Development of Ultimate Seamless Positioning

System based on QZSS IMES

Dinesh Manandhar, Kazuyuki Okano, Makoto Ishii, Hideyuki Torimoto

GNSS Technologies Inc., Japan

Satoshi Kogure, Japan Aerospace Exploration Agency (JAXA), Japan

Hiroaki Maeda, Lighthouse Technology and Consulting, Japan

BIOGRAPHY

Dinesh Manandhar is a Senior Researcher at GNSS

Technologies Inc. He is also a visiting researcher at the

University of Tokyo. He received Ph. D. from the

University of Tokyo, Japan in 2001. Currently, he is

involved in indoor signal analysis. He is one of the

members to design and develop IMES.

Kazuki Okano is chief engineer at GNSS Technologies

Inc. He is involved in hardware design and development

for Pseudolites and IMES.

Makoto Ishii is Director of Strategic Marketing and Sales

of GNSS Technologies Inc. He is coordinating GNSS

based technologies in the business department including

IMES.

Hideyuki Torimoto is president of GNSS Technologies

Inc. He is one of the pioneers of satellite navigation

related application businesses in Japan. He established

Trimble Japan in 1986. In 2002, he founded GNSS

Technologies Inc. to promote R&D as well as marketing

in the field of GNSS in Japan. He is managing GNSS

R&D including IMES to promote the technology

internationally. He served as Satellite Division Member of

ION for 2003-04.

Satoshi Kogure is an associate senior engineer of JAXA.

He received MS in aerospace engineering from University

of Colorado in 2001. He has been working for satellite

positioning system as a satellite systems engineer since

2001. He is one of the members to design and develop

IMES.

Hiroaki Maeda is the founder and the managing director

of Lighthouse Technology and Consulting Co., Ltd

(LHTC). He received Ph. D from the Tokyo University of

Marine Science and Technology in 2008. Since 2003 he

has been a technical lead of the Japanese satellite

navigation system, QZSS. He is one of the members to

design and develop IMES.

ABSTRACT

Satellite-based navigation systems generate huge location

based service markets and it is expected that the market

will drastically increase in near future due to necessity of

GNSS enabled mobile phones for emergency services in

USA, Japan and many other countries. In order to realize

such huge growth, it is necessary to address the problems

of seamless navigation which are limited by the current

GNSS based applications alone. These systems are not

strong enough to provide navigation in indoor and deep

indoor environments with required resolutions. In order to

solve these problems and provide indoor position, we

have developed Indoor Messaging System (IMES).

The basic concept of the IMES is to transmit position data

and/or unique ID and/or other user defined data from the

transmitter while keeping the similar signal structure as of

QZSS/GPS signal. IMES has the same RF properties as

GPS/QZSS and PRN IDs from 173 to 182 are assigned

for IMES. The only difference is in the contents of

navigation message which can be set as per user’s

necessity and application fields. It is not necessary to

compute pseudorange, hence a single unit is enough for

position data.

We have developed prototype IMES device based on

QZSS-IS document. Experiments have been conducted

using software receiver and prototype IMES capable

mobile phone devices. The seamless navigation capability

has been demonstrated by using the IMES capable mobile

phone device. The mobile phone shows the user position

in seamless fashion when the user moves indoors and

outdoors. The interference analysis results showed that

with proper separation of other GNSS devices from IMES

transmitter, there is no harmful interference to the GNSS

user. Based on these experiments, minimum threshold

distance has been estimated, which is about 1.6m from the

IMES antenna at transmission power of -70dBm. This

paper discuss about IMES concept, signal structure,

prototype device, various experiments and their results.

INTRODUCTION

Localization or the problem of estimating spatial

relationships among objects has been a classical problem

in many disciplines. Positioning types range from

proximity indication, crossing of boundary, or precise

positioning. The requirement for positioning systems

depends on types of applications. However, positioning

alone does not help much unless the position data are

associated with other related database. The use is limited

if a user knows only his position. However, this limitation

can be expanded if the position data are related with local

database and ultimately to the global database. The rooms

in a building may have the same physical characteristics

but they are associated with different information. Thus

just knowing the building location is not enough for many

applications. The user always need to know it’s location,

direction and relation with other objects and database

regardless of whether the user is outdoors or indoors.

