LightAPRS is the most affordable, smallest, lightest, powerful and one of few open source APRS trackers available. It makes tracking pico balloons, weather balloons, model rockets, RC aircraft, and anything else that flies simple and easy. It is able to report location, altitude, temperature and pressure to the internet (APRS-IS) or direct to an amateur radio once a minute with a solar panel/super capacitors or just 4xAAA batteries. Because LightAPRS is open source you can add your own custom sensors via I2C/SPI pins. 

Key Features

• Ready-assembled PCB: all components are SMD and already soldered
• Open Source: Sofware is open. You can easily modify the source code. The code is shared via GitHub for different use cases (pico balloon, HAB, car, etc.)
• Arduino Compatible CPU: Since CPU is Atmega based, you can use Arduino IDE and libraries to programme LightAPRS.
• Extendable: Using I2C and SPI pins you can add extra sensors ( and their librarires) to LightAPRS and programme it as you wish.
• 1 Watt Radio: In fact you don't need 1 Watt radio module for balloon projects but for your other projects you definitely will :)
• High Quality GPS: No need to comment :) Ublox is one of the best GPS manufacturers.
• Price: LightAPRS is more affordable than any other APRS tracker (see comparison at http://qrp-labs.com/lightaprs)

Full details: http://qrp-labs.com/lightaprs