What is known as the vertical angle below which GPS signals are less reliable?

The correct answer is Mask Angle. The Mask Angle refers to the vertical angle at which GPS signals begin to become less reliable. Signals that come from satellites positioned at low elevations (approaching the horizontal plane) can be affected by obstructions such as buildings, trees, or terrain features, which can lead to multipath errors or attenuation of the signal.

By defining a Mask Angle, surveyors can set a threshold for acceptable satellite elevations to ensure the accuracy of the GPS data collected. A higher Mask Angle means that only satellites with higher elevations will be considered, thus improving the quality of the signal and reducing potential errors.

The other options, such as Elevation Angle, Declination Angle, and Azimuth Angle, pertain to different aspects of positional and directional calculations in navigation and surveying but do not specifically address the reliability of GPS signals in relation to their vertical positioning. Understanding the concept of Mask Angle is crucial for surveyors who need to optimize their GPS measurements for better accuracy and reliability.