Directions 2022: Now 3 years old, QZSS hits its stride

By Satoshi Kogure
Director, National Space Policy Secretariat Cabinet Office, Japan/QZSS Strategy Office

At 02:19:37 UTC on Oct. 26, 2021, a new satellite in the QZSS constellation — QZS-1R — was launched from the Tanegashima Space Center in Japan. It is the fifth satellite in the constellation and the replacement of the first satellite, launched in September 2010. 

As of December 2021, initial on-orbit testing (IOT) and tuning of the precise orbit determination (POD) function in the ground control segment was ongoing. QZS-1R is the first QZSS satellite that will transmit the L1 C/B signal, splitting the power spectrum at the L1 center frequency by adopting BOC modulation on the existing C/A signal, to mitigate interference into the GPS L1 C/A signal. C/B signal transmission was verified during the IOT phase. QZS-1R will transmit the C/A signal continuously until QZS-5, 6 and 7 are launched and the noise floor increased. 

The launch of QZS-1R was a milestone toward a sustainable national infrastructure for Japan. The Japanese government’s Cabinet Office (CAO) is trying to establish more secure positioning, navigation and timing (PNT) services by deploying seven satellites for the QZSS constellation. It will add three satellites to the current four around 2023. 

This will give QZSS an independent PNT capability and enhance GNSS performance as well as robustness, and cover a broader area in the Asia Pacific region. CAO is still investigating the future of the QZSS constellation, including its final configuration and how to provide assured PNT services corresponding to future user requirements. However, it is thought that the full operational capability for Japan at minimum may be declared after the completion of the initial seven-satellite constellation. 

Today, QZSS is providing ranging signals on L1C/A, L1C, L2C and L5 for all users able to acquire and track those signals. Those signals have the same RF properties and almost the same message format as the corresponding GPS signals — they are interoperable. 

In addition, a unique characteristic of QZSS is that it transmits error correction messages available in Japan on separate channels — L1S, L1Sb and L6 — from those used to broadcast its ranging signals. Messaging functions are also provided through QZSS L1S and S-band two-way communication links on QZS-3 in support of disaster mitigation and relief operations in Japan. 

CAO launched the QZSS operational services using a four-satellite constellation on Nov. 1, 2018. Its first three years of operation have provided much knowledge to improve their performance. The averaged signal-in-space user ranging error, a 95 percentile daily statistic, has been improved and achieved less than 1.0 meter, while the specification is 2.6 meters; the best daily value in the evaluation period (Aug. 31, 2020 to Sept. 1, 2021) was less than 0.5 meter for QZS-1, 2 and 3. 

This remarkable improvement was shown on the Centimeter Level Augmentation Service (CLAS). According to the original design of CLAS, transmitted error corrections were for only 11 satellites in the GPS, QZSS and Galileo constellations. After two years of initial operation, we updated the ground control segment for CLAS to increase the number of augmented satellites from 11 to a maximum of 17. This increase in the number of satellites with error corrections leads to excellent improvement of CLAS performance in more challenging user environments such as urban and mountainous areas.  

To improve the service performance further and measure new observables for satellite orbit clock estimation, inter-satellite ranging and two-way ranging functions between tracking station and satellite will be developed for QZS-5 to -7 and following satellites. The ground control segment will also be updated.

Multi-GNSS ADvanced Orbit and Clock Augmentation (MADOCA) precise point positioning (PPP) will be implemented as a practical service no later than 2024. It is aiming to provide decimeter-level PPP service with broadcast of globally available satellite orbit and clock error corrections as well as code-phase and carrier-phase biases. 

PPP has a well-known disadvantage: long convergence time. By using the marginal L6D channel on QZS-5 to -7, the ionospheric delay correction for wide area will be distributed. CAO will try to evaluate how such ionospheric correction could reduce the initial convergence time for the PPP calculation. In an experiment planned in collaboration with Asian Pacific countries, regional stations in the nationwide CORS network will be used for generating such corrections. 

Early or Emergency Warning Service (EWS) is also expanding its service coverage into the region. The common EWS format is being jointly investigated by India, the European Union and Japan under the UN-ICG framework. The QZSS EWS for the Asia Pacific region through the L1S signal on QZS-1R, 2, 3 and 4 will be established after completion of a ground segment update around 2024. 

Also see First Transmission of L1C/B by QZS-1R.