The global observation of profiles of the atmospheric wind speed is the highest-priority unmet need for global numerical weather prediction. Satellite Doppler lidar is the most promising candidate to meet the requirements on global wind profile observations with high vertical resolution, precision, and accuracy. The European Space Agency (ESA) decided to implement a Doppler wind lidar mission called the Atmospheric Dynamics Mission Aeolus (ADM-Aeolus) to demonstrate the potential of the Doppler lidar technology and the expected impact on numerical weather forecasting. An airborne prototype of the instrument on ADM-Aeolus was developed to validate the instrument concept and retrieval algorithms with realistic atmospheric observations before the satellite launch. It is the first airborne direct-detection Doppler lidar for atmospheric observations, and it is operating at an ultraviolet wavelength of 355 nm. The optical design is described in detail, including the single-frequency pulsed laser and the two spectrometers to resolve the Doppler frequency shift from molecular Rayleigh and aerosol Mie backscatter. The airborne prototype is representative of the spaceborne instrument, and their specific differences are discussed.
Dark mantle deposit compositions derived from SSI data are consistent with Earth-based observations of similar near-side deposits and are interpreted to be pyroclastic materials. However, the modernitc albedo and 1 gin absorption of the dark mantle deposit on the southwest margin of the Orientale basin suggest it is a local pyroclastic deposit contaminated with underlying highland materials from the Orientale impact.
Optical guidance of coherent light in multimode fibers involves the generation of a speckle pattern at the fiber output due to the interference of the modes traveling in the fiber. Since the speckle pattern is sensitive to external perturbations (temperature, stress) as well as polarization and wavelength changes of the guided light, speckle noise is introduced limiting the quality and precision of fiber-coupled devices employed in various spectroscopic and imaging applications such as optical coherence tomography or differential absorption lidar [1]. Speckle noise was also identified as one of the major contributors to the random error of an airborne directdetection Doppler wind lidar [2] which is a key instrument for the validation of the first spaceborne wind lidar Aeolus providing vertical wind profiles of the Earth. It is composed of a frequency-stabilized laser transmitter, a Cassegrain telescope and a dual-channel receiver for analyzing atmospheric backscatter signals from both molecules (Rayleigh channel) and particles (Mie channel). Precise measurement of the transmitted laser frequency and calibration of the frequency-dependent transmission of the receiver spectrometers are prerequisite for accurate wind retrieval. Hence, a small portion of the pulsed laser radiation, referred to as internal reference, is coupled into a multimode fiber via an integrating sphere and co-aligned with the free-path atmospheric signal before entering the spectrometers. The use of a multimode fiber ensures sufficient signal intensity of the internal reference, but creates a speckle pattern consisting of only about 2 • 10 3 speckles incident on the Rayleigh and Mie spectrometers which are based on a Fabry-Pérot and a Fizeau interferometer, respectively. Although the speckle pattern is static over short time scales of a few seconds, slow changes in the intensity distribution of the fiber-coupled internal reference signal are caused by variations in ambient temperature and pressure acting on the fiber and the integrating sphere, which in turn individually modify the spectral responses of the illuminated Mie and Rayleigh channels. The laser polarization and frequency are stable parameters [3] and of minor influence, but in airborne operation the vibrations on the platform additionally contribute to random intensity modulations. Since the internal reference response forms the basis for the determination of the Doppler frequency shift, and thus the derived wind speed in all atmospheric range gates, the speckle-induced fluctuations significantly increase the random error of the entire wind profile. Comparison of the internal reference frequencies derived from the Rayleigh and Mie channel responses against the frequencies measured using a wavemeter [3] showed random variations on the order of 5.2 MHz (Mie) and 9.0 MHz (Rayleigh) on time scales relevant for the wind retrieval, corresponding to wind errors of 0.9 m/s and 1.6 m/s, respectively (Fig. 1a). Significant reduction of the internal reference response fluctuations has been acco...
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