Radar tracking is often simply not an option for most amateurs as we don't have access to the facilities you would need or they are cost-prohibitive.
Amateurs can make doppler tracking devices, but those tend to have issues as well. Nothing unsolvable, but they aren't terribly common.
Almost all GPS we can get our hands on, even the special unlimited altitude ones tend to have a velocity limit (1000 knots or roughly mach 1.5).
As such, the most common way of measuring velocity is through integration of accelerometer readings.
It's fairly common for spaceshot attempts to break or plan to break Mach 5. That said, spaceshot attempts alone are not exactly a common thing, but I know several people and colleges working on them.