to sell the technology to traditional Chinese customers, such as Iran.

China is developing its own “Biedou” satellite navigation system so that it is not dependent on civilian GPS signals, which can be shut off by the United States in times of military crisis. It is a safe bet that U.S. electronic warfare specialists are working to jam the Chinese Biedou system as Chinese engineers work to perfect jamming against encrypted GPS signals.

The Air Force holds regular “Jamfest” sessions to test new jamming technologies developed by commercial operators as well as military programs.

Sometimes overall communications jam testing can have unintended consequences. During 2007, two U.S. Navy ships in San Diego harbor were testing the jamming of regular communications that was not supposed to affect GPS signals.

But after the tests began, civilians in the San Diego area, including general aviation pilots aloft, began to experience GPS outages. The problem was traced to the Navy exercise, which was halted as soon as commanders realized they were jamming civilian GPS receivers in addition to military communications.

This article was first published in The Year in Defense: Aerospace Edition, Summer 2010.