## 1 Introduction  
With the improved satellite measurement techniques and the advent of advanced assimilation schemes, a large number of satellite observations are currently assimilated into the numerical weather prediction (NWP) models, and the forecast impacts now exceed that of conventional observations [1; 2; 3]. Among many types of observations from satellite sensors being assimilated, the infrared (IR) and the microwave (MW) sounder measurements have a significant positive impact on the forecast error reduction. Their spectral measurements contain valuable information on the detailed vertical structure of atmospheric temperature and moisture as well as other atmospheric compounds [4; 5; 6; 7; 8]. Hence, many operational NWP centers have put in significant effort to maximize the use of these IR and MW sounder measurements together with advanced data assimilation techniques in the NWP system to improve weather forecasting. At operational NWP centers, timely delivery of model outputs to the forecasters is critical. Due to practical reasons, the NWP models are scheduled to be triggered at a data cut-off time after the defined analysis time. Observations made before the cut-off time but delivered after this data cut-off time will not be available to the assimilation system. The time gap between the observation time and the data delivery time to the data users such as the NWP centers is defined as data latency [9]. Major factors affecting data latency are the data transmission speed to the ground station, the frequency of data downlink, and the data pre-processing [9,10]. Data latency is one of the key factors in maximizing the usage of satellite measurements in the data assimilation. The data latency issue is related to all observation types. This is a more sensitive issue for the low-earth-orbiting (LEO) satellites that move quickly around the Earth from pole to pole in the low orbit. Moreover, a large number of the meteorological IR and MW sounders whose data are assimilated are flying on the LEO satellite platforms. Different from the LEO satellites, the geosynchronous-orbiting (GEO) satellites are fixed over one longitude at the equator and communicate with the assigned ground stations in real time. In contrast, the LEO satellites transmit the measured data saved in the onboard data storage to one of the assigned local ground stations distributed over many countries, while the satellite passes over the field of view of the local ground station for a short period. In other words, the LEO satellite data collected after the previous datalink time are saved in the onboard stage until it passes over the next local ground station. The visit-time gap between the ground stations over which the satellite passes sequentially can affect the data latency, compared to the GEO satellites that communicate with the ground station in real time. In the past, the LEO satellite data delivery process was dependent on the model type. In the case of the regional NWP model, observations from the LEO satellites within the model domain were received by the assigned local ground station via the direct readout reception system, but not distributed to other NWP users in near real time. However, with improved computation power, regional NWP has increased the model domain significantly. Observations received at a single ground station no longer cover the whole model domain. For the global NWP model, LEO satellites dumped the data collected each time when they passed over the Command and Data Acquisition (CDA) station located on a Svalbard island. These data were then re-transmitted and assimilated into the global data assimilation at the NWP centers. The time interval between each revisit time to the CDA station contributed to delay in the delivery of the LEO satellite data to the NWP centers. The Direct Broadcast Network (DBNet), coordinated by the World Meteorological Organization (WMO) Space Program (WSP) with the global cooperation of satellite-operating institutes [9,11], is set up to speed up delivery of the LEO sounder observations.