These are the key factors for seamless navigation.

Satellite-based navigation systems like GPS and

GLONASS work well outdoors but their use is limited

indoors due to poor visibility of satellites. In near future,

there will be GALILEO, QZSS and COMPASS for

satellite-based navigation systems. However, for good 3D

position estimation at least four visible satellites are

necessary. It’s almost impossible to compute a position

indoors using such satellite signals without any external

aiding of information. Though, there are weak signal

processing capable receivers, they do need information

about the satellite orbits and/or time which are normally

provided by some external means. There are also

positioning systems that are based on the combination of

GPS and cellular phone networks. However, they still do

lag in providing correct 3D information and need

infrastructure setup in the existing cellular networks

which are not always possible. In the case of emergency,

how do we know that whether the victim is in the 50th

Floor, room number 510 or room number 512? Or how do

we know that whether the victim is in the 5th Floor below

the basement in room number 110? The key point here is

to identify precisely the location of the victim without any

mistakes. We do not want to search and rescue the victim,

but we would like to locate and rescue the victim. This is

the fundamental difference between other GNSS based

systems and IMES.

There are many cases when accurate indoor location

information is necessary. In order to overcome these

problems a new signal has been defined in QZSS IS

document which is known as IMES (Indoor Messaging

System). IMES has been jointly developed by Japan

Aerospace Exploration Agency (JAXA), GNSS

Technologies Inc. and Light House Technology and

Consulting Co. Ltd. (LHTC).

The development includes hardware, software, signal

structure and message formats.

IMES CONCEPT

The main concept of IMES is to transmit position and/or

ID of the transmitter with the same RF signal as GPS and

Quasi-Zenith Satellite System (QZSS). IMES will

broadcast position and other information using similar

message format to GPS and QZSS periodically instead of

ephemeris and correction messages that are necessary for

receiver’s position estimation. A single unit of IMES is

enough to get the position data since the position itself is

directly transmitted. If only ID is transmitted, then the

receiver will connect to the server to get further

information including position. Figure 1 shows the basic

concept of IMES.

Figure 1: Basic concept of IMES. IMES transmits

position and/or ID and/or other data

The most significant characteristic of IMES is to provide

seamless positioning and navigation. Figure 2 shows the

concept of seamless navigation using the IMES where the

same receiver is used both indoors and outdoors without

any interruption. GNSS satellites are used for positioning

and navigations outdoors where as IMES is used for

indoor navigations. Since the signal structures of GPS

satellites and IMES is similar except for the navigation

message contents, the same receiver can be used for both

the cases. Current GPS receivers will be capable of

receiving IMES signals with modification of firmware

only to decode the navigation message.

Figure 2: Seamlessness of IMES. The same IMES receiver

can be used indoors and outdoors.

SIGNAL DESIGN CRITERIA OF IMES

The following points were considered during the signal

design of IMES:

- Same hardware will be used for both indoor and

outdoor positioning with minimum modifications on

the firmware in the GPS chipset.

- High precision like sub-meter order is not required.

Precision of position around 10m seems to be enough

for users to know where they are per moderate room

size, each shop or portion in shopping complex.

Accuracy of such order at any time is more important

than higher precision.

- Low power consumption is required for cell phone

use. Smooth switching of indoor/outdoor continuous

positioning is one of the key issues to decrease

additional power consumption.

- Low cost transmitter is essential as well as low cost

installation and maintenance of it.

- Compatibility with GPS, QZSS and other GNSS is an

essential requirement. IMES signal must not cause

harmful interference with other GNSS signals.

