Opening Up Indoors: Japan’s Indoor Messaging System, IMES

May 1, 2011  - By

By Dinesh Manandhar and Hideyuki Torimoto, GNSS Technologies, Inc. Japan

An indoor messaging system (IMES) has been developed to meet the challenges of indoor and deep indoor positioning, as a system that can be implemented in any device that has a GPS/GNSS receiver without hardware modification. IMES can provide reliable 3D position data with a single transmitter device without performing range calculation.

The cost of embedding location data in portable electronic devices is so low that universal penetration can be foreseen in the next five years. Roughly 70 percent of the world’s population now uses approximately five billion cell phones. This number has doubled in the last four years. Future growth is expected at the same or even a higher growth rate.

Due to the emergence of smart phones and location-based services (LBS), mobile phones are used not only for communications but also for many applications related to LBS, entertainment, and games. GPS/GNSS devices are included in mobile phones due to compulsory requirement of E911 and safety-and-rescue services by law in many countries for security and safety.

Access to map data and value-added services using these map data is getting cheaper and eventually will be freely available. Major service providers like Google, Nokia, and Apple already provide access free of cost, and they increasingly focus on location as a core business construct.

GPS/GNSS devices were designed to work outdoors, and most GNSS applications are limited to outdoor environments. However, GNSS reliability, availability, and accuracy have led to development of many new and innovative applications that are designed for use in both outdoors and indoors in a seamless fashion. Today, GNSS receivers are integrated in many other devices like mobile phones, navigation systems, personal navigation devices, game devices, security devices, and many LBS-related devices. These devices are increasingly used in indoor environments. Indeed, people generally spend much more time indoors than outdoors. Hence, it is extremely important to have a reliable system that can provide fairly accurate position data even in indoor and deep indoor locations.


Figure 1. Seamless IMES navigation concept. (Click to enlarge.)

Current GNSS systems do not provide solutions for indoor and deep indoor environment with reliable accuracy of 10–20 meters. New modernized signals such as L5 do provide better position accuracy and better signal reception in indoor areas, but achievable positioning will still vary, and will continue to require more than four visible satellites with some assist data — and still be limited to soft indoors environments such as rooms with glass windows or walls. Limitations remain for hard and deep indoor environments.

To surmount these obstacles and provide indoor navigation, various technologies such as pseudolites, assisted GPS, wireless networks (Wi-Fi), Bluetooth, RF tagging, and so on have been developed. However, these technologies have their own limitations and are not the most suitable tools for seamless positioning and navigation. Except for pseudolite and A-GPS, they are designed for communication, not for positioning or navigation purposes, but are used for navigation purposes since no other suitable technology exist.

Pseudolite systems are currently in use for indoor positioning. While technically sound, a system needs at least four signal transmitting units. To cover a large area, it needs many transmitters suitably located and time-synchronized to one other, or their clock errors must be known. Pseudolite systems provide position data based on range calculation from the receiver to a number of transmitters, and this calculation is heavily affected by signal multipath. Table 1 compares IMES and pseudolites.


Table 1. Comparison between IMES and pseudolite.

A-GPS is widely used in mobile phones to compute position data. A-GPS technology includes high-sensitivity signal processing to acquire weak signals and external assistance of data like time, approximate position, and satellite-orbit related parameters. Provision of assistance data requires a communication link between the receiver and the data source, for example, the mobile phone network itself. Thus, A-GPS will not be possible if there is no communication link.

Normally, A-GPS provides 2D position data. The height data (if 3D output is available) will be highly erroneous. The accuracy of such position data varies from few tens of meters to few hundreds of meters. Also, the position data is heavily affected by signal multipath. Figure 2 compares IMES position and mobile phone position inside an office building. The A-GPS position error is about 300 meters in this case.


FIGURE 2. Indoor position from high-sensitivity GPS and IMES.

Wi-Fi is used for indoor positioning in many mobile phone devices. The phone provides position data from a built-in GPS receiver, a Wi-Fi device, cell ID, or a combination of any of these. Recently, position data from Wi-Fi has become popular for indoor as well as outdoor position, since Wi-Fi signals are so freely available. However, using these Wi-Fi signals requires registering the signal power and availability at reference locations. To do this, a huge number of Wi-Fi devices are registered driving around the city. Since these devices are basically installed for communication purposes, they can be relocated, removed, or new devices may be installed without any information to the users or service providers. Thus, continuous maintenance and updating of all these devices are necessary at certain time intervals. The coverage of Wi-Fi devices is not uniform and may vary widely from area to area, affecting position accuracy.

