Power System Requirements

The overall mission requirements are set by the customer and given to the satellite contractor in the form of mission specifications and statement of work. The customer performance requirement specifies only the launch, transfer orbit, and operational orbit constraints, and the payload power demand over the mission life with specified power margin. This leads to the spacecraft requirements. All systems of the bus derive their requirements from the spacecraft requirements, and all components of a system derive their requirements from the respective system requirements. The flowdown of the requirements along with the design, manufacturing, assembly, and testing is shown in Figure 6.1. The spacecraft requirement specifications generally do not specify the

power system requirements per se. It merely includes a general statement such as ‘‘ the power system shall generate, condition, distribute and store electrical power to meet the requirements of the spacecraft payload and bus systems during all phases of the mission.’’ To meet such a broad statement of work, almost all requirements of the power system are self-derived based on the design analyses and approach taken by the satellite manufacturer. The basic satellite level parameters impacting the EPS design are the orbit

altitude, orbit inclination, and the mission duration. These parameters are used to determine the orbit period, sunlight and eclipse durations, and the solar angle between the orbit plane and the Earth-Sun line. The power system design is then driven by the load power requirement in all phases of the mission, which are as follows:

Launch and ascent. This phase extends from the lift-off to the sun acquisition, during which the battery supplies the needed power.