We have used the GPS Tracker TK 102-2 for all of our stratoflights successfully. This device is extremely reliable and transmits the GPS data with an accuracy of only a few meters. Therefore you just have to insert a SIM-card, call the number and a few seconds later you will receive the coordinates and a GoogleMaps link via SMS. The great advantage of the TK 102-2: In case of freezing every battery stops working but when it unfreezes again it will continue to track its position automatically. Other devices need to be activated by pressing a button.
Our tip: It is possible to use two GPS Tracker simultaneously which run with different SIM-Cards. Thereby you can double-safe your equipment because especially for the tracking you should play safe.