The UHF radio on the balloon must use the Proximity 1 protocol to communicate with the Orbiters acting as relays. This protocol was developed by the CCSDS, an international body. For more about them, see this article: https://en.wikipedia.org/wiki/Consultative_Committee_for_Space_Data_Systems

The Proximity protocol can be found here in wikipedia:

https://en.wikipedia.org/wiki/Proximity-1_Space_Link_Protocol

Radio frequencies used are:

• 437.1000 MHz
• 440.7425 MHz
• 444.3850 MHz
• 448.0275 MHz

A decision must be made on whether the radio code should be added to the main processor, or whether a dedicated processor (possibly another PIC32) would handle radio alone.