I'll do an extensive primary tracker write up very shortly - it has been quite a lengthy process and we're still not completely finished with it.
A key feature of the backup tracker must be that it keeps working whatever happens! We want this to be a completely separate device to the rest of the payload and work in a different way so we have an alternative should the primary fail. Initial thoughts were to put in some kind of mobile phone that had a GPS chip in that could text us it's location coordinates, but these were heavy and setting them up to continually send their position seemed an unnecessarily large hacking task. I thought that there must be some kind of device that did this already so I began searching around on eBay, it didn't take long:
I got a SIM card from the shop and stuck a fiver on it to test it out. This thing is great, in its most simple mode all you have to do is turn it on and leave it to do its thing, you just dial the SIM phone number from another mobile and it will answer and hang up the call. A few seconds later you receive a text message with the location of the device - along with a link to Google maps if you want it! You can also set it so that you automatically receive a text message every 30 seconds or so to a predetermined set of numbers. Interestingly you do all of the configuration of the device from another mobile - sending commands via text.
If you want to get more advanced, you can take advantage of its GPRS functionality. Instead of sending information via SMS, it will use the mobile internet (GPRS not 3G) to send its location to a central server on the Internet. You can then view it on a map from your PC/Mobile. The benefits of this is it probably cheaper than the 12p a text you pay on SMS mode and you get the added Internet tracking functionality. The problem however is that GPRS (2G) signal is not as widespread as the GSM (1G) network so if it landed in some hills or another forest, we need to use the signal that is most likely going to be available.
For us, I think we are going to leave it in SMS mode and have it send us a location every minute or so. We're expecting the mobile signal for the device to be lost at some point during the flight so that is another consideration we need to think about. The main purpose of it is for us to use it when the payload is back on the ground waiting to be found!