It is not needed for that. When you switch the flight mode to "Stabilize", which is assigned to the "S" setting in ST16's mode switch in the .param file I supplied with the binaries, the drone does not use the GPS data for anything (same than turning off the GPS in stock Typhoon, except the ability to get the motors to absolute maximum or idle power). When the mode switch is flipped to "A", the drone goes to GPS-assisted Position mode (which is like stock Typhoon, although a way smoother). "H" is Return to Home, like in stock firmware.
The drone may start in Stabilize-mode, regardless of the switch position. This is especially the case, if you take off while the GPS is still in "Acquiring" mode. You can do it, if you do not need GPS or RTH capability. If you want to take-off in GPS-assisted Position mode, you have to flip it manually to "A", or if you power it on when the switch is in "A" position, it is safer to cycle the switch, just to be sure. I may have to do something for this, like making the RGB led to indicate the active flight mode.
[ Stabilize/Sport | Assisted | Home ] ;-)
(The Acro/Rattitude modes, where the Typhoon is capable of doing full aerobatics, are not mapped anywhere by default, for safety reasons. It is like the arming of the drone, I am still not going to tell which the super-secret stick gesture is. You have to take a look into PX4's documentation, which hopefully encourages one to study about the PX4 a bit. And if you want to fly aerobatics, you have to enable/assign the modes by yourself (those selections on the screenshot above). Also, there is NO kill switch assigned at the moment as I do not like those. AUX is a good candidate for that. That red button has caused a little unnecessary harm here too I think. ;-)
Edited: I forgot to mention this before, I'll write it now here as this is pretty important. The reason why the Rattitude mode is not there by default, is that it really requires you to handle the drone carefully but promptly. It is a kind of an assisted acro mode, where a sudden and sharp stick deflection initiates a flip, which then gets auto-stabilized when you release the stick. It's like the Stabilize/Stock TH without the GPS, but this time nothing stops you from tumbling the drone. When the stick is deflected, the drone will flip until you release the stick, or the planet Earth spoils your fun, whichever happens first. If you are flying it like a stock Typhoon, you may will flip it accidentally and this may lead to a crash. It will not take that much altitude to recover, the Typhoon/Thunderbird is very aggressive in this mode, but still, the risk is there. My advice with this mode is to perform a smooth takeoff, with slow and steady stick movements (both sharp AND near-full stick deflections will flip it), gain plenty of altitude (room to play and recover from "uncommanded" aerobatics) and then... fly it like it's stolen! It handles very nice, and it is fun to fly. But before this, remove the CGO3+. It
WILL hate you, life, your aerobatics, particularly me for developing this firmware and the world in general, if you fly acro with it. If you are able to keep ~1 G loading on the drone while doing aerobatics (steady large loops and barrel rolls etc.), it will be fine, but sudden and sharp movements are no good for it. ;-)