Taking account of above requirements, the main

characteristics of the current IMES design concepts were

defined as follow:

SAME RF PROPERTIES AS GPS AND QZSS

In order to share same RF front-end as GPS and QZSS,

IMES uses same L1 center frequency, 1575.42 MHz and

BPSK modulation, 1.023MHz Gold code family as

GPS/QZSS C/A signal which is defined on IS-GPS-200D

[3] and the Interface Specification for QZSS Users (IS-

QZSS) [2] respectively. Table 1 shows the signal

properties of IMES. The transmitter power is set based on

location environment and separation between the

transmitters keeping the maximum power level below the

threshold value set by Japanese radio regulation. The

power level at the IMES receiver will be maintained

approximately within -126dBm to -130dBm.

PRN codes from 173 to 182 of 210 C/A codes are

assigned for the dedicated usage of QZSS-IMES [1].

Receiver can distinguish the tracking signal from IMES

transmitter and from satellite easily and smoothly by

acquiring and tracking these dedicated IMES codes in

parallel with codes assigned for satellites of GPS/QZSS.

It should be noted that the use of IMES PRN codes is

currently limited in Japan. However, there is a possibility

to extend their use to worldwide in future, since IMES

transmitter covers small area in indoor and PRN code is to

be re-used without overlapping coverage zone of the

transmitter with the same PRN code.

Table 1: Signal properties of IMES

RF Centre Frequency 1575.42Mhz

PRN Code Rate 1.023Mhz

PRN Code Length 1ms

PRN ID 173 – 182

Navigation Message Rate 50bps

Modulation BPSK

Polarization RHCP

NO TIME SYNCHRONIZATION AND

PSEUDORANGE

The usage of IMES does not require measuring the

pseudorange between transmitter and receiver. In the case

of QZSS/GPS it includes almanac and ephemeris related

messages in the navigation message. Pseudoranges are

estimated from more than three satellites and position

data are computed using pseudorange. However, in the

case of the IMES, position is directly transmitted

embedded in the navigation message and hence it is not

necessary to compute pseudorange.

Therefore, the time synchronization among all

transmitters as well as synchronization with outside GPS

time is not required. This leads to simple system

architecture rather than pseudolite system which require

more than four time synchronized transmitters. This

enables to build low cost transmitters.

NO OVERLAP WITH OTHER TRANSMITTERS’

COVERAGE

The coverage zone of one transmitter should not overlap

with other transmitters. The IMES cell-coverage area is

designed with ten assigned PRN Code with one guard cell

between IMES cells. The resolution of the position data is

limited by the coverage of the transmitter signal. The size

of each coverage zone is equivalent to maximum

positioning error. Usually, one coordinate for a moderate

sized room can be transmitted by one transmitter. Large

areas such as underground malls, metro stations, and

department stores, the transmitters may be distributed at

10-15m span.

RF COMPATIBILITY WITH OUTSIDE GNSS

SIGNALS

IMES signals are transmitted at low power to avoid

interference with live GNSS signals. In addition to this

low power transmission, the separation between

transmitters contributes to avoid increasing noise floor by

multiple IMES signals. Moreover, an appropriate

separation distance between an IMES transmitter and an

outside GNSS receiver should be maintained. The

separation distance is to be defined in the annex of IS-

QZSS as an installation standard. Results of interference

analysis are discussed in the later section of the paper.

MESSAGE STRUCTURE

The message structure of IMES is similar to QZSS or

GPS L1C/A and is defined in the annex of IS-QZSS. The

message consists of words of 30bits. Depending upon

message types, the number of words in a frame can vary.

Currently, there are four types of IMES messages defined

that are named as type “0”, “1”, “3”, and “4” as shown in

Figure 3 to Figure 6 respectively. Message type “0”

contains 2-D position data using three words of 30bits

each. This is the shortest message length to transmit

absolute position data using latitude, longitude and floor

data. Message type “1” contains 3-D position data using

four words of 30bits each and the resolution is as twice

fine as the position transmitted in message type “0”.

