LightAPRS 2.0 is one of the most affordable, smallest, lightest, powerful and open source APRS trackers available. It makes tracking pico balloons, weather balloons, model rockets, RC aircrafts 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/supercapacitors or just 3xAAA batteries. Because LightAPRS 2.0 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
• Arduino Compatible MCU: Since MCU is Arduino compatible, you can use Arduino IDE and libraries to programme LightAPRS 2.0.
• Extendable: Using I2C and SPI pins you can add extra sensors ( and their libraries) to LightAPRS 2.0 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 2.0 is more affordable ($125) than any other APRS tracker

Full details: http://qrp-labs.com/lightaprs2