Securing military GPS from spoofing and jamming vulnerabilities

Electrically steerable directional antennas are “the best bet to combat jamming.”

2The military community is in the midst of navigating a transition from selective-availability anti-spoofing module (SAASM) to modernized M-code encryption and adjusting to the major shift to international satellite constellations; the military is also on a quest for better methods to secure the Global Positioning System (GPS) from spoofing and jamming threats.

The U.S. NAVSTAR and global satellite system (GNSS) are essential to military navigation, but how secure is military GPS to and threats?

The short answer is that there are numerous changes underway to make military GPS more robust to these threats, as well as efforts to develop supplemental technology capable of operating even within GPS-denied environments.

Within the military realm, secure GPS has traditionally been defined as “encrypted GPS, military GPS, or ,” says Al Simon, marketing manager for in Cedar Rapids, Iowa. “While there are a number of terms associated with it, it all has to do with the military code being used. We generally describe ‘secure GPS’ as military-encrypted GPS that uses military P(Y) code.”

New terminology is starting to emerge from within the military community. “The U.S. Department of Defense () is now talking about assured position, navigation, (),” Simon adds. “This introduces other elements of , even though they don’t call it ‘secure GPS.’ Instead, it’s usually referred to as ‘assured PNT,’ and uses the latest encrypted GPS technology or , as well as high levels of anti-jam or resiliency … and eventually other sensors will be used to protect the PNT. So we’re starting to move the navigation conversation beyond just GPS.”

PNT approaches to provide backup coverage, such as inertial navigation systems (INS), are appealing options. “These systems use inertial measurement units (IMUs), a collection of precise accelerometers and gyroscopes that measure movement without any outside reference and are immune to jamming,” says John Fischer, chief technical officer for Spectracom in Rochester, New York. IMUs tend not to be used as a standalone approach, however, because, Fischer says, they “drift over time and become inaccurate quickly.”

GPS vulnerabilities: spoofing and jamming

Military GPS must be capable of withstanding or working around vulnerabilities to fend off spoofing and jamming attacks, which are both increasing.

Jamming is the intentional or unintentional interference of the signal that prevents it from being received, which is relatively simple to do.

Spoofing, on the other hand, is more challenging. “It requires simulating the GPS accurately and capturing the user’s receiver away from the true signal to steer it off course,” Fischer says. “Doing this requires some very complex signal-generation equipment to track the vehicle first to exactly match its trajectory before you can start spoofing. So if you’re engaging an enemy vehicle and have acquired it and are tracking it, there are simpler ways of engaging – like shooting it down – than spoofing.”

How secure is military GPS?

Military receivers use encrypted GPS signals to ensure that they are receiving an authentic signal – so these are secure in that they can’t be spoofed, Fischer points out.

A common misconception, however, is that a secure military GPS receiver is immune to jamming. “It’s easy to jam even the encrypted signal,” he adds. “Signals from satellites are so weak that even a one-watt to 10-watt jammer can deny GPS coverage for a large area of both military and civilian signals.”

The U.S. military is in the midst of transitioning from the current SAASM technology to a newer technology known as M-code. “This offers a few measures to be more jam resistant, but with high jammer-to-signal ratios so easy to implement by the enemy, M-code won’t alleviate the problem with jamming,” Fischer says. (Figure 1.)

Figure 1: The SecureSync from Spectracom is a SAASM GPS receiver in a GPS time server with a backup atomic clock in case of GPS disruption.
(Click graphic to zoom by 1.9x)

While points of vulnerability always exist, the industry is designing systems to provide the maximum protection possible. “One of the reasons the U.S. government is spending a lot of money on the GPS modernized M-code system is because it provides a substantial increase in the level of protection from an encryption and security standpoint,” Simon says.

When dealing with and spoofing, it’s essential to put solid protection in place and use levels of redundancies to prevent and identify attacks. “Even if one level of protection falters, redundancies or crosschecks need to be built into systems to ensure that you catch these attacks – even if your system is shut down for one reason or another – so that it’s not taken off the air completely and you can restore it yourself,” Simon says. “It’s analogous to any national infrastructure trying to protect itself from the whole cyberenvironment.”

Rockwell Collins is a forerunner within the anti-jam realm because the initial need for anti-jamming was driven by the weapons community. “The weapons community requires high-precision anti-jamming and the ability to maintain it until an end target is hit,” Simon points out. “I think the highest levels of the DoD would say that as long as there are satellites in the sky, we’ve got the level of anti-jam that’s going to keep forces protected where needed.”

