## 6 Discussion  
As mentioned in the introduction section, with such a smaller antenna size for Spire, one might expect the detected SNR to be much smaller than that from a larger antenna, which may lead to a higher measurement and retrieval uncertainty. However, this study did not find that the lower SNR Spire RO data led to less accurate retrieval results. The penetration height and precision of Spire were even better than those from COSMIC-2 at mid-latitudes (45\({}^{\circ}\)N-30\({}^{\circ}\)N and 30\({}^{\circ}\)S-45\({}^{\circ}\)S). These were also consistent with previous studies from [34; 35; 36], which indicated that the Spire bending angle, refractivity, temperature, and moisture profiles were compatible with those of COSMIC-2.  
In this study, we concluded that the accuracy and uncertainty of the retrieved water vapor and refractivity profiles for COSMIC-2 from higher SNR (\(>\)2000 \(v/v\)) signals were compatible with those from lower SNR (less than 1000 \(v/v\)) signals. Even with a smaller SNR, the retrieval uncertainties from COSMIC (SNR \(\sim\)1200 \(v/v\)) were almost identical to those of COSMIC-2. However, the SNR may not be the only factor that affects RO retrieval uncertainty. For example, a simulation study (JPL, personal communication) demonstrated that horizontal water vapor irregularity and turbulence may also affect RO retrieval uncertainty, especially in the lower troposphere. That study showed that the retrieval biases and uncertainty were identical for all SNR groups while increasing the turbulence effect. A detailed analysis will be in a future study.  
In this study, we also noticed that the antenna's viewing geometry affected the SNRs' latitudinal distribution, affecting the observation error distribution. As discussed in Section 2, the factors that affected the SNR included (i) the GNSS emitter's signal power, (ii) the receiver intermediate frequency bandwidth, (iii) the RO antenna design, (iv) the antenna gain pattern related to the viewing geometry, and (v) the azimuth angle, where the antenna viewing geometry directly affected the SNR's latitudinal distribution. With the high-gain side-mounted antenna for both the L1 and L2 frequencies, the Spire SNRs for GPS, GLONASS, and GALILEO were uniformly distributed at all latitudes. Unlike Spire, the COSMIC-2 RO antenna points to the nadir, and the position-tracking antenna points to the sideway. As a result, the COSMIC-2 SNR was smaller in the mid-latitude. Generally, COSMIC-2 SNR had a broader distribution (from 200 \(v/v\) to 2000 \(v/v\)) from 30\({}^{\circ}\)N to 30\({}^{\circ}\)S. The COSMIC-2 event distribution as a function of the antenna view angle was also shown by [32]. The geolocation distribution of the COSMIC-2 SNR may affect the COSMIC-2 penetration depth at different latitudes. We demonstrated that, owing to the viewing geometry, the COSMIC-2 SNR was lowest (\(\sim\)1000 \(v/v\)) at mid-latitudes (45\({}^{\circ}\)N-30\({}^{\circ}\)N and 30\({}^{\circ}\)S-45\({}^{\circ}\)S). As a result, the COSMIC-2 retrieval uncertainty at mid-latitudes was higher than that from Spire and other RO missions.  
These results will provide confidence for the RO community to use commercial Cute-Sats Spire data for climate [4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27] and weather applications [29], just as those using previous RO missions. Ref. [30] also summarized the recent preliminary studies using COSMIC-2 and Spire data for climate analyses, atmospheric studies, weather forecasts, gravity wave detection, and atmospheric applications.