Although many aircraft operators complain that the advent of Automatic Dependent Surveillance-Broadcast (ADS-B) Out mandates do nothing to improve flight operations, that might soon change. Apart from the benefit of better surveillance compared with radar, the ADS-B signal broadcast from participating aircraft contains data that could help forecasters spot turbulence earlier, allowing them to share that information with other aircraft.
ADS-B signals provide a wealth of data, such as the aircraft’s GPS position, velocity, identity, barometric and geometric altitude, and, most important for the turbulence concept, horizontal and vertical velocity relative to the Earth. The vertical velocity information is key to sussing out turbulence.
The FAA has long been studying how aircraft sensors can contribute valuable data to weather forecasters under its Weather Technology in the Cockpit (WTIC) program. In fact, during the coronavirus crisis, the vast reductions in airline flying have had a significant effect on forecasting models, as forecasters had been depending on airborne methods to supplement traditional ways of gathering weather information.
ADS-B data might further aid the WTIC program, which seeks to “determine whether an operationally useful turbulence detection algorithm using routine ADS-B reports is feasible.”
Larry Cornman, a project scientist at the National Center for Atmospheric Research, gave a presentation at the American Meteorological Society’s annual meeting in January, introducing the concept of using ADS-B information for turbulence detection and summarizing research on developing the algorithm.
“Part of the problem with turbulence is it’s highly dynamic, both spatially and temporally,” he explained in the presentation. “Lack of observation that supports understanding of that dynamic nature is one of the big problems.”
The current method for obtaining information about turbulence is through pilot reports (pireps) and in-situ eddy-dissipation rate (EDR) reporting. In-situ EDR gathers data from software installed in aircraft monitoring systems, such as information about acceleration, pressure, wind, angle of attack, and pitch and roll angles, “to calculate a measure of the turbulent state of the atmosphere,” according to the University Corporation for Atmospheric Research (UCAR).
The problem with pilot reports, UCAR explained, is that they tend to be subjective, with large variations in location accuracy and reported time as well as different severity assessments depending on the size of the aircraft.
In-situ EDR has its own limitations, in that it has to be installed on aircraft operated by a willing operator. According to Cornman, only about 1,400 aircraft in the U.S. are so equipped. And just about 1,200 pireps are generated each day. These numbers are tiny compared with the nearly 140,000 aircraft fitted with ADS-B Out in the U.S. that are automatically broadcasting data-rich signals every time they fly. (Obviously, not as much flying as normal is taking place during the pandemic.)
Cornman pointed out another problem with existing turbulence detection methods, and that is that the scale for the current models is four to eight times larger than the scale of actual turbulence affecting an aircraft. This number ranges from tens of meters to two kilometers. “There is a mismatch there,” he said.
It is hard to gain an understanding of how “these large-scale processes in the atmosphere pull down to the small scales that impact a vehicle,” he explained. “That’s why we have trouble with forecasting turbulence because we don’t have information at the small scale, and we don’t have observations to support better forecasting. Getting more observations is key.”
What ADS-B could do for this effort is provide “precise information about the location and time,” he said. “It gets over a lot of the problems we have with the pireps. If we can get good information out of it, it’s going to be a game changer. It’s going to be really important information.”