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Disruptions of GPS timing signals can be either intentional or unintentional, but many right now are unintentional, said Richard Linzy an architect engineer for carrier Windstream Communciations during a webinar hosted by the Alliance for Telecommunications Industry Solutions (ATIS).
Windstream Communications performed a study on GPS vulnerabilities and found that intentional attacks comprised just 3 percent of the cases studied. Most instances of GPS disruption came from what Linzy described as collateral disruptions, such as when drivers have a jammer intended to block a specific signal and end up blocking all signals they pass.
The Windstream study also found environmental disruptions from factors such as malfunctioning equipment or nearby antenna fields.
In terms of disruptions to the GPS signal, there are two main types — jamming and spoofing, said Richard Funderburk, a timing system architect for Trimble, which manufactures GPS receivers. In general, jamming of a GPS signal is not intentional but comes collaterally from someone else attempting to block another signal.
“The GNSS signal is a very low signal; it doesn’t take much to actually block that signal,” he said.
Spoofing, however, is always intentional, Funderburk said. In spoofing, a signal is overlaid on top of the GPS signal and used to modify the time or location information of that signal.
“There is not currently a known system that can accidentally spoof GPS,” he said.
Trimble has seen spoofing of a GPS signal occur only in a controlled lab environment and not in the real world, Funderburk said.
While GPS spoofing attacks are not common, the threat space for spoofing is growing as the knowledge and equipment becomes more widespread, said Sarah Mahmood, program manager for the Department of Homeland Security (DHS) First Responder Group.
Information on how to spoof is becoming more public at hacker conferences and on blog posts, and some mobile-phone users are using GPS spoofing to gain an advantage in location-based mobile games such as Pokemon Go, Mahmood said.
“The information and know-how and equipment is all much more prevalent than it has been in the past,” Mahmood said. “So, that’s one of our driving concerns as well.”
Because of GPS’ importance to many timing applications, disruptions to the signal can cause a variety of negative impacts. For example, loss of a sync clock for a carrier’s central office can result in a drop-off in voice services, leading to a loss of access to emergency services for customers, Linzy said.
Because most timing methods are dependent on global navigation satellite systems (GNSS) , protecting GPS and other GNSS signals is critical to keeping important systems running, Funderburk said.
“GPS allows the transfer of time across large distances and have that time be quite accurate,” he said. “Currently, no other method is capable of doing that.”
Therefore, taking steps to mitigate and prevent disruptions to the GPS signal is important for critical infrastructure and other applications that rely on timing synchronization.
One key to mitigating disruptions is knowing the environment that a receiver is in. Because environmental factors, such as nearby antennas or malfunctioning equipment, can cause disruptions, knowing what’s in the environment can help users locate and fix any issues with a receiver.
Proper antenna selection and installation also play a key role in preventing disruptions to the GPS signal. An antenna with multiple layers of filtering helps ensure that the GPS signal comes through, Funderburk said.
Another effective mitigation technique is having a secondary reference signal, such as a different GNSS signal, that can provide the time should the main GPS signal experience any disruptions. A variety of other GNSS systems, including Europe’s recently launched Galileo are available now, providing plenty of options for secondary sources, Funderburk said.
To help critical infrastructure deal with potential GPS disruptions, DHS has taken a multifaceted approach to addressing the issue, Mahmood said.
The first layer is testing the devices, finding vulnerabilities and working with the industry to raise awareness of and address those vulnerabilities.
To this end, DHS has held and will hold field exercises focused on raising awareness of issues around jamming and spoofing and making systems more resilient against those threats. One such event took place last July at White Sand Missile Range in New Mexico and focused on how public-safety organizations can deal with jamming.
To help raise awareness of the issues around jamming and spoofing, DHS also released a series of best practice documents for using and positioning GPS equipment.
A third prong of DHS’ strategy includes developing improved equipment for GPS applications. One example of this is a horizon ring nulling antenna that limits the access points for signals. DHS is working to make the antenna available via a no-cost licensing agreement to manufacturers that might want to commercialize it, Mahmood said.
The final layer of DHS’ efforts involves looking at positioning, navigation and timing (PNT) service requirements and searching for other diversified timing sources and potential new technologies that can meet those requirements.
“We want to make sure we’re doing what we can so we’re resilient enough to face these growing threats,” Mahmood said.
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