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High-Altitude Balloon Controller

This will be the second balloon controller I make[^1]. Features:

• RP2040-based controller (Pi Zero, probably).
• RFD900 radio link (probably limited to 64 kbps).
• APRS backup link.
• HA GPS.
• Pressure sensor.
• Internal and external temperature sensors, as well as sensors on the GoPros.
• Current & voltage sensor (measure power and battery voltage).
• Coulomb counter?
• On-board SD card for logging (downlink is unreliable).
• Camera for downlink (hopefully we can get some low-res images).
• MAVLINK protocol (not sure if I can find a way to send images).
• GoPro cameras, one up, one out.
• Power distribution to controller & GoPros. APRS has separate supply (depending on weight).

A high-altitude balloon flight puts unique requirements on sensors. On the way to 31,000 m, the payload will encounter the following:

• 31 km maximum altitude (very few GPSs can operate this high)
• -60 °C minimum temperature, encountered between 11 and 20 km (very few sensors can read this low)
• 183 Pa minimum pressure (very few sensors can read this low)

[^1]: The first was based on an old Make Controller board and a custom PCB with Microhard radio, pressure sensor, thermistors, and GPS. It also had a backup APRS transmitter and two GoPro cameras.