**2. Typical satellite major subsystems**

In general, the space mission dictates the type of orbit<sup>2</sup> , satellite design and its expected life cycle, and its operational scenarios. The PL design includes dimensions, interfaces, weight, physical characteristics, and basic utility needs (e.g., power consumption), which usually influences spacecraft (SC) bus design. The PL is often a unique and one-of-a-kind design tailored to meet specific mission requirements, frequently relying heavily on newer technology, while the satellite bus has the supporting function, and as such relies largely on existing or modified hardware such as batteries, inertial devices, and star trackers. Since PLs and their missions vary widely, so is this satellite bus supporting role.

Traditionally, the PL is considered a subsystem of the satellite bus that is designed to generally satisfy the corresponding mission requirements. The PL operational requirements sometimes impose specific requirements on the satellite bus that must be satisfied for the PL to accomplish its mission. This interdependence between satellite bus and PL subsystems has historically resulted in many nonstandard interfaces developed and implemented by the incumbent spacecraft builders. As a result, the aerospace industry has been moving toward a more standardized and commodity satellite bus framework that can potentially result in a tremendous cost saving approach.

As shown in **Figure 3** below, a satellite bus typically consists of the following subsystems: command and data handling subsystem (C&DHS); communications subsystem (CS); electrical power subsystem (EPS); propulsion subsystem (PS); thermal control subsystem (TCS); attitude control subsystem (ACS) also known as guidance, navigation and control (GNC) subsystem; structures and mechanics subsystem (S&MS); and life support subsystem for manned missions if required. The C&DHS will be described in detail below. The CS provides the satellite bus with the necessary communication functionalities to connect the user and ground segments to different satellite subsystems. The EPS provides the electrical power generation and distribution for various spacecraft subsystems. The PS provides maneuvers

<sup>2</sup> There are three main types of satellite orbits: low earth orbit (LEO) of 2000 km in altitude or less; geostationary (GEO) with altitude around 35,786 km; and medium earth orbit (MEO) with altitude between LEO and GEO.

#### **Figure 3.**

*A typical satellite bus and payload subsystem.*

necessary for altitude, inclination adjustment, and momentum management adjustments. The TCS provides active thermal control from electrical heaters and actuators to control temperature ranges of equipment within specific ranges. The ACS provides proper pointing directions for the satellite subsystems, such as sun pointing for EPS to the solar arrays and earth pointing for CS. The S&MS provides the necessary mechanical structure to withstand launch loads by the launch vehicle, during orbital maneuvers, as well as loads imparted by entry into the atmosphere of earth or another planetary body.

On the other hand, a PL is tailored to a specific mission type. For example, a remote sensing satellite can have as its payload an electro-optical (EO) camera to take day-time pictures of the earth and then convert them to electrical signals that can be captured. Alternatively, the camera may also have infra-red (IR) sensors that enable the PL to see the earth at night, or microwave sensors that will let the PL "see" radio frequency (RF) signals from the earth at several radio frequencies

**Figure 4.** *A typical and generic sensor payload.*

*Communication Subsystems for Satellite Design DOI: http://dx.doi.org/10.5772/intechopen.93010*

(RFs). These sensors can be classified as passive or active, and each of them can be further classified as imaging or sounding3 . **Figure 4** below illustrates a generic imaging PL that will convert the sensor analog data into electrical signals that can be captured and transmitted to a ground station. Note the existence of a communication subsystem as part of this imaging payload.
