## 2 Unified Modeling of LEO Satellite Constellation
### Model Implementation Based on Modelica
#### 2.3.2 Satellite Model  
The satellite model is composed of four subsystem models, namely, the GNC subsystem model, propulsion subsystem model, payload subsystem model and power subsystem model. Among them, the GNC subsystem consists of a satellite body model, a solar wing sail model, a sensor model, a flywheel model and a controller model, as shown in Figure 4, which is mainly responsible for the attitude and orbit control of the satellite. In order to maintain the satellite's payload to orient or track a specific target, attitude control is required. There are two ways to achieve attitude control. Small-scale attitude control is achieved by the controller generating a control signal to the flywheel, while large-scale attitude control is realized by the controller generating the switch signal of the attitude control engine.  
The main task of the propulsion subsystem is to cooperate with the GNC subsystem to complete the attitude and orbit control during the life of the satellite. According to the function, it can be divided into a gas pressurization module, a propellant storage module and a thruster unit module, as shown in Figure 4. The gas pressurization module is composed of three gas cylinder models, a gas orifice model and a pressure-regulating valve model. The main function is to provide the gas required for constant pressure operation to maintain working pressure for the rail-controlled engine; the propellant storage module  
Figure 4: Four main subsystems that make up satellite model in MWorks.  
consists of two storage tank models and liquid orifice models. The main effect is to store, distribute and supply the propellant required by the engine; the unit module mainly includes one orbit control engine model and sixteen attitude control thruster models, which provide propulsion for orbit control and attitude adjustment.  
The power subsystem, as shown in Figure 4c, includes two solar wing models, a battery model, a power controller model and load models. The solar wing models on the left and right sides are composed of a certain number of solar cell models combined in series and parallel, according to the design needs, and can generate full power under the condition of standard illumination of 1360 W/m\({}^{2}\). The battery model receives the electric energy output by the shunt regulator in the sunlit area for charging and discharges when the solar wing power is insufficient for the load to maintain the stability of the bus voltage. The main function of the power controller model is to realize the distribution and regulation of solar wing power generation, battery charging and discharging and load power and to shunt according to needs. The load model is divided into two types; one is the fixed power load given in Figure 4c, and the other is the load coupled in the other three subsystems. The second load is related to the working state of some equipment. For example, the power consumption of the electric load in the propulsion subsystem is related to the working status of seventeen engines.  
The payload subsystem is generally composed of antenna models and transponder models, as shown in Figure 4d. For the LEO communication satellite constellation, it is mainly used to establish ISL [19] and satellite-to-earth links. The user's communication data is received by the receiving antenna of the corresponding frequency band and then sent to the transponder. After the transponder performs gain adjustment and signal amplification processing on the data signal, it is transmitted to the transmitting antenna and then to the next communication node through the transmitting antenna.