## 4 Discussions
### Zero Differencing versus Single Differencing  
COSMIC-1 has high-rate linked datasets available, different from COSMIC-2. Generally, the single differencing from high-rate link observations is superior to the zero-differencing given the uncertainties in the Clock bias. While we present the results using single differencing for COSMIC-2, we have also tested the zero differencing results using  
Figure 10: Using different POD solutions can yield different bending angle profiles. The POD difference (**a**) is up to 50 cm and changes with time for the case: 2 October 2019, 18:46 UTC for G15 of C2E1. The jump in the excess phase (**b**) is due to the cycle slip removal difference. The relative bending angle bias (**c**) can be about 10% around 50 km.  
POD resolved clock bias. There are uncertainties in the LEO clock errors due to less screening of the dataset. Noise in the clock solutions presents in the Bernese outputs (Figure 2). The single differencing requires the best reference GNSS satellite regarding SNR and GNSS satellite zenith. The signal delay between the POD and OCC antenna to the onboard central processing system might differ.  
Figure 11 shows the bending angle difference using these two clock removal methods for 2 October 2019 for C2E1. Zero differencing can cause more considerable variations than UCAR towards the upper layer. This is understandable given that the UCAR datasets are also processed using the single differencing method. The zero differencing method did not consider the small corrections, such as the antenna signal delay differences, and the POD screening solution from Bernese may not completely remove those outliers since the Clock solution is not from the final kinematic screening, and hence noise in the clock bias evaluation can cause more significant deviations. The zero difference results can cause a 2% larger standard deviation of the relative bending angle at 35 km than single differencing results. However, the difference is usually a small addition in the Doppler domain; hence, it hardly affects the results at a lower altitude.  
A low pass filter in the Doppler frequency domain is applied for the L3 ionosphere-free phase combination at a 1 Hz rate to remove the significant discontinuities and reintegrate them to form a smoothed L3. Moreover, the carrier phase ambiguity in the RINEX file (used for single differencing) must be removed. This can be further improved later, engaging in the Bernese software's full usage for ambiguity solutions or other GNSS utility tools for carrier phase smoothing.  
Carrier phase observations from multiple GNSS satellites can be used as the reference link for the single differencing clock bias removal. In our process, we have ensured that each RO event's reference satellite has the highest SNR and high zenith angle viewed from  
Figure 11: Bending angle comparison with UCAR for 2019.275, for all non-L2P observation profiles, the outer envelope indicates one standard deviation; the middle two lines indicate relative differences. The green color represents the results of a single differencing, and the blue color for zero differencing.  
the COSMIC-2 satellites. However, depending on the zenith angle, residual atmospheric delays can contaminate the clock bias solution when the zenith angle is away from 90\({}^{\circ}\).