RETRIEVAL ALGORITHMS FOR ATMOSPHERIC ATTENUATION IN THE FREQUENCY BAND 15 – 52 GHz FROM TWO-CHANNEL MICROWAVE RADIOMETER OBSERVATIONS Peter Forkman, Patrick Eriksson, and Gunnar Elgered Department of Space, Earth and Environment, Chalmers University of Technology, Gothenburg, Sweden Abstract Retrieval algorithms for atmospheric transmission, signal propagation path delay, atmospheric water vapour and liquid water contents have been produced within the context of a prototype development of a dual-channel microwave radiometer. The errors found match what is achieved by existing radiometers. The requirement formulated by ESA for the transmission retrieval is met. In fact, this requirement is met with some margin allowing for additional offset errors in the atmospheric attenuation coefficients and in the two radiometer channels which have not been included in the simulations. Key words: atmospheric transmission, microwave radiometry, signal delay, water vapour, liquid water. 1. INTRODUCTION Ground-based microwave radiometers operating in the frequency range 20–40 GHz have several applications using at- mospheric remote sensing techniques. Often the basic idea is to measure the atmospheric sky brightness temperature on and off the water vapour emission line centred just above 22 GHz. These temperatures may be used to infer atmospheric transmission (or absorption) [1] as well as the signal propagation path delay through the atmosphere in the direction of the observation [2][3]. Because the main atmospheric constituents determining the sky brightness temperatures are the integrated amounts of water vapour (IWV) and liquid water (ILW) also these quantities may be estimated [4], [5] and [6]. We will refer to this instrument as a water vapour radiometer (WVR). In this study the focus is on the estimation of atmospheric transmission along earth-space paths although we also present example results for the other applications mentioned above. We have developed these algorithms for a site at the Onsala Space Observatory on the Swedish west coast. A prototype WVR, developed by Omnisys Instruments, Gothenburg, Sweden, for which the derived algorithms are intended is shown at the test site in Figure 1. Here we first, in Section 2, describe the simulations of sky brightness temperatures and the approaches used in the modelling work needed to estimate the relevant parameters. The obtained results, in terms of the expected accuracy, are presented in Section 3. Finally, in Section 4, we give the conclusions of the study. 2. SIMULATIONS 2.1. Input data and assumptions The input data describing the atmospheric properties were taken from ERA-Interim via its web interface [7]. The quan- tities used are: surface pressure, temperature profile, geopotential altitude, humidity profile, liquid water content profile (LWC) and low cloud fraction (LCF). Data were downloaded for the years 2000–2013, for 00, 06, 12 and 18 UT at the highest available resolution. The position was selected to match the location of the Onsala Space Observatory, OSO. This atmospheric database contained 20,456 cases. TTC 2019