High Altitude GPS operation
There are several considerations when choosing a GPS for high altitude rocketry flights. The first is proper implementation of the CoCom limits. These regulations place a limit on GPS tracking devices that disables tracking when it determines that is is moving faster than 1,000 knots, and at an altitude higher than 18,000m. Some manufacturers interpret this requirement as an or function, which means that it stops working at altitudes above 18,000m. The high altitude balloon crowd has been aware of this for years, and maintains a list of affected devices. Devices targeting high altitude flights have been advised to avoid these GPS Modules.
The u-blox 'MAX' gps modules correctly implement the CoCom limits. This correct implementation has been observed on many flights. During ascent if the velocity exceeds 1,000 knots above 18,000m, tracking abruptly shuts down. When the speed drops below this limit (during the coast phase on its way up) tracking resumes. There will be a period where the GPS stops tracking mid-flight, but resumes correct operation prior to apogee and then all the way back down to the ground. For these flight profiles, the GPS can be used for recording peak altitude, and to aid in recovery of the rocket.
There has been some disagreement regarding the absolute altitude limitations. The u-blox documents guarantee operation up to 50km, but it wasn't clear what happened when that limit was exceeded. Requests to u-blox support yielded differing answers, with some suggesting that normal operation would continue (but accuracy could not be guaranteed) while others indicated that the device would cease to function. Furthermore, it was unclear if the GPS would resume normal operation when it fell back below 50km.
In 2018, the speculation ended with data gathered during the PHX4 flight at Blackrock. Data captured during the flight showed the GPS module shutdown above 50km. It did resume normal operation when it fell below 50km, and worked to the ground and provided coordinates for recovery.
What was once regarded as a rarity, this altitude is now becoming more common. Organized events such as the Base 11 Space Challenge now require contestanst to fly (and verify) altitudes that greatly exceed 50km. Barometric pressure sensors, accelerometers and gyroscopes are too error prone to be used for verification. Estimates can be made by combining available flight data with timestamps from video, for example, but these are only indirect approximations.
Space rated GPS modules have been available to the cube-sat community for years. However, the approval process can be lengthy, costs high (starting at $5000 for an unlocked GPS module), and some manufacturers are reluctant to approve unlocked devices for rocketry projects.
u-blox has also been approached, but requests to relax their arbitrary 50km have not succeeded.
So, what is the rocketry community to do? The CoCom limits can be tolerated, but a low cost solution is needed that doesn't require unlocked modules and works above 50km.