All existing and previously available embedded Global Positioning System (GPS) receivers are implemented with an implicit assumption that the GPS receiver is generally powered-down, disabled or turned-off for a majority of the time that the equipment in which the GPS receiver is embedded, or operating in conjunction with, is enabled. Therefore, legacy GPS receiver architectures can be viewed as “mostly-off”. Under such a usage model, architectures of embedded GPS receivers are significantly influenced by a criterion to rapidly determine a current location when enabled or powered-up. The rapid location determination is commonly known as the “Time To First Fix” or TTFF of the GPS receiver.
A strength of normal GPS signals at the surface of the Earth is typically no less than −129 dBm. However, the normal GPS signal strengths are observed with a direct, unobstructed line-of-sight to each of several GPS satellites. When the strengths of the received signals are lower than −142 dBm, there is no positive signal-to-noise ratio of direct sequence spread spectrum codes in the GPS signals. Therefore, signal acquisition below the −142 dBm power level becomes complex. When a GPS receiver is located in an obstructed region, such as inside a building, within a car or in a coat pocket, the received signal strengths can fall to signal levels of −160 dBm and lower.
At very low signal strengths, phase lock between the GPS receiver and one or more of the GPS satellites is often lost. If the GPS receiver loses phase lock with a particular GPS satellite, the receiver is no longer “tracking” with the particular GPS satellite and thus attempts to “reacquire” phase lock. Conventional GPS receivers can remain phase locked to (i.e., track) GPS signals while the received signal power is below a minimum threshold at which the GPS receiver can acquire phase lock. Once phase lock with the signal is lost, reacquiring phase lock is either (i) significantly expensive in terms of die area of an integrated circuit and/or total power consumed by a solution or (ii) not possible because the signal is too weak for the solution.
Operation of conventional GPS receivers in weak signal conditions, such as within a building, has been a serious problem. Before 2007, acquisition sensitivity exceeding −162 dBm has not been possible. The issue is not if the GPS receiver would work indoors, but rather when would the GPS receiver stop working once moved indoors. In a weak signal environment, GPS signal acquisition is either (i) difficult, (ii) expensive or (iii) impossible. The indoor scenario is a major issue for GPS receivers architected around the assumption that the receiver is generally disabled, powered-down or off. When “mostly-off” receivers are activated in the weak signal environment, the ability to determine location varies with GPS satellite position, signal strength and receiver location. Under such circumstances, the “mostly-off” embedded GPS receivers fail to determine the current location and reports “location unknown”.