To date, observations with vertically resolved moisture have been carried out with ground-based stations [8] with limited spatial coverage. A dense space-based profiling capability in the tropical latitudes would offer the opportunity to perform convective transition analysis globally, to address key questions such as:  
1. Is high lower free tropospheric moisture a prerequisite to heavy deep-convective precipitation events, and to which levels does the convection appear to be the most sensitive? 2. Are there substantial differences in the above relationships amongst different oceanic basins, where forcing effects such as surface wind convergence may be different, or between tropical land and ocean regions? 3. How closely does the suite of current climate models replicate the observed convective transition statistics? Recent years have witnessed rapid miniaturization of millimeter (mm)-wave and sub-mm wave electronics and hardware including passive microwave (MW) sounders [9], and Ka- and W-band radars [10; 11], suitable for constellations of small low Earth-orbiting (LEO) satellite atmospheric science investigations [12; 13]. The Global Navigation Satellite System (GNSS) radio occultation (RO) measurement has unique advantages when deployed in a small satellite constellation. Rather than sense clouds by backscatter or thermal emission, the RO measurement is a precise-time measurement technique providing the environmental temperature, pressure, and moisture (\(T,p,q\), respectively) structures at a fine-scale vertical resolution (\(\sim\)100 m to 1 km, depending on height) necessary to separate water vapor within different vertical layers. When the GNSS signal propagates through a region of large enough aspherical hydrometeors such as heavy precipitation, a propagation differential (horizontal minus vertical, or H-V) phase shift is induced, denoted as \(\Delta\varphi=\varphi_{\mathrm{H}}-\varphi_{\mathrm{V}}\)[14], measurable with a polarimetric RO (PRO) receiver with dual orthogonal (H/V) polarimetric receive antenna (in this manuscript, the acronym RO refers to the radio occultation measurement, and the acronym PRO is used to refer specifically to an RO with polarimetric receive capability). The Radio Occultations through the Heavy Precipitation (ROHP) demonstration receiver onboard the Spanish PAZ satellite (deployed in February 2018) has successfully demonstrated the PRO concept [15]. Essentially, \(\Delta\varphi\) complements the typical RO processing, providing direct complementary indication of heavy precipitation at each profile level. That is, the moisture profile and its corresponding precipitation detection profile are provided directly by the same observation. The attenuation by clouds and precipitation at GNSS frequencies is insignificant, providing a unique capability of PRO to probe the moisture structure through extreme precipitation weather systems and in the nearby environment with similar accuracy, during all stages of cloud development. Despite its fairly coarse along-ray resolution of 200 km, the \"across-ray\" resolution of an RO is fine scale (\(\sim\)1-km). Coupled with its innate fine vertical resolution, an RO measurement is an inherently fine-scale measurement in two of the three spatial dimensions (Figure 1). In this graphical depiction, an on-Earth projection of the lowest 200-km segment of the RO ray paths (corresponding to the low levels where the majority of the water vapor is present) is shown intersecting precipitation, centered upon the corresponding on-Earth tangent point. We note that this is an idealized representation of actual conditions, which can exhibit sharp gradients near the top of the boundary layer [16]. However, current operational RO constellations are typically configured such that opportunities for near-simultaneous sampling capability of moisture in and near convection are limited [17; 18; 19].