Since, floor number is considered more important than

height (for inside the building data), floor number is given

higher priority than height data in the current message

type design. Message type “3” and “4” contains only IDs

and are one and two words long respectively. The first of

each frame has an eight bit preamble followed by

message type. The corresponding words of a frame have

three bit counter at the beginning of each word. Since,

message type “3” has one word only, it does not have

counter bits. It takes 0.6 sec to read one word at 50bps.

Message type “3” and “4” do not contain any position

data or coordinates. The position data are retrieved from

database server based on the unique ID to get the latitude,

longitude, height, floor number as well as other value-

added information such as guidance map, advertising and

so on. Medium IDs are unique IDs assigned to each

operator, for instance, department store, underground mall

and etc. Medium ID works for connecting each IMES

transmitter to local server established by each operator to

provide LBS services to their customers. Short IDs are

defined and maintained by each operator. It is possible to

define up to eight different types of message structures.

The messages can be transmitted as required. Currently,

absolute position in message type “0” or “1” and unique

ID combining message type “3” and “4” are planned to be

transmitted with time divided multiplex manner. The

absolute position is important for emergency use since

user can know their position without server assist. Above

procedure requests that absolute position should be

broadcasted in some intervals. For example, the

frequency of transmission of message type “0” or “1”

may be higher in the public area than in commercial area.

Message containing ID may be transmitted more

frequently in commercial area for LBS information. The

sequence of message transmission can be changed by the

IMES operator using the GUI developed for IMES.

One more point to be emphasized is “BD bit” included in

the message type “3” and “4”. “BD” stands for boundary

and this one bit indicates that the transmitter is located in

the boundary between indoor and outdoor locations.

When user move from indoor location to outside, it is

more effective in the case that receiver starts searching

PRN code for GPS referring this BD bit rather than the

case that receiver continue to search satellite signal to

acquire them. It can help reducing power consumption of

receiver. Low power consumption is significant

requirement for cell phone handset.

Figure 3: IMES L1C/A message type”0” as defined in

QZSS-IS document.

Figure 4: IMES L1C/A message type “1” as defined in

QZSS-IS document.

Figure 5: IMES L1C/A message type “3” as defined in

QZSS-IS document.

Figure 6: IMES L1C/A message type “4” as defined in

QZSS-IS document.

INTERFERENCE ANALYSIS

It is of utmost important to analyze the impact of the

IMES signal on existing GPS signals. Since IMES

compatible receivers are supposed to work at the same

signal level as GPS receivers, there shall be no

interference from IMES signal. The expected signal at the

receiver will vary from -126dBm to -130dBm which are

similar to GPS signal at the receiver. However, IMES

transmitters are ground-based transmitters that may be

located indoors or outdoor locations where users cannot

obtain enough accuracy heavy multipath circumstance

like sidewalk in the urban canyon. In such case, it is

necessary to transmit the signal at the transmitter at much

higher power level so that the signal at the receiver will

be between -126dBm to -130dBm at a distance of about

five meters from the transmitter antenna. The power level

at the transmitter decides the IMES coverage zone. Larger

coverage zone requires higher transmission power.

However, the maximum power is limited by regulation

for license free signals. In Japan this limits the signal

level at 35microVolt/m at 3m distance. This means that

the transmit power we would like to use for the IMES

(e.g. -70dBm) is lower than the allowed value. .

We have conducted experiments to analyze the

interference of the IMES signal on GPS signal. The

experiment setup is shown in Figure 6. IMES transmitter

is set outdoors on a pole with adjustable antenna height.

The transmitter antenna is a standard GPS passive patch

antenna. The height of the transmitter antenna can be

varied from few tens of centimeters to four meters. Data

are logged by changing the vertical distance between the

IMES transmitter antenna and GPS receiver antenna at

every 20cm interval from 20cm to 320cm. These data are

logged for three different transmission power settings at

-64dBm, -70dBm and -76dBm.

Two GPS receivers are set vertically beneath the IMES

transmitter antenna. One of the receivers is software GPS

receiver and another is a commercial GPS receiver (Rx1).

