Development of QZSS

The ENRI has been participating Japanese national project developing QZSS (Quasi-Zenith Satellite System) since FY 2003. The first space vehicle for QZSS named "Michibiki" has been launched successfully in September 2010, and since then, various experiments are conducted by participating organizations.

QZSS Functions

The QZS (quasi-zenith satellite) is circling on the inclined geosynchronous orbit (IGSO) which draws ground track of "8"-figure. Employing such an orbit, the QZS is able to broadcast navigation signals from just above Japan as shown in Figure 1. Figure 2 illustrates that users under conditions which GPS and GEO signals are blocked by tall buildings and trees still can receive QZS signals broadcast from the zenith.

A combination of three QZS space vehicles makes it possible to broadcast navigation signals over the whole Japan from the zenith at all times. The QZSS means this combination of satellites and QZS-1 "Michibiki" launched in September 2010 is planned to be the first satellite of this system.

Figure 1: Ground Track of QZS

Figure 2: Signals from QZSS

The QZSS will provide roughly two functions as follows:

• Supplement to GPS

The QZSS broadcasts GPS-like navigation signals and increases navigation satellite available for users. Note: The existing GPS receivers cannot receive supplement navigation signals from "Michibiki." You need a new receiver capable to receive QZS signals.

• Augmentation to GPS

The QZSS broadcasts augmentation signals and improves navigation performance of users. Note: The existing GPS receivers cannot receive augmentation signals from "Michibiki." You need a new receiver capable to receive QZS signals.

In order to achieve these functions, "Michibiki" broadcasts navigation signals listed in Table 1. All of these signals are broadcast simultaneously.

QZS-L1-C/A, QZS-L1C, QZS-L2C, and QZS-L5 are supplement signals. Each of them is very similar with a GPS signal of the same name, however, existing GPS receivers cannot receive QZS signals.

Both augmentation signals, QZS-L1-SAIF and QZS-LEX, offer supplement function as well as augmentation function. QZS-L1-SAIF (submeter-class augmentation with integrity function) signal broadcasts augmentation information of submeter-class accuracy. The RF specification of QZS-L1-SAIF signal is identical with the SBAS (satellite-based augmentation system) which is the international standard augmentation system defined by the ICAO (international civil aviation organization). However, PRN code is different from the SBAS, so existing GPS and/or GPS/SBAS receivers do not receive QZS-L1-SAIF signal. On contrast, QZS-LEX (L-band experiment) signal is defined originally for QZSS and of course existing GPS receivers do not aware of this signal. The purpose of this augmentation signal is enabling highly accurate positioning for survey application.

All signals broadcast from "Michibiki" are defined and explained by the IS-QZSS (interface specification) document issued by JAXA.

L1-SAIF Augmentation Signal

L1-SAIF (submeter-class augmentation with integrity function) signal is designed to broadcast augmentation information of submeter-class accuracy. The RF specification of QZS-L1-SAIF signal is identical with the SBAS (satellite-based augmentation system) which is the international standard augmentation system defined by the ICAO (international civil aviation organization). However, PRN code is different from the SBAS, so existing GPS and/or GPS/SBAS receivers do not receive QZS-L1-SAIF signal. The ENRI has been responsible of development of L1-SAIF signal.

Augmentation information of L1-SAIF is broadcast as a sequence of messages. L1-SAIF carries one message per one second. Each message is idenified by Message Type which is a number between 0 to 63. The order of messages is not defined. Each message consists of 250 bits and synchronizes with beginning of every seconds of GPS time. Figure 3 illustrates the structure of the message which contains preamble of 8 bits, Message Type of 6 bits, data contents of 212 bits, and CRC parity of 24 bits. Meaning of each bit within data contents is defined with respect to message type, see IS-QZSS here.

Figure 3: Message structure for L1-SAIF

There are two kinds of L1-SAIF messages: SBAS-compliant messages (Message Types 0 to 28, 62, and 63) and extended messages (Message Types 52 to 60). SBAS-compliant messages are identical with the messages defined in SBAS standard. Submeter-class augmentation is achieved by using only SBAS-compliant messages. Extended messages are used for advanced augmentation and receivers may or may not apply extended messages. We are conducting experiments and validation of extended messages and their formats are, so far, tentative.

Existing receivers need to be modified to receive and process L1-SAIF messages. If you already have SBAS-capable receivers, you need very small software modification to make it also capable of L1-SAIF because basic augmentation information is carried by SBAS-compliant messages.

Statistics of Position Error System Horizontal Error Vertical Error Without L1-SAIF 95% 2.90m 5.84m Max 6.02m 8.45m With L1-SAIF 95% 0.58m 0.78m Max 1.56m 2.57m Note: This result is based on survey-grade receiver and antenna. Location: GEONET 940058 (Takayama) Period: 2008-01-19 to 23 (5 days)

Figure 4: Example of L1-SAIF Augmentation

Figure 4 shows example of statistics of user position error with and without L1-SAIF augmentation (Only SBAS-compliant messages and no use of supplement function of "Michibiki"). Red plots indicate results of GPS standalone positioning while Green ones associate with L1-SAIF augmentation. Large errors which appear frequently in case without augmentation are reduced within roughly 1 meter by applying augmentation information. Note that this result is obtained with survey-grade receiver and antenna, and generally the performance of augmentation depends upon the surrounding environment and characteristics of receiver and antenna.

Schedule of L1-SAIF Experiment

L1-SAIF broadcast capability of "Michibiki" is shared by ENRI

Table 2 lists experiment periods assigned for ENRI. On the usual operation, broadcast of L1-SAIF begins before noon of the first day of each experiment period and stops in the evening the last day. However, broadcast may stop frequently whenever necessary due to needs of the experiment. Note that broadcast information is generated for experimental purpose and there is not any guarantee on the augmentation performance.

Recording format is idential with $FRMA record NovAtel OEM-3 receivers.

Directory QZS-1 L1-SAIFiPRN183j

Observation of QZS-1 "Michibiki"

ENRI just began continuous observation of "Michibiki" as well as other GNSS satellites including GPS and GLONASS with triple frequencies. Raw observation is converted into the RINEX format and stored here. Equipment and conditions of observation are described below.

In addition, navigation messages broadcast on ranging signals of L1 frequency are decoded and stored here. This information is archived in EMS format explained below.

Directory RINEX Files (Range measurements)
Directory EMS Files (Broadcast message)

Note 1: Sometimes there is missing data due to some reason such as power outage.
Note 2: Before 2012-01-23, a little part of data may be missing due to some limitation of the receiver. This problem is fixed on data collected after 2012-01-23.

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