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发表于 2010-1-9 16:03:33
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Assisted GPS (A-GPS) improves on standard GPS performance by providing information,
through an alternative communication channel, that the GPS receiver
would ordinarily have received from the satellites themselves. Figures 1.1 and
1.2 show overviews of an A-GPS system. Note that A-GPS does not excuse the
receiver from receiving and processing signals from the satellites; it simply makes
this task easier and minimizes the amount of time and information required
from the satellites. The A-GPS receiver still makes measurements from the satellites,
but it can do so more quickly, and with weaker signals, than an unassisted
receiver.
GPS was originally designed to guide bombs, aircraft, soldiers, and sailors. In
all cases, the GPS receiver was expected to be outside with a relatively clear view of
the sky. The system was designed to require a start-up time of approximately 1 min,
and after that it would operate continuously. Today GPS is used for many more civilian
than military purposes. Counterintuitively, the system demands of these civilian
applications far exceed those seen before. GPS is now expected to work almost
anywhere, even, sometimes, indoors; push-to-fix applications have emerged where
a single position is expected almost instantly; and all of this must be delivered in a
way that adds little or no cost, size, or power consumption to the host device. These
requirements are what drove the development of A-GPS. |
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发表于 2010-1-9 16:02:49
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