A pair of antennas emerging from a Styrofoam cooler suspended atop a weather balloon 80,000 feet above New Mexico are listening carefully for signals that could improve air travel safety. From this height, the transition between Earth’s blue atmosphere and the darkness of space is visible, but the focus of these antennas is not the landscape but an experimental navigation technology being tested by researchers at Sandia National Laboratory and Ohio State University.

The goal is to develop a backup system that can keep planes on course when GPS signals are unavailable or unreliable.

Cell phone signals for GPS assistance

The idea behind this research is to use alternative signals (radio frequencies from cell towers and non-GPS satellites) to calculate a vehicle’s position and speed.

“We’re not trying to replace GPS,” says Jennifer Sanderson, Sandia’s principal investigator. “We’re just trying to help him in situations where he’s humiliated or in danger.” If this situation is not addressed, it can pose a danger to pilots and passengers.

Why should you consider GPS backup?

GPS remains the gold standard for navigation thanks to its speed, accuracy and reliability; So why look for alternatives? “I’m afraid of relying on it too much without a backup,” said Sanderson, an expert in navigation algorithms.

GPS is well-established in the modern world and impacts a variety of activities, from aviation to agriculture and financial operations.

This high dependency raises concerns about the impact of potential failures. “Loss of GPS could impact the entire community,” Sanderson said.

Service interruptions and flight safety

GPS outages are not uncommon, especially near conflict zones where pilots may find GPS unreliable or unusable. Long-term GPS outages increase the risk of accidents. One threat is GPS jamming; that is, devices that emit disruptive signals at GPS frequencies.

“Commercial GPS receivers are exposed to many different threats, one of which is jamming,” Sanderson said, adding that although such devices are illegal, they can still be purchased.

Another problem is spoofing, where false signals are sent to fool GPS receivers. Although fraud is often associated with gaming, Sanderson points out that it can have serious consequences in aviation.

“Pilots may not be able to tell whether a signal is fake or real, which can lead them in the wrong direction,” he says.

Navigating “opportunity signals”

The concept of using non-GPS signals for navigation, known as “opportunity signals”, is not entirely new, but has often been studied close to the ground, such as in urban areas where buildings block GPS signals.

Unlike GPS, where location information is embedded in the signal, opportunity signals often require measurement of physical properties of radio waves, such as the Doppler effect.

When the satellite moves towards the receiver, the radio waves are compressed, and when it moves away they are stretched. Using these changes, researchers can calculate their location relative to the source of the signal.

Sanderson’s team is pioneering the study of signals of opportunity at much higher altitudes. By collecting data on signals from the stratosphere, they hope to develop a method of navigation that uses a network of radio frequency waves at these altitudes.

“So we attach our payload to these weather probes and launch them into the air,” Sanderson explained. This payload consists of electronic equipment housed in an insulated foam cooler, containing antennas that collect data above the clouds.

Unstable cell phone signals

This new method faces challenges such as identifying and characterizing signals from satellites and base stations at such altitudes. Although satellite signals are strong, gaps can occur due to the cone shape of the beam pattern, which tapers as it gets closer to the source.

Coverage in rural areas, such as some parts of New Mexico, may be spotty. The signal level of base stations can be predicted theoretically, but it needs to be tested in real conditions.

“The highest altitude we have reached so far is approximately 80,000 feet. By comparison, the other studies we saw focused on altitudes between 5,000 and 7,000 feet,” Sanderson said.

Accuracy in navigation work

The team is focusing on overcoming technical challenges and improving their methods as they continue to analyze their initial data.

Sanderson emphasized the importance of accuracy in navigation research. “An unpleasant but very important aspect of navigation is understanding all sources of error.”

Its goal is to create a comprehensive data set that can be used to create algorithms for real-time navigation systems, leading to hardware testing with real-world data collected during these high-altitude flights.

The future of flight safety

A key part of the upcoming project is the development of a system that can automatically match incoming signals with its transmitters and calculate the vehicle’s position and speed in real time.

Currently, the team manually matches signals with satellites, which takes time. “It can be quite boring. Therefore, one of the important aspects we need to work on is the automation of this process,” says Sanderson.

Despite these challenges, scientists remain optimistic about the potential of their work.

“While we are still processing flight data, we believe our preliminary findings indicate that we detected cell tower beacons at our peak altitude of approximately 82,000 feet,” Sanderson said.

“If these signals are clear enough for navigation, it will significantly change what we think is possible for alternative navigation.”

The team’s efforts could pave the way for a reliable backup navigation system that would provide a safety net for aircraft in situations where GPS is compromised, ultimately improving aviation safety and resilience. The team’s preliminary findings were presented at the GNSS+ Navigation Institute conference in Baltimore, September 16-20.