7-satellite constellation is planned to be implemented in around 2023

Feb 2018 | No Comment

Takeyasu SAKAI

Principal Researcher, Navigation Systems Department, Electronic Navigation Research Institute, National Institute of Maritime, Port and Aviation Technology, Japan while sharing the status and prospects of QZSS and MSAS programmes

Please update us with the status of QZSS.

Following QZS-1 launched in 2010, additional three satellites has been launched in 2017 to complete the initial constellation of QZSS. QZS-2, -3, and -4 satellites were launched in June, August, and October, respectively, from Tanegashima Space Center by Japanese H-IIA launcher. Among these, QZS-3 is put into geostationary orbit at 127E while other three are IGSO (inclined geosynchronous orbit) with inclination angle of around 43 degrees. The operator of QZSS is determined as QZSS System Service Inc. (QSS), a private company, and the operation of QZS-1 was transferred from JAXA (Japan Aerospace Exploration Agency) to them. They are now working for final preparation to begin the service in April 2018.

What are the key features of QZSS?

First of all, QZSS has two missions: Positioning Service and Messaging Service. For Positioning Service, it provides ranging service and augmentation service both on L-band signals. For Messaging Service, QZSS provides mobile communication services for disaster and crisis management including (i) Q-ANPI: S-band two-way data communication service for information on status of evacuation shelters, number of evacuees in shelters, and evacuee condition during a disaster; and (ii) DC-Report: Downlink short messages related to disaster warnings such as about earthquakes, tsunamis, volcanic eruptions, floods, and crisis management information on L1 frequency.

As the ranging service, QZSS transmits GPS-like ranging signals on L1, L2, and L5 frequencies to improve signal availability in urban and mountain areas.

Additionally, QZSS has augmentation service including: (i) Submeter Level Augmentation Service (SLAS) on L1, code-phase differential correction service for mobile users; (ii) Centimeter Level Augmentation Service (CLAS) on L6, carrier-phase differential correction service for precision applications; (iii) PPP Service by RTCM SSR messages on L6; (iv) L1 SBAS service by QZS- 3 GEO; and (v) Transmission of test signal for DFMC (dual-frequency multi-constellation) L5 SBAS.

It should be noted that QZSS employs both IGSO and GEO satellites so that it transmits signals from high elevation angle to the intended users. This feature is actually the origin of the word “Quasi-Zenith”. Anytime, at least one or two QZSS satellites serve to you from the zenith.

What kind of applications QZSS is being used for?

Many applications related to positioning are investigated for harmonization with QZSS. For example, ranging service will improve availability of position fix in urban areas for vehicles and pedestrians by signals from the zenith, and the CLAS service will be quite useful in agriculture and construction applications.

Would you like to share any application in disaster management?

Yes. QZSS has DC-Report which is a small-capacity downlink channel using a part of the L1S message stream for disaster and crisis management potentially used by other States in the coverage area.

How QZSS is compatible with other GNSS systems?

QZSS transmits GPS-like signals for ranging service, i.e., L1C/A, L1C, L2C, and L5IQ; It is quite easy to support QZSS ranging service signals in addition to GPS signals. The QZSS system time is maintained to align with GPS time so that users can ignore the time offset between QZSS and GPS.

The L1 SBAS service will be, of course, fully compliant with the international standards for civil aviation.

The detail of each signal is specified by the IS-QZSS document issued by the QSS and available online.

Does QZSS have any advantage over other systems?

It has been confirmed ranging accuracy of QZS-1 reaches the level of 0.4m. The initial constellation will follow this performance and further improvements are expected for QZS-5 and later satellites.

CLAS and PPP service might be also advantage of QZSS; These precise positioning services will be provided from the space for free of charge without any other radio links.

How can other countries of the region can take benefit of QZSS?

QZSS signals are available for any users in the service area. Simply receive and use them for free of charge. In other words, choose QZSS-supported receivers.

How to address the issues like interference, jamming and spoofing?

QZSS transmits GPS-like signals so the ability against the issues is the same level with GPS in terms of signal interface. The location of QZSS ground facilities are chosen with careful survey for RFI and continuously monitored. There is some early discussion to implement signal authentication by L1 SLAS signal or L5S Q-channel but it has not been decided yet. It also takes some time for standardization of such an idea.

What are the future plans for QZSS?

Following the initial constellation with four satellites already on the orbit, 7-satellite constellation is planned to be implemented in around 2023. Ranging performance will be improved with additional satellites and it will be enabled to have position fixes independent from GPS.

Please share the status and future plans of MSAS?

The current MSAS, MSAS V1, has been operational since September 2007. Currently it is transmitting SBAS signals with PRN 129 and 137, generated based on measurements at 6 monitor stations, via the MTSAT-2 geostationary satellite. MSAS serves for horizontal navigation only, i.e., currently vertical guidance is not available.

In 2020, the MSAS mission will be taken over by MSAS V2 with QZS-3 GEO satellite. At the same time, MSAS ground facilities will be fully replaced with modernized MCS facility and 13 monitor stations. After that, the update to MSAS V3 which provides vertical guidance service is planned to be done in 2023, along with the launch of the second GEO for QZSS.

Currently, the DFMC L5 SBAS is being discussed for finalization of the international standards. QZS-2 and following satellites have L5S augmentation signal for testing L5 SBAS; Actually we have been transmitting L5 SBAS test signal with QZS-2 since August 2017. It is expected to realize MSAS V4 as an implementation of the DFMC L5 SBAS.

What are key sectors where MSAS being used? Could you highlight its role in aviation sector?

In Japan, there are a lot of small remote islands. Some regional airlines flying to airports equipped with less navaids are using, or planning to use, MSAS to improve their availability. Emergency response aircraft including helicopters are also using, or expecting, MSAS services.

Using MSAS, airlines do not need to conduct RAIM prediction before taking off. For this reason, some major airline has decided to take the SBAS option for their brand-new airplanes. Implementing vertical guidance service will further push them to use MSAS.