Flightradar24 is powered by ADS-B data fed directly from thousands of aircraft around the world. The aircraft’s onboard equipment communicating with our global network of ADS-B receivers allows us to display a variety of detailed parameters such as speed, weather information, and altitude.
How do we measure altitude on Flightradar24?
For every flight, we display the barometric altitude, which is sent by all aircraft. Many aircraft now also transmit their GPS altitude, which may be different from the barometric altitude.
What is Barometric altitude?
Aircraft measure barometric altitude using their altimeter, essentially a pressure-based instrument that measures the air pressure outside to determine the vertical position of the aircraft. These are calibrated in a unit of measurement called Hectopascals ‘hPa’ in Europe and Inches of Mercury ‘InHg’ in other locations, mainly the United States.. To accurately measure altitude, aircraft need to use a datum reference, either height above the ground (known as QFE) or height above mean sea level (known as QNH).
Above certain altitudes, commercial aircraft set their altimeters to the same setting of 1013 hectopascals (or 29.92 inHg). Referred to as the ‘Standard Pressure Setting’, this ensures that all of the aircraft’s altimeters are referenced to the same datum. Altitudes reported on this setting are referred to as ‘Flight Levels’. This is the ‘Barometric Altitude’ reading you see on Flightradar24.
Below these altitudes, aircraft fly with reference to local pressure, commonly referred to as ‘QNH’. This is essentially a localised measure of altitude above mean sea level. ADS-B only reports altitude values based on the standard pressure of 1013 hectopascals; this is why altitude values near the ground can sometimes appear unrealistic.
This is particularly prevalent at high altitude airports such as Denver (DEN/KDEN), which sits at an elevation of 5,433 feet above mean sea level. As such, aircraft can shift from reading altitudes of 5,400 feet on final approach to o feet upon landing.
What is GPS altitude?
GPS altitude values are measured based on a different datum – a consistent line drawn around the earth called an ellipsoid, which for different locations can be up to 100m offset from true altitude (mean sea level). The deviation of the ellipsoid from mean sea level can range from +85 meters in Iceland to −106 meters in Southern India. GPS altitude is not sent by all aircraft, and may not always be displayed.
Which altitude readout isn’t included in ADS-B data?
A key omission from altitude readings reported by ADS-B is radio altitude. During the final stages of an instrument approach, most commercial aircraft switch to using a device called a radio altimeter. According to the International Civil Aviation Organisation (ICAO), the radio altimeter:
“Uses a pulse of radio frequency energy transmitted towards the earth to measure the absolute height above the terrain immediately underneath the aircraft”.
This is more accurate than using a pressure-based altitude reading, and helps with terrain avoidance and awareness when the aircraft is on final approach. Whilst the radio altimeter was often a separate gauge on older aircraft, on modern aircraft its reading is typically integrated into the primary flying display (PFD). The radio altimeter kicks in around 2,500 feet above ground level (AGL) and prompts the aural callouts of ‘50’, ‘40’, 20, ‘10’ and ‘retard’ in Airbus aircraft.
