Optical air temperature and density measurement system for aircraft using elastic and Raman backscattering of laser light

Abstract

A novel concept of an optical system for remote temperature and density measurements from aircraft is presented. The system shall monitor local air data being critical for aircraft control. The measurement method overcomes the drawbacks of conventional probes, which is mainly the vulnerability to mechanical damage caused in harsh weather conditions or e.g. by volcanic ash. The measurement system is based on LIDAR technology used to extract air temperature and density information out of the elastic and Raman backscatter, generated by scattering of a laser beam from air molecules and airborne particles. Four optimized interference filters constitute the core of the four measurement channels. The measurement method, the setup of a prototype, and first results of laboratory test measurements with a single laser pulse energy of 145 mJ at a wavelength of 532 nm are presented. The results confirm our simulations of the expected system performance. With regard to temperature measurements, the measurement precisions with current experimental settings like the central wavelengths of the rotational Raman interference filters amount 0.7 K at 1000 hPa to 2.9 K at 165 hPa for one pulse detection and 0.25 K at 1000 hPa to 1.1 K at 165 hPa for an average over 10 pulses (equal to 1 s measurement time). With regard to density measurements the corresponding errors are ranging from 0.4 % (1000 hPa) to 1.5 % (165 hPa) for one pulse detection and from 0.15 % (1000 hPa) to 0.6 % (165 hPa) for an average over 10 pulses. Further optimization is expected to reduce the required laser power and improve the precision further.