Telecom service providers are considering the possibilities of seamless positioning technologies. They would like to have one single device that can provide 3D position data both indoors and outdoors, without additional power or cost, and with satisfactory 3D position information. If such a seamless positioning technology is available, it will undoubtedly generate a huge global commercial market. The availability of such technology will also aid development of new applications in location-based services, advertising, marketing, entertainment, and gaming.

We have conducted research in indoor positioning for the past few years, beginning with pseudolite systems. We have developed IMES to meet the shortcomings of the technologies described earlier for indoor and deep indoor positioning. IMES for a seamless positioning environment can be implemented in any device that has a GPS/GNSS receiver, without hardware modification. IMES can provide satisfactory and reliable 3D position data with a single transmitter device without performing range calculation.

Table 2 compares IMES with other indoor-position capable devices. IMES can provide the same accuracy even in deep indoor locations, whereas cell tower, A-GPS, and GPS cannot work in such areas. All other systems except IMES provide only 2D position data indoors. The height data from A-GPS is very unreliable and hence cannot be used.


Table 2. Comparison of IMES with other indoor positioning systems.

IMES Concept

The main concept of IMES is to transmit position and floor ID of the transmitter with the same RF signal as GPS. IMES transmits latitude, longitude, height, and floor ID by replacing the ephemeris and clock data in the navigation mes
sage of GPS. A single unit of IMES is enough to get the position data, since the position itself is directly transmitted.

Figure 3 shows the concept of seamless position data using IMES, where the same receiver can be used both indoors and outdoors without interruption. GNSS satellites provide positioning and navigations outdoors, while IMES provides indoor navigation. Since the signal structures of GPS satellites and IMES is the same except for the navigation message contents, the same receiver can be used for both cases. Current GPS receivers will be capable of receiving IMES signals with modification of firmware only to decode the navigation message. Figure 3shows the concept of seamless 3D route guidance.


Figure 3. Seamless 3D route guidance using IMES.

Signal Properties. The IMES signal is designed much like the GPS signal. It uses the same center frequency as GPS with an offset of +/– 8.2 kHz to minimize the possible interference from IMES to GPS signal. Ten PRN codes from 173 to 182 are assigned for IMES. These codes are provided by the U.S. government. Other signal-related parameters are the same as the GPS L1 C/A code signal. Table 3 shows IMES signal properties with respect to the GPS signal.

 Table 3. IMES signal properties with respect to GPS.

Table 3. IMES signal properties with respect to GPS.

IMES has four different types of navigation message. The most significant is Type 1 as shown in Figure 4. It transmits latitude, longitude, height, and floor ID. The transmission of floor ID is a key factor for perfect 3D position data. Other message types are Type 0 (2-D position data with floor ID), Type 3 (short ID), and Type 4 (medium ID).

Figure 4. IMES Message type 1, 3D position, and floor.

Figure 4. IMES Message type 1, 3D position, and floor

Interference Issue

Since IMES shares the same frequency as GPS L1 band (1575.42 MHz), there is an interference level that IMES may have on GPS signals. This interference has been studied in detail by conducting experiments and simulations. Based on these studies and analysis, various methods have been considered to avoid harmful interference to GPS signal. To avoid such interference, IMES center frequency is shifted by +/– 8.2 Khz from GPS L1 band. This will have the least impact on the GPS L1 band signal. For example, if the IMES signal is –110 dBm (very strong) and the GPS signal is –142 dBm (very weak), the loss of GPS signal (C/N0) due to IMES is less than 2 dB. If the IMES signal is –120 dBm and the GPS signal is –142 dBm, there is no loss of GPS signal (C/N0). Based on this analysis, the IMES transmitter power must be controlled such that the maximum power to the receiver does not exceed –110 dBm at a distance of 3 meters from the transmitter. Figure 5 shows the guideline specified in the QZSS IS document for setting the transmitter effective isotropic radiated power (EIRP)based on location.

