## 4 Sentinel-3A SRAL and MWR Cal/Val Results
### Transponder Calibration Results  
Absolute coordinates for the transponder were supported by two continuously operating GNSS stations and by periodic geodetic and local surveys at the CDN1 Cal/Val site. Final coordinates for the transponder reference center were delivered after diverse GNSS processing (different receiver makes, processing, and procedures) was carried out, along with a weighting mechanism depending on component uncertainties. Figure 18 shows the two GNSS stations of CDN0 and CDN2 and the way final positioning results were produced for the transponder reference center. Atmospheric delays were derived through GNSS processing and at the time of satellite overpass. The absolute range bias of Sentinel-3A Pass No.14, and for the period under consideration and the dataset available, is illustrated in Figure 19. Level-0 data products, with Non-Time Critical orbits in SAR mode have been used for cycles 3 to 32. Figure 17: Differences between the S3A Radiometer and the GNSS-Derived delays for the Wet Troposphere as a function of cycle number. It seems that there is an upward trend with a slope of \(+0.0603\) cm/cycle (S3A descending Pass No.335). The standard error for this estimated slope is SE = \(\pm 0.0245\) cm/cycle and its \(p\)-value is 0.0137 which determines the significance of these results (98%). The plot, below the first diagram above, is the boxplot of these delay differences. It can be seen that the location estimate (horizontal red line in the middle) is the median (slightly above zero). The average value, however, comes to \(-0.09\) cm, a value also close to zero. The standard deviation for this average is \(\pm 2\) mm. Uncertainty Budget for Transponder Calibration  
This uncertainty analysis attempts to standardize practices for reporting uncertainties in altimetry calibration. To express the uncertainty for the results in range bias, as previously presented, a two-stage procedure was required. Specifically, this depended upon: (1) the identification of all the parameters, components and constituents that contribute to the transponder error budget, and (2) the quantification of each error contribution, following international units and standards [38]. As a first step in this work, we started to approximate procedures for analyzing uncertainty by assuming residuals were to be random and normally distributed, with no autocorrelation. Figure 19: Absolute range bias results for the ascending Sentinel-3A Pass No.14, Cycles 3–32 at the CDN1 Cal/Val site. Figure 18: An example of data processing and procedures, in a Block diagram, to arrive at the transponder absolute coordinates and their uncertainties. The International Standards Organization Guide to Uncertainty in Measurement [38] classifies elemental uncertainties in two categories: Type A and Type B. Uncertainties that can be evaluated by statistical methods are treated as Type A uncertainties, and those evaluated by any other means (scientific judgment, previous measurement data, experience of the behavior of instruments, manufacturer's specifications, data provided in calibration and other reports, uncertainties assigned to reference data taken from handbooks, etc. ), are treated as Type B uncertainties. Type B error constituents [38; 39], are commonly built up step by step and gradually based on knowledge accumulated from prior measurements, instrument certificates, scientific knowledge, etc. The standard uncertainty for each error constituent is formed by combining uncertainties quoted in different ways (Type A and Type B errors, see for details [40]), and by starting off, at this initial stage in transponder calibration, with the following assumptions: (a) The error constituents for the transponder internal delay and the GNSS antenna reference point were to follow a normal distributions, and (b) all the rest error constituents followed a uniform distribution. All constituents of error sources in transponder calibration were identified, and thus, an estimation of their variance is presented.