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GPS availability and continuity are challenged indoors and cannot be ensured when a traditional receiver fails to track enough GPS satellites for a complete navigation solution: i.e., when less than four satellites can be tracked. The sensor diversity approach described in this article was designed to support precise navigation and timing capabilities in any environment and at any time, but GPS is still a critical part of sensor diversity even in challenging signal conditions. This article describes an advanced GPS signal processing architecture and illustrates how it can be applied to extend the GPS coverage into difficult environments such as indoors, urban canyons, and under dense foliage canopies. The article also reports the results of a feasibility study that evaluates the cm-level carrier phase availability and signal quality using real deeply integrated receiver hardware using live GPS data collected indoors without the need for external network assistance. This article focuses primarily on the carrier phase measurement as it is the most useful for inertial calibration. The test results presented herein indicate that deep integration enables reliable GPS measurements for many dynamic indoor scenarios.