When was selective availability turned on

SA is a purposeful degradation in GPS navigation and timing accuracy that controls access to the system's full capabilities. SA is accomplished in part by intentionally varying the precise time of the clocks on board the satellites, which introduces errors into the GPS signal. This component of SA is known as dither.

A second component of SA, known as epsilon, can also add error to the signal by providing incorrect orbital positioning data. PPS receivers with the appropriate encryption keys can eliminate the effects of SA. SA-induced errors can be varied by the DOD at the request of the National Command Authority or eliminated altogether, as was the case during the Persian Gulf War and the initial 25 The process of sending incorrect information to an adversary's radio equipment in this case a GPS receiver without their knowledge, using mimicked signals, is known as spoofing.

PPS accuracy is normally represented in this manner. SPS accuracy, however, is normally represented using a horizontal 2 drms measurement, or twice the root mean square radial distance error. Normally, 2 drms can be graphically represented as a circle about the true position containing approximately 95 percent of the position determinations. These include unintentional clock and ephemeris errors, errors due to atmospheric delays, multipath errors, errors due to receiver noise, and errors due to poor satellite geometry.

Each of these error sources is discussed below and summarized in Table C Many GPS receivers currently in use are able to achieve better results than the specifications call for.

From July , amidst growing pressure from bodies such as civil aviation, SA was turned on constantly until May As opposed to totally limiting its usefulness, this period probably actually accelerated the development of high-accuracy GPS technology. Developers and users were forced to innovate to negate the impact of the error. Techniques such as differential GPS — using a surveyed location and providing correction messages to nearby equipment — and utilising the carrier phase of the encrypted L2 signals form the basis of the precision techniques used in the present day.

With regards to SA, the DoD had one last surprise card to play. Whilst many had predicted that it would eventually be switched off, the majority of predictions were for or later. The more optimistic were hoping for a date in By turning it off, the President immediately improved GPS accuracy for the entire world. The United States has no intention of reactivating SA ever again.

Users in the U. In theory, civil receivers should now match the accuracy of PPS receivers under normal circumstances. We are in the process of collecting data to verify whether this is true. PPS still gives advantages to the military beyond accuracy. Rather than focus on the accuracy levels achievable on the ground, the document provides guarantees on the characteristics of the signal in space and the satellite constellation. This allows end users in different parts of the world to make more accurate predictions of GPS performance in their particular location, taking into account regional effects of the atmosphere and satellite geometry, as well as the effects of their particular user equipment.

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