Investigating GPS signals indoors with extreme high-sensitivity detection techniques

High-sensitivity GPS and assisted GPS are being extensively researched as methods to improve positioning indoors, where weak, multipath-affected signals are often difficult or impossible to use. To improve knowledge of indoor GPS behavior, this paper presents details of a raw GPS processing technique that enables extremely long coherent integrations, thereby providing extremely high detection sensitivity for indoor signals. The technique is used to evaluate signal characteristics in a pair of datasets gathered indoors, with carrier-to-noise density ratios as much as 40 dB or more below nominal open-sky signals. Results show that weak signals such as these can be used to provide reasonably accurate positioning if a sufficient number of signals can be detected to ensure good positioning geometry. Signal degradations caused by multipath are shown to be less damaging to posi- tion than the loss of availability caused by low signal strength. In addition, the high-sensitivity techniques based on precise tracking loop control demonstrate the potential for improved high-sensitivity GPS-based technologies using ultra-tight integration with additional sensors.