The status of QZSS

Space Policy and QZSS Plan

Japan’s Cabinet Office (CAO) is currently developing the Quasi-Zenith Satellite System (QZSS) seven-satellite constellation (7SV-const.)

Japan’s cabinet revised our new basic space policy plan on June 13, 2023. It states that the number of QZSS satellites should be increased from seven to 11 to improve stability and reliability as one of the social infrastructures by securing backup function for coming full-scale use and to reduce the dilution of precision (DOP).

On June 12, 2024, the National Space Policy Secretariat revised the action plan for the satellite positioning system for the first time in three years. According to this plan, the completion of the QZSS 11-satellite vehicle constellation is anticipated by the late 2030s.

Brief History of QZSS

Since 2006, the Ministry of Education, Culture, Sports, Science and Technology (MEXT), the Japan Aerospace Exploration Agency (JAXA), the Ministry of Internal Affairs and Communications (MIC), the Ministry of Economy, Trade and Industry (METI) and the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) have collaborated to develop the world’s first centimeter-level positioning satellite.

The first QZS satellite was launched in September 2010, for what was then envisioned as a four-satellite regional satellite navigation system, also known as MICHIBIKI. In September 2011, the Cabinet approved a plan to build the four-satellite constellation (4SV-const.) in the late 2010s, with the goal of building a 7SV-const. in the future. The budget was approved in JFY2012 to promote this as a national project.

In 2017, QZS-2, 3 and 4 were successfully launched and a 4SV-const. was established. On Nov. 1, 2018, QZSS officially began service. On Oct. 26, 2021, the successor to the first MICHIBIKI was launched.

Development is underway to launch three additional satellites for the 7SV-const. sequentially from JFY2023 to JFY2024, depending on the development status of the H3 launch vehicle.

To improve resilience for more stable positioning and improved DOP, CAO has started a study to transition from a 7SV-const. to an 11SV-const. (See Figures 1 and 2)

Service Overview

The 4SV-const. provides GPS Complementary Service (PNT service); GNSS Augmentation Service, i.e. Sub-meter Level Augmentation Service (SLAS), Centimeter Level Augmentation Service (CLAS), MADOCA-PPP and satellite-based augmentation service (SBAS), as well as a messaging service. 

The QZSS constellation consists of a geostationary orbit (GEO) satellite, QZS-3 and three QZO satellites.

QZSS’s performance of the positioning, navigation and timing (PNT) service and signal-in-space user range error (SIS-URE) was much better than specified. Evaluation results are shown in Table 1. 

The evaluation period was “08/01/2023＃07/31/2024.” The specification is less than 2.6 m (95%).

Toward Seven-Satellite Constellation

Three additional satellites will be positioned in an inclined geostationary synchronous orbit (IGSO) at 90.5° east longitude and in a quasi-geostationary orbit at 175° west Longitude. This constellation is designed to ensure that multiple satellites are visible at high elevation angles, with the goal of having more than four satellites visible for extended periods of time, ultimately improving the DOP.

The positioning signals of 7SV-const. are shown in Table 2. QZS 5 to 7 transmit L1C/B.

After the QZS-5 service starts, QZS1R’s signal will switch from L1C/A to L1C/B. The QZSS Navigation Message Authentication (QZNMA) service for other GNSS is included in L6E signals.

QZS5-7 development is ongoing. A system-level test is in progress. The plan is to launch QZS 5-7 around 2025. QZS-5 and 6 have almost finished the system-level test. QZS-7 finished the electrical test, and now it is in the environmental test phase.

QZSS antenna patterns for SSV users, published since Aug 2023, correspond to the ICG WG-B recommendation “Release of GNSS Transmit Antenna Patterns Including Side Lobes,” submitted on Dec. 11, 2019. 

QZSS satellite information for precise orbit determination (POD) users was also updated online.

The QZSS ground system, capable of operating with seven satellites, was completed in August 2023. The only remaining task is to conduct compatibility tests between the satellites and the ground radio frequency. The QZSS ground system consists of two master control stations located in Hitachi-Ota and Kobe, as well as 10 satellite tracking and control stations (telemetry, tracking and command stations), primarily situated in the southwestern islands. Additionally, there are more than 30 monitoring stations worldwide, established in cooperation with various countries.

The following additional services  began this year or will begin next year:

Users obtain position, velocity, and time (PVT) solutions using ranging signals provided by the 7-SV const. The Navigation Message Authentication (NMA) began in April 2024.

Multi-GNSS Advanced Orbit and Clock Augmentation — Precise Point Positioning (MADOCA-PPP) covers a major part of the Eastern Hemisphere and provides PPP error corrections on the L6 signal. It enables highly precise positioning by providing augmentation data to correct some GNSS errors and provides two service levels, i.e., (a) global-PPP and (b) fast-PPP with ionospheric correction.

Operational service started on April 1, 2024 and Internet distribution of MADOCA-PPP in July 2024.

Expanded Satellite Report for Disaster and Crisis Management (DC report), Early/Emergency Warning Satellite Service (EWSS).

In addition to the current DC report service generated by the Japan Meteorological Agency, other disaster-related reports, called L-alert and J-alert, such as an evacuation order, began on April 1, 2024.

The Global PPP service provides highly precise positioning where QZSS L6E signals can be received.

It provides satellite-dependent error corrections (orbit, clock, code/phase bias) by QZSS L6E signals.

Faster PPP using observation data from regional/nationwide Continuously Operating Reference Stations (CORS). In addition to global PPP, QZSS L6D (QZS-6 and 7) signals will provide regional error corrections (ionospheric correction).

MADOCA-PPP began operational service on April 1, 2024.  For MADOCA’s stable operation, the GNSS Integrated Data Sharing System (MIRAI) was released in April 2022. The ionospheric correction data will be broadcast to reduce the initial convergence time starting in 2025.

MADOCA-PPP service was defined and described on PS-QZSS 003, published on March 17, 2022.  Internet distribution of MADOCA-PPP, including ionospheric correction data, began in July 2024 to promote MADOCA-PPP utilization. MADOCALIB, test library of MADOCA-PPP, has been open-sourced and made available on GitHub this April, and updated to process ionospheric correction data in July 2024. MADOCA-PPP performance results from May to August 2024 using IGS monitoring station are shown in Figure 3.

Better initial convergence time than the defined specification and approximately 10 cm of accuracy are confirmed.

Thanks to the cooperating countries, ionospheric correction data can be generated, and the initial convergence time reduction was confirmed in all areas.

CAO started the Internet distribution, including the ionospheric correction data for these areas.

QZNMA

QZNMA began on April 1, 2024, to enhance the constellation’s resilience against spoofing attacks. 

Navigation messages in the following signals are authenticated using the Elliptic Curve Digital Signature Algorithm (ECDSA P256).

■ QZSS signals (L1C/A(C/B), L1C, L5) are directly protected by self-authentication.

■ GNSS signals (GPS: L1C/A, L1C, L5, Galileo:E1b, E5a) are protected by cross-authentication (L6E).

The interface specification (IS-QZSS-SAS-001) is now available on our website. All the verifications were successful.

QZSS Applications

As of September 2024, approximately 434 products are compatible with QZSS (See Figure 4). New applications such as autonomous driving, drone operations and buoys for real-time ocean tide monitoring are emerging. We expect that many applications and devices using QZSS will emerge. We appreciate your cooperation and support of QZSS.