Intermodulation Component-Based Authentication for Civilian GNSS Signals

We propose a navigation signal authentication scheme for civilian GNSS receivers that exploits the intermodulation components introduced by constant envelope modulation without relying on modernized authenticatable signals or direct access to authorized spreading codes. Since the product of multiple authorized spreading codes is unpredictable and does not reveal the original codes, the spreading code of the intermodulation component can serve as an authentication feature similar to that of authorized signals. The receiver obtains the intermodulation spreading code via a communication link, then authenticates the GNSS signal by detecting the presence of this code through a correlation-based hypothesis test. We analyze the scheme using the operational GPS L1 signal as an example and compare its performance with Chimera (proposed for GPS), OSNMA (proposed for Galileo), and authorized spreading code authentication in PROSPA. Results show that the proposed scheme achieves robustness and security comparable to Chimera, while avoiding the authorization restrictions associated with authorized codes. Under a 63.82 kbps communication rate and a civilian signal C/N0 of 30 dB-Hz, its authentication efficiency exceeds Chimera's fast channel, with substantially lower data storage requirements.