Development of DFMC SBAS Receiver Supporting QZSS L5S Signals

Abstract

In Japan, DFMC(Dual-Frequency Multi-Constellation) SBAS(Satellite Based Augmentation System) messages can be received via satellites, and the environment for conducting demonstrations is already in place. However, there are only a few reports showing its usefulness so far, because no receiver supporting DFMC SBAS has been available in Japan. For this reason, we have developed a receiver that supports DFMC (Dual-Frequency Multi-Constellation) SBAS (Satellite Based Augmentation System) messages transmitted from QZSS (Quasi-Zenith Satellite System). ICAO (International Civil Aviation Organization) is working on the standardization of the next generation SBAS. The next-generation SBAS called DFMC SBAS solves the ionospheric delay on the receiver side by using two frequencies. The conventional method provides grid-by-grid ionospheric delays so that positioning errors occur depending on the position within each grid. Also, it cannot support sudden ionospheric changes because the ground station estimates corrections and uplink the data to satellites before broadcasting. On the other hand, the next-generation DFMC SBAS solves the ionospheric delay on the receiver side, so that stability of positioning accuracy is achieved. Since it supports multiple constellations, the positioning rate is improved with the larger number of satellites it uses. As a result of these, the overall benefit of improved availability is achieved with the next generation SBAS in terms of time and locational “continuity,” “availability of SBAS service even in areas with high ionospheric activity,” and “resilience to frequency interferences and ionospheric storms.” CORE Corporation, with ENRI (Electronic Navigation Research Institute), has been developing receivers that support the augmentation information broadcasted from QZSS. During the trial phase, we used a programmable software receiver for establishing specifications of DFMC receivers and confirmed the capability of DFMC SBAS. QZSS is currently in the verification phase, and we have been working on hardware that support various configurations to carry out more tests. Currently, in Japan, ENRI broadcasts the next-generation DFMC SBAS messages generated from L5S-I signals of QZSS Units 2-4 on trial. With the current QZSS, it is difficult to broadcast messages in the full DFMC SBAS format due to functional restrictions, so the existing SBAS format is temporarily adjusted for use. Broadcasting of messages in the complete format of the next generation DFMC SBAS will start with the QZSS satellites to be launched in and after 2020. The prototype we have developed receives the next-generation DFMC SBAS messages generated by ENRI via QZSS L5S-I signals and realizes the next-generation DFMC SBAS positioning in real-time. This receiver supports not only the specifications of the above-mentioned messages broadcasted on trial but also the specifications of the next-generation DFMC SBAS messages which are 4-bit preamble and Manchester encoding. Also, in order to flexibly respond to the spreading codes used by the satellites, PRN can be changed to any number between 1 to 210. In order to enable the ex-post verification, three types of data are output: “SBAS messages broadcasted,” “observation data used for positioning,” and “ephemeris data.” The prototype provides three positioning modes, NPA (Non-Precision Approach), APV (Approach Procedure with Vertical Guidance), and PA (Precision approach). The positioning results include locations, horizontal and vertical protection levels, and the DOP information. The information on the satellites used to calculate protection level and DOP information are also output. Continuity and availability of DFMC SBAS are confirmed with this information. In this presentation, we will describe the development of the DFMC SBAS receiver for the above QZSS L5S signals, discuss the results of our evaluation of the DFMC SBAS using the developed receiver, and show the improvement of availability and continuity based on the test results in areas with high ionospheric activity and in the environment affected largely by multiple constellations.