## 2 RO Measurement Requirements
### Polarimetric RO Concept  
However, conventional RO provides no unique capability for distinguishing the presence or absence of precipitation along each RO ray path. A large collection of close-time coincidences of RO-based soundings with Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) and CloudSat radar observations was examined by [34] to differentiate the vertical distribution of atmospheric layers stable to convection during precipitating events. In this manuscript, a combination of such RO soundings with the polarimetric RO (PRO) concept is proposed for directly distinguishing RO profiles whose propagation ray paths traverse regions of heavy precipitation (Figure 2), characterized by increasingly larger numbers and sizes of aspherical (oblate-shaped) hydrometeors as the rainfall rate increases. PRO enhances standard RO by observing the GNSS signals in two orthogonal polarizations [35]. The co- and cross-polarized radio signals propagating through heavy precipitation media will experience different phase delays (phase delay is denoted by \(\phi\)), measurable by the receiver's polarimetric antenna. Realistic scattering simulations performed using collocated COSMIC RO and TRMM precipitation retrievals have shown that the phase difference between the horizontally (H) and vertically (V) polarized signals (defined as \(\Delta\phi\)) is large enough to detect a path-averaged rain rate exceeding 2 mm h\({}^{-1}\) with over 70% probability [35]. The PRO concept has recently been proven by the ROHP instrument onboard the Spanish PAZ satellite launched in February 2018, a proof-of-concept experiment [25; 36]. Further calibration and validation accounting for the antenna pattern, Faraday effects and other factors is detailed in [37]. As an example of ROHP data in the presence and absence of heavy precipitation, Figure 3 shows five ROHP observations (labeled A through E) on 23 May 2018 near tropical cyclone Mekunu. Large \(\Delta\phi\) exceeding 25 mm were obtained up to 10 km in height in regions associated with heavy precipitation  
Figure 2: A depiction of the polarimetric RO measurement concept. As the right-hand circularly (RHC) polarized transmitted GNSS signal propagates through a region of heavy precipitation characterized by aspherical hydrometeors, an orthogonal (i.e., cross-polarized) left-hand circularly (LHC) polarized component is induced. The co- and cross-polarized signals are measured by a GNSS receiver at two linear (horizontal and vertical, denoted H and V) orthogonal polarizations. surrounding the eyewall near 55E/10N (label A), whereas regions of no precipitation (labels B, C and D) exhibit no significant \(\Delta\phi\) anywhere in the profile. \(\Delta\phi\) near 10 mm (label E), associated with thin cold clouds, suggests that ROHP may also be sensitive to frozen crystalline ice shapes, similar to the differential phase observed by the ground-based polarimetric radars [38]. The locations of strong convection and associated heavy precipitation have implications for the orbital design strategy outlined in Section 3. ROHP is currently (Iate 2019) providing about 200 observations per day. To show geographically where the strongest observed \(\Delta\phi\) occur and how this correlates with known precipitation climatologies, Figure 4 shows the locations of the upper percentile (upper 2%) of ROHP observations that results after \(\Delta\phi\) is averaged from the RO rays whose tangent point intercepts three different vertical regions (0-5, 5-10 and 10-15 km height). The top panel includes only data from June-July-August (JJA), and the lower panel from December-January-February (DJF). The color background map depicts the average rain rate during these months derived from the GPM IMRG precipitation product. During JJA, there is a good agreement of \(<\)\(\Delta\phi\)\(>\) in the 10-15 km region (red symbols) with known areas of convection, notably the Maritime continent, the Pacific Intertropical Convergence Zone (ITCZ) and the African Sahel.