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# Power Out

A BasicX-24 requires 20mA itself, plus up to 40mA per I/O channel used. An Arduino requires 50mA, plus the same 40mA per I/O channel used. The bulk of sensors used will be passive, which is to say they require no additional power past that provided by the I/O connections. Finally, a Radiometrix UHF transceiver uses 27mA to receive and 110mA to transmit a 100mW signal; boosting it to 500mW (e.g., Radiometrix AF2S amplifier) requires 2mA when receiving but a hefty 250mA additional power when transmitting.

We can now calculate our power budget, as shown in Table 4-1.

Table 4-1. Power budget

 CPU 50mA Radio (waiting) 30mA Sensors (assuming four) 4 x 40mA = 160mA Radio (transmitting) 750mA Power usage, nominal 240mA Power usage, transmitting 990mA

Recalling our “power in” calculation, we anticipated a 500mA average power availability. What we first realize is our nominal (gathering data and open to receiving signal but not transmitting) power rate is half our average power, and we are safe.

The second realization is that we don’t have enough power to be in transmission mode all the time. For our typical DIY picosatellite, this is acceptable. We have burst usage, where for brief periods we will exceed our average power, but on a per-orbit basis, we can still come in under budget.

Max transmission time calculation: 90 min × 500mA power in, minus 90 min × 240mA nominal usage, yields (23400mA ÷ 750mA transmitter usage) yields 31 minutes per orbit available for full-power, happily transmitting operation.

So the situation isn’t too bad. Using these initial profiles, we can still transmit for up to one-third of a given orbit and remain within our expected power budget.

You can run the numbers yourself very easily for alternative configurations. This is an essential activity well worth spending a day on, as your power budget is a fundamental entity that you cannot exceed.

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