One of the coolest projects I have done so-far with my kids is a "near-space" balloon. That's not quite into space itself (100Km+) but so high that the sky looks black and you can start to see the curvature of the earth below you.
The Flickr set with the photos from our first launch is here: http://flic.kr/s/aHsjK22nDc
This is a quick-fire slideshow of the photos from the balloon, formatted as a 2.5 frames/sec video. I'm having trouble embedding it so the link is here
In order to take this kind of photo you need to send a camera up to the stratosphere. Ours went 38Km (124,000 feet) straight up. This is easy enough: you attach it to a massive balloon, let it rise until the balloon bursts (due to the v. low pressure at the edge of the atmosphere) and then it will fall back to earth.
In order to see the photos that you have taken, you then need to find the camera afterwards. This is the trick.
Fortunately, at least in the UK and increasingly across Europe, the very helpful guys at the UK High Altitude Society (ukhas.org.uk) have developed a distributed network of trackers who will receive a signal from your balloon, upload the data to a server and plot the position for you on a Google Maps based page (spacenear.us/tracker/).
In order to take advantage of this wonderful network of helpers, we need to build a tracker that will communicate with their equipment. That is what I will outline in this instructable.
There are plenty of rules and regulations regarding what you can fly and which parts of the radio spectrum you can use for various tasks. The approach I will use in this instructable is suitable for the UK under the rules prevailing in 2013. If you live elsewhere or you are doing this significantly after I write this instructable, please check the rules that apply to you. The guys at UKHAS are fantastically helpful.
So - let's build a radio-tracker.