# Satellite Edge Computing for the Internet of Things in Aerospace
## 3 The Architecture and Application Scenarios of Satellite Edge Computing
### iSat System
#### 3.1.1 Hardware Structure  
In terms of hardware structure, traditional satellites usually consist of two parts: platform and payload. The platform is responsible for satellite management, including attitude determination and control (ADCS), communication (Comm), electrical power (EPS), telemetry and telecontrol (TM and TC), thermal control (TCS), etc.  
The payloads perform specific tasks with the support of the platform. However, this structure leads to the independence of each subsystem and a complicated interface relationship. There are several problems in the following aspects:  
1. Low system integration. Each subsystem has substantial weight, high power consumption, large volume, and complicated interfaces. The optimization design is limited to the subsystem level. The overall performance cannot be guaranteed to be optimal.
2. Low resource utilization. There are a lot of redundant designs in each subsystem, but these idle resources cannot be effectively utilized.
3. Low bus bandwidth. The data transmission capability between subsystems is limited. It is impossible to achieve effective coordination and integrated design between systems.
4. Low reusability. Each subsystem needs to be customized, resulting in poor compatibility and interchangeability. The development and expansion of the system are costly.  
In order to make the most use of the resources on the satellite, iSat changes the hardware structure of the traditional satellite. It uses an integrated structure to realize the re-division of the function and structure of the satellite platform and payload. It combines the same or similar parts of each subsystem. The internal data interaction is achieved via a standardized bus. The comparison between the two structures is shown in Figure 2.  
Figure 2: The hardware structure of the traditional satellite and iSat.  
The hardware structure of iSat consists of computing network, storage network, interface/bus controller, system monitoring module, sensors network, and actuators network. The computing network is a distributed heterogeneous computing platform, which is the core of iSat. The computing network not only needs to complete the computing tasks of traditional satellite platforms but also needs to complete the general calculation function of various payloads. The storage network adopts a distributed structure to realize the storage of various types of information and software to maximize the utilization of limited storage resources. The interface/bus controller provides standardized data interfaces for different devices. The system monitoring module monitors all essential parameters to ensure the regular operation of the satellite. When there is a problem with the satellite, a reset signal is given in time to help the satellite restart. During the restarting, the system monitoring module will maintain the basic parameters of the satellite, such as attitude and temperature, etc. The sensors network contains all the sensors in the traditional satellite platform and payload, such as magnetometers, IMUs, star sensors, cameras, etc. Through the sensor network, it is possible to collect the information for satellite operation, monitor the operating status, and facilitate uniform processing analysis. The actuator network includes the actuators required to perform a series of tasks such as satellite attitude control, orbital change, and payload adjustment.