Print The shortwave time service system is a vital land-based wireless time service solution, serving as a supplement and backup to the global navigation satellite system. It ensures that time users can access reliable timings, especially in extreme situations
With the continuous development of the economy and society, the electromagnetic environment of shortwave channels has become increasingly complex, and the existing amplitude modulation (AM) of shortwave time signals has been difficult to guarantee the performance of the time dissemination service of the shortwave time dissemination system.
In order to enhance the anti-interference receiving ability of shortwave time signals and improve the performance of shortwave time service system, researchers from the National Time Service Center(NTSC) of the Chinese Academy of Sciences(CAS) began the research on the new system of shortwave time signals. The researchers proposed to introduce spread spectrum technology into BPM shortwave time service system and achieved a new shortwave time signal modulation scheme (DCTCDM scheme) based on linear frequency modulation (Chirp).
The results were published in the academic journal Remote Sensing under the title "Research and Design of BPM Shortwave Time Signal Modulation Technology Based on Chirp".
Researchers added the DCTCDM modulation signal on the basis of retaining the original AM modulation signal to achieve the compatibility of the old and new signal systems. On the premise of not affecting the normal work of existing users, the DCTCDM scheme has higher signal detection accuracy, stronger anti-multipath fading ability and anti-interference ability. The spread spectrum gain can reach 24 dB, and the multipath resolution ability is at least 125 μs.
Researchers carried out extensive field measurement and comparison experiments in Xi'an, Luonan, Xuyi, Sanya, Changchun, Kashgar and other places to verify the effectiveness of the DCTCDM scheme. Figure 1 shows some test results in Sanya. It can be seen from it that the timing accuracy of the DCTCDM scheme is 10 μs, which is significantly better than that of the AM scheme. Figure 2 shows the test results of Xuyi for one month. It can be seen from it that the availability rate of the time signal of the DCTCDM scheme is significantly better than that of the AM scheme.
This research has improved the availability rate of shortwave time signals and the timing accuracy of users, and is of great significance for promoting shortwave timing technology and supporting the construction of the next-generation BPM shortwave time service system.
Figure 1: Part of comparison test results in Sanya (Image by NTSC)
Figure 2: The results of a one-month comparison test in Xuyi (Image by NTSC)