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 position 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.