## 1 Introduction
### Mission Overview of GPM  
Global Precipitation Measurement (GPM) constellation satellites are an international mission to provide next generation observations of rain and snow. NASA and the Japanese Aerospace Exploration Agency (JAXA) launched the GPM Core Observatory satellite on 27 February 2014, carrying advanced instruments that will set a new standard for precipitation measurements from space. GPM constellation satellites provided by the American National Aeronautics and Space Administration (NASA), the Japanese Aerospace Exploration Agency (JAXA), Eumetsat's MetOp-B and planned MetOp-C, the NASA-NOAA (American National Oceanic and Atmospheric Administration) Suomi National Polar-orbiting, France and India's Megha-Tropiques, NOAA's Polar-orbiting Operational Environmental Satellites, Japan's first Global Change Observation Mission-Water, U.S. Defense Department meteorological satellites and NOAA's Joint Polar Satellite System. The data they provide will be used to unify precipitation measurements made by an international network of partner satellites to quantify when, where, and how much it rains or snows around the world. The GPM Core Observatory satellite flies at an altitude of 407 km in a non-sun-synchronous orbit and continues the TRMM sampling strategy and will extend the observations to higher latitudes, covering the globe from the Antarctic Circle to the Arctic Circle [6]. However, as you see in Table 1, at present, this version covers a latitude from 60\({}^{\circ}\)N to 60\({}^{\circ}\)S.  
The increased sensitivity of the Dual-frequency Precipitation Radar (DPR) and the high-frequency channels on the GPM Microwave Imager (GMI) will enable GPM to improve forecasting by estimating light rain and falling snow outside the tropics, even in the winter seasons, which other satellites are unable to measure [6; 7]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Products** & **Temporal Resolution** & **Spatial Resolution** & **Regions** & **Availability Period** \\ \hline IMERG & half-hourly & 0.1 degree & 60\({}^{\circ}\)N–60\({}^{\circ}\)S & March 2014–present \\
3B42 & 3-hourly & 0.25 degree & 50\({}^{\circ}\)N–50\({}^{\circ}\)S & 1997–April 2015 \\ ERA-INTERIM & daily & 0.125 degree & 90\({}^{\circ}\)N–90\({}^{\circ}\)S & 1979-present \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Characteristics of Satellite/Model Precipitation Products. Researchers and scientists use models for analysis and forecasts of the atmospheric state. The models may have different spatial and temporal resolution. One of the best models, which is used around the world, has been developed by the European Center for Medium Range Weather Forecasts (ECMWF). They have produced a global reanalysis for the last decades of ERA-40 and ERA-Interim, 12-hourly and daily precipitation fields [8]. Moreover, ERA-Interim is the latest global atmospheric reanalysis produced by ECMWF. The ERA-Interim project was conducted in part to prepare a new atmospheric reanalysis to replace ERA-40, which will extend the data to the early part of the twentieth century. Another tool to determine precipitation at relatively fine temporal and spatial scale is satellite observation. During the last decade, many researchers have evaluated and used satellite data sets and some have found that the TMPA-3B42 post real-time product performed better than other products compared with the rain-gauges [9; 10; 11; 12; 13; 14; 15]. One of the disadvantages of TMPA-3B42 was the limited area of observation; it covered only the tropical and subtropical belts. Another disadvantage was the ability to estimate heavy rainfall while light rainfall and snowfall were not detected properly [6; 7; 8; 9; 10; 11; 12]. As seen in Table 1, IMERG is presently available from mid-March 2014 to the present (with access delay in the order of about three months for the final run version). Based on the preliminary analysis during beta testing by [PERSON] _et al._ (2015), IMERG is smoother than 3B43 over oceans and at higher latitudes.