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The Horizontal Protection Level (HPL) is the radius of a circle in the horizontal plane, with its centre being at the true position, which describes the region assured to contain the indicated horizontal position.
The Vertical Protection Level (VPL) is the half length of a segment on the vertical axis with its centre being at the true position, which describes the region assured to contain the indicated vertical position.
In other words, the HPL bounds the horizontal position error with a confidence level derived from the integrity risk requirement. Similarly, the VPL bounds the Vertical Position Error.
The figures present HPL and VPL over the selected day for the operational GEO satellites and their combination.