<Emphasis Type="Italic">sub</Emphasis>ATTO indoor GPS – pitfalls,solutions and performance using a conventional correlator

A key requirement for emergency call location (e.g. E911), for robust operation of location-based m-commerce systems and for telematics systems is that the location technology be able to operate in urban canyons and inside buildings. We start from a definition of the target environments, which includes multi-level parking garages, office buildings and homes, but not underground parking garages or tunnels. Based on experience in these target environments and understanding of typical applications we derive specific requirements for sensitivity and acquisition speed. The primary problems associated with weak signal operation are as follows. (1) In conventional GPS receivers sampling at the correlator output typically occurs at a sampling interval of the order of 1 ms. With weak signals, however, the signal-to-noise ratio of these samples is too low to support lock-in of a phase-locked or frequency-locked loop. (2) With weak signals, the signal-to-noise ratio is too low to support the extraction of the 50BPS navigation message from the signal. Therefore, aiding data is required from an external source. (3) Because the data cannot be extracted, it is not possible for the receiver to synchronize to the incoming bits, words or subframes. Therefore, it is not possible to construct pseudoranges without prior information. (4) The paper describes Sigtec Navigation's subATTO technology. This technology provides sensitivity down to –185 dBW (19 dBHz assuming NF of 1.5 dB and no other implementation loss). This is 5 dB below an attoWatt (10^–18 W) and has been shown to provide reliable positioning inside buildings, multi-level parking garages and in urban canyons without any aiding at all. The paper describes the patented signal processing scheme, how ambiguity resolution and time synchronization are achieved, the wireless assistance technique, the acquisition strategy and the use of scanning channels. Results are presented from trials in a multi-level parking garage. The results obtained in most parking garages are similar to these in terms of availability of fixes, signal strengths received and location accuracy achieved. The performance achieved in multi-level parking garages is rarely worse than this. One of the major impediments to practical application of weak signal-processing schemes is the limited dynamic range imposed by the GPS C/A code signal structure. This problem is discussed along with the problems of multipath distortion in the context of telematics operation in urban canyons. A realistic urban accuracy goal of 20 m for 95% of fixes is proposed based on experience with GPS and dead reckoning. Enhancements under development will provide sensitivity of –188 dBW, which will provide continuous availability within a broader range of indoor environments. For practical applications, this will require the use of modern 'search engine' hardware for acceptable acquisition speed. As the paper shows, this sensitivity is near the practical limit of sensitivity with acceptable acquisition times and dynamic capability. Electronic Publication