# A NavCom Signal Authentication Scheme Based on Twice Two-Way Satellite Time Transfer
## 3 Performance Analysis
### Security  
To mitigate generated spoofing interference, the authentication scheme can enhance information authentication or employ digital signatures on the navigation signal messages. This effectively blocks unauthenticated signals at the receiver on the ground, thus preventing generated spoofing interference. There are well-established methods to counter generated spoofing interference in the field of navigation, such as the navigation message authentication method OSNMA and the spreading code encryption method Chimera. However, these methods are not the central focus of the proposed authentication scheme, so they will not be evaluated here.  
For repeater spoofing interference, it can be seen from the analysis in the previous section that, in order to optimize the detection performance of the present NavCom signal authentication scheme, it is necessary to increase the magnitude between the twice TWSTT signal transmitting elevation angles as much as possible. It is recommended to set them to \(30^{\circ}\) and \(90^{\circ}\), respectively. When the values of the two transmitting elevation angles are fixed, the detection probability is affected by the vertical distance between the satellite and the user terminal, the signal CNR, the pseudo-code rate, the code-ring parameters, and the false alarm probability.  
Let the spreading code rate of both uplink and downlink signals be 1.023 MHz, the loop noise bandwidth be 1 Hz, the coherence accumulation time be 1 ms, and the front and rear correlator spacing be 1 chip. When the false alarm probability is taken as \(10^{-3}\) and \(10^{-6}\), respectively, the relationship between the detection probability and the signal CNR of the proposed authentication scheme at different satellite-to-user vertical distances is shown in Figure 9.  
As shown in Figure 9, the authentication scheme proposed in this paper can achieve highly reliable authentication even with a low signal CNR and can effectively resist repeater spoofing interference. Additionally, the proposed method is minimally impacted by the vertical distance between the satellite and the user terminal, making this factor negligible.  
To further analyze the effect of the spreading code rate on the system performance, let the vertical distance between the satellite and the user terminal be 1000 km, and keep the code loop parameters of the receiver unchanged. Figure 10 illustrates the relationship between the detection probability and signal CNR of the proposed method at different spreading code rates. This relationship is graphed for two cases of false alarm probability, specifically, \(10^{-3}\) and \(10^{-6}\).  
When examining Figure 10, what is striking about the figure is that the NavCom signal authentication scheme exhibits enhanced reliability as the spreading code rate increases. For instance, when the spreading code rate is set to 1.023 MHz, the signal CNR of 28 dBHz is necessary to attain a detection probability of 100%. Surprisingly, when the spreading code rate is increased to 10.23 MHz, a considerably lower signal CNR of 18 dBHz is sufficient to achieve the same 100% detection probability.  
In summary, the analysis results demonstrate that the proposed authentication scheme exhibits highly reliable authentication even with a low signal CNR. Moreover, it effectively addresses repeater spoofing interference, further enhancing its robustness and security.