Figure 5. IMES transmitter power setup guideline in QZSS IS document.

Figure 5. IMES transmitter power setup guideline in QZSS IS document.

Figure 6 shows the signal propagation loss for transmitter power of –70 dBm for various propagation loss-factor values of n. Figure 7 shows path loss for various transmitter power for the same loss factor, n = 2.5. These graphs shows the maximum power that shall be used to cover an area without exceeding the maximum power level. If a single unit of IMES cannot cover the complete area, then multiple IMES units will be deployed to cover the entire area with suitable power level. These graphs serve as a guideline for setting transmitter power.

Figure 6. Signal path loss for –70 dBm signal for different path loss coefficient, n.

Figure 6. Signal path loss for –70 dBm signal for different path loss coefficient, n.


Figure 7. Signal path loss for path loss coefficient, n = 2.5, for different transmitter power levels.

The signal propagation loss is calculated using the following equation; the gain of transmitter and receiver antennas is considered as unit gain (0 dB).


Hence, the equation depends on distance from the transmitter, d, and the propagation loss factor, n. The value of n is 2 for free space and increases for areas with objects that obstruct the signal. An office with soft partitions may use n = 2.5. The graphs can be used as a guideline to estimate the transmitter power to cover an area within the allowed power levels.

Application Areas

IMES can be used wherever indoor position data is required. It depends upon the application for that particular location as well. For example, an infrastructure-related safety application should have IMES installed at all elevators, escalators, staircases, emergency exits and routes, fire-fighting unit locations, and so on. Here are some of places where IMES might be used:

  • Every room of a building, to provide exact room location.
  • At entrances, exits, elevators, escalators, staircases, public facilities, and corridors for indoor navigation.
  • At every emergency exit for guidance.
  • Along hallways and lobbies at set intervals to guide the user.
  • In front of shops for advertising and information.
  • In sign posts to provide user’s location and guidance.
  • Complement other positioning systems like Wi-Fi, RF Tag, UWB, and so on.
  • As an indoor ground control point for surveying of large and multi-storey buildings.
  • With security cameras to provide accurate position data.
  • In factory production lines for automated control of moving objects.

Business Perspective

IMES technology was developed with the guiding concepts of low-cost global implementation and ease of installation and use. Low cost on the transmitter side is achieved by developing large-scale integratin (LSI) chips and IMES installation, setup, and database management tools. At the receiver side it is achieved by design of IMES signal so that existing GPS receivers in mobile phones, PDAs, or any other devices can use IMES by modifying only the firmware. The signal is designed so that it can adapt to other GNSS signals available in the future, for example, Galileo, QZSS, or Compass signals, requiring only firmware modification. Global implementation is made possible by signal design compatibility with existing GPS or GNSS signals. Ease of use is achieved again by signal design: one IMES transmitter can provide 3D position data, including floor information, with reliability and accuracy of a few meters even in deep indoor locations.

The development of IMES LSI chips (IMES transmitter) will also lead to development of value-added products for many consumer household appliances. For example, the green energy concept produced low-power LED lightbulbs. IMES chips can be installed in LED bulbs at very low additional cost. Similarly, it can be built in many other products like power socket devices, security devices, timing devices, and sensors where position data is also critical. This will provide an opportunity for the manufacturers to provide value-added products to users with indoor positioning devices. Not only electrical products but some construction materials or interior decoration materials like gypsum (dry
wall) boards can be made with built-in IMES chips. Installation of one piece of wallboard with an IMES built-in chip can provide position data in the room, reducing installation cost while not affecting the interior design of the room.

Implementation of IMES will also lead to new applications in the field of location-based services and applications where position data are necessary. It can also lead to new applications using IMES as an indoor electronic ground control point (GCP) in large buildings and indoor areas.

Chip Development. To reduce IMES transmitter cost, the IMES LSI chip has been developed and will be available by the end of the third quarter of 2011. This will reduce overall cost and size, and create platforms to develop value-added products integrating with other devices and systems. The chip is designed for global communications systems like personal handy-phone system (PHS, a mobile phone communication system developed in Japan), CDMA, and GSM. Figure 8 shows a block diagram of the chip transmitter.

Figure 8. IMES large-scale integration chip block diagram.

