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How does an aircraft measures (or calculates) its speed while in air? Which equipment or sensors are used for measurement of speed? Does it takes Earth as a reference frame for its speed measurement so its speed is relative to earth surface OR does it calculate's absolute speed?

Does the aircraft altitude above sea level or ground level effects the measurement of its speed? I mean if the aircraft is flying low vs high then will it use same sensor to measure its speed?

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The speed most relevant to flight is airspeed. It is measured by using the aircraft’s pitot-static system. The speed most relevant to navigation is ground speed. It can be calculated from True Airspeed adjusted for airmass movement (wind drift). Or, it can be measured using GPS.

There are 4 types of airspeed, 1 type of relative airspeed, and 1 type of ground speed commonly used in aviation. They are:

  1. Indicated Airspeed - indicated on the instruments
  2. Calibrated Airspeed - IAS adjusted for sensor placement
  3. Equivalent Airspeed - CAS adjusted for air compressibility
  4. True Airspeed - EAS adjusted for density altitude
  5. Mach Number - a ratio of airspeed over speed of sound
  6. Ground Speed - ground position change over time

Absolute Altitude is another commonly used term in aviation. Absolute airspeed is not a commonly used term.

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