I have a GPS tracker/transmitter I use in rockets. I use it to locate and recover a rocket. I realize that Altitude is not the strong suit in GPS, and don't expect it to be really accurate. Neither do I expect the lock on a typical GPS unit to be maintained during the rapid acceleration of a rocket.
However, at apogee, and under parachute, I find that the re-locked-on GPS altitude varies enormously from the verifiable height of the rocket trajectory. A rocket descending from 5000 feet might show between 800 and 1000 feet in height, calculated as an offset from the launch pad altitude (so no, this is not local terrain elevation causing a difference). The GPS-reported altitude will rise, but slowly compared to its ability to track lateral (relative to ground) movement.
I've been told that this might be an intentional limitation of capability, to help prevent malicious use of GPS chips, but I can't find documentation of this, and think instead the chip might just be designed for terrestrial uses of GPS, and not need to track altitude change rapidly (or think that rapid altitude change in a land or sea vehicle must be in error).
Does anyone know what limits altitude rate-of-change?