Mumbai University > EXTC > Sem 8 > Satellite Communication and Networks

Marks: 10M

The technical constraints which limit the maximum available DC power from a satellite

The voltages generated from solar cells undergo both short- and long term variations. Short-term variations are caused by rapid changes in temperature on the spacecraft. The voltage decreases with an increase in the ambient temperature. When a satellite emerges from an eclipse, the temperature can change from -1800C to 600C in a few minutes, leading to array voltage change by a factor of about 2.5.

Long-term variations occur because of the periodic yearly variations in the intensity of solar radiation. The average DC power of a solar array varies over the year because of changes in the incident solar power caused by variations in the distance between the Sun and the Earth. Further long-term reductions in voltage occur owing to gradual deterioration in the efficiency of the solar cells over time.

For reliable and accurate functioning of spacecraft equipment it is essential to regulate such voltage variations. The two regulation approaches commonly used are:

a. Centralized

b. Decentralised

Centralised regulation

In a centralised regulation shown in figure, the solar array voltage is regulated centrally and the regulated voltage is available to all the loads on the bus. A fraction of the generated voltage is used for charging the battery. The amount of charge can be controlled through ground commands. The output of the battery can be switched to a reconditioning mode or to the main bus. The regulator circuit can take form of a series or shunt regulator. The monitor and control sub-system provides various control signals, send monitored data to ground via the telemetry sub-system and receives commands from the command sub-system.

The advantages of this scheme are:

1. Availability of a well-defined voltage;

2. Simplified power conditioning for loads

The main disadvantages of the scheme are:

1. Possibility of a single point failure;

2. Large demands on the regulator circuit because of high variations in load requirements.

Decentralised regulation

In a decentralised power regulation scheme, each load regulates its own voltage. To avoid dangerously high voltage when a satellite emerges from an eclipse, a voltage limiter is used with each load.

The advantages of this scheme are:

1. Simplicity in the overall power regulation requirements;

2. Lower overall mass.

The main disadvantages of the scheme are:

1. Complex power conditioning requirements on individual loads;

2. Need to provide over-voltage protection on each load.

In a multiple bus concept, both regulated and unregulated bus structures are used. This approach avoids the complete satellite failure should one section of the power sub-system fail. As an example, the TT&C system, which is vital for the success of a mission, could be grouped on a separate bus with a more reliable configuration.