###### Abstract
In this manuscript, the radio occultation (RO) measurement concept is proposed to address these observational shortcomings [15; 16]. An RO occurs when a receiver in low-Earth orbit (LEO) views a GNSS satellite (e.g., the US Global Positioning System, GPS) as it sets or rises behind the Earth's atmosphere. The measured signal phase and amplitude are analyzed to derive atmospheric refractivity, from which profiles of atmospheric state variables (p, q and T) are derived [17]. A unique aspect of the RO is that the L-band (near 1.4 GHz) GNSS signals suffer minimal attenuation when propagating through even the most severe precipitation events. Owing to the orbital and observation geometry, these measurements represent integrated quantities along each \(\sim\)150 km propagation ray-path, which is considerably coarser than the spatial scales associated with clouds. Traditionally, RO measurements are collected from GNSS receivers onboard disparate operational and research-based LEO satellites to assimilate into global numerical weather prediction (NWP) models at each update cycle. In order to maximize and homogenize the global sampling density, no particular preference is given to one location or type of weather being observed. Using common metrics such as geopotential height anomaly correlation scores, assimilation of these RO data into NWP models has been demonstrated to lead to an improved overall forecast predictive skill [18; 19]. However, the ability of these same RO observations to capture dense vertically resolved (p, T and q) profiles in and near regions of heavy precipitation may provide insight to reveal the role of free tropospheric water vapor influencing underlying cloud-convective processes [20; 21]. The idea of using RO for examining the thermodynamic makeup of convective clouds is not new [22; 23] and has recently been summarized [24]. In this manuscript, we describe the potential advantages of collecting time-sequential RO from a constellation (or \"train\") of closely spaced low Earth-orbiting satellites tracking the same transmitting GNSS satellite. This strategy is driven by the fact that in the acrossray direction, the horizontal resolution of an RO measurement is of a much finer scale than its orthogonal alongray scale, which is essentially limited only by the effective Fresnel zone of \(\sim\)1 km. A \(\sim\)2 minute spacecraft separation between up to five small LEO satellites would enable each RO receiver to observe the same GNSS transmitter, such that successive RO ray paths are offset from the previous and lie \"parallel\" to one another. When the RO train orbits near a region of heavy precipitation, there is a possibility that one or more of the RO ray paths will intersect the region of heavy precipitation, whereas others would lie outside of heavy precipitation (Figure 1). The presence of heavy precipitation can be discerned by the use of the polarimetric RO (PRO) technique, recently demonstrated by the Radio Occultations through Heavy Precipitation (ROHP) receiver onboard the Spanish PAZ spacecraft [25], which is further detailed below. This sampling strategy provides unique, near-simultaneous observations of the water vapor profile inside and in the environment surrounding heavy precipitation, which is not possible from current RO data. Sections 2 and 3 describe the RO measurement requirements and a proposed PRO instrument suitable for small satellite investigations, respectively. Section 4 examines several possible orbital strategies for implementing such a closely spaced RO constellation. To estimate how frequently RO measurements onboard a constellation \"train\" of orbiting satellites will encounter heavy precipitation in nature, orbital simulation experiments were carried out where the RO ray paths from a four-satellite constellation were superimposed on actual 30 minute precipitation fields from the Global Precipitation Measurement (GPM) [26] Integrated Multi-satellite Retrievals for GPM (IMERG) precipitation product [27].