###### Abstract  
The Alpha navigation system is the only operating radio system based on very-low-frequency (VLF) signals that can be used to research VLF navigation, timing, and ionospheric characteristics. The detection of the Alpha navigation signal is the key step in the Alpha receiver; however, the received Alpha navigation signal is susceptible to noise and mutual interference, which deteriorates signal detection performance. This paper presents a multichannel orthogonal correlation method for Alpha navigation signal detection. Once the three frequency signals of the Alpha navigation system are obtained using a notch filter, station identification is realized using a multichannel orthogonal correlation method and signal format. The selection of key parameters and the detection performance under noise and mutual interference are analyzed. This method's detection probability exceeds 90% when the signal-to-noise ratio (SNR) is greater than \(-10\) dB. The influence of mutual interference on the signal correlation peak is less than 1% when the signal-to-interference ratio (SIR) of the mutual interference is greater than \(-28\) dB. The proposed method is verified using an actual signal collected using an Alpha receiver. The results show that an Alpha signal can be detected at an extremely low SNR. This method has strong practicability and satisfies the application requirements of an Alpha receiver. VLF radio signals; alpha navigation system; correlation detection; station identification +
Footnote †: journal: _sciences_  
0000-0001-6500-7999-22000]Yan  
0000-0002-3300-3300]Yan  
0000-0002-3300-3300]Yan  
0000-0002-3300-3300-33000]Yan  
0000-0002-3300-3300-3300-3300-33000-33000-33000-33000-33000-33000-33000-33000-330  
operating frequency (low- and high-frequency), operating mode (land- and satellite-based), and signal strength (high- and low-level). In general, such signals do not lose their service capabilities when subjected to intentional or unintentional interference. Therefore, many countries are now researching VLF-based radio navigation and timing systems to reduce the risk of relying on GNSS [7, 14]. The most representative is the Spatial, Temporal and Orientation Information in Contested Environments (STOIC) project released by the United States Defense Advanced Research Projects Agency (DARPA) on 12 June 2014, which aims to find a positioning, navigation, and timing (PNT) system for various military services that does not rely on a GPS but has the same level of accuracy as a GPS, where the signals used are VLF signals [15, 16]. According to the test results published in 2019, the positioning accuracy after the correction of the propagation delay of the VLF signals can reach 40 m.  
The Alpha navigation system is a good source of signals for research on VLF technology. Its signal transmission frequency ranges from 10 to 15 kHz, and the frequency reference of the system is generated by a highly stable cesium atomic clock [17]. Therefore, Alpha can not only be applied to navigation but also to research VLF timing technology, new VLF navigation technologies, solar activity monitoring, and signal propagation characteristics [5, 17]. Related applications and research based on Alpha must accurately measure the field intensity and phase signal parameters of different stations with different frequencies using receiving equipment. Signal detection is the first step in the signal-receiving equipment of an Alpha navigation system. The purpose of this is to automatically detect the expected station signal and determine the starting position of the signal at different frequency points of the expected station. However, owing to the large number of stations in the Alpha navigation system and because each station broadcasts three different frequency signals, it is possible to simultaneously receive different frequency signals from multiple stations, which causes mutual interference with the expected stations and makes signal detection difficult.