## 3 Trial Experiment Design  
To assess the impact of reduced data latency for LEO satellite sounders on the global weather forecasting, three assimilation experiments were conducted: (a) a control run with the operational GPS cycle, (b) \"latency_20m run\", where all LEO satellite sounder observations have the data latency of 20 min, and (c) \"latency_5m run\", when the data latency is 5 min. The experiments were conducted from 1800 UTC 30 June to 1800 UTC 22 August 2018. This trial includes a seven-day spin-up period (1800 UTC 30 Jun to 1800 UTC 6 July 2018) to adjust for a spatial resolution change. The initial startup files were created from operational GPS output at T1534 (~13 km). For this study, the NCEP GFS was run at T670 (about 25 km in mid-latitudes). In addition, different from the operational configuration, instead of running the early cycle (GPS run) for all four analysis cycles, it was run only on a 0000 UTC cycle to produce the long-range forecast up to 168-h forecast hours due to the limited computation power. Apart from the modification made to the LEO IR and MW sounder measurements, all other observations, listed in Tables 1 and 2, were assimilated.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Observation Type** & **Atmospheric Parameter** \\ \hline Rawinsonde and Dropsonde & Temperature, humidity, and wind profiles \\ PIBAL and Wind profiler & Wind profile \\ Ground radar (NEXRAD) & Wind and precipitation \\ Wind profiler & Wind profile \\ Land surface (METAR) & Surface pressure \\ Ship and Buoy & Temperature, humidity, and surface pressure \\ Aircraft (AIREP and AMDAR) & Wind \\ BOGUS & Wind of tropical cyclone \\ \hline \hline \end{tabular}
\end{table}
Table 1: Conventional observation types used in the GPS data assimilation system. See the Abbreviations for an explanation of acronyms.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Observation Type** & **Instrument** & **Platform** \\ \hline \multirow{3}{*}{\begin{tabular}{c} Infrared sounder \\ (LEO) \\ \end{tabular} } & CrIS & S-NPP and NOAA-20 \\ \cline{2-3}  & IASI & MetOp-A and MetOp-B \\ \cline{2-3}  & AIRS & Aqua \\ \hline \multirow{3}{*}{\begin{tabular}{c} Microwave sounder \\ (LEO) \\ \end{tabular} } & ATMS & S-NPP and NOAA-20 \\ \cline{2-3}  & AMSU-A & NOAA-15, NOAA-18, NOAA-19, MetOp-A, and Aqua \\ \cline{2-3}  & MHS & NOAA-18, NOAA-19, MetOp-A, and MetOp-B \\ \hline GPSRO bending angle & GPS sensor & COSMIC, TerraSAR-X, and TanDEM-X \\ \cline{2-3}  & GRAS & MetOp-A and MetOp-B \\ \hline \multirow{3}{*}{\
\begin{tabular}{c} AMV \\ (GEO) \\ \end{tabular} } & Imager & GOES-15 \\ \cline{2-3}  & AHI & Himawari-8 \\ \hline \multirow{3}{*}{
\begin{tabular}{c} AMV \\ (LEO) \\ \end{tabular} } & MODIS & Aqua and Terra \\ \cline{2-3}  & VIIRS & S-NPP \\ \cline{2-3}  & AVHRR & MetOp-A, MetOp-B, NOAA-15, NOAA-18, and NOAA-19 \\ \hline Sea surface wind (LEO) & ASCAT & MetOp-A and MetOp-B \\ \hline Microwave imager (LEO) & SSMIS & DMSP-17 \\ \hline Infrared sounder (GEO) & Sounder & GOES-15 \\ \hline Radiometer (LEO) & AVHRR & NOAA-18 and MetOp-A \\ \hline \hline \end{tabular}
\end{table}
Table 2: Satellite-based observation types used in the GPS data assimilation system. See the Abbreviations for an explanation of acronyms.