How Does a Plane Know Its Altitude?

A plane primarily determines its altitude using a barometric altimeter, which measures air pressure. As altitude increases, air pressure decreases due to the lower air density at higher elevations. The altimeter compares the measured air pressure to a pre-set pressure value, typically representing sea level pressure, to calculate the altitude.

• Barometric Altimeters: These are the most common type of altimeter, relying on the relationship between air pressure and altitude. They measure the ambient air pressure via a static port on the aircraft and use this to calculate altitude relative to a reference pressure set by the pilot. This reference pressure is usually the sea level pressure obtained from weather reports. [Source: Hartzell Propeller, AOPA, National Geographic]

• Radar Altimeters (Radio Altimeters): Larger aircraft often employ radar altimeters which measure the time it takes for a radio wave signal to travel from the aircraft to the ground and back. This provides a highly accurate measurement of altitude above ground level (AGL). [Source: Reddit (r/explainlikeimfive), Quora]

How the Altimeter Works

1. Air Pressure Measurement: The altimeter's sensor measures the ambient air pressure surrounding the aircraft.
2. Pressure Conversion: This pressure reading is converted into an altitude reading based on a pre-defined relationship between pressure and altitude.
3. Altitude Display: The calculated altitude is displayed on the altimeter's gauge, typically showing altitude above sea level (ASL).

Different Types of Altitude

It's important to note that there are different types of altitude used in aviation, and the methods for measuring them vary:

• Altitude Above Sea Level (ASL): The height above mean sea level, primarily determined using barometric altimeters.
• Altitude Above Ground Level (AGL): The height above the ground directly below the aircraft, usually measured using radar altimeters.

Accuracy Considerations

While altimeters provide a crucial measurement, their accuracy depends on several factors, including:

• Air Pressure Variations: Changes in weather patterns and atmospheric conditions can affect air pressure and thus the accuracy of barometric altimeters.
• Calibration: Regular calibration of altimeters is crucial for maintaining accuracy.
• Reference Pressure: The accuracy depends on the accuracy of the reference pressure used (usually sea level pressure).