Atmospheric Radiation Group
The AisaEAGLE is an imaging spectrometer, which contains of a single-line sensor with 1024 spatial pixels. The instrument measures radiances in three dimensions: space, time, and wavelength. The spatial and spectral dimensions are resolved by a special set of optics that displays the image onto a two-dimensional (2D) sensor chip. The third dimension, time, corresponds whether to the motion of the clouds passing over the sensor (ground-based application), or in the case of airborne measurements to the sensor movement itself. An optical schematic for the path of the electromagnetic radiation detected by the center spatial pixel is shown in Figure 1.
Figure 1: Optical scheme of an imaging spectrometer. DellEndice et al. (2009)
The incoming solar radiation within the field of view (FOV) of AisaEAGLE is collected by a lens (objective optics) and an entrance slit. A collimating optics direct the radiation to a grating (dispersing element) where it is split into its spectral components. The spectral components are focused on the detector which consists of a charge-coupled device (CCD) element for the spatial and spectral dimensions.
The FOV of the AisaEAGLE depends on the lens that is used for the measurements. Figure 2 shows the size of a single image pixel and the swath of the entire image as a function of cloud height using a 36.7° lens.
Figure 2: Characteristic (a) swath and (b) pixel width for AisaEAGLE using the 36.7° lens.
Figure 3 additionally illustrates the measurement geometry which is needed to derive the detected scattering angles ϑ for each spatial pixel. The scattering angle ϑ is derived by the scalar product between the vector of the incoming solar radiation (SC) and the vector of the radiation scattered into the sensor direction (CD):
Figure 3: Illustration of the AisaEAGLE measurement geometry in a Cartesian coordinate system (x, y, z) with position of the Sun (S), a scattering cloud particle (C) and the AisaEAGLE detector (D). θ0 is the solar zenith angle, φ0 the solar azimuth angle, ϑ the scattering angle, and βi the viewing angle of the corresponding pixel.
The AisaEAGLE has been already used successfully for cloud measurements in an airborne and ground-based application during the following campaigns:
The main objectives by using AisaEAGLE during those/ further measurement campaigns were/ is to investigate cloud microphysical properties such as the cloud optical thickness and in the special case of cirrus the most probably ice crystal shape. Cloud macrophysical properties such as cloud inhomogeneities and cloud shadows are also a big part of interest during the performed investigations.
Due to the mentioned scope of application, AisaEAGLE is an useful instrument to support all the other remote sensing instruments during measurement campaigns and to extend the range of scientific findings.
Last modification on 2013/3/04 by Michael Schäfer