ENRI Int. Workshop on ATM/CNS. Tokyo, Japan. (EIWAC 2010)[EN­108] Highly­Accurate Positioning Experiment System Using
QZSS at ENRI
(EIWAC 2010)+

K. Ito*, T. Sakai*, S. Fukushima**CNS Department
Electronic Navigation Research Institute (ENRI)
Tokyo, Japan[itoken | sakai | fks442]@enri.go.jp
Abstract: QZSS (quasi-zenith satellite system) is a Japanese satellite navigation system with regional service coverage.
The orbit of a QZS (Quasi Zenith Satellite) is 24-hour elliptic one inclined at 45 degrees in order to broadcast radio signals
into urban canyons at high elevation angle. ENRI (Electronic Navigation Research Institute) has been developing a highlyaccurate positioning experiment system using QZSS since 2003. The purpose of this experiment system is to develop
technology to determine a position of a mobile user with high accuracy and high integrity. In this experiment system,
integrity information and error correction information required to achieve highly accurate positioning are generated at a
ground station using GPS data measured at reference stations. This information is transmitted to a QZS. A GPS-like signal
with the information on error correction and integrity will be produced on a QZS and sent to mobile users on GPS L1
frequency. This GPS-like signal is named as a L1-SAIF (Submeter-class Augmentation with Integrity Function) signal by
ENRI. This paper gives the present status of the experiment system of ENRI.
Keywords: QZSS, GPS augmentation, L1-SAIF
One of satellites in QZSS is visible near zenith any time.
Users can always receive a signal from that satellite in
QZSS near zenith.

1. INTRODUCTION
Positioning with GPS is widely used in Japan in the areaof car navigation, man navigation and survey.

In 2003, the development of QZSS started undercollaboration between the Japanese government andprivate sectors in Japan.

GPS is said to have some difficulties. The first one is thatusers in mountainous or urban area cannot receive signals
from sufficient number of GPS satellites for positioning
because of shadowing by mountains or tall buildings. Thiscannot allow users provided with positioning service at
any time and any place. The second one is that
positioning accuracy with GPS may not be sufficient for
high-speed mobile users such as trains or cars.

In contrast to the global systems such as GPS, GLONASS
and the Galileo systems, QZSS has a more focused andregional objective, which is to complement and augment
GPS coverage in Japan by continuously providing an
additional satellite at a high elevation angle.

These difficulties require the development of new satellite
positioning technology. QZSS (Quasi-Zenith Satellite
System) is considered to be a possible candidate to solvethese difficulties.

The design policies for QZSS are to: a) preserve andimprove existing GPS user benefit and convenience, andb) to develop and demonstrate highly accurate and reliable
satellite positioning technology. [2]QZSS [1] is the constellation consisted of several satellites
orbiting in inclined orbital planes with GEO-synchronous
period. Each satellite is allocated on a different orbit so as
to pass over the same ground track with constant interval.

ENRI (Electronic Navigation Research Institute) has
started an experiment plan to study on correction
technology for highly accurate positioning with a highelevation satellite, QZS, combined with GPS since 2003.
An ENRI experiment system is developed in cooperation
with Japan Aerospace Exploration Agency (JAXA),
Geospatial Information Authority of Japan (GSI) andrelated institutes.

Eccentricity and inclination are selected so that theminimum elevation angle is higher than about 70 degrees
through 24 hours in service areas.

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K.Ito, T. Sakai, S. Fukushima
This paper gives the present status of the ENRI
experiment system [3], [4].

QZS
GPS

As the service area of QZSS is regional, QZS will belaunched into 24-hour geosynchronous elliptic orbit andkept within a certain longitude range. At least threesatellites are necessary to provide continuous navigation
service from high elevation angle.

Measured
Data

GEONET
GSI

F
AI
-S
L1

L1SMS

al
gn
Si
ClosedLoop

L1-SAIF
MessagendbaK-

2. QZSS PROGRAMQZSS MCS

ENRI

JAXA

Figure 1. Experimental system configuration of QZSSL1-SAIF augmentation.

QZSS broadcasts both complement and augmentation
signals. The former consists of GPS civilian signals, i.e.,
L1C/A, L2C, L5, and L1C with the minimum
modifications.

at GPS reference stations such as ‘GPS continuous
observation stations’ in GEONET (GPS Earth Observation
Network System) of GSI located throughout Japan. Thisdata is sent to L1SMS (L1-SAIF Master Station) andprocessed to generate differential GPS corrections andintegrity information. As shown in Figure 1, messages
including this information are uploaded to QZS via QZSSMCS (Master Control Station) of JAXA, then modulated
and broadcast to users in an experimental area. Users canuse these differential GPS corrections and integrity
information to improve positioning error and carry out
integrity monitoring. A signal including differential GPS
corrections and integrity information is named as L1-SAIF
by ENRI.

