Investigation of the ground reflectance for spaceborne IPDA lidar measurements of greenhouse gases

Active remote sensing using lidar shows high potential for the measurement of atmospheric greenhouse gases like carbon dioxide or methane from spaceborne platforms. Due to the weak atmospheric backscatter in the near IR the IPDA lidar (Integrating Path Differential Absorption Lidar) technique is preferred over the range resolving DIAL method. IPDA shows much better performance with respect to systems of comparable size. Sensitivity studies reveal, that this technique promises to match the stringent sensitivity requirements. The earth´s surface reflectance becomes an important issue, since an IPDA lidar uses the laser return of the ground. Gradients of the ground reflectance could introduce noticeable retrieval errors in the column content of the measured gas. Therefore airborne lidar measurements at 1.6 µm wavelength were performed to investigate this type of error source. A lidar system was deployed on the DLR research aircraft Cessna Caravan to measure the variations of the ground return. Data from different regions and various terrains across Europe were collected including sea surfaces. In order to simulate a satellite system the data were upscaled to a larger ground spot corresponding to a conceivable spaceborne setup. The focus of the analyses was on the small-scale reflectance variability as well as the overall dynamic range. Comparisons to MODIS reflectances are performed additionally. It is shown that the impact of this error source on spaceborne CO2 measurements is on the order of 0.2 ppm

An optical study of single pentacene molecules in n-tetradecane

We report the spectroscopic observation of single pentacene molecules in the matrices n-tetradecane and n-hexadecane, using a confocal microscope operating at liquid-helium temperatures. A maximum detected photon emission rate of only 30 counts per second (cps) is found for pentacene in n-hexadecane and 160 cps for pentacene in n-tetradecane. For the latter system, the low count rate is shown to be caused by a high S1 → T1 intersystem crossing yield of about 40% in combination with a triplet lifetime of 33±3 µs. Pentacene molecules in this polycrystalline host are found to show little spectral diffusion on a timescale of seconds.

Validation of the ALADIN Airborne Demonstrator A2D by end-to-end simulations and measurements

An airborne prototype of ALADIN (Atmospheric Laser Doppler lidar Instrument) was developed to analyse the measurement concept, to validate and optimize the data processing algorithms, and to examine atmospheric backscatter and internal signals at the receivers. The ALADIN Airborne Demonstrator A2D is designed to perform wind measurements in a downward viewing geometry, similar to how ALADIN will operate in space. To validate the A2D, an end-to-end simulator was developed at DLR (Deutsches Zentrum für Luft und Raumfahrt, or German Aerospace Centre), representing the properties of the A2D and various atmospheric models, and including the signal processing modules. This offers the possibility to study wind measurements for different atmospheric and instrumental parameters, and to analyse the performance of the A2D from ground and aircraft. The results of the simulations are used furthermore to develop and optimise the signal processing algorithms, knowing the properties of the modelled signal. Various algorithms were developed, analysed, and validated and the most suitable are used to determine the atmospheric wind speed. The ALADIN receiver consists of two spectrometers: one to detect aerosol (Mie) backscatter and one to detect molecular (Rayleigh) backscatter, and the Doppler shift is determined from these measurements. The Rayleigh receiver is a radiometric detector, whereby the Doppler shift is determined from a change in intensity. It employs the principle of the double-edge method in a new implementation of the Fabry-Perot interferometer, called the sequential technique. The Mie receiver consists of a Fizeau interferometer, which was not used for atmospheric wind measurements before to our knowledge. The Doppler shift is determined from the spatial location of the Mie signal at the detector by employing the fringe imaging technique. First measurements of atmospheric backscatter with the A2D were performed at DLR from ground and aircraft, and it was the first time that a direct detection Doppler wind lidar had been deployed on an aircraft. Signals between the aircraft and ground, along with backscatter from clouds, and signals of the Earth’s surface, were detected by the instrument, showing the capability to identify the ground and cloud return at the receivers. Atmospheric wind was not derived during these first functional tests, but the wind speed measurement accuracy was assessed by the Doppler shifted signal of the surface of a building