Speed Values in ADS-B Data: GS, IAS, TAS, and Mach

Let's discuss speed values that are reported in ADS-B data. SkyAware can show you up to four different speed values (subject to the availability of that data from a particular aircraft in your area). Speed data is transmitted from the aircraft in units of knots. SkyAware will convert to Imperial or metric units based on your selected unit of measure preference.

What do these values tell us?

Ground speed - This is the speed everyone can understand. It is simply how fast the aircraft is moving over the ground, regardless of wind, and is the most important for measuring travel time.  For example, an airplane with a 600 miles-per-hour ground speed will travel 600 miles in one hour.

Indicated Airspeed - This is the speed of the aircraft through the air as read directly from the airspeed indicator (which is a part of the aircraft's avionics). This is measured by the amount of air passing into an instrument on the airplane called a pitot tube. At higher altitudes where the air is less dense, the Indicated Airspeed will be much lower than the ground speed or True Airspeed because there is less air entering the pitot tube than at sea level.

True Airspeed - This is the actual speed of the aircraft relative to the airmass in which it is flying. To calculate true airspeed, the indicated airspeed is adjusted based on the outside air pressure and temperature.

Mach Number - This is the ratio of the aircraft's true airspeed to the speed of sound, i.e. at Mach 1 true airspeed is equal to the speed of sound, values less than 1 are subsonic speeds and values greater than 1 are supersonic speeds. It is calculated using the outside air temperature and the true airspeed. Mach 1 is approximately 340 meters per second at mean sea level but varies based on temperature.

Fun fact: speed data can be used with heading and ground track to mathematically derive in-flight weather data such as wind direction, wind speed, air temperature and static air pressure!

For those interested in joining the ADS-B community, FlightAware offers step-by-step instructions to build a PiAware Receiver for under $100.

ADS-B hosts located in areas needing additional coverage might be eligible for a FlightFeeder, a free, prebuilt ADS-B receiver.

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