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Precise Positioning Without GPS

October 1, 2019

LET’S GO AHEAD and call this one the "Really Advanced Flight Technology" column. This isn’t a system you’ll likely see on a commercial UAS in the next few years, but it’s an extraordinary technical achievement, providing GPS-like guidance to uncrewed aircraft in flight without relying on GPS, and without adding to the size, weight, or complexity of the aircraft.

Before I discuss how to accomplish this formidable goal, let’s pause to consider why we would want to do it in the first place. In our daily lives, GPS seems as regular and predictable as the tides.

If you need to find a location that you have never visited, you enter the address into your smartphone and it provides you with turn-by-turn instructions, fixing your starting point, and progress along the route by using radio signals from a constellation of satellites that are orbiting 12,500 miles overhead.

Enable GPS position-hold on your drone and it will hover at a single point in the sky, as docile as a lamb, until the battery runs low—at which point, it will return to the starting point and land at your feet.

GPS, however, does not always function as advertised. In 2012, Liberty International Airport in Newark, New Jersey, began to experience sporadic GPS outages that interfered with commercial flights. A pattern eventually emerged, and the FCC deployed investigators to the vicinity, who tracked down a man in a pickup truck registered to a local construction company.

The man was using a jamming device to prevent his employers from monitoring his whereabouts by using a GPS tracking system installed in the truck. The device cost him $100, but his fine for using it was $32,000.

Earlier this year, ships that were operating in the Black Sea reported spurious GPS readings, a phenomenon that has since been detected trailing Russian president Vladimir Putin around the globe.

As Matt Hederstrom, the chief development officer for Coherent Technical Systems, Inc. (CTSi)—the man hired by the U.S. military to find an alternative to GPS—said, "We jam GPS. The bad guys jam GPS. GPS can be quite unreliable in-theater."

Secret Squirrel

An authority no less eminent than the father of GPS himself, Dr. Brad Parkinson, said, "Reliance on satellite navigation and timing systems has become a single point of failure for much of America and is our largest, unaddressed critical infrastructure problem."

the two green nodes are tuned to the frequency
This isn’t a system you’ll likely see on a commercial UAS in the next few years, but it’s an extraordinary technical achievement, providing GPS-like guidance to uncrewed aircraft in flight without relying on GPS, and without adding to the size, weight, or complexity of the aircraft.

Matt put it another way. "GPS is like electricity at a grocery store: It’s great when its working, but when it’s not, the whole system is basically shut down. Everything you need is right there, but you can’t buy it."

A graduate of Virginia Tech with a degree in aerospace engineering, Matt is a veteran of the famed Lockheed Martin Skunk Works in Palmdale, California. Along with the team at CTSi, his solution to GPS jamming is the Enhanced Link Navigation System (ELNS).

The team began by reasoning that all UAS have at least one thing in common: the data links that connect them with their remote operators. Command and control signals are sent to the aircraft, which responds with video, telemetry, and other sensor data.

It seemed to the development team that those signals could be used to provide information about the location of the aircraft. They took an example from a down-to-earth source. "We won a Department of Defense contract to track endangered ground squirrels on a military base, so we put out microphones so that we could hear them. Then, by measuring how long it took for a sound to reach two different microphones, we could triangulate the location of the squirrel," Matt explained. "By doing this on an ongoing basis, we were able to come up with an accurate estimate for the number of squirrels and where they are living."

Marco Polo

Like his squirrel sonar system, Matt reasoned that by setting up two receivers (called nodes) tuned to a drone’s downlink frequency, he could determine the direction and distance of the aircraft from the receiving stations. For added reliability, he introduced a third node into the system, essentially a dummy aircraft at a fixed location. "That way, if the system shows that the target is moving, it knows something is wrong," he said.

These nodes can be installed at a military airfield or onboard a naval vessel. Working together, they yield location data for the drone relative to its home base. This information is then sent back to the aircraft, which uses it for navigation in lieu of GPS coordinates. The only modification to the aircraft that is required is a software tweak of programming the flight management system to recognize and process the incoming ELNS data.

In its initial testing, the system proved to be extremely robust. As the distance between the drone and its base increased, so did the error in determining its precise direction. The error was, however, small enough that it was not problematic during long-range operations and it resolved as the aircraft got closer to home—when precision was required to make a safe landing.

"We even introduced deliberate errors into the ranging system, and the platform landed within two inches of the predicted touchdown point," Matt said. "In 2018, we performed 152 approaches at St. Mary’s [County Regional] Airport in Maryland and had a 100% success rate."

In an environment where multiple drones are operating simultaneously and potentially alongside crewed aircraft, ELNS can become even more accurate. "Together, all of these different aircraft form a mesh network," Matt explained. "Each aircraft is constantly sending ‘Marco Polo’ ranging signals to every other aircraft, and if even one participant in this network gets a solid GPS fix, it can share that accurate location information with everybody else."

Every aircraft within range of the GPS jammer (red) is joined into a mesh network established by the ELNS system. If even one participant in the network achieves a solid GPS lock, it is able to share accurate position data with all of the other aircraft.

ELNS for All?

Another way to enhance the accuracy of the ELNS system is to take advantage of what Matt calls "signals of opportunity." These are transmissions from sources that have not been explicitly established to facilitate navigation, such as cellphone towers, commercial radio stations, and telecommunications satellites, but which nevertheless can be used to derive location information.

"Google does a version of this with their Street View vans," he said. "When they drive through your neighborhood, not only are they taking photographs, but they are also sniffing for Wi-Fi signals. By comparing the signal strength of various routers, they are able to make extremely accurate estimates as to the van’s location."

Of course, UAS downlinks are nothing more than radio signals, like the transmissions from GPS satellites, which makes it reasonable to ask whether they could also be jammed. Confronted with this question, Matt demurs that he is fully confident that it will work, even on a noisy electronic battlefield, but the actual answer explains why that can’t be published.

Although this technology is strictly the purview of military users at this point, Matt believes that a version of it could be available to civilian users sooner rather than later. "You can extrapolate directly from this to the 5G infrastructure that cellular carriers are rolling out right now to permit high-speed, low-latency internet across their networks," he said.

Spend time talking with drone experts these days and you’ll hear 5G mentioned as a potential solution for remote identification, uncrewed traffic management, beyond visual line of sight operations, and real-time data acquisition—meaning that ELNS might not be such a far-fetched concept for civilian UAS after all.

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