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It's All in The Processing
The first thing to understand about high sensitivity GPS is that this high sensitivity does not, in fact, come from using a different material for the receiver, or some different design. Rather, it has to do with the processing involved at the signals that the device receivers. “High sensitivity” is sometimes used somewhat more loosely in advertising than its strict sense of the term with GPS technology, so it's important to clarify.
Think of how far away those satellites are, thousands upon thousands of miles. The signal they transmit is incredibly weak after traveling all that distance, and are thus incredibly difficult to pick up from the normal background noise.
The difference between conventional and high sensitivity receivers is that the high sensitivity receivers will integrate the signal for far longer periods of time, basically more time in which they will piece together those weak, degraded signals to put together one that can then be used to triangulate your position. The result is that high sensitivity GPS units may pick up signals up to 1,000 times weaker than their conventional counterparts, resulting in 1,000 times more sensitivity. Technically speaking, this amounts to approximately 30dB of increased sensitivity.
In developing this technology, extensive research was done into the exact nature of the signal degradation that goes on in signal multipathing, which is the degradation that happens when a signal bounces off of other surfaces, be it the side of a skyscraper or the trunk of a tree, as well as into signal attenuation, the process by which the signal degrades as it passes through a material such as a roof of a building. For an interesting, if somewhat technical read, check out this article on testing high sensitivity GPS receivers from GPS World.
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Working Where No GPS Has Worked Before
As a result of this, high sensitivity GPS units are faster at acquiring satellites, and may acquire satellites that conventional receivers may never be able to pick up on in the first place. Note that this does not inherently mean greater accuracy, just that you're more likely to pick up more satellites and thus increase the number of satellites involved in triangulation.
This has proven to be invaluable additions in environments especially prone to GPS signal degradation, from so-called “urban canyons” to indoors to heavy foliage cover to closed containers. This opens up a whole host of possible locations for GPS units, from the underside of a car to inside a glovebox.
High sensitivity GPS technology is already widely available, usually in “premium” devices, though they are slowly becoming the standard. Every major company on the market currently has high sensitivity GPS products that are available for purchase.
High sensitivity GPS is not technically a type of assisted GPS technology, as is sometimes advertised. A-GPS relies on external infrastructure, whereas high sensitivity GPS is entirely internal. However, many people tend to lump high sensitivity GPS technology in which A-GPS technology, as they are often used in tandem for even greater effectiveness.