Galileo NMA signal unpredictability and anti-replay protection

Galileo is studying the addition of Navigation Message Authentication (NMA) to its Open Service. NMA cryptographic data is mostly unpredictable, and this unpredictability can provide certain protection against replay attacks, depending on the user environment and how the signal is processed in the receiver. This paper first characterises Galileo NMA symbol unpredictability, which depends on how the NMA data is packaged within the I/NAV message, and how the bits are coded into symbols and subsequenly interleaved. Next, a replay protection method is proposed and preliminarily characterised. By storing the first chips of every unpredictable symbol, a receiver can create a synthetic sequence whose correlation gain will be low if the tracked signal is being spoofed.