NASHUA, N.H. — Aircraft were navigating long before GPS became widely available, and they still don't rely on it alone. Modern aircraft can use many sources of information to determine where they are, including systems that don't depend on GPS.

That matters as GPS interference becomes a bigger concern for aviation. Jamming can prevent a receiver from picking up usable satellite signals, while spoofing can feed it misleading information. Either problem can affect the navigation data pilots and onboard systems normally expect to receive.

Related: Final GPS III satellite launch sets stage for next-generation GPS IIIF

GPS is only one piece of navigation

GPS belongs to the broader Global Navigation Satellite System (GNSS) family. A receiver determines its position using signals transmitted by satellites, giving an aircraft highly accurate information about its location, speed, and time.

Airliners and many other vehicles combine that information with additional navigation equipment. Instead of asking one sensor to handle everything, the avionics can compare or blend information from several sources. 

Inertial navigation provides one of the most important backups. Rather than looking outside the aircraft for a signal, these systems use accelerometers and gyroscopes to measure movement and rotation. The system starts from a known position and continuously calculates how the aircraft has moved from there.

That gives an aircraft another way to keep track of where it is when satellite signals disappear. Because the measurements happen onboard, the system doesn't need an outside signal to keep working.

It does have a limitation, though. Small measurement errors build over time, causing the position to slowly drift away from the actual location. GPS can usually help correct that drift. Without it, crews and avionics need other sources to keep the position accurate.

Ground-based navigation still has a role

VHF omnidirectional range, better known as VOR, uses radio signals from ground stations to help pilots determine their position relative to a beacon. Distance measuring equipment (DME) tells vehicles how far they are from a ground station.

Using information from multiple stations can help an aircraft understand its position without GPS. While these systems aren't new, they provide another navigation method that doesn't depend on satellites.

Air traffic control can provide additional support. Controllers track aircraft using surveillance systems and can issue headings or other instructions when needed. Pilots also have procedures for handling unreliable or unavailable navigation equipment rather than simply continuing to follow information they no longer trust.

The exact response depends on the situation. Losing GPS during cruise over an area with other navigation coverage is very different from interference during an approach where crews need precise positioning.

Jamming and spoofing create different problems

Jamming overwhelms relatively weak satellite signals with radio frequency interference. A receiver may lose GPS altogether or report that satellite navigation is unavailable. In that situation, the problem may be fairly obvious because the system knows it has lost a source of navigation information.

Spoofing, on the other hand, can be trickier because instead of blocking GPS, it can make the aircraft think it's somewhere it isn't. Instead of just going offline, the receiver may initially produce information that looks believable. That puts more emphasis on cross-checking.

If GPS suddenly places an aircraft somewhere that doesn't agree with its inertial system, ground-based navigation, or other onboard information, the disagreement itself can show that something isn't quite right. Pilots can stop relying on the questionable source and use the navigation equipment and procedures available to them.

Navigation redundancy is built around disagreement

Backup systems can also help a vehicle figure out when something doesn't look right. Modern avionics compare data coming from satellite receivers, inertial sensors, and other navigation equipment. When those sources agree, the system has greater confidence in its calculated position. When they don't, pilots and onboard systems spot the inconsistency rather than blindly trusting one source.

That distinction has become very important as navigation threats have changed. While a complete loss of GPS is one problem, a GPS receiver confidently reporting the wrong information presents another.

For aircraft designers, the challenge is bigger than keeping a position marker on a map. Navigation systems need enough independent information to recognize when a source becomes unreliable and continue operating without it.

New backups are getting more attention

The aviation industry is also exploring more ways to navigate when GNSS isn't available. Some technologies use signals that already exist in the environment as references. Others look at terrain or visual features and compare them with stored maps. Improved inertial sensors can lessen how quickly position estimates drift.

No single technology has to become a universal replacement for GPS. Different aircraft and missions have different requirements, and a system that makes sense for an airliner may not fit a small uncrewed aircraft. 

That's really the idea behind resilient navigation. Aircraft don't have to assume GPS will always be available. They just need another way to figure out where they are when it isn't.