## 4 Fractal Reflectarray Interference Analysis in Satellite Simulation Environment Scenario
It plays an important role in stabilising the satellite to track the target receiver as well as to point the solar panel towards the sun while rotating around the earth. Now, the main beam is reoriented towards the receiver. Hence, in Figure 15b, the downlink is now possible after using the gimbal. The scenario is created at 22:52 UTC, where three LEO satellites are close to the path between an MEO satellite and the ground station. However, exactly 2 min later, at 22:54 UTC, a LEO satellite moved into the path, creating interference. During that period, a sudden drop in the downlink is visible in Figure 16. Figure 16 depicts the downlink closure status from the desired MEO satellite over a period both with and without interference from LEO satellites. Whenever the downlink is closed successfully, the status is set to true; for any failure in the downlink, it is set to false. The status is indicated in terms of 1 and 0 for true and false cases, respectively. Our sole purpose is to avoid this interference at all times to maintain the downlink. This interference can be avoided either from the transmitter satellite end or from the receiver ground station end. However, due to space and modification constraints at the satellite end, it is challenging to modify any parameter once the satellite has been launched. Hence, the scope is limited to modifications at the ground station without altering anything at the satellite. Figure 14: (**a**) LEO/MEO satellite and the ground station and (**b**) interference path between LEO and MEO satellite at a time frame. Figure 15: (**a**) Mounting of reflectarray to MEO satellite and visualisation of its pattern and (**b**) the pattern reoriented towards receiver after using gimbal at the transmitter. Hence, to avoid interferences, the Gaussian antenna is replaced by a rectangular array at the ground station receiver and its radiation pattern is shown in Figure 17. An \(8\times 8\) uniform rectangular array is designed at the same 12 GHz frequency, with both row and column element spacings equal to half of its operating wavelength. It creates a more pointed beam in a particular direction. It can orient its main beam towards desired directions while placing null or side lobes with a low gain pattern towards interference directions. The manoeuvring of pattern can be carried out by carefully adjusting the amplitude and phase weight of excitation for each element in the array. The same concept is used here to orient the main beam towards the desired MEO satellites while placing nulls or side lobes towards the interference from LEO satellites. The ground station continuously tracks the position of its nearby LEO/MEO satellites. It adjusts its weights accordingly when the LEO satellite path comes close to the MEO satellite. Figure 18 shows the radiation patterns of the rectangular array at the receiver with respect to different positions of LEO satellites at different points in time. The array automatically adjusts its weights to produce different radiation patterns at various times. In all the cases, the main beam is oriented towards the MEO satellite and nulls are oriented towards the LEO satellite to avoid interference. Figure 16: Downlink closure status from the MEO satellite with and without interference. Figure 17: A uniform rectangular array mounted at the ground station receiver and visualisation of its radiation pattern. Now, the same downlink closure status is re-evaluated after replacing the array with a Gaussian antenna in Figure 19. Previously, the downlink could not be closed at 22:54 UTC due to excessive interference from the LEO satellites. However, the downlink link status is now always up, even in the presence of interferences. The plots in Figure 19 can visually confirm this. The performance of the proposed reflectarray is now evaluated in a MATLAB real-time simulation environment by creating a custom antenna replica from measured reflection and radiation characteristics.