Hemisphere GPS MBX-4, Руководство пользователя

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MBX-4 User Guide
3
GPS Services
The positioning accuracy offered by GPS varies depending upon the
type of service and equipment available. For security reasons, two GPS
services exist: the Standard Positioning Service (SPS) and the Precise
Positioning Service (PPS). The US Department of Defense (DoD)
reserves the PPS for use by its personnel and authorized partners. The
DoD provides the SPS free of charge, worldwide, to all civilian users.
In order to maintain a strategic advantage, the US DoD has a policy to
artificially degrades the performance of the SPS. Currently the level of
this degradation has been set to zero, however, in years past, this
intentional error limited the positioning accuracy of the SPS to 100
meters 95% of the time. This policy is called Selective Availability (SA).
Without SA, autonomous positioning accuracy is currently about 10 to
15 m 95% of the time.
For many positioning and navigation applications, an accuracy of 10 to
15 meters is insufficient, and differential positioning techniques must be
employed.