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# Chapter 4. Power

Power is one of the primary budgets for your satellite, and is a hard limit that must be obeyed. If you exceed your bandwidth budget, you lose some (but not all data), but if you exceed your power budget, your entire satellite cannot function.

Fortunately, the calculations for your power budget are relatively straightforward. Solar cells will charge the batteries so they are X full on average, with a minimum power of Y (during dark periods, at maximum predicted discharge). Your satellite will use Z power.

• If Z < X, you live.
• If Z < Y, you thrive.

# Power In

There are two parts to your power calculation. First and foremost is your maximum and average solar cell power capability. This sets the limit on your energy budget. You can have 10kg of batteries that can potentially store 3 Amps of power, but if your solar panels have a maximum total load of 310mA (per second), you will never have more than 310mA sustained power available.

Put another way, you cannot charge batteries with power you don’t have. The batteries exist only as a storage device to keep excess solar power during the night portion of your orbit. Your power is solar.

Unless, that is, you aren’t using solar (see Alternative Power for Satellites).

Let’s look at the TubeSat reference architecture:

• Solar cells: (48) 2.5V, 62mA
• Lithium-ion 3.5V, 900mA

An estimate of maximum solar charging is (ignoring voltage conversion for the moment) simply the number of cells times their output. 62mA * 48 cells yields 1488mA. Predicting that your orbit will spend half the time in sunlight and half outside, and that only one-third of the satellite will have a line of sight to the Sun at any point, you have a reasonable expectation of an average charging ability of 250mA.

For a rule-of-thumb figure, assume that one-sixth your total solar panel output will be available as an average power during the entire mission.

Your power budget is affected by your anticipated power usage profile. If you intend for your satellite to be on most of the time, with a sustained consistent power usage, you typically want usage Y to be at or under one-half your solar capacity X.

Under that steady stage plan, the satellite operates using half the flowing solar power while the sun is shining, while the other half of the solar power charges the battery. During dark, the battery fully powers the satellite and slowly discharges, but it does not reach zero before the next sun passage.

This is a typical plan for a sensor-driven or communications satellite mission. You are on at all times, and periodically dump your data back to Earth.

## Burst Mode

An alternative mode is burst mode, where you charge the satellite but do little to no power usage for most of each orbit. Your satellite will have a minimum power usage: processor on, radio transmitter able to receive signals, heaters (if needed).

Then, when there is enough accumulated charged battery power, you can engage in a higher-energy experiment, such as testing an electric ion propulsion system or deploying a folding structure.

This model requires either direct commanding from the ground or very intelligent programming for autonomous operations that takes into account power available. It is most suitable for engineering projects that exceed your expected per second solar power input.

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