# The Effect of Space Objects on Ionospheric Observations: Perspective of SYISR
## 3 Preliminary Attempts to Eliminate Space Object Pollution  
A large amount of space object pollution reduces the quality of the ionosphere results. A special process to handle the effect of space objects on observations is necessary. If the echo is lower than the detection ability of radar, the corresponding space objects cannot be found in the observations, and this type of pollution is neglected. In this study, we implemented the commonly used smallest of constant-false-alarm-rate (SO-CFAR) detector algorithm to identify and eliminate the space objects in our data process [24; 25; 26; 27]. Specifically, we found the abnormal echoes of space objects at different ranges. The detection of the space objects was completed in the step when we obtained the power profiles of the pre-integration pulses, which is a part of the SYISR data process to obtain the ionosphere parameters. If the space objects were detected in a certain range, the raw IQ data at the according range would be replaced. The neighboring information of the range is used to  
Figure 2: Autocorrelation function (ACF, (**a**,**c**)) and the corresponding power spectrum (**b**,**d**) of SYISR ionospheric observation with obvious space object pollution on 16 April 2022 at 11:23:15. The top panel shows the results using the long pulse (LP) signal, while the bottom panel shows the results using the alternating code (AC) signal. estimate the detection background. For space object detection in ionosphere observations, the number of reference units used is important. Instead of the usual space object detection in a stable background, the background includes ionosphere variability. The fluctuation of the ionosphere affects the background estimation in different reference unit numbers. From the results in Figure 1, the ionosphere has a different spatial distribution from space objects. Ionospheric echoes appear over a wide and continuous range, while the appearance of objects in space is usually within a range resolution. Therefore, the width of the reference units should not be too wide. In our process, we chose a detection reference slightly wider than one pulse to simultaneously avoid the multi-target ambiguity and the misjudgment of the ionosphere. However, at the beginning and end scope, these reference units are not sufficiently wide because of the lack of the upper or lower side, which causes missed detection at the start and the end of the signal. We increased the reference units by two times in this situation. Another important factor that affects the correctness of space object detection in the ionosphere observations of SYISR is the detection threshold. The detection threshold is related to the false alarm probability. The false alarm probability means the rate at which the false detection of space objects occurred. Different detection thresholds and reference units were tested to find the balance between protecting the ionosphere echo and picking out the space objects. A higher unsuitable threshold will increase the possibility of missed detections, especially when the space objects occur at the altitude of the F layer. Because the space objects are mixed with the ionosphere echo, the difference between the power of space objects and the background of the ionosphere is small. With a high detection threshold, the missed detection of space objects is probable. A lower unsuitable threshold will increase the number of false alarms. In a false alarm, an ionosphere echo with strong fluctuation is regarded as space object pollution and incorrectly removed. To avoid this situation, we consider the fluctuation level of the ionosphere to obtain a suitable threshold. The background is divided into high background and low background according to the altitude of 600 km. In the low background, there is mainly environmental noise, which hardly changes with the range, and its power is relatively stable and small. We set the detection threshold as a lower value to reduce missed detections.