However, there is always the potential issue of a satellite that stops working or is taken out for one reason or another. “So, within the past several years, there’s been a lot of discussion about GPS being ‘denied’ or ‘challenged’ … there’s a need to ensure some sort of backup capability to sustain temporary GPS or GNSS outages,” Simon says.

Eventually, “the Holy Grail will be an alternative navigation system that can do everything and is as widespread as GPS is today,” Simon adds. “Those systems don’t exist yet, but for several years down the road, the military community will be seeking to develop them.”

GPS “smart” antenna tech advances

A conventional antenna has full hemispherical coverage so it can see the entire sky and listen to as many as 12 satellites at once to determine the best navigation solution. “A ‘smart’ antenna is capable of focusing multiple narrow beams directly at the satellites and tracking them as they pass overhead, while pointing nulls – regions where the antenna doesn’t receive a signal – at any interference,” Fischer explains. “These devices drastically improve GPS reception within the presence of jamming, but are expensive and larger than conventional antennas.”

Electrically steerable directional antennas are “the best bet to combat jamming,” according to Fischer. These are also known as controlled-reception pattern antennas (CRPAs) or “smart” antennas.

In the past, fixed-reception pattern antennas (FRPAs) were used, but now “users are migrating to multi-element CRPAs,” Simon says. “For several years, we’ve operated with FRPAs and four-element CRPAs but some of the more sophisticated systems are beginning to operate with more elements. This means more flexibility or degrees of freedom to deal with the signal environment, which has to do with the number of signals being tracked or jammers you’re trying to reject.”

As the level of multi-element CRPA antennas in the world increases, it is “enhancing interoperation with the more sophisticated GPS receivers with advanced anti-jam capabilities that can track more channels,” Simon adds.

Another area of antenna advances has to do with size, weight, and power (SWaP). “In some cases, we’re seeing the potential integration of antennas and antenna electronics,” Simon says. “It used to be that the more sophisticated antenna electronics were either placed on a GPS card or on standalone antenna electronics units. In isolated cases, we’re even seeing antenna electronics integrated with the antenna. So there’s a slight shift in flexibility in terms of how any particular integrator or user can now start integrating all of this capability.”

Future of navigation

GPS is currently undergoing changes on several fronts, including the introduction of international satellite constellations, the transition from SAASM to M-code, and a search for technologies capable of augmenting GPS.

So, what does the future of navigation hold? With the shift to GNSS, “there’s value to be gained by looking at other constellations’ signals, in terms of signal availability, cross-checking, and using these added signals to ensure system robustness,” Simon asserts.

The shift from SAASM to M-code is well underway – nearing the final stages of development – and the weapons community is busy trying out figure out how to get M-code into their systems to make the transition by fiscal year 2018.

In terms of the future of navigation systems, expect to see supplemental technologies used as backup, depending on the user and platform. “There will be definite levels of GPS and GNSS assistance, which may involve going to other constellations or the highest level of navigation-grade inertials,” Simon says. “Or it could involve very enhanced timing capabilities … there are a number of different levels of protection and assistance that different users’ systems may need, depending on who the user is and exactly what they’re using GPS for.”

Whether used exclusively for navigation, positioning, targeting, sensor support, or timing support – or all of the above – these factors “will likely dictate a different set of sensors and capabilities,” Simon points out. “But the common denominator is GPS.”

Nearly everything that moves will have a GPS receiver in it in the future, according to Fischer. “The signal covers the globe, receivers are cheap, and the accuracy is superior to anything else. Its two biggest drawbacks are the weak signal that’s easy to jam and its unavailability indoors,” he adds.

Fischer says he also envisions the future of navigation as a hybrid blend of GPS and other alternatives. “ () technology is greatly improving inertial measurement unit (IMU) accuracy at a fraction of the cost of conventional components and is revolutionizing INS systems,” he points out. “And graphical processing, led by the gaming sector, is growing at a rapid pace, making vision-recognition systems practical.”

and systems “are advancing with millimeter-band RF semiconductor and electro-optic advances being fueled by the driver-assisted and driverless car industry,” Fischer says. “‘Crowdsourced’ navigation – in which a device on a network that doesn’t know its position can infer it by communicating with many other nodes that might know theirs, and then measuring the proximity distance to them by radio transmission delay – is an emerging technology ideal for smartphones.”

Spectracom is working with all of these technologies to create a comprehensive navigation system. “Our mission is to simplify the integration of PNT technology into our customers’ systems, so we’ll bring all of these technologies – not just GPS – to the solution,” Fischer notes.