Figure 8. IMES large-scale integration chip block diagram.

The basic specifications of the LSI chip are: size, 12 x 12 millimeters; power, to be determined; maximum transmit power, –30 dBm or –60 dBm (user selectable); frequency, L1 band, 1575.4282 MHz or 1575.4118 MHz (user selectable); PRN codes, 173–182 (user selectable); signal type, GPS L1C/A, with upgrade capability to other GNSS signals.

Installation and Management

An IMES installation, setting, and management system has been developed to facilitate deployment. The main purpose of the system is to provide IMES transmitter position data (latitude, longitude, height) without conducting precision surveys, thus reducing installation, setting, and management costs. The system helps locate optimum locations for IMES transmitter siting, control transmitter EIRP power, set PRN IDs, and assign position data. The system can also use various types of map data sources to generate necessary floor data or indoor maps in 3D. The inputs can be either 3D vector data or 2D raster images, or even paper maps.

The overall system consists of four sub-systems:

IMES Setup Tool (ISET). This tool is used to set up the IMES transmitter. It provides two basic functions: to set up signal-related data (setting PRN code, transmitter power, navigation message rate, and so on) and to set up message-related data (position data, floor data, message types and their contents, message sequence, and so on). The R&D version of IMES also allows transmitting some special data for research and development purpose. It is possible to change the preamble value different from GPS, load a different PRN code table than IMES, change the navigation message data rate, generate a BOC(1,1) signal to test L1C-like signals, and change the RF frequency. The setup tool also has user-access management so that only authorized users can change certain sensitive data like PRN code, position data, and transmitter power.

IMES Database Management Tool (IDBM). This tool simplifies installation and management by providing a necessary database including a building-related database, a service-provider database, a device-related database, other integrated sensors database (if any), and a signal-related database. Since IMES is controlled and managed, guaranteed and authorized services can be provided for dedicated applications. This enhances the reliability of an IMES-based positioning system for infrastructure, security, and safety-related applications.

3D Mapping Tool (IMAP). This tool, shown in Figure 9, provides a 3D map database for IMES either for implementation or end-user applications. The mapping tool can use 3D vector data (for example, existing DXF files), raster image data, or direct user input. A laser scanning system with CCD camera is used to generate 3D data if existing data is not available. The tool creates walls, windows, doors, ceilings and other smaller objects from the laser data. If data are available in paper drawings, they are scanned to create raster images before digitizing them into vector format.


Figure 9. 3D Map Database Development System.

Figure 10. Concept of IMES database for implementation, setting and management.

Figure 10. Concept of IMES database for implementation, setting and management.

The system will ultimately create a 3D database of a building at floor level that can be linked with external databases. Figure 10 shows the overall concept of the IMES database system that includes both IMES database and 3D map database. The two database systems are linked by a relational database system. Any update in the map database can be reflected into the IMES database.

Signal Propagation Loss Tool (IPMODEL). This tool simulates the signal level where IMES will be set up. It is necessary to have optimum deployment of the transmitter to cover the area as large as possible within the allowed power level. Although the allowed maximum EIRP power level is –64 dBm for Japan, the approach is always to use the least power possible to cover the area, to avoid any possible harmful interference to other systems as well as to limit the availability of the signal to only the desired area.

The following equation is used to calculate the signal path loss which is based on Frii’s free-space path-loss model.


GT is the transmitter antenna gain. The receiver antenna gain is assumed to have unit gain (0 dB) and hence not included in the model.

L0 is the power loss at 1 m distance and is given by 20 x log10(signal wavelength) — 20 x log10(4*pi).

N is the path-loss factor, which is 2 for free space, 2.5 for office room with soft partition, and 3.0 for rooms with hard partition.

Ri is loss due to i number of reflections by objects.

Pj is loss due to j number of penetrations through objects.

Figure 11 shows the propagation-loss tool flowchart. It uses 3D map database provided by the 3D mapping tool and database from the database management tool. It also uses antenna gain pattern and material electrical properties to compute the power loss due to reflection and penetration. Figure 12 shows the signal propagation output from the model for a building lobby. Figure 13 and Figure 14 show the output from the propagation loss results from the actual measurement and model output, respectively. The results match within a difference of few dBs.


