## 3 Effect of Fixed-Altitude Heterogeneous LEO Constellations on CIT
### Results of CIT
According to the statistical approach in Figures 6 and 7, we counted the results of adding different numbers of LEO satellites at 500 km, 800 km, and 1000 km, but only the RMS distributions of the different plans at the altitude are shown here, as in Figure 8. As can be seen from Figure 8, after adding different numbers of LEO satellites observations at 300 km altitude, the RMS decreases significantly at 250-600 km altitude, the RMS decreases gradually with the increase of the number of LEO satellites, and the maximum value of the RMS is close to 2.0 \(\times\) 10\({}^{11}\) el/m\({}^{3}\) after increasing the number of LEO satellites observations up to 192 satellites; this is in line with the results obtained in Figure 7. After increasing LEO satellite observations at 500 km altitude, the RMS decreases with the increase of the number of LEO satellites at 200-400 km altitude and above 500 km altitude, the RMS decreases to the minimum value of about 0.3 \(\times\) 10\({}^{11}\) el/m\({}^{3}\) at 550 km altitude, and there is the same degree of increase of the RMS in all the altitudes from 400 to 500 km. After increasing the number of LEO satellites at 800 km altitude, there is no significant improvement in IED below 750 km altitude, and the RMS decreases to different degrees above 750 km altitude, and the RMS decreases to different degrees above 750 km altitude. Figure 6: Bias distribution of CIT results from GNSS and GNSS + 300 km altitude with varying numbers of LEO satellites compared to the NeQuick2 model, over time and altitude, on 19 March 2024. Figure 7: Root mean square (RMS) distribution of CIT results from GNSS and GNSS + 300 km altitude with varying numbers of LEO satellites compared to the NeQuick2 model, over time and altitude, on 19 March 2024.  
with a minimum value of about 0.4 \(\times\) 10\({}^{11}\) el/m\({}^{3}\). Increasing the number of LEO satellites at 1000 km has no significant improvement on the CIT results. Based on the statistical results of CIT by adding different numbers of LEO satellite observations at different altitudes, we calculated the percentage of RMS gain for each plan versus using only GNSS observations, as shown in Figure 9. It can be seen that the percentages of RMS gain are positive after adding LEO satellite observations at 300 km and 500 km altitude, indicating that the CIT results are significantly improved after adding LEO observations compared to using only GNSS observations. After adding the 300 km altitude LEO satellite observation, the maximum altitude interval of RMS gain is 300-500 km, the maximum gain plan is 192 LEO satellites with a maximum gain of 34.20%, while the overall maximum gain is 21.58% of the 192 LEO satellites. After adding 500 km altitude LEO satellite observation, the RMS gain is maximum at 500-800 km, the maximum gain scheme is 96 LEO satellites, the maximum gain is 48.26%, and the overall maximum gain is 19.42% of the 96 LEO satellites. After increasing the LEO satellite observations at 800 km and 1000 km altitude, the RMS gain at 100-300 km is negative while the RMS gain is smaller with the increase in the number of LEO satellites. However, in all other altitude intervals, the RMS gain is positive, and after adding the LEO satellite observation at 800 km, the maximum RMS gain is at the altitude of 800-1000 km, while the maximum gain plan is 96 LEO satellites, with a maximum gain of 20.89%. Figure 8: The RMS distribution of the CIT results from GNSS and GNSS combined with varying numbers of LEO satellites at different altitudes, compared with the NeQuick2 model, across altitudes on 19 March 2024. The instability of the IED above the altitude of each LEO constellation can be found in Figures 8 and 9. Taking the LEO constellation at 300 km altitude as an example, although the observation rays from the GNSS stations to the LEO satellites increase, the statistics in Figure 3 show that, in the 100-300 km altitude range, the number of voxels with rays passing through them is 12,865 when only GNSS observations are used.