Pi In the Sky is a simple and reliable GPS receiver and radio transmitter designed for tracking high altitude balloon flights utilizing a Raspberry Pi+.
It uses a UHF radio transmitter which is license-free in Europe (for other areas check your local regulations).
The board is designed for either the Raspberry Pi A+ or B+ boards and has the physical HAT dimensions (its not a HAT due to the lack of EEPROM), it won’t fit the original boards. Its expandable with additional boards that can be stacked on and give additional functionality such as APRS. To track the location of flight you also need a radio receiver, aerial and PC on the ground. Suitable radios include TV dongles.