15.10.2013
Figure 1: The infrared Fourier spectrometer GLORIA (left) which performs - mounted on the body of a research aircraft (right) - measurements of particle densities and temperatures in the Earth's atmosphere.
Figure 2: Measurement principle of the horizon investigation. The infrared spectrometer "looks" along its sight line through the atmosphere. It observes the infrared radiation of all trace gas molecules which lie on this sight line. The viewing direction runs just above the Earth's horizon. Due to the density of the Earth's atmosphere which decreases with the altitude, the largest part of the measurement signal comes from the region of the point closest to Earth. The array detector of GLORIA allows spectra from an extended region of approx. 4 km to the flight altitude to be recorded simultaneously.
Figure 3: Basic design of one of the two in-flight reference radiators for the GLORIA infrared Fourier spectrometer.
Figure 4: The Reduced Background Calibration Facility of PTB which allows the radiometric and thermometric calibration of infrared radiators and infrared detector systems for remote sensing of the Earth to be performed under conditions which come close to the conditions during application.
Figure 5: GLORIA with reference radiator (red circle) on the altitude research aircraft HALO (left) and on the Geophysica (right).
Figure 6: A flight route of GLORIA on Halo within the scope of the TACS/ESMVal- (Transport and Composition in the UTLS/Earth System Model Validation) measurement campaign in September 2012.
Quantitative spectroscopic measurements in the infrared spectral range are essential for the exact determination of the density distribution of trace gases and of temperature curves in the Earth's atmosphere. The traceability of such radiometric measurements to the International Temperature Scale via Planck's radiation law is an indispensible basis to achieve the smallest measurement uncertainties and to render reliable proof of long-term tendencies. For this purpose, PTB has – in cooperation with the University of Wuppertal (Bergische Universität Wuppertal, BUW), the Research Center Jülich (FZJ) and the Karlsruhe Institute of Technology (KIT) – traced the calibration radiators of the ultra-modern instrument for atmospheric research GLORIA (Gimballed Limb Observer for Radiance Imaging of the Atmosphere, Fig. 1) back to the radiation temperature scale of PTB with measurement uncertainties of smaller than 100 mK.
GLORIA is an airborne infrared Fourier spectrometer, combined with a spatially resolving infrared detector which can perform more than 16,000 spatially and spectrally resolved atmospheric observations at the same time. By means of limb sounding (Fig. 2), the instrument generates 3-dimensional images of the atmosphere in the spectral range from 7 µm to 13 µm with a resolution unachieved so far. Of particular interest is the dynamic transition region of the Upper Troposphere and the Lower Stratosphere (UTLS region) at a height of approx. 10 to 15 km which is highly relevant to the climate on Earth.
For the regularly repeated in-flight calibration sequences of GLORIA, the BUW has developed large-area black-body radiators which – due to an innovative surface structure – exhibit an extreme blackness and a very good isothermality and, thus, provide, during the flight, black-body radiation of precisely known radiation temperature (Fig. 3). To meet the very high requirements for the exact knowledge of the radiation temperature of these reference radiators, the radiators were extensively radiometrically characterized and calibrated at the Reduced Background Calibration Facility (RBCF, Fig. 4) of PTB. The RBCF is a measuring facility which is unique in Europe and which has been developed especially for the calibration of instruments for remote sensing of the Earth in the infrared spectral range under conditions which come close to the conditions during application.
GLORIA has already carried out first successful measurement flights on the Russian high-altitude research aircraft Geophysica during the ESA measurement campaign ESSenCe and on the German research aircraft HALO during the TACS/ESMVal campaign in which the instrument could demonstrate its function and calibration (Figures 5 and 6). Further flights, in particular on HALO for which GLORIA has been developed, will follow. To pursue possible changes in the radiation temperatures of the reference radiators, the radiators were calibrated at the RBCF each time before and after the respective aircraft campaigns.
It is planned to continue the very successful cooperation to obtain – for this important region of the atmosphere – high-precision data which are relevant to the climate and which have been traced back permanently to the International Temperature Scale. The experience gained by the BUW and by PTB on the aircraft-borne reference radiators is intended to be taken into account also for the development of calibration radiators for a balloon-borne GLORIA instrument and, thus, also serve as a test for a subsequent satellite-borne version of the Fourier spectrometer.
C. Monte, B. Gutschwager, A. Adibekyan, M. Kehrt, A. Ebersoldt, F. Olschewski, and J. Hollandt, Atmos. Meas. Tech. Discuss. 6, 5251 (2013)Contact:
J. Hollandt, 7.3, e-mail: 
Joerg.Hollandt(at)ptb.de