From the laboratory to the skies – measuring flight data with lasers
August 24, 2022 | Optical Air Data
From the laboratory to the skies – measuring flight data with lasers
Sensors for laser absorption spectroscopy mounted on the rear of the aircraft
Laser absorption spectroscopy sensors are positioned on the rear of DLR's Falcon research aircraft. They measure the degree to which oxygen molecules in the atmosphere absorb the laser beam passing between the sensors. This analysis enables the precise calculation of air pressure and, consequently, altitude, as well as the rate of ascent and descent.
Three laser-based instruments, two of which were mounted on the underside, were installed onboard the DLR Falcon research aircraft. In a first flight campaign, DLR tested how laser sensors could enhance the precision and reliability of flight data measurements.
DLR is developing laser-based methods to determine flight data more reliably.
This includes measuring key parameters such as altitude, speed and rates of ascent and descent.
Three such instruments recently flew for the first time aboard DLR’s FALCON research aircraft, successfully passing their first practical test.
Focus: Aviation, laser technology
The German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) is developing the 'Optical Air Data' system for measuring essential flight data with laser-based methods. This includes measurements of key parameters such as altitude, speed, and rates of ascent and descent. This innovative approach enables more reliable flight data measurements while significantly reducing the time and effort needed for sensor installation and calibration. Another key advantage of laser sensors is that they provide an immediate indication of whether they are functioning correctly. This is not always possible with traditional sensors, which can sometimes deliver inaccurate readings if compromised by dirt or ice build-up. Currently, many aircraft rely on mechanical methods that have remained largely unchanged since the early days of aviation.
Three laser-based instruments for more accurate and reliable flight data
During a series of six flights, DLR researchers collected approximately 20 hours of data, observing the sensors in action. The three instruments onboard included a laser Doppler anemometer, a scattering measurement device and a laser spectrometer. "The laser Doppler anemometer measures relative wind speed, allowing us to determine the aircraft's speed and angle relative to the airflow," explains project manager Oliver Kliebisch. "Using a specialised laser sensor, we measure filtered Rayleigh scattering to determine the temperature outside the aircraft. Meanwhile, laser absorption spectroscopy enables us to measure the air pressure and thus the altitude, as well as the rates of ascent and descent." With the first two methods, the instruments direct a laser beam into the air surrounding the aircraft. Laser Doppler anemometry examines how particles in the air scatter the laser beam, while Rayleigh scattering analysis observes how the beam scatters off air molecules. In the third method, the instrument sends a laser beam along the rear of the aircraft, then analyses how strongly oxygen in the atmosphere absorbs the laser beam.
Before the instruments could move from the laboratory bench to the research aircraft, a significant amount of preparation was required. In collaboration with the development team at the DLR Flight Experiments Facility and engineering firm SII Deutschland, the researchers refined the design of the experiments. This involved scaling down the instruments to fit within the aircraft's measurement racks. The new sensors also had to be robust enough to endure temperature and pressure fluctuations, as well as vibrations experienced during flight. Moreover, the laser-based instruments were designed to operate without interfering with the aircraft's own systems. The team also devised specialised mounts for the sensors, which had to be meticulously documented, described and tested on a separate flight for regulatory approval.
"When we finally took off with our instruments for the first time, after all the preliminary work, it was a very special moment," recalls Oliver Kliebisch, capturing the atmosphere. Three members of his team were onboard each flight, ready to activate the instruments once they received clearance from the cockpit. They made minor adjustments as needed and closely monitored both the sensors' performance and the data recording. Such experiments are only possible within an air corridor specifically cleared by air traffic control and closed to other aircraft. This campaign was also particularly significant for the two pilots. Sensor technology lies at the technological heart of every aircraft, and testing a novel approach using laser-based, optical instruments – promising considerably more precise and reliable data – was an exciting experience for them too.
"We gathered an extensive amount of data during the flights and demonstrated that the principle underpinning this technology is sound," summarises DLR researcher Oliver Kliebisch. Notably, the laser Doppler anemometry performed exceptionally well from the outset – even in clouds and at very high altitudes, where there are fewer particles to scatter the laser light. The team was also pleased with the initial results from the laser absorption spectroscopy. "The instrument functioned well, and we were able to measure the flight altitude accurately. We did encounter some issues when passing through clouds, which we aim to investigate further in the next phase," explains Kliebisch.
Following the initial flight campaign over the Allgäu region, the next campaign is scheduled for October 2022. This time, the team will venture to northern Germany and over the sea, allowing them to test the laser-based methods under a broader range of conditions. The objective is clear: "We aim to further develop the technology, make it even more accurate and advance its miniaturisation," says project manager Kliebisch, outlining the team's goals. A follow-up project is also in the pipeline.
