Generation of the QZSS L1-SAIF Message from the MADOCA Realtime Products

Japan has been developing QZSS (quasi-zenith satellite system) as a regional satellite navigation system operating together with the US GPS. The QZSS satellites broadcast GPS-like ranging signals on L1, L2, and L5 frequencies including the wide-area augmentation channel on L1 (L1-SAIF signal: submeter-class augmentation with integrity function). QZSS satellites are launched into a 24-hour elliptic orbit inclined 43 degrees in order to broadcast signals from the high elevation angle into urban canyons. With a full constellation of QZSS consisting of three or more satellites on the same ground-track, one or more QZSS satellites always exist at around the zenith so that they provide users additional ranging signals as well as augmentation information from the zenith all the time. So far, the first QZSS satellite, QZS-1, is on the orbit and available for the experimental purposes. The authors have developed L1-SAIF Master Station (L1SMS) which generates the wide-area differential correction regarding orbit and clock of the ranging satellites as well as ionosphere corrections. Generated L1-SAIF message is transmitted to the QZSS MCS and thus broadcast from the QZS-1 satellite. Meanwhile, JAXA (Japan Aerospace Exploration Agency) has been conducting the MADOCA (Multi-GNSS Advanced Demonstration Tool for Orbit and Clock Analysis) project. MADOCA is a software tool which estimates clock and orbit of navigation satellites including GPS, GLONASS, and QZSS with the highest accuracy. Recently MADOCA became capable of generating its products in realtime and began broadcast via the Internet. The primary purpose of MADOCA is PPP processing. The products of MADOCA, in fact, include clock and orbit corrections to broadcast ephemeris. This means it is possible to generate the L1-SAIF message, which includes WADGPS corrections, based on the products of MADOCA. Currently the L1SMS generates the L1-SAIF message based on dual frequency measurements of a monitor station network separate from MADOCA. Using MADOCA, the L1SMS operates without the network. In terms of the perspective of users, the L1-SAIF might work, in this strategy, as a kind of simple version of the MADOCA-PPP on the QZSS L1 SBAS-like signal. Here the authors have conducted the trial to apply the products of MADOCA to generation of L1-SAIF WADGPS correction messages. Because MADOCA operates for the ionosphere-free combination rather than the L1 single frequency measurement, in this case, L1-SAIF message is generated for dual frequency mode users. The performance is tested and it is verified that MADOCA is available for generating L1-SAIF messages.