One of the things I had been experimenting with in 2012 was using a PICAXE and RFM22B as a balloon tracker.
I built a tracker that was light enough to use party balloons, heavy payloads need large balloons and lots of expensive helium (or hydrogen!). Small balloons are low key, simpler to launch and don't need CAA approval. These are known as Pico balloons and some of the solar powered ones have gone hundreds and thousands of kilometres.
My original tracker design was called 'PongSat';
It worked, but I got distracted on other 'higher' projects, and never had a chance to fly it more than a few hundred meters.
I have been looking at this again recently, wanting to make a more versatile tracker that you could fit with one of the very small digital cameras.
My idea was a prototype PCB about 100mm x 25mm and see if the basics of a tracker can be fitted with additional connectors\pads to allow for extra PCBs to be added to hold cameras or experiments. The radio and antenna would be on the bottom of the PCB, the GPS on the top where the balloons are attached.
The basics of the tracker to include;
[I]PICAXE, 28X2 or 40X2, because there are 4 program slots, and dual SPI\I2C interfaces
GPS, Ublox 6 or 7 because they are small and light, PCB antenna
RFM22B transceiver for up and down link, small light cheap, frequency agile, proven to work in space !
A small Lithium Ion or Polymer battery, good range of sizes and available.
Regulator, to ensure stable operation of Radio.
Watchdog and power down circuit, to protect against program crashes or radiation problems.
Current monitor, to detect possible latch ups.
Memory for storing balloon track, if no-one but you is tracking the balloon, you would want to know the track when you get it back.
Expansion connector with I\O and SPI\I2C bus connections, for 'experiments' and other power supply options.
The PICAXE basic code and other stuff can be found here;