# A Survey on Satellite Communication System Security
## 3 Background
### Satellite Communication System Architecture  
Generally, a satellite's hardware is specific for its purpose. In Figure 3, the five types of payload generally used in satellite systems are shown [21; 22].  
**Onboard Computer System:** The onboard computer system (OCS) is a central component that controls the satellite and is often referred to as the satellite's processor [23]. The satellite's OCS plays a crucial role in coordinating various sensors and hardware components. Firstly, the processor controls the sensors contained within the satellite's payload, and it performs computations to determine the satellite's orientation and position by adjusting actuators. Additionally, the OCS handles the encryption and decryption of data packets during communication.  
**Sensors:** Sensors play a fundamental role in the operation of satellites, performing key functions such as measuring the velocity, acceleration, orientation, and tracking position. Examples of these sensors include antennas, solar sensors, star trackers, Earth sensors, inertial units, RF sensors, laser detectors, angular rate sensors, magnetometers, and temperature sensors. In addition to these, there are special-purpose sensors that, unlike the ones listed above, do not influence the basic operation of the satellite but are included in the satellite payload for specific objectives. These special-purpose sensors often include cameras, SAR (Synthetic Aperture Radar), and Light Detection and Ranging (LiDAR) systems [24].  
**Actuators:** Actuators play a primary role in controlling the orientation of the satellite and the direction of its sensors. Actuators are crucial in space environments due to the virtually resistance-free nature of space, meaning satellites continue to move under the influence of inertia. In other words, even weak forces such as solar wind can alter a satellite's orientation or altitude, necessitating actuators for correction. For instance, in communication satellites, precise antenna positioning is crucial [21], and actuators are used to counteract the inertia moment. Typically, actuators produce torque to change angular velocity, but there are also actuators that use propellants. Key components of these actuators include angular momentum devices, thrusters, magnetic coils, and solar sails.  
**Power System:** The power system is a component that supplies energy to the satellite. Since satellites are limited in mass and volume, it is essential to precisely design the power system to fit the payload. The power system not only supplies electricity to sensors, the OCS, and actuators, but also needs to be precisely controlled and managed to avoid issues like power shortages and system downtime [25]. In the past, onboard power supply devices like nuclear fission devices were used, but in recent times, especially in power-intensive satellite systems like SCSs, external power supply devices such as solar panels are almost exclusively used. Solar panels are typically used in conjunction with batteries, which serve as auxiliary power supplies, ensuring the availability of power even when solar radiation is not available.  
**Communication System:** The communication payload is the component used for communication in satellites. It is utilized for various spatial communications, including ground-to-ground, satellite-to-satellite, and satellite-to-ground communication. The primary components of a communication payload typically include repeaters and antennas [26].