Understanding Extended Mode S Data in Flightradar24
With the release of the latest version of the Flightradar24 website we introduced new flight details called Extended Mode S data. We’re proud to be the first flight tracking service to offer this additional flight data. We receive the Extended Mode S data directly from the aircraft we are tracking and decode the data for display on the website. Extended Mode S data can include additional information about the speed and altitude of the aircraft as well as weather information. Extended Mode S data is available to subscribers with a Gold or Business subscription.
Extended Mode S speed data can include values for Indicated Airspeed, True Airspeed, and Mach. Not every aircraft broadcasts each value. Airspeed values differ from ground speed, which is displayed on Flightradar24 for every aircraft tracked, as they measure the aircraft’s speed through an air mass rather than how quickly it is moving over the ground.
Indicated Airspeed is provided by the aptly named airspeed indicator and is a pressure-derived value. Indicated Airspeed is relied upon for important reference points, such as takeoff, approach, and landing speeds. Used primarily in the cruise portion of flight for navigation purposes, true airspeed is the speed of the aircraft relative to the air mass through which it is flying. The true airspeed value takes the density of the air mass in which the aircraft is flying into account, as opposed to Indicated Airspeed, which assumes sea level atmospheric conditions. The Mach number is the speed of the aircraft relative to the speed of sound in current conditions, which changes based on pressure and temperature.
For each flight we track we display the calibrated altitude reported from the aircraft, which is a pressure-derived value. Extended Mode S data received from some aircraft also includes the GPS-derived altitude of the aircraft. Calibrated Altitude and GPS Altitude values may differ based on the local pressure of the air in which the aircraft is flying. Note that calibrated altitude values reflect the altitude above Mean Sea Level, a constant value used in aviation and other applications. 0 feet/meters above Mean Sea Level does not necessarily reflect an aircraft’s altitude above the ground. GPS altitude values are based on an ellipsoid of the entire earth, but are also not necessarily indicative of altitude above the ground.
From an increasing number of aircraft, we are also receiving wind and temperature data. Wind data includes speed and direction from which the wind is blowing. The temperature reported is the temperature of the air outside the aircraft.
Limitations and Future Enhancements
As Extended Mode S data is received directly from the aircraft it is subject to reporting error as there is no checksum sent to verify values received. The most common error in incorrect Mode S data is an unrealistic Mach value. Erroneous values in other data sets are also possible. We are also limited by the number of aircraft broadcasting Extended Mode S data. While the number of aircraft broadcasting full or partial Extended Mode S data is increasing, it is generally limited to aircraft with a newer transponder. Reception of Extended Mode S data also requires that the air navigation service provider where the flight is operating is interrogating the aircraft’s transponder to request the data. While this is increasingly common, it is not yet universal.