An interview with Bart Klein Obbink, NLR – Royal Netherlands Aerospace Centre
As part of the Green-GEAR project, researchers from a variety of organisations have been investigating innovative ways to make aviation more efficient while maintaining the highest safety standards.
The Green-GEAR project consists of three solutions: GeoAlt, Separation Minima, and Green Route Charging. The Separation Minima work package, led by NLR, is assessing the theoretical feasibility of reducing vertical separation in en-route airspace after replacing barometric with geometric altimetry. We spoke with Bart Klein Obbink, researcher at NLR in Amsterdam, about his role in the project and what the research results could mean for the future of aviation.
Q: Bart, could you start by telling us a little about your background and your role within Green-GEAR?
A: My background is in mathematics, and at NLR I mainly focus on the safety of air traffic control operations. Within Green-GEAR, I worked as a researcher on the solution concerning the reduction of vertical separation between aircraft. My role was to carry out the mathematical analysis—essentially doing the maths to determine whether this could be done safely.
Q: What exactly does “reducing vertical separation” mean, and why is it important?
A: Currently, aircraft are separated vertically by 1,000 feet. We explored whether that distance could be safely reduced to 500 feet, enabled by geometric altimetry, which uses satellite data instead of traditional barometric measurements. More accurate altitude data means aircraft could fly closer together.
The benefits are twofold: firstly, it could allow aircraft to fly at their most fuel-efficient altitudes, reducing fuel consumption and environmental impact. Secondly, it can potentially increase airspace capacity, which might help reduce delays in busy regions.
Q: What were the biggest challenges you identified at the start of this research?
A: There are three main challenges. Firstly, the technology itself—geometric altimetry is essential, but not yet in operational use. Secondly, the traffic collision avoidance system (TCAS) would currently trigger constant alerts if aircraft flew only 500 feet apart. And thirdly, safety: we had to analyse both normal operations and non-normal situations, such as satellite failures or jamming and spoofing, to ensure risks remain acceptably low.
Q: Did your view on the feasibility of reducing separation change as the project progressed?
A: Yes. At first, I was sceptical. Aircraft are large, turbulence exists, and satellites are not optimised for vertical accuracy. But as we analysed the data in more depth, it emerged, that the main risks fall within the acceptable range. This was something I hadn’t expected. Both the accuracy of the satellite systems and the height-keeping performance of aircraft are good enough that reduced separation seems feasible under normal conditions.
Q: What were your key results and conclusions?
A: We made an important step toward showing that reduced vertical separation is feasible in theory. The next challenge is to look at operational aspects—how this could actually work in practice, across global aviation systems.
Q: And what are the next steps for this line of research?
A: This is not something that Europe can decide and put into operation single-handedly. Any change would need worldwide agreement, and even introducing geometric altimetry on its own would be a huge undertaking. When vertical separation was last reduced—20 years ago with RVSM—it took 10 to 15 years to implement. So, while our results are promising, it will take international collaboration and further development before this becomes reality.
Q: Finally, could you briefly describe the mathematical work you did for this research?
A: In simple terms, I calculated how different deviations in aircraft positioning add up, using stochastic distributions and complex analysis. We also studied the accuracy of satellite systems such as GPS and Galileo in vertical positioning at higher flight levels. This gives other researchers a clearer picture of what is possible and where more work is needed.
Looking ahead
The NLR’s work within Green-GEAR demonstrates that reducing vertical separation could open new possibilities for aviation efficiency and sustainability. While implementation will be a long and complex journey, the research conducted within the project has laid valuable groundwork for future developments.
