Description
A Quasi-Zenith Satellite (QZS) is a key component of the Japanese regional satellite navigation augmentation system. Unlike traditional geostationary satellites, QZS satellites operate in highly inclined, elliptical orbits known as Quasi-Zenith Orbits (QZO). This specific orbital geometry ensures that at least one satellite is positioned nearly overhead (at a high elevation angle) over Japan and the Asia-Oceania region for approximately 8 hours per day. This high elevation angle is crucial because it significantly reduces signal blockage from buildings, mountains, and other obstacles compared to satellites lower on the horizon, which is a common problem in urban canyons.
From a 3GPP perspective, QZS satellites are integrated into mobile networks primarily to provide enhanced positioning data. They transmit standard Global Positioning System (GPS) compatible signals (L1C/A, L1C, L2C, L5) as well as unique Japanese augmentation signals (L1S, L5S, L6). These augmentation signals carry correction data and integrity information to improve the accuracy, availability, and reliability of positioning services for User Equipment (UE). The system is designed to interoperate seamlessly with other Global Navigation Satellite Systems (GNSS) like GPS, Galileo, and BeiDou.
In the 3GPP architecture, support for QZS is defined within the protocols for Assisted GNSS (A-GNSS). The network can provide assistance data to the UE, which includes precise orbital information (ephemeris) and clock correction data for QZS satellites, reducing the time-to-first-fix and improving positioning sensitivity. The specifications detail the message formats and procedures for the UE to receive and utilize QZS signals, either standalone or in combination with other GNSS constellations. This integration allows mobile operators to offer highly accurate location-based services, emergency caller location, and other applications dependent on precise positioning.
Purpose & Motivation
The Quasi-Zenith Satellite system was created to address the significant limitations of traditional GNSS, particularly GPS, in the specific geographical and urban environment of Japan. Japan's topography features dense urban centers with skyscrapers creating deep 'urban canyons' and mountainous regions that frequently block signals from satellites near the horizon. Standard GNSS constellations often do not provide sufficient satellite visibility in these conditions, leading to degraded accuracy, long positioning times, or complete service outages.
Historically, reliance solely on GPS posed challenges for critical applications in Japan, including vehicular navigation, disaster management, and precision agriculture. The QZS concept was developed to provide a regional augmentation and complement to global systems. By ensuring a satellite is almost always near the zenith over Japan, the system guarantees a strong, unobstructed signal source. This directly solves the problem of signal availability. Furthermore, the QZSS transmits augmentation signals that provide correction data, improving positional accuracy from the meter-level to the centimeter-level for authorized services, and integrity information that alerts users if the system should not be used for safety-of-life applications. Its development was motivated by national requirements for resilient, high-precision positioning infrastructure independent of sole reliance on foreign GNSS systems.
Classification
Detected Changes Across Releases
from 3GPP Change RequestsSpecific changes extracted from the „Change history“ tables of 3GPP specifications (19 CRs across 4 releases). Complements the general historical overview above with the evidence-based evolution of this function.
Studied in Rel-9, normative work from Rel-16.
In Release 16, support for the Quasi-Zenith Satellite System (QZSS) was formally integrated into the A-GANSS testing framework, specifying its signal power levels and test scenarios. The release introduced the QZSS L1C signal alongside the existing L1 C/A, L2C, and L5 signals, defining its relative power level for conformance testing. Furthermore, it established that in supported test cases, a GPS satellite can be replaced by a QZSS satellite with the respective signal support.
In Release 17, the QZS (Quasi-Zenith Satellite) function was updated with clarifications and corrections for High Accuracy GNSS (HA-GNSS) support. This included providing NMEA GGA sentence information in high accuracy location estimates and correcting field descriptions for metrics and the GNSS-SSR-URA. The release also clarified the alignment of SSR (State Space Representation) orbit and clock integrity bounds and corrections with external standards like RTCM.
- NMEA GGA sentence info in high accuracy GNSS location estimates [HA-GNSS-NMEA] TS 37.355CR0349
- Correction on the GNSS Orbit and Clock Integrity Bounds in TS 37.355 TS 37.355CR0377
- GNSS SSR BDS orbit emphemeris reference clarification to align with RTCM TS 37.355CR0461
- Field description correction for HA-GNSS metrics TS 37.355CR0474
- Correcting field description and definition of GNSS-SSR-URA TS 37.355CR0400
- Clarifying Galileo NAV message in the GNSS Navigation model to clarify SSR clock correction signal reference TS 37.355CR0412
+ 1 more changes
In Release 18, the QZS (Quasi-Zenith Satellite) function was enhanced with new assistance information for GNSS Line-of-Sight/Non-Line-of-Sight (LOS/NLOS) conditions and the provision of SSR Satellite PCV (Phase Center Variation) Residuals data. Furthermore, corrections were made to A-GNSS positioning support elements, including the GNSS-AlmanacSupport and GNSS-UTC-ModelSupport, to ensure proper interoperability. These updates refined the assistance data framework to improve positioning accuracy and reliability for terminals utilizing QZSS signals alongside other constellations.
- GNSS LOS/NLOS assistance information [GNSS LOS/NLOS] TS 37.355CR0446
- SSR Satellite PCV Residuals [Rel18PCV] TS 37.355CR0465
- Miscellaneous RIL corrections for GNSS LOS/NLOS [GNSS LOS/NLOS] TS 37.355CR0495
- Correction on GNSS-AlmanacSupport and GNSS-UTC-ModelSupport in A-GNSS positioning TS 37.355CR0518
- Correction on NavIC almanac set IE, and field descriptions under KlobucharModelParamater and GNSS-SystemTime. TS 37.355CR0534
In Release 19, the new QZS (Quasi-Zenith Satellite) function introduced support for the L1C and L2C civil navigation signals, as defined in the system's signal power level tables for testing. This complemented the existing L1 C/A signal support and was integrated into the A-GANSS minimum performance requirements for sensitivity and nominal accuracy test scenarios, where a QZSS satellite could replace a GPS satellite in the constellation allocation.
Explore further
Broader topics and technologies where QZS plays a role.
Defining Specifications
3GPP specifications that define or reference QZS, with the latest known release. Sourced from the 3GPP document catalog — see methodology.
| Specification | Title | Release |
|---|---|---|
| TS 25.172 vj00 | A-GANSS UE Minimum Performance Requirements (FDD) | Rel-19 |
| TS 25.173 vj00 | A-GANSS Performance Requirements (TDD) | Rel-19 |
| TS 36.355 vj00 | LTE Positioning Protocol (LPP) | Rel-19 |
| TS 37.355 vj20 | LTE Positioning Protocol (LPP) | Rel-19 |