As a backup in case my GPS transmitter fails for any reason, I have one of these:
It is a GPS Tracker fitted with GSM technology. It has been fitted with a Virgin Mobile sim card and set to automatically send an SMS message containing it's GPS co-ordinates every 5 minutes. It will sit in the payload and once the altitude drops below 60,000 feet and it comes within range of a GSM cell, will start sending SMS messages, at 5 minute intervals, containing the payloads current location. In the event of the main GPS/Transmitter system failing this should act as a backup system to allow the landing spot of the payload to be found and for the recovery team to locate it.