Browsing by Subject "Laser system"

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Aircraft air data system based on the measurement of Raman and elastic backscatter via active optical remote-sensing
(2012) Fraczek, Michael Darius; Wulfmeyer, Volker
Flight safety in all weather conditions demands exact and reliable determination of flight-critical air parameters. Conventional aircraft air data systems can be impacted by probe failure caused by mechanical damage or impairment due to different environmental influences. In this thesis, a novel measurement concept for optically measuring the air temperature, density, pressure, moisture and particle backscatter for aircrafts is presented. The detection of volcanic ash is possible as well. This concept is independent from assumptions about the atmospheric state and eliminates the drawbacks of conventional aircraft probes. The measurement principle is based on a laser emitting pulses into the atmosphere from inside the aircraft and a receiver detecting the light signals backscattered from a defined region just outside the disturbed area of the fuselage air flow. With four receiver channels, different spectral portions of the Raman backscatter of dry air and water vapor, as well as the elastic backscatter are extracted. Measurements at daytime and in any atmospheric condition, including very dense clouds, are possible. In the framework of this thesis, a first laboratory prototype of such a measurement system using 532 nm laser radiation was developed, comprising all relevant theoretical and experimental studies. These were notably the comparative feasibility assessment of the measurement methodology, the computational modeling of the measurement concept, the laboratory setup and the experimental validation. Detailed and realistic performance and optimization calculations were made based on the parameters of the first prototype. The impact and the correction of systematic errors due to solar background and elastic signal cross-talk appearing in optically dense clouds were analyzed in computational simulations. The simulations supplement the experimental results for measurement scenarios that are not generable in the laboratory. The laboratory experiments validate the predictions from the simulations with regard to systematic errors and statistical measurement uncertainties. Where possible, the experimental setup and the signal and data analysis were optimized. Residual differences between the experimental and the model results were analyzed in detail. Concrete further hardware optimizations were suggested. The resulting experimental systematic measurement errors at air temperatures varying from 238 K to 308 K under constant air pressure are < 0.05 K, < 0.07 % and < 0.06 % for temperature, density and pressure, respectively. The systematic errors for measurements at air pressures varying from 200 hPa to 950 hPa under constant air temperature are < 0.22 K, < 0.36 % and < 0.31 %, respectively. The experimentally achieved 1-σ statistical measurement uncertainties for the analysis of each single detected signal pulse range from 0.75 K to 2.63 K for temperature, from 0.43 % to 1.21 % for density, and from 0.51 % to 1.50 % for pressure, respectively, for measurement altitudes from 0 m to 13400 m. In order to meet measurement error requirements specified in aviation standards, minimum laser pulse energies were experimentally determined to be used with the designed measurement system. With regard to 100-pulse-averaged temperature measurements, the pulse energy at 532 nm has to be larger than 11 mJ (35 mJ), when regarding 1-σ (3-σ) uncertainties at all measurement altitudes. For 100-pulse-averaged pressure measurements, the laser pulse energy has to be respectively larger than 95 mJ (355 mJ). Based on these experimental results, the laser pulse energy requirements were extrapolated to the ultraviolet wavelength region as well, resulting in much lower laser pulse energy demand. The successful results of this thesis do not only prove the viability of the concept implementation, but also demonstrate its high potential for aircraft air data system application.