351 1 Eric Pottiaux, René Warnant, Royal Observatory of Belgium, Eric.Pottiaux@oma.be, R.Warnant@oma.be First Experiences with a Water Vapor Radiometer at the Royal Observatory of Belgium E. POTTIAUX, R. WARNANT 1 Abstract In 1998, the Royal Observatory of Belgium decided to set up a reasearch program on the wet component of the troposheric error affecting GPS observations. In the frame of this study a Water Vapor Radiometer has been installed at Brussels. The paper describes our first experiences with this instrument. 1. Introduction In 1998, the Royal Observatory of Belgium (ROB) decided to set up a research program on the wet component of the tropospheric error affecting GPS observations. In the frame of this study, a Water Vapor Radiometer (WVR) jointly built by the Swiss Federal Institute of Technology in Zurich (ETHZ) and the CAPTEC GmbH has been installed at Brussels in February 2001. This instrument measures the sky brightness temperature at two radio frequencies - namely 23.8 Ghz and 31.5 Ghz - characteristic of the Water Vapor (WVR) and Liquid Water (LW) emission lines. Precipitable Water Vapor (PWV) can be retrieved from these sky brightness temperatures applying radiative transfer methods. A meteorological logger is attached to the radiometer and provides surface observations of pressure, temperature and humidity. Using these meteoro- logical observations and a suitable mapping function, Zenith Wet Delay (ZWD) and Zenith Total Delay (ZTD) can be retrieved from pointed PWV. The Figure 1 shows the WVR of Brussels. Figure 1: Picture of the generation III Water Vapor Radiometer jointly built by the Swiss Federal Institute of Technology and the CAPTEC GmbH. The paper describes first results obtained at Brussels with this instrument. We cover different topics such as the funda- mentals of WVR observation (section 2.1) and the descrip- tion of raw observations (section 2.2). We also devote a part of the paper to the sensitive problem of the data quality (section 3). Finally we briefly describes first results obtained with the Water Vapor Radiometer of Brussels. 2. Principe of a Water Vapor Radiometer In this section we describe the fundamentals of WVR obser- vation. The first paragraph (section 1) will cover observation principle and answer the question "What does a WVR observe?". Then in the next paragraph, we answer the questions "What kind of observable do we really have?", "What are the raw observables of a WVR?". Finally, in the section 2.3 we describe how to compute the Path Delays (PDL) from the raw observables. 2.2 Fundamentals of observation: The Sky Brightness Temperatures The Water Vapor contained in the atmosphere behaves as a black body in term of electromagnetic emissions. The observation principle of a WVR is therefore based on the radiative transfer methods as explained in [SOLHEIM, 1993]. The main Water Vapor molecular emission line is located at 22.235 Ghz. Nevertheless the atmospheric presure and the presence of Liquid Water also affect microwave emission at this frequency. Observing at 23.8 Ghz can minimize the pressure dependency and a second observation at 31.5 Ghz allows compensation for liquid water emissions. Therefore, the WVR measures the atmospheric Sky Brightness Temperature at these two frequencies. From the observation of the Sky Brightness Temperature at both frequencies, it is possible to compute the Integrated Water Vapor Content (IPWV) above the observation site. 2.2 Raw observations and reference signals From the technical point of view, the WVR has two antennae which measure the Sky Brightness Temperature at these two frequencies. These antennae output electric signals (voltages) corresponding to our observables (i.e. the Sky Brightness Temperature at both frequencies). Nevertheless, to analyse these signals, the WVR needs to tune its antenna measure- ments to reference signals: the so-called Calibration Targets (CT). These Calibration Targets (one per channel) have an emission power spectrum similar to a black body and can be settled in two different states called "cold" and "hot" corresponding to two different brightness temperatures