A Study of Reduction of Time-to-CED in GPS CNAV Message Structure With Reduced CED

Legacy GNSS requires several minutes to obtain accurate user position, velocity, and time information under cold start. However, with the advancement of navigation aiding systems and telecommunication systems, fast positioning is increasingly preferred by general users in many areas, even if the initial positioning accuracy is somewhat lower. From this perspective, several methods are proposed for fast positioning and the Galileo system has adopted reduced clock and ephemeris data (CED) in the I/NAV navigation message. This approach shortens the time required to read navigation data, thereby reducing the Time-to-First-Fix (TTFF). In a similar manner, this paper proposes an improved GPS CNAV message structure that incorporates reduced CED, in which reduced CED is allocated to each time slot, thereby significantly reducing the TTFF compared to the original GPS CNAV. Along with this, a functional TTFF calculation method is presented, compared to the conventional approach. This method enables easy performance evaluation of new GNSS/RNSS signals while meeting users’ requirements.