## 5 Conclusions and Discussion  
Currently, with the near-real-time DBNet system coordinated by the WSP, the delivery speed of the LEO satellite sounder (IR and MW) measurement to the operational GFS model is significantly enhanced, increasing the number of the satellite data assimilated in the GFS. However, the current data latency of LEO satellites is still longer than the data latency of 20 min recommended by the WSP due to the limited spatial coverage of DBNet stations and the data processing time. Thus, in this study, the impact of reduced data latency of LEO IR and MW sounders on the global weather forecasting was examined using the NCEP GFS under the assumption that the WSP-recommended 20-min latency is met. This study also evaluated the forecast benefit of further reducing the latency to 5 min. To assess the forecast impact of reduced data latency, the satellite data input files to GFS were created depending on the different data latency requirement (i.e., 20 min and 5 min) for the IR and MW sounder data being assimilated in GFS. Three trial experiments (i.e., control, latency_20m, and latency_5m runs) were conducted from 30 June to 22 August 2018. This trial included a seven-day spin-up period for model resolution change. For the control run, the original LEO satellite data used in the operational GFS were assimilated. Apart from the LEO IR and MW sounder data, the same baseline observations were assimilated for all three trial runs. The assimilated data ratios of the IR and MW sounders reach an average of about 92% and 95% for the latency_20m and latency_5m runs, respectively, compared to less than 90% for the control run. In particular, for the IASI onboard MetOp-A and AIRS onboard Aqua satellite, the difference in data usage between the control run and two experiment runs is larger than 20%. The increase in the number of assimilated satellite data from reduced data latency is mainly presented on the spectral channels sensitive to the middle and upper troposphere, away from surface and cloud contaminations. Forecast verification results show that reducing the data latency to 20 min has a large positive impact, particularly in the Southern Hemisphere. The improvement is likely due to the increased number of assimilated IR and MW sounder measurements by the reduced data latency of 20 min as forecast improvements in this part of the globe are driven by satellite observations. However, the forecast impact analysis shows that a further reduction of data latency to 5 min has an overall neutral impact, implying that the 20-minute data latency recommended by WSP is optimal for maximizing the forecast impact from the IR and MW sounder measurements. In this study, it was demonstrated that reducing the data latency for the LEO IR and MW sounder has a positive impact on the global forecasts, but it could be achieved if currently operating LEO IR and MW sounder data are uniformly delivered to the GFS assimilation system within 20 min of data latency. Unfortunately, as of now, it would be difficult to reduce the data latency to 20 min because the DBNet ground stations cannot be built over the ocean that occupies most of the Earth's surface, in particular, in the Southern Hemisphere. However, by building extra DBNet ground stations on the areas that current DBNet stations do not cover and improving the advanced data processing system, current data latency can be further reduced. In addition, it is likely premature to mention, but the data relay system between the LEO satellites and the GEO satellites can be one of the solutions to further reduce the data latency of LEO satellites. With this data relay system, the satellites flying over the ocean can transmit the observed data to the user communities in real time. However, to make such plans (i.e., expanding the DBNet station coverage and building the data relay system) feasible, huge investments are required in terms of cost and technology. Therefore, continuous cooperation of global satellite-operating institutes is necessary, aiming to meet the WSP recommendation.