Researchers are advancing experimental navigation systems that aim to keep airplanes on course when global positioning system (GPS) satellites are unavailable.
Suspended from a weather balloon soaring 80,000 feet over New Mexico, two antennas peek out from a Styrofoam cooler. At this altitude, the dark expanse of space contrasts sharply with Earth’s vibrant blue sky. However, the antennas are not distracted by the stunning view; their focus is to capture signals that could enhance aviation safety.
Scientists from Sandia National Laboratories and Ohio State University are innovating navigation technology in the air, creating a backup system to ensure that aircraft stay on track without relying solely on GPS satellites.
Below the floating cooler, cell phone towers send out a continuous stream of radio frequency waves from more than 15 miles away. At an even higher altitude, communication satellites that do not use GPS provide additional signals.
The goal is to utilize these alternative signals to determine a vehicle’s position and speed.
“Our aim isn’t to replace GPS,” stated lead researcher Jennifer Sanderson from Sandia, “but rather to supplement it in situations where it may be compromised or degraded,” which can create potentially hazardous conditions for both pilots and passengers.
The research team shared its initial findings at the Institute of Navigation GNSS+ conference that took place from September 16-20 in Baltimore. This research is funded by Sandia’s Laboratory Directed Research and Development program.
The case for a GPS backup
There’s no doubt that GPS is the primary standard for navigation due to its speed, accuracy, and reliability. This raises the question: Why are new navigation methods being explored?
“I have concerns about relying too heavily on it without having a backup,” remarked Sanderson, who specializes in navigation algorithms.
She noted that GPS has become integral to our technologically advanced society. We rely on it for everything from landing aircraft and navigating city streets to managing crop yields and synchronizing financial transactions. This dependency makes researchers like Sanderson anxious about the possible repercussions if the GPS connection fails.
“The effects of a GPS failure could ripple through society,” she said. GPS disruptions are increasingly frequent. Pilots navigating near conflict zones often lose access or find GPS unreliable. The longer the period without GPS, the greater the risk of an accident.
“Commercial GPS devices face several threats, including jamming,” Sanderson explained. Jammers, which inundate receivers with irrelevant signals, are illegal but often accessible for purchase.
Another issue is spoofing, where a false signal misleads receivers into thinking they are in a different position. This tactic, widely known, is sometimes utilized by gamers to cheat in location-based games like Pokémon Go.
“There are actual apps that can alter your location, even tutorials in forums dedicated to explaining these methods for different games,” she added.
While spoofing in gaming may seem harmless, Sanderson highlighted that it can have serious consequences when applied to actual vehicles. Pilots could be misled by a spoofed signal, directing them off course.
Project investigates high-altitude signals of opportunity
Sanderson’s concept of using nearby non-GPS signals for navigation is not entirely new. Researchers refer to it as “signals of opportunity,” but investigations have primarily focused on ground or near-ground scenarios. This method has been proposed for helping autonomous vehicles navigate dense urban environments where GPS signals may be obstructed by skyscrapers.
Nonetheless, this task is not straightforward. Unlike GPS, where time and location data are extracted straightforwardly, receivers of signals-of-opportunity often assess physical properties of the radio frequency waves.
For instance, they utilize the Doppler effect; radio waves from a satellite travelling towards a receiver appear compressed, while waves from a satellite moving away appear stretched. Using advanced math and multiple signals, scientists hope to identify the signal sources and determine the receiver’s position.
Sanderson and her group are examining signals-of-opportunity at high altitudes. If they can successfully gather signal data from the stratosphere, they may develop a method to guide vehicles like aircraft using a network of atmospheric radio frequency waves. “We attach our instruments to weather balloons and launch them,” she explained.
The equipment comprises electronic units linked to two antennas, all insulated within a foam cooler, which are crucial for analyzing signals high above the clouds. While satellite signals are typically robust, dead zones can arise along the cone-shaped transmission pattern that narrows nearer to the source. Furthermore, satellite coverage may be inconsistent over sparsely populated regions, such as much of New Mexico. The potential strength of cell tower signals can be theorized but needs real-world characterization to be effective.
“Up to this point, we have reached altitudes close to 80,000 feet; other research has mainly targeted elevations of 5,000 to 7,000 feet,” she noted.
Data processing: The next step in the research
As the team processes their initial data set, they anticipate both new achievements and hurdles.
“The less glamorous but crucial part of navigation is identifying all your sources of error,” Sanderson observed. “My aim is to have a comprehensive dataset to create algorithms for real-time systems, enabling hardware tests using actual live-sky data.”
Ultimately, a functional navigation system will need to synchronize signals with their transmitters in real time, subsequently calculating position and velocity in relation to those sources. However, currently, her team is manually correlating received signals with nearby satellites using reference data.
“It can be quite an involved process. Thus, a significant aspect we need to work on is streamlining this to an automated system,” she stated.
Despite the complexities, she remains hopeful.
“While we are still analyzing the flight data, our preliminary findings suggest that we have detected cell tower signal beacons at our peak altitude of around 82,000 feet. If these signals prove reliable for navigation, it could revolutionize our understanding of feasible alternative navigation options,” Sanderson remarked.
Sandia National Laboratories is a multipurpose facility operated by National Technology and Engineering Solutions of Sandia LLC, a fully owned subsidiary of Honeywell International Inc., on behalf of the U.S. Department of Energy’s National Nuclear Security Administration. Sandia Labs is responsible for significant research and development initiatives in nuclear deterrence, global security, defense, energy technologies, and economic competitiveness, with primary facilities located in Albuquerque, New Mexico, and Livermore, California.
