ADS-B Turbulence Reports: How Do They Work?
By Scott Spangler on May 18th, 2020
Weather.gov
The FAA recently posted a fascinating story on Medium, Taking the Turbulence Out of Flight that said ADS-B turbulence reports offer the possibility of more accurate reports on the bumps in the sky. What the story never fully explained is how the ADS-B system would generate the reports.
Like driving on a bumpy road, turbulence is what happens when atmospheric forces result in rapid changes in the vehicle’s horizontal and vertical planes. Its intensity depends on the size of the bump or pothole in the sky. And until this article, reporting such turbulence was a subjective evaluation made by the individual filing a pilot report.
Given a baseline understanding of the system, it is easy to intuit without explanation how the ADS-B turbulence position reports used its GPS WAAS engine. And its extended squitter bandwidth certainly had room for turbulence reports, but it never said how ADS-B would know it was in turbulence.
An FAA ADS-B FAQ page offered clues. ADS-B reports barometric pressure altitude as well as the airplane’s GPS-computed geometric altitude, “the height of the aircraft above the earth ellipsoid.” The two altitudes are not the same, the FAQ explained, “but having both allows for applications that require one or the other as an altitude source and provides a means of verifying correct pressure altitude reporting from the aircraft.”
FAA.gov
The next paragraph provides another assumed part of the ADS-B turbulence report puzzle. “ADS-B does not report vertical or horizontal airspeed. Instead, ADS-B reports horizontal and vertical velocity relative to the earth. This velocity is useful for air traffic control functions and ADS-B applications. Airspeed can be provided by other aircraft sensors.” (And speaking of sensors, how will ADS-B separate atmospheric turbulence from the roiling vortices created by other aircraft?)
Might ADS-B turbulence reports be one of the system’s new applications? It seems that the necessary ingredients—position, altitude, and horizontal and vertical velocities—are available for a turbulence-sensing algorithm. But for the reports to have any value, the system must be communicating constantly with a ground station. With an update interval of once a second, that could be considered constant, even with uncompensated latency.
But this is merely an exercise in supposition, a guess of how ADS-B turbulence reports might work. If this guess is anywhere near to close, the best part of it, besides more granular turbulence reports, which any pilot would appreciate, is that that capability might become part of the ADS-B system with a software update, and not the installation of another piece of hardware.
Weather.gov
Another benefit to pilots would be buried in the ADS-B turbulence report algorithm. It would consistently define different aircraft-appropriate levels or categories of turbulence with standardized variations or changes in altitudes and horizontal and vertical velocities. So before making a Pirep, pilot would not have to recall the Turbulence Reporting Criteria Table in the Aeronautical Information Manual. –Scott Spangler, Editor
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