In QZSS, GPS augmentation messages will be broadcast
on the GPS L1 frequency. For this purpose L1-SAIF (submeter-class augmentation with integrity function) signal
has been developed based on SBAS standard [5] because
both QZSS and SBAS will offer similar function to muchthe same service area. The target of user position accuracy
is set to 1 meter RMS. As QZSS provide the integrity
function necessary for safety of mobile users, integrity
algorithm developed for SBAS is also adopted in QZSS.
The frequency plan of QZSS is summarized in Table 1.
The detail specification is defined by IS-QZSS document[6].

JAXA is in charge of developing the QZSS space segment(i.e., QZS) and deploying ground facilities including MCS
and regional tracking stations.

3. ENRI EXPERIMENT SYSTEM

3.1 Overview of QZSS L1-SAIF signal

The ENRI experiment system has the function of GPS
augmentation and integrity monitoring. Figure1 shows theconfiguration of the ENRI experiment system.

The structure of QZSS L1-SAIF signal is defined to havefull compatibility to the SBAS L1 signal. The signal
frequency is equal to GPS/SBAS L1 (1575.42 MHz). The
generator of pseudorandom noise code for spreading
spectrum is the same as the one for the GPS gold code;
PRN numbers from 183 to 187 are allocated to QZSS L1SAIF signal.

In the ENRI experiment system, satellite data is collected
Table 1 Frequency plan of QZS.
Signal

L1
C/
A,
L2
P

2P
,L
/A
C
L1

Frequency

QZS-L1CQZS-L1-C/A

1575.42 MHz
Transmitted FirstQZS-L1-SAIF
QZS-L2C

1227.6 MHz

QZS-L51176.45 MHz

QZS-LEX

1278.75 MHz

Preamble
8 bitsMessage Type
6 bitsData Field
212 bits

CRC parity
24 bits

250 bits

Figure 2 L1-SAIF message structure.

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The data rate for the L1-SAIF signal is 250 bits per second
with 1/2 rate FEC (forward error correction) encoding,and a message consists of 250 bits, so QZS will transmit
one message per second. The message contents are uppercompatible to the SBAS. The only difference from SBAS
signals except for message type definitions is Doppler andpower variation due to elliptic orbit. Correction to therelativistic effects should be also applied.

The primary mission of L1SMS is to generate a L1-SAIF
message stream and deliver it to MCS in real-time.
L1SMS is developed by and set up at ENRI (Chofu City,
Tokyo) 90 km away from QZSS MCS. L1SMS and MCS
are connected to each other by ISDN and othercommunication lines.
In the worst case, it possibly takes 10 seconds fromtransmission of a message from L1SMS to reception of themessage by a user receiver. This latency is caused bybuffering at each step because an L1-SAIF signal is notbroadcast via a bent-pipe transponder.

Each L1-SAIF message (a message included in the L1SAIF signal) consists of 250 bits, which divided into (1) 8
bits preamble pattern; (2) 6 bits message type ID; (3) 212
bits data field; (4) 24 bits CRC parity, like the SBAS
message. Figure 2 summarizes L1-SAIF message format.
The MSB is transmitted first in each field. Note that FEC
encoding is applied to the 250 bits raw message so that a
preamble pattern could not be used to capture thebeginning of a message unless FEC is decoded.

3.2.2 System Configuration
The L1SMS consists of several subsystems as shown in
Figure 3. They are connected to each other via Ethernet
LAN and work on Linux OS.
- GEONET Server

Some of message types are assigned compatible with
SBAS. QZS will not transmit message types which contain
information dependent upon GEO orbit. Message types for
SBAS network time and service message are also not used.
Other SBAS-compatible messages are formatted to be
identical with SBAS specification. There is no change ofmessage format for these messages.

- Interface Processor (I/F)- Message Generator- Ionosphere Processor- Troposphere Processor
The function of each subsystem is explained below. All
subsystems are operated in realtime.

Extended messages are assigned to message types not usedin SBAS. Some of extended messages are used foratmospheric corrections, intersignal bias corrections, QZS
ephemeris and almanac.

