## 6 Conclusions  
We described a new strategy for generating advanced RT and NRT tropospheric products exploiting the PPP method and, simultaneously, the Kalman filter and the backward smoothing supported with the IGS real-time products. The strategy can be used to provide RT and NRT products synchronously, while it can be further optimized either for the latency or the accuracy of the NRT product. Although the NRT solution is generated as a side product of the real-time analysis, in terms of the precision, it is comparable to the traditional NRT ZTDs using the LSQ adjustment and batches of double-difference observations in a network mode, still commonly used within the E-GvAP. In addition, both the RT and NRT products can produce a consistent set of all parameters, namely the ZTD, horizontal tropospheric gradients and slant delays, all provided in high resolution and using optimally all observations from multi-constellations if precise products are available. A long-term assessment of the IGS RTS in terms of the quality demonstrated that the products were available over 90% during the period 2013-2017 with 2-3 longer gaps only, and reached the 3D orbit accuracy of 4-6 cm and the clock precision of 2-4 cm. All the products were stable and were usable for the purpose of the troposphere monitoring, though some temporal variability was observed in the quality. An assessment of 1-year ZTDs generated routinely in the real-time PPP solution reached a stable precision of 6-8 mm over 18 global stations with about 10% improvements when additionally including GLONASS observations. A simulated analysis of the impact of the backward smoothing on the ZTD and horizontal tropospheric gradients in NRT were evaluated within the GNSS4 SWEC Benchmark campaign. Improvements of 20% for NRT ZTDs compared to RT ZTDs were demonstrated using the new strategy, i.e., the same improvement as observed within the piece-wise linear NRT ZTD estimation using the LSQ data processing. The overall improvements of the backward smoothing algorithm  
Figure 15: Carrier-phase post-fit residuals from the Kalman filter (a) and the backward smoothing (b) using COM (**top**) and GBM (**bottom**) products at ZIM2 station. reached over 20%, independently if the final or real-time precise products are used. The impact of RT precise products on ZTD estimates indicates a systematic error of 2.4-2.8 mm and a degradation of 13-17% when compared with the use of final products, however, up to a factor of 2 worse in the accuracy when comparing parameters from collocated stations. We further evaluated differences of ZTDs and horizontal tropospheric gradients from two collocated GNSS stations. We observed a significant impact of the backward smoothing on estimated tropospheric parameters when applied with 30-min latency in near-real time. A similar effect was observed for single- and multi-constellation solutions. An improvement of about 25% was then achieved when using multi-constellation compared to the standalone GPS, though observations from GLONASS and Galileo systems were down-weighted and their precise products and models are less accurate. Models were verified by visualizing carrier-phase post-fit residuals at individual stations. It showed that the backward smoothing improves mainly adjusted parameters, but does not affect essentially the distribution of post-fit residuals, however, retrieved slant delays still benefit from improved estimated parameters. Main authors acknowledge the Ministry of the Education, Youth and Science of the Czech Republic for co-financing the study with the project No LO1506, [PERSON] acknowledges the project No. SP2017/25 of the VSB-Technical University of Ostrava, Ostrava, Czech Republic. Authors further acknowledge CODE and GFZ for providing multi-GNSS products, IGS and its analysis centers for real-time products and, finally, all data and product contributors to the GNSS4 SWEC Benchmark campaign [31].