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High Sensitivity GPS receivers use large banks of correlators and digital signal processing to search for GPS signals very quickly. This results in very fast times to first fix when the signals are at their normal levels, for example outdoors. When GPS signals are weak, for example indoors, the extra processing power can be used to integrate weak signals to the point where they can be used to provide a position or timing solution.
GPS signals are already very weak when they arrive at the Earth’s surface. The GPS satellites have transmitters that only deliver 27 Watts from a distance of 20,200km in orbit above the Earth. By the time the signals arrive at the user's receiver, they are typically as weak as –160dBW, equivalent to one tenth of a millionth billionth of a Watt. This is well below the thermal noise level in its bandwidth. Outdoors GPS signals are typically around the -155dBW level.
Conventional GPS receivers integrate the received GPS signals for the same amount of time as the duration of a complete C/A code cycle which is 1ms. This results in the ability to acquire and track signals down to around the -160dBW level. High Sensitivity GPS receivers are able to integrate the incoming signals for up to 1,000 times longer than this and therefore acquire signals up to 1,000 times weaker. A good High Sensitivity GPS receiver can acquire signals down to -185dBW, and tracking can be continued down to levels approaching -190dBW.
High Sensitivity GPS can provide positioning in many but not all indoor locations. Signals are either heavily attenuated by the building materials or reflected in as multipath. Given that High Sensitivity GPS receivers may be up to 30dB more sensitive this is sufficient to track through 3 layers of dry bricks or up to 20cm of steel reinforced concrete for example.