## 2 Principles and Technical Architecture of the Satellite Navigation Signal Authentication
### Principles  
Satellite navigation signal authentication technology aims to add encrypted authentication marks to satellite navigation signals to prevent satellite navigation signals from GNSS spoofing attacks. It is a new GNSS anti-spoofing technology that combines information security and navigation signal design. The sender (navigation satellite) uses cryptography technology to generate an \"authentication symbol\", which is embedded in the existing satellite navigation signal and broadcast to users. The receiver (GNSS user terminal) verifies the \"authentication symbol\" to confirm whether the received navigation signal is from a real satellite in orbit, and whether the navigation message has been forged or tampered with [21]. Satellite navigation signal authentication technology has the following characteristics:One-way broadcast.  
The satellite navigation signal uses the navigation satellite broadcast signal to provide PNT services for terrestrial users, and its signal characteristics have the characteristics of one-way broadcast. Therefore, satellite navigation signal authentication technology should be based on the broadcast system authentication framework.  
Signal disclosure transmission.  
Satellite navigation signals use the public signal structure to broadcast signals, and their signal authentication needs to have the characteristics of public signal transmission.  
The authentication of satellite navigation signals will not affect existing GNSS services, so its authentication signal design should be compatible with an existing signal structure.  
Satellite navigation signals include the carrier, pseudocodes, and message. The newly added authentication mark can be added to the navigation message [22] and spreading spectrum codes [23]. Figure 1 shows the generation of the navigation message including authentication message and the spreading spectrum code including authentication code. Therefore, the satellite navigation signal authentication type is divided into Navigation Message Authentication (NMA) and Spreading Code Authentication (SCA) [24].  
NMA uses message bit-level authentication to realize navigation source authentication. Its advantage is that the modification of the existing signal system is small and the signal modulation method is not changed--it is just used to upgrade the software of the user receiver. The engineering realization cost is small. The Galileo E1 OSNMA structure is shown in Figure 2. Galileo reserved a 40-bit message in the early ICD, and the ICD announced in 2021 clarified that the 40-bit message is the navigation authentication message [25].  
SCA adopts the characteristics of unpredictable authentication spreading chips, and implements authentication processing in the power domain, which can provide spoofing protection in the pseudorange domain. The typical SCA is the CHIMERA signal, as shown in Figure 3. Based on the TMBOC (Time-Multiplexed Binary Offset Carrier) signal, the 1 ms sector is divided into 31 segments via a combination of time division and time hopping, and different authentication channels (fast channel and slow channel) are assigned for each  
Figure 1: Satellite Navigation Signal Authentication.  
segment. The authentication codes are randomly replaced for 29 BOC(1,1) in each segment of 33 chips, and the 4 BOC(6,1) chips are never modified [26].  
Compared with NMA, SCA can provide spoofing protection in the pseudorange domain, and it has higher security. However, the SCA authentication chip needs to be delayed to the user receiver; the receiver needs to buffer the sampled data so the implementation cost of the receiver is relatively costly. Table 1 shows the comparison of NMA and SCA.