One more commercial GPS receiver (Rx2) is set at 30m

away from the IMES transmitter antenna (as reference for

GPS signal) so that the IMES signal will not have impact

on this receiver. Receiver Rx1 and Rx2 are of the same

type and have the same configurations. Since the distance

between Rx1 and Rx2, we can observe the same GPS

satellites during the experiment period.

Figure 8 shows the results for C/No with respect to

vertical distance for visible GPS satellites including the

IMES and vertical separation threshold distances where

the first fix observed. These graphs show the minimum

distance required for a GPS receiver to provide a fix. The

graphs on top, middle and bottom of Figure 8 shows the

results for -64dBm, -70dBm and -76dBm respectively.

At -64dBm, the GPS receiver provides first fix when the

vertical separation between the IMES and GPS antenna is

240cm. This means IMES has impact on GPS signal when

the GPS receiver is very near to the IMES transmitter

antenna. Thus, for the GPS receiver to work properly at

least a separation of 240cm is necessary. This distance is

100cm for -70dBm transmitter power. Thus, at least

100cm of separation is necessary for a GPS receiver to

work properly. At -76dBm, the GPS receiver provides

first fix when the vertical separation is 60cm.

Figure 7: Experiment setup for interference study

Figure 8: TTFF vs. Vertical distance separation at

different RF power output level

Figure 9: TTFF at different vertical distance for different

RF power levels

Figure 9 shows TTFF at different vertical distance for -

64dBm, -70dBm and -76dBm cases. Figure 8 shows the

minimum distance for the first fix which is called

threshold value. However, this first TTFF is much longer

than normal GPS operation TTFF. As seen in Figure 9 the

first TTFF is two to four time higher than normal TTFF.

There seems to be some strong IMES signal’s impact on

acquisition process for GPS receiver. Thus, it is necessary

to consider the distance for normal TTFF when an

appropriate separation threshold distances is investigated

as a range there is no harmful impact on live GPS signals.

Hence, we conclude that at least 160cm shall be the

separation threshold distances between the IMES

transmitter and GPS receiver antenna to avoid possible

interference from the IMES transmitter to GPS receiver.

This distance is for -70dBm transmitter power level.

However, if the transmitter power is higher, e.g. -64dBm,

then the separation shall be at least 300cm as results from

experiment currently.

Vertical threshold distances for the TTFF could be found

even if a GPS receiver is used beneath the IMES

transmitter, i.e. it could be a worst case. Actual transmit

powers of IMES shall be set based on the antenna location

so that the distance between the IMES transmitter’s

antenna and the edge of zone, which outside GPS receiver

exist potentially, is maintained less than above threshold

distances and the user received power of the IMES is

between -110dBm to -130dBm. The final figures for this

separation threshold distances according to IMES

transmitting power levels have not yet fixed. Before

finalizing values, several field tests and demonstrations

will be conducted.

IMES HARDWARE AND SOFTWARE

Figure 10 shows picture of IMES signal generator and

power spectrum of IMES L1C/A signal. The signal

generator complies with the signal specifications defined

in QZSS-IS document. The IMES signal properties and

message types can be controlled by using GUI based

software shown in Figure 11. The user can select PRN

code, input device position data, message types, message

sequence combination and control the RF power output.

A prototype IMES software receiver has also been

developed. The receiver can process IMES signal and

decodes navigation message. The receiver provides

position data for message type “0” or “1”. If the message

type is “3” or “4”, it is necessary to access to a database

where the position of the device is available based on the

device ID. Since there are different message formats, the

receiver output may vary depending upon the message

types and applications. However, inn all cases, the

position of the receiver will be available either directly

from the message or through the access to the database.

The position data can be linked or displayed on other

applications like Google Earth as shown in Figure 12.

This is one of the differences between the conventional

GPS receivers and IMES receivers. The main approach of

IMES is to use it’s position data and ID to link the user

position to the outer world for more LBS and many other

spatially related applications and database. Such

integration of IMES data with other database will expand

applications related with GNSS that ultimately will create

huge LBS related business.

