What is the mask angle in satellite navigation?

• A. The maximum angle below the horizon for usable satellites.
• B. The angular separation between satellites in the sky.
• C. The angle between the aircraft and the satellite at nadir.
• D. The lowest angle above the horizon at which a satellite can be used.

Answer: (D)

The mask angle is the minimum elevation angle above the horizon at which GNSS satellites are considered usable. Signals from satellites that are very low on the sky are more likely to be degraded by the atmosphere, multipath reflections, and obstructions, so receivers exclude those satellites to improve accuracy and reliability. If you raise the mask angle, you’ll use fewer satellites but with better signal quality and geometry; lowering it gives more satellites but can introduce weaker signals and higher error potential. Typical values are in the 5 to 15 degree range, depending on the required integrity and flight phase.

The other options aren’t what this term describes: it isn’t about how far apart satellites appear in the sky, nor about the angle between the aircraft and a satellite at nadir, nor about the maximum angle below the horizon for usable satellites.