Payload Subsystem

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The articles in this segment of the Satellite 101 Wiki have been contributed by Dhrumil Shah, Manu Srivastava, Riya Singh, Yash Shrivardhankar and Sagar Kumar, all of whom have been in the payload subsystem of Advitiy, the Second Student Satellite of IIT Bombay.

The subsystem that looks into the science of the payload and its working in the satellite is called as the payload subsystem. This subsystem has to have interacted with various subsystems for understanding their capabilities as well as know what all requirements it can set on the other subsystems to achieve the payload. The subsystem and its expertise is an important aspect when it comes to the above factors. Thus, a payload subsystem member has to “pay-for-the-load” for being in this subsystem.

Meaning of Payload

The question that comes first to mind when hearing about a satellite is that - "What does it do?" Payload is essentially the answer to this question. It refers to the actual function of the satellite in space. In other words it is the purpose the satellite is built for. Most satellite teams have a payload subsystem that works on deciding this purpose and the methodology of achieving this aim. Once decided, the payload puts requirements on other aspects of the satellite. It decides what power will be required, the size of the satellite, which attitude control mechanism should be used etc. The payload affects almost everything required to build a satellite. For example, Pratham had ‘Total Electron Count (TEC) measurement in the ionosphere ’ as its payload.
Usually the bigger and more advanced satellites built by ISRO, NASA or some other space agency are given a payload around which the teams have to build a satellite. To illustrate, consider that a country wants to build a telecommunication network using satellites, say for broadcasting channels on television. This is a predefined payload given to the satellite building agency and the agency can go forward to build the satellite according to the requirements. In other words, they have a purpose for which they have to build a satellite. This is not the case in most student satellite projects.
What happens in a typical student satellite project is that the team wants to build a satellite and they have to decide what their payload should be. This can be a very tricky task. The payload should not be so complex to implement that it goes beyond the team’s capacity. It should be relevant to the scientific community or have some larger social good. It should also not be so trivial such that it doesn’t get the team members excited about it. It should also be kept in mind that apart from implementing the payload, the satellite has to perform several other functions that are equally important for it to be a success. You have to ensure that the control mechanism works well, you have to make sure that the communication works well etc. The payload should be compatible with all these while being the heart of the satellite project.

Composition of the Payload Subsystem

Diversity is one factor that is crucial in defining and deciding the payload finalized for the satellite. The subsystem has to incorporate all the factors that every other subsystem has to think upon to decide whether a particular payload is feasible. Thus, having a subsystem which has at least one member from each subsystem would be helpful in getting a quick feasibility check and deciding upon the setting the requirements.
The members, at the stage of payload selection, do not need to have any experience in the field of satellite technology and what can be achieved with a student satellite. Rather, this inexperience could help getting some new and wild ideas. However, do note that generally a subsystem with no experience could take a longer time in getting their work done than a subsystem with experience. Thus, it would be a tradeoff between time and availability of experience and learning opportunities for the subsystem. Once the payload has been selected, if the payload requires special instrumentation, then it would be a good idea to have a person having good technical knowledge in that field is brought on-board the payload subsystem.
Generally, for a team which is starting a Student Satellite Project, almost all the members would contribute to the payload subsystem till the payload is finalized

Selection of Payload

This can be a bit challenging for a new team. Please go through Planning the mission and finalizing the Payload to get an overview of the process. The process after the mission statement is finalized will be explained in detail here. It can be split in the following sections:

The role of the subsystem post payload selection

Once the payload has been selected, the payload subsystem can have varying roles depending on the type of the payload. However, one function is independent of the type of payload selected: Documentation of the rejected payload ideas. This is crucial for the future teams. The functions dependent on the type of the payload are listed below:

  • If the team has selected a scientific payload, the payload subsystem is responsible for understanding the literature and laying down the foundation of the payload. The payload subsystem is responsible for bringing clarity within the team about the science behind the payload and establishing the theoretical feasibility of the concept and generating the relevant parameters for the rest of the subsystems
  • If the team has selected a payload which requires a specific instrument, say a camera, then that instrument is the responsibility of the payload subsystem. Procuring it, calibrating it, testing it, interfacing it with the rest of the satellite, establishing that the instrument is suitable for the intended applications, developing and enforcing proper handling and storage practices for the instrument etc. are the responsibilities of the payload subsystem
  • If the payload can fall under the purview of another subsystem, then the payload subsystem can be disintegrated post the selection of the payload and the members can be segregated in different subsystems. For instance, if the payload is the development of a repeater satellite, then the payload falls under the purview of the Communications Subsystem and hence the payload subsystem can be disintegrated.

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