The GEONET Server receives data in dual frequency
measurement from GEONET real-time stations. Currently
most stations equip Trimble receivers and output ameasurement data stream in RT17 format. This server
consists of five independent processors and the number of

3.2 L1-SAIF Master Station
The ENRI has been developing an L1SMS (L1-SAIF
master station) system. This system consists of several
subsystems operating in parallel. The configuration of thesystem is described in this section.

I/F

Message
Generator

Ionosphere
Processor

Storage

3.2.1 Function and Requirements
QZSS is a program supported by several agencies andlaboratories in Japan. The satellite will be launched andoperated by JAXA, and experiments will be conducted byprogram member agencies and laboratories. The master
control station for QZSS is placed at Tsukuba Space
Center of JAXA with communication lines to/fromprogram members at remote sites. The GEONET realtime
receiver network is available as monitor stations for QZSS.
GEONET is operated by GSI. L1SMS receives
measurement data stream from GEONET stations andgenerates
L1-SAIFmessagesusing
receivedmeasurements.

Storage

Router

GEONET
ServerStorage

UPS
UPS

Figure3 L1-SAIF Master Station (L1SMS)

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K.Ito, T. Sakai, S. Fukushima
monitor stations to be processed is limited to 200 for eachprocessor.

The Troposphere Processor is also an optional processor
for tropospheric correction. This processor receives realtime GPS observations from the GEONET Server andestimates atmospheric condition over Japan with a latency
less than 1 hour. Tropospheric delay estimates areformatted into L1-SAIF message by the Message
Generator. In case that this processor is not used, atropospheric correction message is not broadcast, andstandard tropospheric delay model is used in both the
Message Generator and user receivers.

The output rate of a receiver in GEONET real-timestations is 1 sample per second (1 Hz), so transfer rate
between GSI and ENRI reaches about 0.5 MB (Mega
Bytes) per second for 1000 stations or 0.1 MB per second
for 200 stations.
The Interface Processor distributes measurement data
stream received from the GEONET Server. Other
subsystems obtain necessary measurements from thisprocessor. The Interface Processor also relays L1-SAIF
message packets from the Message Generator to QZSSMCS. All interfaces are based on TCP/IP socket
connection.

3.2.3 Real-time Monitor Stations
A GPS augmentation system need monitor stations
distributed uniformly over its service area. Each station
should equip one or several dual-frequency and surveygrade GPS receivers and a communication line to transmit
measurements to a master station.

L1SMG (L1-SAIF Message Generator ) is the mostimportant subsystem of L1SMS and explained based on
WADGPS (wide-area differential GPS) [7].

Currently three networks are available for L1SMS as
monitor stations; (i) GEONET real-time stations, (ii)
ENRI real-time sites, and (iii) JAXA monitor stations.
Each network has different properties as described below.
The L1SMS Interface Processor is capable of handling
any set of receivers of these three networks.

The WADGPS basically needs 6-10 monitor stations forclock and orbit corrections over the L1-SAIF experimental
area. Additional stations might help not only to generatebetter estimation of clock and orbit errors but also toenhance ionospheric correction and integrity performance.

(i) GEONET real-time stations

The L1SMG is developed for the experiment purposes.
Therefore, the number of monitor stations for generationof correction information is not limited. The only
limitation is caused by processor speed and memory
capacity.

GEONET consists of 1200 reference stations in Japan forsurveying purposes. All of reference stations output realtime data streams to GSI and several surveying companies
provide real-time surveying services. This national
resource is available for the QZSS program.

The Message Generator receives measurement data stream
from the Interface Processor and generates L1-SAIF
messages every second. It is capable of handling input
data stream in the form of RINEX and format used in
NovAtel, Trimble, and JAVAD receivers.

L1SMS has its own GEONET Server and acommunication link to GSI. The GEONET Server iscapable of receiving measurement data streams from all
1200 stations. Each GEONET station equips a dualfrequency and survey-grade receiver without an atomic
clock. Each station transmits measurement data every
second in the format intrinsic to its receiver. We can try alarge variety of monitor station configurations. A
disadvantage of this network is data transmission latencyof a few seconds, up to 2 seconds measured in the current
environment.

Generated messages are in NovAtel OEM-3 FRMA record
format.
The Ionosphere Processor is specialized to generate
ionospheric correction and/or augmentation information
based on vast number of monitor stations. The processor iscapable of receiving and processing real-timemeasurement from at least 200 stations. Ionosphericinformation is delivered to the Message Generator to be
integrated with clock and orbit corrections. This processor
is optional. In case that this processor is not used, standard
ionospheric augmentation algorithm implemented in the
Message Generator is employed for message generation.

