## 3 New Adaptable Strategy for RT and NRT troposphere Monitoring
### Retrieving Slant Tropospheric Delays from Both RT and NRT Processing  
The G-Nut/Tefnut supports SINEX_TRO format Version 2 [46] which was newly designed for the exchange of all tropospheric parameters including slant tropospheric delays (SD), meteorological parameters or other auxiliary parameters for reconstructing all components of the slant tropospheric delay model for a single epoch, station and satellite. Using the PPP method, the SDs are retrieved by adding undifferenced carrier-phase post-fit residuals (RES) and, optimally, by subtracting any systematic errors (MPT) such as carrier-phase multipath and errors in antenna phase center variations [47]  
\[\text{SD}=\delta\text{T}+\text{RES}-\text{MPT} \tag{2}\]  
The estimation of systematic errors, e.g., by generating the so-called multipath maps [48], is possible only through the stacking of carrier-phase post-fit residuals from multiple days for a single station. The post-fit residual stacking hasn't been completed yet in our software, however, we shortly discuss a procedure planned for the continuous production of all tropospheric parameters in both RT and NRT. Due to the temporal stability of the multipath maps, and a necessary presence of time series of post-fit residuals over two weeks at least, it is practical to separate the procedure of post-fit residuals processing from the actual RT and NRT data processing. For this purpose, we are developing a dedicated tool that will be regularly triggered over archived NRT products for updating multipath maps for all available stations and GNSS constellations. The results will be stored in the SINEX_TRO file and regularly ingested to the new RT/NRT operational processing. The retrievals of the raw slant tropospheric delays consisted of the model slant delay and raw carrier-phase residuals, i.e., the first two terms from (2), could be provided immediately from the RT/NRT analysis. The systematic term in (2) will be introduced later for the stations and satellites/constellations supported from independently provided multipath maps. All parameters, corrections, mapping factors and other auxiliary information such as satellite number, elevation and azimuth angles, are provided in the SINEX_TRO format too, thus a user can decide how to use the product optimally. In order to demonstrate the impact of the backward smoothing on the slant tropospheric delay retrievals, we exploited a dual GNSS station (POTM, POTS) from the benchmark campaign. We compared differences in slant delays from these two stations located only 2.5 m from each other when considering three retrievals strategies [47]: (1) model slant delays estimated from model parameters, \(\delta\text{T}\) (model SD), (2) raw slant delays, \(\delta\text{T}+\text{RES}\) (raw SD), and (3) clean slant delays, \(\delta\text{T}+\text{RES}-\text{MPT}\) (clean SD). The estimated slant delays at both stations should theoretically represent the same tropospheric effects for each particular satellite, while non-zero differences reflect mainly the uncertainty from the data processing or station-specific systematic errors. Figure 10 shows the elevation dependence of slant delay biases, standard deviations and the normalized statistics calculated from the differences divided by actual slant delay; all provided for the period of May-June 2013. The three variants of slant delay retrievals are plotted by solid lines (model SDs), dotted lines (raw SDs), and dashed lines (clean SDs). Additionally, different colors of solutions depict the utilization of real-time analyses (F--Kalman filter) or post-processing (S--backward smoothing) and the use of IGS precise products (PP) or real-time products (RT). Generally, performances of biases for the model and the clean SDs are in a very good agreement and normalized biases do not vary with the elevation. On the other hand, a clear elevation-specific pattern is visible for the normalized biases of raw slant delays indicating systematic effects at one of the stations [47].