SBAS Reference Stations are deployed throughout the region of service at pre-surveyed locations to measure pseudo ranges and carrier phases on L1 and L2 frequencies from all visible satellites.

The reference stations send these measurements to SBAS Master Station, which calculate clock and ephemeris corrections for each GPS satellite monitored, ephemeris information for each GEO, and Ionosphere grid points (IGPs) at an altitude of 350km above the earth's surface.� 

In addition to the corrections, the Master Station calculate error bounds for Ionosphere corrections call grid Ionosphere vertical errors (GIVEs) at each IGP , and also combined error bounds for clock and ephemeris corrections for each visible satellite, called user differential range errors (UDREs)

The Master station sends these corrections and error bounds to the users through GEO communication satellites with a data rate of 250bits/s User avionics apply these corrections to their pseudo ranges obtained from GPS measurements, in order to improve the accuracy of their position estimates.� They also use the UDREs and GIVEs and other information to calculate error bounds on position error called the Vertical Protection Level (VPL) and Horizontal Protection Level (HPL).For the integrity of the system, these protection levels must bound the position errors with probability must bound the position errors with probability greater or equal to 0.9999999 in one hour for en-route through Non Precision Approach operations and for Precision Approach in 150 seconds.


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