## 3 The PFAC Measurements and the FRM Uncertainty Analysis
### Uncertainty Budget for Transponder Calibration
#### 3.3.1 Atmospheric Propagation Delay Constituent
Figure 10: The wet troposphere (up) and total troposphere (down) delays coming out after GNSS processing with precise point positioning (GIPSY) and relative differential (GAMIT) positioning during Sentinel-3A Pass No. 14 transponder calibrations at the CDN1 Cal/Val site. transponder Cal/Val site to fulfill this FRM requirement (Figure 12). This instrument measures the brightness temperature of microwave radiation from the atmosphere and determines the respective contribution/attenuation from water vapor and liquid water. Preliminary results indicate an agreement of the wet troposphere delays from the radiometer and the GNSS-derived delays. At this preliminary stage, dispersion of wet delay is in the order of \(\pm\)1-2 cm (maximum) (Figure 12). Research and field measurements are ongoing. As more data are built up, more robust and reliable results on the final wet troposphere estimation will be estimated. An alternative technique to determine wet tropospheric delay can be derived from measurements of the imaging spectrometer flown aboard the Sentinel-3 mission. It relies upon the Integrated Water  
Figure 12: The microwave radiometer operating at the CDN1 transponder Cal/Val site (upper image) and preliminary assessment of GNSS-derived wet tropospheric delays against radiometer’s measurements (middle). The lower picture shows the setup of the radiometer and the two GNSS stations. Vapor (\(IWV\)) products derived from the Ocean and Land Color Instrument (OLCI). There is a direct relation between the zenith wet delay (\(ZWD\)) and the \(IWV\), Equation (1). \[IWV_{OLCI}=[F]\cdot ZWD=\left[\frac{10^{6}}{\rho\cdot R_{V}\cdot\left(\frac{C_{ 1}}{T_{m}}+C_{2}\right)}\right]ZWD \tag{1}\]  
where: \(F\) is a temperature-dependent factor given in the right-hand equation, \(IWV\) is the atmospheric water vapor content in (kg m\({}^{-2}\)), \(\rho\) is the mass density of water, \(R_{V}\) the gas constant for water vapor in (N M K\({}^{-1}\) gr\({}^{-1}\)), \(C_{1}\) and \(C_{2}\) are refractivity coefficients in (K\({}^{2}\) hPa\({}^{-1}\)) and (K hPa\({}^{-1}\)), respectively, \(T_{m}\) is the weighted temperature of the atmosphere in (\({}^{\circ}\)K) and \(ZWD\) is the zenith wet delay caused by the wet troposphere in (mm). The OLCI instrument of Sentinel-3 has a swath of 1270 km and thus permits global coverage within 2 days. OLCI is an optical instrument that captures images during daytime. The spatial resolution of these images is 300 m (at nadir). Both the SAR radar altimeter (SRAL) and OLCI are collocated but OLCI measurements are only available during periods of solar illumination. For example, Sentinel-3A SRAL measurements over the CDN1 transponder Cal/Val site are available at 20:00 UTC but no OLCI images are captured at this time of day (actually night). On the other hand, simultaneous measurements of Sentinel-3B SRAL and OLCI instruments are made over the CDN1 Cal/Val site at 08:49:15 UTC every 27 days. Therefore, it is possible to directly compare the OLCI-derived IWV value at the 300m pixel, when the CDN1 site is observed, against the one GNSS-derived by the ground operating GNSS stations (Figure 13). In Figure 13, this comparison is made for the CDN0 GNSS station using three independent techniques: relative differential, precise point positioning and OLCI imagery. In GAMIT, the IWV is computed as a secondary product in the GNSS processing. In GIPSY, the IWV value has been estimated using Equation (1). In this present analysis, only cloud-free conditions are considered in this OLCI investigation when a Sentinel-3B transponder calibration takes place. The comparison, as shown in Figure 3, starts after the Sentinel-3B satellite reached its nominal orbit. It seems that, at the moment, there are four different techniques for determining the wet troposphere delays during transponder calibrations of the CDN1 Cal/Val site in West Crete. As more observations are accumulated, more reliable results will be reported.