Public and commercial services such as search-and-rescue, firefighting, and location-based services (LBS) require accurate indoor position determination. Users face the fundamental problem of knowing location at the room level, requiring accuracies on the order of a few meters. Because of the need for floor determination, vertical accuracy can be more important than horizontal.
The United States and Japan have passed emergency-call legislation requiring third-generation cell phones to have functions determining their positions. Rapidly increasing numbers of cell phones with built-in GPS capabilities stimulate the LBS market, which is expected to be the dominating market for GNSS receivers in the future.
Significant problems remain, however. Standard GPS receivers do not work well indoors, giving position errors up to several tens of meters - if a position fix is possible at all. Assisted GPS (AGPS) improves the situation, but makes equipment more complicated and expensive, while reliability and accuracy may still not suffice. Local solutions based on RFID, Wi-Fi, ultra-wideband, pseudolites, and others have been and are being tried, but with no definitive solution in sight.
The IMES concept
The Japan Aerospace Exploration Agency, GNSS Technologies, and Lighthouse Technology and Consulting have proposed and developed an indoor messaging system (IMES) to solve the availability issue. Composed of transmitters, GPS receivers with modified firmware embedded in cell phones, and systems of servers, IMES aims to provide seamless positioning anywhere in a covered area. Hitachi Ltd., Systems Development Laboratory is also working on such a system.
IMES uses satellite signals outdoors in the usual way, while using signals from IMES transmitters indoors, where satellite signal quality is strongly reduced. IMES signal structure is similar to that of GPS satellite signals, except for the contents of the navigation message. Thus, the same receiver can be used for both.
An IMES transmitter sends an RF signal similar to that of GPS and the Japanese Quasi-Zenith Satellite System (QZSS), giving its 3-D position, the position of the center of its cell coverage zone, or linking the receiver to a database corresponding to an identifier.
Instead of the ephemeris data, clock corrections, ionospheric parameters, and so on contained in the GPS message, the IMES navigation message periodically dispatches position broadcasts and additional information in a similar format.
The concept for seamless transition between indoor and outdoor use is built on three further requirements:
- low power consumption,
- low transmitter cost, and
- no harmful interference to other users of GPS or QZSS.
IMES assumes that positioning accuracies on the order of 10 meters will satisfy users, who just want to know where they are in rooms of moderate size, shopping areas, parking garages, and so on. Stable and reliable position information means more than high accuracy.
Signals and codes
IMES uses the same L1 center frequency as GPS and QZSS, and the same BPSK modulation. IMES dedicated spread-spectrum codes are from the same family of Gold codes as GPS and QZSS (numbers 173-182 from the C/A-code assignment table). It is assumed that signals similar to L1C can be used in the future. These dedicated codes are re-used repeatedly but without using the same code as neighboring transmitters.