# The Effect of Space Objects on Ionospheric Observations: Perspective of SYISR
## 2 Space Objects in the SYISR Ionospheric Observations  
The ionosphere observation of ISR is based on Thomson scattering from the randomly distributed free electrons in the ionosphere [17; 18]. Although the scattered signal power from a single electron is low, scientists have predicted and confirmed that a powerful radar with high gain and a large aperture can measure the ionosphere by integrating the signal from a scattering volume at a certain time period [19; 20]. When the space objects pass through the ISR radar beam, the abnormal echo will pollute the power profile and autocorrelation function (ACF)/power spectrum of the measured signal and consequently pollute the inverted ionosphere parameters (including the electron density, electron temperature, ion temperature, and line-of-sight velocity). In the ISR signal process, incoherent integration is usually employed to improve the signal-to-noise ratio (SNR) and the statistical accuracy of the estimation. There is some flexibility with concern to the integration time because SYISR saves the original I/Q data. A longer integration helps to obtain a better statistical accuracy of the measurements under the assumption of a stable ionosphere. If the temporal resolution is smaller than the time that the space object passes through the radar beam, the space object will appear in several consecutive time intervals. For the space objects among the ionosphere observations, the echo power that we receive is related to the size of these space objects and the range in which they are located. Figure 1 shows an overview of the appearance of space objects in the RTI plot of SYISR ionospheric observations in three experiments with different typical waveforms. The long pulse (LP) and alternating code (AC) signal experiments occurredsimultaneously because we used combined coding to execute both in one experiment. To demonstrate the effect of integration time, we have shown the results with 3.2-s and 32-s of integration for comparison in Figure 1. There are \(\sim\)20 (10) obvious space objects identified with the 3.2-s (32-s) integration time in both experiments. Generally, the SNR is \(\sim\)10-30 dB higher around the space object occurrence location, and obvious pollution can be identified. The ionospheric echo power is higher with the 32-s integration than with the 3.2-s integration because echoes from more electrons are added. For the SNR around the space object occurrence, the value does not change with the integration time. With 32-s of integration, some smaller space objects become nonvisible because their SNRs are comparable with the background ionosphere after a longer integration time, especially around the ionospheric peak altitude. The range extension of the space object is related to the pulse width, since we integrated before decoding, which is \(\sim\)72 km for the LP/AC results and \(\sim\)58.5 km for the barker code (BK) results in Figure 1. The temporal extension of the space object in Figure 1 is related to the temporal resolution, which of the LP/AC combination experiment is \(\sim\)2 times larger than that of the BK experiment. Figure 1: Range-time-intensity (RTI, unit: dB) of the SYISR observations with different integration times (3.2 s in the left column, 32 s in the middle column) during half-hour intervals of 16 April 2022 (upper and middle panels) and 12 March 2022 (lower panel). The right column is the corresponding power profile case with a space object pollution during a selected time (blue line: results with the 3.2-s integration, red line: results with the 32-s integration). The upper (**a–c**), middle (**d–f**), and bottom (**g–i**) panels are the long pulse (LP, 480 \(\upmu\)s), 16-bit alternating code (AC, 480 \(\upmu\)s) signal, and 13-bit barker code (BK, 390 \(\upmu\)s) signal, respectively. The radar beam steered to the zenith direction ranging from 120 to 1200 km in all three experiments. During the ionospheric parameter retrieval from the ISR signal, the ACF is calculated after decoding [21; 22; 23].