# Performance Evaluation for China's Planned CO\({}_{2}\)-Ipda
## 3 Results  
### Critical Orbit Parameters  
Among all the orbit-related parameters, the orbit altitude and solar radiation were identified as the critical parameters that differed from those for other space-borne CO\({}_{2}\)-IPDAs. To improve the performance of other passive sensors, the transit time of the satellite was set to noon and the orbit height was set significantly higher than that of the ASCENDS satellite. It is worth noting that there is no relationship between solar radiance and orbit height. Moreover, the relationships between the random errors and the solar radiance, and between the random errors and the orbit height, are linear. In Figure 3, the relative random error is shown to be a function of the solar radiance and orbit height, highlighting the critical orbit parameter. Figure 3 demonstrates the results of the simulation when the solar radiance was set to vary from 5 to 60 mW/(m\({}^{2}\) nm sr) and the orbit height from 300 to 850 km. The assumed maximum solar radiation is significantly higher than the most extreme possible case (the mean solar radiation at 1.6 \(\upmu\)m is approximately 5 mW/(m\({}^{2}\) nm sr)), exaggerating the possible effects of solar radiation. This simplifies the simulation because accurately calculating the solar radiation for given conditions is a complex procedure. Nevertheless, the effect of solar radiation is almost negligible compared to that of the satellite orbit height, according to Figure 3. Compared to the results for a track height of 405 km, results obtained using a value of 700 km showed that the relative random error almost doubled. Fortunately, using an orbit height of 705 km will not result in a relative error exceeding the ideal target as long as other parameters meet or surpass the original hypothesis, as shown in Table 3. We think that the current orbit parameter configuration can fulfill the demand for accurate and precise global XCO\({}_{2}\) measurements. Figure 3: The effect of orbit parameters on the relative random error. (**a**) The relationship between the orbit height and the relative random error. (**b**) The relationship between the solar radiance and the relative random error. (**c**) The combined effect of the two factors. The orbit altitude influences the distribution of samples as well as the influence on the random error. Unlike passive remote sensing of CO\({}_{2}\), the IPDA yields a uniform coverage in terms of latitude regardless of the satellite's altitude. This feature is significant for determining carbon fluxes in high-latitude regions, where the uncertainty of the carbon flux is relatively high because of the insufficient coverage of in-situ and passive remotely sensed measurements. Furthermore, samples from both ascending and descending tracks should provide useful datasets because the IPDA relies on reflected laser signals but not solar signals. In terms of the cloud effect, the cloud fraction should be low for IPDA data compared with passive sensor data, because the diameter of the laser footprint on the ground is 45 m (70.5 m) for a satellite altitude of 450 km (705 km) with a divergence angle of 0.1 mrad. Additionally, the IPDA can utilize reflected signals from thick clouds, particularly low altitude clouds. Therefore, IPDA samples are more useful than those from passive sensors such as GOSAT and OCO-2. The different settings for the orbit altitude will also influence the distribution of samples, as shown in Figure 4. In this experiment, the observation period was set to one month. Using an orbit altitude of 705 km, samples are more evenly distributes than when using an orbit altitude of 450 km. There are evident gaps in Figure 4b. Therefore, we show a close-up image of the Jing-jin-ji area, where China's capital and another municipalities are located, to investigate the details of the sample distribution. Figure 4d shows that a group of tracks, taken using an orbit altitude of 450 km, are clustered together to give a high-density sample area. However, there are also large blank areas between different clusters.