Global Navigation Satellite Systems (GNSS) are the invisible backbone of modern navigation, enabling everything from aircraft routing and maritime shipping to smartphone maps and precision agriculture. But as our dependence on GNSS has grown, so too have the risks of malicious interference. One of the most concerning threats is GNSS spoofing, which is the deliberate broadcast of false satellite signals designed to mislead receivers about their true position. Unlike simple signal jamming, which blocks positioning entirely, spoofing deceives devices into reporting inaccurate locations, sometimes hundreds of kilometers away.
The unintended consequences of GNSS spoofing
Unfortunately, GNSS spoofing has become a regular occurrence with the potential for severe consequences when precise and reliable positioning is critical. Recently, GNSS spoofing has been mostly observed in and around conflict areas such as international borders. Some countries even use these techniques to safeguard their infrastructure, but there can be unintended consequences.
Unlike small errors measured in centimeters or millimeters, a spoofing attack can cause a receiver to report a position that is meters, tens of meters, or even hundreds of meters away from its true location. In conflict regions, vessels are being spoofed to output positions across the Black Sea, hundreds of miles away.
Legacy GNSS signals are the primary target for bad actors, as most precise positioning relies on these signals, and it’s constantly getting easier and cheaper for people to fake the message.
“Spoofing of the ‘legacy’ GNSS signals as a primary target is due to the fact that the GPS L1 C/A is the most used navigation message for civilian applications, meaning you can impact the largest number of GNSS users. Additionally, the legacy signals are broadcast from most of the satellites in the constellation. The more modern messages, such as GPS CNAV-2 and GAL I/NAV and F/NAV are more resilient with features such as error detection and authentication - but not all satellites in the constellation are necessarily capable of broadcasting the more modern signals”, explains Jason Evans, Portfolio Manager for Real Time Corrections at Trimble.
Solution to mitigate spoofing attacks on GPS and satellites
To combat GNSS spoofing, Trimble has introduced Trimble RTX-NMA (Navigation Message Authentication), the first solution on the market to mitigate spoofing attacks on the GPS and BeiDou satellite constellations. Trimble RTX-NMA leverages the Trimble RTX correction service and enhances the security and integrity of GNSS navigation messages for all Trimble ProPoint receivers. Used in conjunction with Galileo OSNMA, Trimble customers now have three constellations protected from spoofing attacks.
Effective protection against spoofing requires multiple layers of defense (as illustrated in the graphic below) that are implemented in Trimble receiver firmware ranging from authentication of data messages, receiver autonomous integrity messages, GNSS autocorrelation, signal-to-noise ratio correlation to multi-antenna checks. RTX-NMA is the next layer of defense, not only adding protection from bad actors spoofing GNSS signals, but also identifying Signal In Space Errors.
How the solution works
Trimble RTX-NMA seeks to detect both fake GNSS signals and faulty ephemeris data through real-time authentication that ensures navigation messages from multiple RTX reference station receivers are genuine and trustworthy.
Evans: “Using the global network of RTX reference stations, Trimble is able to continuously observe all GNSS satellites from multiple locations. These observations provide multi-frequency code and carrier phase measurements, along with the demodulated raw navigation messages for all GNSS satellites. This information is streamed to Trimble’s RTX control centers, where RTX server software validates and authenticates the broadcast ephemeris data, and evaluates the range residuals to the broadcasting GNSS satellites.”
Within the RTX correction stream, faulty GNSS satellites are flagged, and authentication data for the NAV messages of healthy GNSS satellites is provided. Trimble receivers can interpret this information from the RTX correction stream and weight observed signals accordingly.
The rover receiver performs the following checks in real-time. It simultaneously tracks the live navigation message directly from the GNSS satellite and receives the RTX correction stream from Trimble. Using the same cryptographic algorithm as the control center, it generates its own tag from the live navigation message received, then compares this locally generated tag with the authenticated tag it received from the RTX stream.
“If the two tags match, the receiver knows the navigation message is authentic and has not been spoofed or corrupted. If the tags do not match, the receiver knows the message is invalid. This could be due to a faulty satellite or a malicious spoofing attempt. In either case, the receiver immediately rejects the message and its measurements, preventing a false position from being calculated”, concludes Evans.
GNSS spoofing presents challenges that can affect both critical systems and everyday users, but solutions are steadily advancing. Tools like Trimble RTX-NMA and Galileo OSNMA show how authentication and monitoring can strengthen positioning reliability, and a layered approach provides additional protection. As these defenses become more widely adopted, GNSS users will be better equipped to maintain accurate, dependable navigation without significant disruption.
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