Definition
The minimum elevation angle above the horizon at which a GPS receiver will accept a signal from a satellite. Satellites below this angle are ignored because their signals travel through too much of the atmosphere and are too easily blocked or distorted to be reliable for position fixing.
Plain English
A cutoff line above the horizon. The GPS only listens to satellites that are higher in the sky than this line. Anything lower gets ignored because the signal is too weak or unreliable.
Context Anchor
Seen in GPS discussions, especially when explaining which satellites a receiver accepts or rejects for navigation.
Derivation
The word 'mask' comes from the idea of covering or blocking something out. The mask angle 'masks off' the lower part of the sky, hiding satellites that sit too close to the horizon to be trusted.
Why Pilots Care
Setting an appropriate mask angle prevents weak or obstructed satellite signals from degrading position accuracy and integrity.
Grounding Statement
Picture a low line above the horizon: satellites below that line are ignored, and satellites above it may be used.
Intuition Check
“Mask” does not mean a physical covering here. It means the receiver is electronically screening out satellites below a set angle.
Example Sentence 1
Most GPS receivers use a mask angle of around 5 degrees, so satellites sitting just above the horizon are not used in the position calculation.
Example Sentence 2
With a 5-degree mask angle selected, the GPS receiver ignored satellites near the horizon during the approach.