Satellite communications allows over-the-horizon communications with your balloon, and sometimes two-way communications. Satellite modems are radios that directly communicate with data communication satellites. Some consumer tracking products will work as-is when flown on a balloon, but most fail to function above a certain altitude limit.

For most projects, ready-made circuit boards for the modems with power and antenna hardware are recommended by those experienced with trying to make DIY satellite modem PCBs (-Dan Bowen, White Star). Bare satellite data modems have very strict power quality and antenna requirements, thus PCB development requires a lot of circuit calculation or many trial-and-error revisions. Testing hardware becomes cost prohibitive because you are usually charged by the message.

Due to the small size and low cost, most satellite modems typically have complex command sequences and communication protocols, are usually unable to deal with storing messages and retrying transmissions as would be desired on balloon systems. The code to communicate with the modem may require the use of state machines, external message storage chips, and interrupts to avoid data loss.

Some existing microcontroller satellite modem control software are listed below that can help in your development, or just do the job for you.

Several different networks of satellites exist, relevant ones are:

• Iridium
• Orbcomm
• Globalstar

• + Full global coverage, continuous service pole to pole
• + Inexpensive service
• + Inexpensive pre-assembled trackers available
• + Inexpensive components to build trackers
  • Bare Iridium 9602 Modem
  • RockBlock, Iridium 9602 Modem + Power and USB-Serial PCB - This is recommended over the bare modem

• - Significant length service dropouts regularly due to missing satellites
• + Two-way data
• + Inexpensive modem available: Digi m10 ~$150US ~115€
  • m10 has high power consumption - up to 12 milliwatt-hours per message session
• - Radio Frequency is VHF, requiring a VERY large heavy antenna
• - Very complex binary serial protocol
• - The primary messaging method uses live relay to ground station, requiring that the satellite passing overhead be able to see your modem AND the Orbcomm ground station at the same time
• - Difficult to obtain service for single modems

• - Coverage only near continents, not over much ocean
• - One-way data only - “modem –> internet”
• + Many cheap consumer devices available
  • FindMeSpot Position Tracker

Project Page on github

• Makes the Iridium 9602 and 9603 modems easy to use
• Queues telemetry messages so you don't have to!
• Arduino/C/C++, on Arduino Mega 2560, or Wiring ATmega1281 boards.
• This will run on a standalone ATMega chip, and your main flight computer will send messages by I2C or serial to/from this chip.
• Fully implemented Iridium transmit retry algorithm means you won't get service cut off or disturb others.
• Interactive setup and commands accessed by serial terminal

Basic Digi m10 satellite modem interface functions work fine, use to build more complex messaging code on. THIS IS NOT FUNCTIONAL FOR FLIGHT AS-IS. We stopped this development due to the failure to get a large lightweight ORBCOMM antenna built.

• Zip file of code
• Github commit to fork from
• Arduino (0023 IDE), FIO board

• Hacking the FindMeSPOT to accept external telemetry
• Hacking the SPOTConnect