This thread is showing a big lack of understanding of GPS and atti mode.
I'll try to clarify some of it.
When you have no GPS satellites, atti mode isn't a friend, it's your only option.
Your Phantom is in atti mode whenever it has insufficient GPS satellites for a GPS position fix regardless of the position of your flight mode switch.
Less than enough sats = Atti mode.
LOL indeed ... you may have flown for a while but you haven't learned anything about GPS.
In a cave there is no way to receive GPS satellite signals.
His Phantom didn't lose GPS .. it never had GPS but mysteriously it never spun out, hit the rocks and crashed into the water.
Perhaps it had something to do with the pilot knowing what he was doing with the controls and there being no wind in a cave?
Your Phantom flies in the house with one sat because it is in atti mode.
Being in atti mode doesn't mean you automatically crash.
It just means you don't have the benefit of GPS position holding.
You are in your house and the Phantom is outdoors where it can receive GPS sat signals as you'd see on your screen.
Your controller indoors doesn't need GPS.
Ground-based satellites ??? What does that even mean?
If you'd asked them they would have also told you that DJI sell striped paint and left-handed screwdrivers or that submarines have screen doors and helicopters have ejection seats.
With your handheld GPS you need to receive signal from 4 sats to get a proper 3D fix.
It was easy to understand GPS sat numbers with the earlier models like the Standard because they only received one kind of satellite (the US GPS system).
Because it's possible to get a poor fix with on;y 4 sats, DJI programmed the Phantom to only consider a fix good when it came from 6 or more sats.
That's no problem up in the sky where your Phantom will usually find more than 6 sats anyway.
With Phantoms after the Standard it gets confusing because they receive US GPS sats and Russian Glonass sats.
However they need at least 6 sats of one type before a fix is considered valid.
You could have 5 GPS sats and 4 Glonass sats for a total of 9 sats but still be in atti mode until you find a 6th GPS sat.
This isn't often a problem unless you fly from a location where your Phantom only sees a part of the sky.
You won't get the green GPS acknowledgement until you get enough sats for a GPS fix.