Improvement and modernization of the next generation CNAV-2 navigation message: CNAV-2M

Global Satellite Navigation Systems (GNSS) play a critical role in vital areas including especially aviation, artificial intelligence, autonomous land vehicles, location detection in case of emergency situations and unmanned aerial vehicles. Errors in the navigation message that is sent from a satellite occur usually owing to the channel effects such as noise, attenuation, fading, interference, and physical defects. As in many systems, GNSS uses Forward Error Correction (FEC) mechanism, which is capable of correcting the aforementioned errors that take place on noisy channels without any re-transmission request, to increase the reliability of the navigation message. From the past to the present, four different methods, namely Hamming, Bose–Chaudhuri–Hocquenghem (BCH), Convolutional and Low Density Parity Check (LDPC) codes, have been utilized to correct errors and to check data integrity in GNSS. Apart from these four methods, Turbo, especially Polar coding and its advanced versions, which have been very popular in recent years, have been attracted significant attention from a number of researchers. However, it has been not coincided with any study that compares the performance of methods such as QC-LDPC, Turbo, Polar and CRC-Polar on the message structure of satellite systems and especially the next generation CNAV-2. CNAV-2 differs from the conventional navigation message since it will be broadcasted by all satellite systems consisting of GPS, GLONASS, Galileo and BeiDou in the near future within the framework of interoperability and compatibility program. In this framework, Convolutional, Turbo, LDPC, QC-LDPC, Polar, and CRC-Polar methods that are used in innovative communication systems and have more accuracy and robustness against noise are holistically and elaborately investigated for the new generation CNAV-2. Depending on the light of seven benchmarks under different noise levels, CNAV-2 message is improved and modernized, respectively. For this purpose, the CNAV-2 performance of Convolutional, Turbo, LDPC, QC-LDPC, Polar and CRC-Polar is evaluated based on the benchmarks using the developed simulation model in the Matrix Laboratory (MATLAB) environment. In the light of the obtained results, when the CRC-Polar instead of the available channel coding methods without modifying the message structure, the proposed CNAV-2 message exhibits more robust against the noise and has a better performance. Along with the use of the CRC-Polar, the gain of the sub-frame 1 at 10^−3 BER is measured to be 0.9 dB in comparison with the reference value while those of the sub-frame 2 and 3 at 10^−2 BER are found to be 0.72 dB and 1.63 dB, respectively. In the next step, the number of parity bits used in the CRC-Polar method is reduced and then the CNAV-2M message structure having a lower message length and lower duration is proposed. The analysis results indicate that the CNAV-2M message including 1800 −bit and 18 seconds duration can be reduced to 1500-bit and to 15 seconds duration. The 300-bit reduction in message length and the reduction of 3 second duration mean that there is a significant improvement of approximately 16.6% in message length. Moreover, the CNAV-2M message needs only 10 epoch time instead of 12 epoch time. Since the CNAV-2 message is broadcasted together by all satellite systems, the results obtained from this study will contribute significantly to the modernization studies of the available satellite-based positioning systems. Furthermore, it will play a vital role in the system design for the countries such as Turkey, which aim to develop their own regional satellite-based navigation system in the future in line with domestic and national targets.