Figure 10: IMES signal generator and power spectrum of

IMES L1C/A signal

Figure 11: GUI to control IMES signal generator

Figure 12: IMES message display with database over the

Google Earth Data

IMES EXPERIMENTS

IMES experiments have been conducted in different areas

to demonstrate it’s capability of seamless navigation.

Standard mobile phones with IMES capability and

software receivers are used for analysis. The analysis

includes signal availability, propagation properties, effect

of different types of transmitting antennas, interference to

GPS, location of receivers such as inside the pocket or

bag etc. The experiments have been conducted at

underground parking areas, underground train stations,

office and building rooms, open spaces etc. The IMES

receiver worked well when the receiver is moved from

indoors to outdoors or vice-versa. GPS signals are used

when the receiver is outdoors and IMES signal is used

when the receiver is indoors. The detail results will be

published in separate papers. Figure 13 shows experiment

conducted inside an underground parking area. In this

area normal GPS receiver does not work. The mobile

phone GPS with assisted data shows a position with

accuracy level of 100m to 200m. Using IMES signal the

same receiver provides the accurate position transmitted

by the device which enables the user to his location with

accuracy of few meters only.

Figure 13: IMES experiment inside underground parking

area. The IMES capable cell phones were kept in the pole,

pocket and handbag.

Figure 14: IMES experiment inside underground railway

station area. The mobile phone map shows the exact

location of the user.

CONCLUSIONS

We have designed and developed IMES for seamless

navigation. IMES will be used for indoor navigation

purpose where GPS signals are not available. The current

version of IMES is based on QZSS/GPS L1C/A signal

structure. It can be modified to use L1C signal structure in

future. Various experiments have been conducted to

verify and applicability of the system for indoor

navigation. The flexibility of using different types of

navigation messages and their combinations are quite

powerful tools for the IMES to suit the needs of different

applications. Implementation of the IMES on existing

GPS receivers does not require hardware modifications

since IMES and GPS use the same signal structures. It

only needs modification in the software to decode the

navigation messages as specified in the QZSS-IS

document. The preliminary interference analysis tests

have shown that there are no adverse effects on GPS

receivers provided that the receivers are beyond the

threshold values from the IMES transmitter antenna.

Experiments using mobile phone with IMES and GPS

receiver have been conducted to show the seamlessness of

IMES when the user moves from indoor to outdoor or

outdoor to indoor. One of the concepts of IMES is to link

to the external database so that the user location data can

be expanded to various applications, information and

services. We hope that with these characteristics of IMES,

it would be a strong tool for seamless navigation.

ACKNOWLEDGMENTS

The authors would like to thank Dr. John W. Betz,

MITRE and Mr. Tom Stansell, Stansell Consulting, for

discussions and suggestions related with IMES signal

design.

REFERENCES

[1] C/A code assignment table can be downloaded from

GPS PRN code assignment web site;

http://www.losangeles.af.mil/library/factsheets/factsh

eet.asp?id=8618

[2] Japan Aerospace Exploration Agency, "Interface

Specifications for QZSS" (IS-QZSS Ver.1.0) available

from the following site: http://qzss.jaxa.jp/is-

qzss/index_e.html

[3] NAVSTAR GPS Joint Program Office (2006),

ISGPS-200D with IRN-200D-001 Navstar GPS

Space Segment/Navigation User Interfaces;

http://www.navcen.uscg.gov/gps/modernization/IRN-

200D-001%207Mar06.pdf

[4] Kogure, S., M. Kishimoto, M. Sawabe, K. Terada,

“Introduction of IS-QZSS (Interface Specifications

for QZSS) in Proceedings of the ION GNSS 2007

[5] Kogure, S., H. Maeda, M. Ishii, D. Manandhar, K.

Okano, The Concept of the Indoor Messaging

System, ENC-GNSS 2008