Figure 11. Path Loss Tool flowchart.

Figure 12. 3D view of signal power in a building lobby.

Figure 12. 3D view of signal power in a building lobby.

Figure 13. Actual signal power measured at different locations in the lobby shown in Figure 12

Figure 13. Actual signal power measured at different locations in the lobby shown in Figure 12

Figure 14. Signal power output from the propagation loss tool at the same location shown in Figure 13

Figure 14. Signal power output from the propagation loss tool at the same location shown in Figure 13

Experiments and Demonstrations

Experiments and demonstrations have been conducted to validate the IMES concept, uses, and applications. Early experiments validated the
concept, message design, and interference analysis. Later experiments focused on actual implementation for infrastructure, and social-network and location-based applications. Pilot projects have been conducted in collaboration with the Japanese government to test IMES capabilities for seamless positioning and navigation and for social infrastructure platform.

The Free Mobility Project in Kobe is the biggest social experiment using IMES for seamless navigation under the sponsorship by the Ministry of Land, Infrastructure, Transport, and Tourism. The project was conducted in an underground shopping mall of Kobe railway station. Shopping mall visitors were asked to participate in the navigation using IMES-capable mobile phones. Most visitors could follow the route they had chosen or find the destination point using the IMES set-up.

A total of 70 IMES transmitter units were installed at locations including ticket counters, elevator entrances, emergency exits, fire-extinguisher locations, staircases, station entrances, and alleys of the shopping mall. Figure 15 shows a part of the IMES transmitter location map. It covers one of the sections of the shopping mall. Figure 16 shows various locations where IMES transmitter devices were installed. As shown in Figure 17, intelligent 3D route guidance can be performed based on user preference. For example, a user in a wheelchair must be guided by a route that has no staircases, shown by green route in the figure, to reach the destination. A pedestrian can be guided by red route, which is the most direct route to the destination.

Figure 15. IMES transmitter location map to cover the underground shopping mall in Kobe Station.

Figure 15. IMES transmitter location map to cover the underground shopping mall in Kobe Station.

Figure 16. Installation of IMES near the station entrance and emergency exit.

Figure 16. Installation of IMES near the station entrance and emergency exit.

 Figure 17. Intelligent 3D route guidance using IMES.

Figure 17. Intelligent 3D route guidance using IMES.

Figure 18. Seamless navigation by mobile phone using GPS and IMES.

Figure 18. Seamless navigation by mobile phone using GPS and IMES.

The distribution of each IMES transmitter is done in such a way that it covers a radial distance of 10 to 20 meters. The deployment density of IMES depends on the location environment. If an IMES device is located near the entrance, the coverage distance will be around 10 meters to minimize transmitted power. IMES devices in deep indoor locations can cover a radial distance of about 15 to 20 meters.

Commercially available mobile phones with a firmware update for IMES were used to receive the IMES position data. The phones also included the shopping mall and station map including related databases for various applications.


IMES can provide reliable and guaranteed 3D position accuracy, including floor information. IMES signal design is done in such a way that it can use existing as well future GPS/GNSS receivers without any hardware modifications. Necessary implementation, setup, and management tools are also developed to facilitate IMES installation and to minimize the cost so that large-scale global implementation is possible. IMES LSI chips are being developed for large-scale implementation. IMES will also help in developing many other location-based applications and services. IMES evaluation kits will soon be available for joint R&D projects.

IMES technology-related patents have been filed in Japan and many other countries. The basic patents have already been approved in Japan. GNSS Technologies invites academic institutions to participate in joint R&D projects.

Dinesh Manandhar is a visiting researcher at the University of Tokyo, where he received his Ph. D, and a senior researcher at GNSS Technologies Inc. He is one of the designers of IMES message structure and involved in developing indoor navigation system based on IMES for seamless navigation environment. He can be reached at

Hideyuki Torimoto is the president of GNSS Technologies Inc. Japan. He established Trimble Navigation Japan and Weathernews Inc. in 1986. He also established the Research Forum on Social Infrastructure for Advanced Positioning (NPO) in 2003 and the Satellite Technology Laboratory in Tokyo University of Marine Science in 2004. He served as Satellite Division Member of ION for 2003-04. He can be reached at torimoto @

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