## 3 Assessment of Spire Data Characteristics
### The Lowest Penetration Height  
In RO inversion procedures, it is essential to use RO tracking information and the derived excess phase to determine the cut-off height for RO retrievals. This ensures the optimal retrieval of the RO bending angles and refractivity profiles [33]. Therefore, the lowest penetration height is an essential indicator of the RO data quality. The lowest penetration height of RO tracking is usually related to the data's SNR and the atmosphere's dryness. Generally, observations with a higher SNR usually penetrate deeper than those with a lower SNR [31]. Figure 7a,b depict the global mean of the lowest penetration height in June 2022 binned into a \(5^{\circ}\times 5^{\circ}\) grid for COSMIC-2 (from 45\({}^{\circ}\)N-45\({}^{\circ}\)S) and Spire (from 90\({}^{\circ}\)N-90\({}^{\circ}\)S), respectively. With a 200 km horizontal resolution, the RO's lowest penetration height may be affected by mountains and uneven topography along the tracks. Here, we showed the penetration height only over oceans. Figure 7 shows that, although, in general, COSMIC-2 has a relatively more uniform spatial distribution than Spire, the pattern of the lowest penetration height for COSMIC-2 is similar to that for Spire. The Spire and COSMIC-2 data generally penetrate the lower troposphere at higher (and drier) latitudes. The Spire and COSMIC-2 penetration heights are from around 0.6 to 0.8 km near the tropical region. The lowest penetration heights for Spire are from approximately 0.2 to 0.3 km altitude at mid-latitudes and high latitudes (Figure 7a). Figure 8 depicts the penetration heights for different RO missions over oceans within [45\({}^{\circ}\)N, 45\({}^{\circ}\)S] during the DO3 period. It shows that the Spire data penetrate to a similar height as those of COSMIC-2 over oceans within this latitude range. Although not shown, the Spire data penetration heights are also very close to those of COSMIC-2 over land. Figure 6: Two-line element (TLE)-based SRO event prediction for two LEO receivers with the same GNSS satellite. Table 1 summarizes the lowest penetration heights of 80% of the total data (defined as the 80% penetration height) for different RO missions in different latitudinal zones during the DO3 period. The Spire 80% penetration height is slightly higher than that of COSMIC-2 in latitudinal zones within [10\({}^{\circ}\)N, 10\({}^{\circ}\)S], but they are the same for the latitudinal zones of [30\({}^{\circ}\)N, 10\({}^{\circ}\)N] and [10\({}^{\circ}\)S, 30\({}^{\circ}\)S]. Spire has lower penetration heights than COSMIC-2 in the latitudinal zones of [30\({}^{\circ}\)S, 45\({}^{\circ}\)S] and [45\({}^{\circ}\)N, 30\({}^{\circ}\)N]. This may be owed to COSMIC-2's SNR being lower in those latitudinal zones (see Figure 5). Because of the frequent occurrence of super-refraction over most of the stratocumulus cloud-dominated areas (i.e., the southeast of the Pacific, Atlantic, and South China seas), the COSMIC-2 and Spire penetration heights are from about 0.5 to 0.8 km altitudes over those regions. Although not shown, the SRO pairs of COSMIC-2 and Spire also showed that the Spire penetration height is about 100 m lower than that of COSMIC-2 in the latitudinal zones of [30\({}^{\circ}\)S, 45\({}^{\circ}\)S] and [45\({}^{\circ}\)N, 30\({}^{\circ}\)N]. The lowest penetration heights of 80% of the total data for the COSMIC-2 and Spire SRO pairs are very similar to those in Table 1 and are not further detailed here. Figure 8: (**a**) The RO penetration percentage (defined as the observation number at each penetration depth relative to the observation number at 8 km) over oceans within [45\({}^{\circ}\)N, 45\({}^{\circ}\)S] during the DO3 period and (**b**) the corresponding numbers of observations from surface to 14 km altitude for COSMIC-2, Spire, KOMPSAT5, and PAZ. Figure 7: The global mean of the lowest penetration height in June 2022 binned into a 5\({}^{\circ}\times 5^{\circ}\) grid for (**a**) Spire (from 90\({}^{\circ}\)N–90\({}^{\circ}\)S) and (**b**) COSMIC-2 (from 45\({}^{\circ}\)N–45\({}^{\circ}\)S).