## 1 Introduction  
### Overview  
To specify the current thermodynamic state of the atmosphere and to predict its future state, one must know the flow field in addition to the mass field [1]. The latent heat present in the water vapor field is one of the most dominant forms of energy storage in the atmosphere. Transport of this stored energy by the three-dimensional wind field is one of the most fundamental processes driving our weather, and is a key element in both the water and energy cycles [1]. Observations of the thermodynamic state of the atmosphere from low earth orbit (LEO) using infrared (IR) and microwave spectral sounding methods have enabled significant advances in our understanding of atmospheric processes [2]. These observations currently have a stronger impact on global numerical weather forecast model accuracy than any other type of observation [3] (p.307). However, it has long been recognized that more accurate, vertically resolved, wind field observations in the troposphere would provide a further significant improvement in our ability to characterize the processes that lead to high impact weather events and more generally, improve weather forecast accuracy [4]. The horizontal wind field has been observed from space since the introduction of the geostationary weather satellite in the 1960s using the method of atmospheric motion vectors (AMVs) [1]. This method has been refined substantially over the years, and is currently employed operationally using the observations from multi-spectral meteorological imagers worldwide. AMVs provide an important positive impact on numerical weather prediction (NWP) [5]. However, their full impact on NWP has been constrained by the limitations on height assignment accuracy inherent in multi-spectral meteorological imaging observations [6]. Over the years, several approaches that would provide vertically resolved wind field observations have been advanced, including Doppler wind light detection and ranging (LIDAR or DWL) [4], and geostationary hyperspectral infrared sounders (GEO hyperspectral) [7], with sufficient spatial resolution to enable motion-vector wind observations. Each of these approaches has received substantial investment, and technical progress has been made, but the cost of these approaches remains very high. Hundreds of millions of dollars were spent on GEO hyperspectral sounding developments during the last few decades, but these efforts in the US were halted prior to fully developing and launching an instrument. Space-based Doppler wind LIDAR has been under development for an even longer period than has hyperspectral IR sounding. After two decades of development, a single vector component/single curtain DWL [8] was recently launched by the European Space Agency and is undergoing check-out at a very high overall program cost of more than $550M. The National Research Council (NRC), in the recently released Decadal Survey for Earth Sciences [3] (p. 10, p. 138), highlighted once again the importance of vertically resolved tropospheric winds as well as the need for thermodynamic vertical profiles at higher spatial resolution, vertical resolution, and observation tempo. The purpose of this paper is to present a new observation approach, MISTiC(r) Winds (Midwave Infrared Sounding of Temperature and Humidity in a Constellation for Winds), which is highly responsive to the observation needs identified by the NRC, and to contemporary fiscal constraints. MISTiC Winds would employ miniature infrared hyperspectral sounders in a LEO microsatellite constellation that will provide vertically resolved wind velocity observations using hyperspectral and imaging methods, together with more frequent, higher spatial resolution soundings of the troposphere, at much lower cost than DWL and GEO hyperspectral implementations. A preliminary and partial presentation of MISTiC Winds was given previously [9; 10; 11]. The remainder of this paper is organized in the following way.