(ii) ENRI realtime sites
ENRI has been operating its own realtime observationsites. Currently 9 domestic stations are available; Sapporo,
Sendai, Tokyo, Toyama, Shigaraki, Kochi, Fukuoka, Naha,and Yonaguni. All sites have no atomic clocks.

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The great advantage of this network is less latency. Most
stations are connected to ENRI directly via ISDN line.
Especially the L1SMS has direct LAN access to Tokyosite receivers.

The size and weight of the prototype receiver is 40(width)x 40(depth) x 20(height) cm and 11 kg.
5. SCHEDULE(iii) JAXA monitor stations

In 2003 and 2004, algorithm for generation of differential
correction and integrity-related information adequate to
QZSS was studied and validated using off-line dataobtained at GPS reference stations in GEONET. In 2005
through 2007, a real-time generation system ofaugmentation messages was developed. This real-timesystem was used to test that algorithm of generation ofaugmentation messages using on-line data obtained at
GPS reference stations in GEONET. The procedure to
broadcast augmentation messages to users was also
estimated. During this period, the prototype receiver wasdeveloped.

JAXA set up several monitor stations for QZSS operation.
Four domestic stations are located at Wakkanai, Koganei(near Tokyo), Chichijima, and Okinawa. Each station will
equip a specialized triple frequency receiver which canreceive L1-SAIF, L1C and L2C signals and synchronize toan atomic clock. L1SMS will receive measurements atthese stations via QZSS MCS.
4. PRTOTYPE RECEIVER
A prototype receiver can receive a L1-SAIF signal from a
QZS and decode augmentation messages contained in thesignal to carry out highly accurate positioning. The
development of a prototype receiver completed in 2007.
The prototype receiver was tested combined with L1SMS
at ENRI.

A QZS is to be launched in 2010. ENRI performed aground test in 2008 and an interface test between JAXA
and ENRI. Validation test using a QZS will be carried out
in 2010.
In the ground test, basic functions of the ENRI experiment
system, that is, highly accurate positioning and integrity
monitoring, were assured in detail by using online dataobtained at GPS reference stations in GEONET. A groundtest used a satellite simulator instead of a QZS and was
carried out in ENRI.

Figure 4 shows the outside of the prototype receiver. The
prototype receiver consists of a main part and monitoring
PC. The main part of the prototype receiver has thefollowing functions:(1) To receive L1-C/A signals from GPS satellites and an
L1-SAIF signal from QZS(2) to measure pseudoranges and decode navigation
messages and augmentation messages(3) to calculate a position of a prototype receiver
A monitoring PC displays measured data and calculated
positions.

An interface test was carried out to make sure datainterface between MCS of JAXA and L1SMS of ENRI
with real configuration of communication lines. The
format of transmitted and received data packets waschecked and compared with log files taken at both stations
bit by bit. This interface test was successfully completed
early in 2010.
A validation test will be performed by using a QZS, GPS
and online data from GPS reference stations in GEONET.
The purpose of this test is to make sure that the ENRI
experiment system has the expected performance when areal satellite (QZS) and GPS reference stations are used inreal-time.
6. REALTIME OPERATION TEST

Main Part

Monitoring PC
A real-time operation test was conducted to verify the
initial performance of the L1SMG. In this test, all data
used for message generation were obtained with reference
stations in GEONET in real-time. Generated messages

Figure 4 Prototype Receiver291

K.Ito, T. Sakai, S. Fukushima
were stored and positioning errors at other stations in
GEONET were evaluated by posterior processing.

Shigaraki, Kochi, Fukuoka, Naha and Yonaguni
11-Station Configuration:
JAXA monitor stations and ENRI real-time sites.

6.1 Configuration of Monitor Stations
The performance of WADGPS largely depends on theconfiguration of monitor stations. For the test, several
configurations of monitor stations were adopted. Note that
GEONET was used for all configurations and only the
number and combination of monitor stations were changed
because we were investigating the performance of L1SMG
not monitor stations.

The operational parameters of L1SMG are fixed except for
configuration of monitor stations. Only the minimum
number of monitor stations required to generate correction
information for a particular satellite was changed for 9 ormore stations configuration. Other parameters are thesame for all configurations. Ionospheric correction is
performed with the standard planar fit algorithm andparameters used are identical to those for the MSAS.

4-Station Configuration: (Red Squares in Figure 6)
JAXA monitor stations at Wakkanai, Koganei (nearTokyo), Chichijima and Naha.

