NearSys LLC now sells a GPS Simulator. It allows you to test a near space APRS tracker or flight computer for its behavior during a mission. It's a way to simulate a mission on your bench top, saving you money and difficulty should something go awry.
The GPS Simulator models all the events of a GPS that's making a trip to near space. These events include GPS lock and loss of lock, launch and ascent of the balloon, balloon float, balloon burst, descent, and landing. The GPS lock can be lost at any time during the mission and can be regained at will. The ascent rate, landing speed, float altitude, and burst altitude of the mission are easy to set and vary by adjusting four well-described variables at the top of the GPS Simulator program.
This level of functionality is useful if you programmed your flight computer to respond to conditions like too slow of an ascent rate, unexpected loss of GPS lock, unplanned for neutral bouyancy, passing specific altitudes, balloon burst, approaching landing, or touchdown.
The NearSys GPS Simulator lets you test and debug these programmable features of your near space mission.
You can view the kit and its instructions at the GPS Simulator page at NearSys.com