The military’s reliance on the Global Positioning System is so great that an obvious question arises on a regular basis: What if it isn’t available?

Soldiers operating in remote or contested environments don’t always get the reliably consistent GPS signal that civilians finding their way to a restaurant have become used to. In the field, warfighters may have to deal with complex, cluttered environments that can interrupt or block signals. They also have to be prepared in case an adversary jams signals or even destroys the satellites necessary for GPS to work.

The Army Research Laboratory has come up with one answer, developing an algorithm that can interpret the source of radio signals in order to locate soldiers or robots without using GPS.

“This capability is critical to help find dismounted soldiers and for humans and robotic agents to team together effectively,” Gunjan Verma, who developed the technique with fellow ARL researcher Dr. Fikadu Dagefu, said in a release.

The researchers set out to develop localization methods that don’t rely on GPS because complex environments impede the propagation of wireless signals, making them unreliable for determining location. For example, objects inside a building that are larger than a wireless signal’s wavelength can weaken and scatter a signal.

Their solution involved using an algorithmic technique for determining what’s called the direction of arrival, or DoA, of a radio frequency signal source. The algorithm statistically models the gradient of the received signal strength (RSS) while compensating for objects that cause interference. Significantly, it doesn’t get fooled in an extremely noisy environment, determining that there is no DoA present instead of arbitrarily guessing at a direction.

“The proposed technique is robust to multiple scattering effects, unlike existing methods such as those that rely on the phase or time of arrival of the signal to estimate the DoA,” Verma said. “This means even in the presence of [obstructions] that scatter the signal in different directions before it is received by the receiver, the proposed approach can accurately estimate the direction of the source.”

The Department of Defense, which created GPS starting in the 1970s, relies on its signals for everything from air, sea, and land navigation, to missile guidance, and keeping track of its personnel and equipment. But officials have always known that it’s vulnerable to attack, as well as unsuitable to some operating environments. Signals can be jammed with inexpensive, low-power devices. They also can be spoofed, as when an Iranian cyber warfare unit reportedly fooled a U.S. RQ-170 drone into landing in Iran in 2011—and two years ago unveiled a new combat drone that appears to be copied from the RQ-170.

The Defense Advanced Research Projects Agency has pursued alternative methods of Positioning, Navigation and Timing (PNT) without GPS through projects such as the chip-scale Micro-PNT and Adaptable Navigation Systems (ANS). The Air Force has developed a radio frequency force-tracking algorithm that can pinpoint someone’s location via a smartphone without using GPS. The Army also is working on several non-GPS PNT programs, including equipping soldiers with miniature cameras that can provide vision-aided navigation when GPS is unavailable.

Other projects have including using “pseudolites”—low orbiting pseudo satellites that resist jamming by generating stronger signals than satellites in orbit—and portable chip-scale atomic clocks that could provide precise location data in the absence of GPS.

And in the interest of navigating the seas amid increasingly jammed GPS signals, the Navy has been pushing to revive the radio-based long-range navigation system (Loran) developed during World War II as a backup for GPS. Scrapped as obsolete in 2010, an updated version called eLoran, for enhanced Loran, would be a reliable alternative to GPS, and has been supported by past administrations. But Congress has steadily resisted spending on its development, and this year took it out of the final 2019 National Defense Authorization Act.

None of the alternatives are likely to ever replace GPS, which has become a constant in everyday life. But the prospects of its signals being scattered, jammed, or unavailable has created the need for the military to have backup systems.

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