GPS and UWB for indoor navigation

A multitude of applications would benefit from precise indoor navigation. Anywhere from automating storage in warehouses to tracking firemen in hazardous environments would make such an endeavor worthwhile. Some server-based GPS systems, like SnapTrack, already claim some navigation capabilities indoors. However, such systems are in general accurate to within a few tens of meters. Furthermore, pseudolites have been deployed for indoor use. Although some experimental setups show decent navigation performance, there is a question of whether GPS has a “good enough” signal structure for such applications in the first place. Spread spectrum pseudoranging is susceptible to multipath that is less than one chip width away from a direct path ray. In the case of GPS C/A-code, the chip length is about 300 m. Obviously, most indoor signal reflection delays would be significantly shorter than that distance. Ultra-WideBand (UWB) technology is built around transmitting short discrete pulses instead of continuously modulating a code onto a carrier signal. Such pulses typically last only 1-2 ns, and one can distinguish pulses that are more than 1-2 ft apart. Thus, making UWB systems robust to multipath delays of more than one pulse width. We measured the impulse response of the RF channel at the Stanford University LAAS Laboratory. This paper quantifies that multipath channel in terms of average delay and delay spread. We found several cases where multipath components were stronger in magnitude than the direct signal. Whereas such an environment would bias pseudorange measurements of GPS C/A-code, a properly designed UWB system could resolve most multipath and accuracy would degrade more gracefully than for GPS.