6.2 Results
GEONET stations indicated as Green Circles in Figure 5
with brackets were regarded as user receivers in theevaluation of positioning error. User position accuracy
was evaluated at these stations. Note that none of theseuser stations was used for generation of augmentation
information.

6-Station Configuration: (Green Triangles)
MSAS domestic ground monitor stations at Sapporo,
Hitachi-Ota, Tokyo, Kobe, Fukuoka and Naha.
9-Station Configuration: (Blue Triangles)
ENRI real-time sites at Sapporo, Sendai, Tokyo, Toyama,

Figure 6 is an example of horizontal position error at Site
940058 Takayama, indicated with (5) in Figure 5, for 5days from Jan/19/08 to Jan/23/08. L1-SAIF messages
were generated based on 6-station configuration. Most of
position errors are less than 1 meter and the RMS accuracy
was 0.292 m.

Figure 5 L1-SAIF experimental area (bordered byorange line; (Red Square) JAXA monitor stations;(Green Triangle) MSAS GMS; (Blue Triangle) ENRI
real-time sites; (Red Circle) Real-time test user
stations numbered with brackets.

Figure 6 User position error at Site 940058 Takayama
for Jan/19/08-Jan/23/08; (Green) Augmented by the
L1-SAIF of 6-Station Config.; (Red) Standalone GPS.

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signal which contains this information will be broadcast
users within a service area by QZS.
The ENRI has been developing L1-SAIF Master Station
which consists of several subsystems including L1-SAIF
Message Generator as the most important part.
Configuration and fundamentals of L1-SAIF Message
Generator were described in this paper. The initial resultsof real-time operation test on the ground demonstrate thetarget accuracy of 1 meter RMS is likely to be achievable
during the normal ionospheric condition.
8. REFERENCES[1] Kawano I., et al: “Japanese Experimental GPS
Augmentation using Quasi-Zenith Satellite System(QZSS)”, pp175-181, Proceeding of ION GNSS 2004
(September, 2004)

Figure 7 User position accuracies at evaluation sites
for four configurations of monitor stations for 4 days;
Solid and broken lines represent horizontal and verticalerrors, respectively.

[2] “GPS-QZSS Technical Working Group Meeting
Report” (December, 2002)[3] Sakai, T. et al: “Implementation of the QZSS L1-SAIF
Message Generator”, Proceeding of ION NTM 2008
(January, 2008)

Figure 7 illustrates the statistics of user position accuracies
with an augmentation at all evaluation sites for 4
configurations of monitor stations for 5 days from
Jan/19/08. Solid and broken lines represent horizontal andvertical accuracy, respectively. In this figure, accuracy isshown in RMS value with respect to the evaluation station
ID numbered from north to south as shown in Figure 6
with brackets.

[4] Ito, K. et al: “Present Status of a Highly Accurate
Positioning Experiment System Using QZSS at ENRI”,
Proceeding of International Symposium on GPS/GNSS
2008 in Tokyo, pp.1079-1086 (November, 2008)[5] International Standards and Recommended Practices,
Aeronautical Telecommunications, Annex 10 to the Convention on Int’l Civil Aviation, vol. I, ICAO, Nov. 2002.
[6] JAXA, Interface Specification for QZSS (IS-QZSS).

Figure 7 indicates that the target accuracy of 1 meter RMS
is likely to be achievable. The number of monitor stations
should be at least 6. Increase of the number of stations
does not automatically ensure an improvement of accuracy.
The results for 9 or more stations configurations seem bestin terms of accuracy among all locations tested here.
However, it must be notified that the ionospheric activity
is relatively quiet for this test period. Much more stations
would be necessary to prevent a large error caused by anionospheric disturbance.

http://qzss.jaxa.jp/is-qzss/index_e.html[7] C. Kee, Wide Area Differential GPS, Global
Positioning System: Theory and Applications, II, Chap. 3,
pp. 81-115, AIAA, 1996.
9. COPYRIGHTThe authors confirm that they, and/or their company or
institution, hold copyright of all original material includedin their paper. They also confirm they have obtained
permission, from the copyright holder of any third party
material included in their paper, to publish it as part oftheir paper. The authors grant full permission for the
publication and distribution of their paper as part of the
EIAC2010 proceedings or as individual off-prints from theproceedings.

7. CONCLUDING REMARKSThe present status of the ENRI experiment system was
explained briefly. The ENRI experiment system generates
information for differential GPS correction and integrity
monitoring using data from GEONET. The L1-SAIF

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