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:
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.