There are loads of GPS systems out there, but many are only rated to 50k-60k feet (such as the Spot Tracker). What units do you use and why?
An undeniably great one is the 3DR uBlox GPS from 3d Robotics. Not only is it rated for 50,000m (>160,000ft) and has an integrated antenna, it also is housed in a case to ensure that it stays protected for repeated use in the field and lab.
Another pretty good one is the Byonics GPS5HAOEM, which is rated higher (64,000m), but is not enclosed and is subject to ESD concerns (Kapton tape can help with that). That extra 14,000m really isn’t necessary for 99.9% of Amateur Balloons - as we usually peak around 130,000 feet.