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 ABSTRACT 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. 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). 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. 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. 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. 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 (ISQZSS) [2] respectively. Table 1 shows the signal properties of IMES. The transmitter power is set based on location environment and separation between the 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 ISQZSS 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 valueadded 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. the transmit power we would like to use for the IMES (e.g. -70dBm) is lower than the allowed value. . 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 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 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 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 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 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 11: GUI to control IMES signal generator Figure 13: IMES experiment inside underground parking area. The IMES capable cell phones were kept in the pole, pocket and handbag. Figure 12: IMES message display with database over the Google Earth Data 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/isqzss/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/IRN200D-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
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