Why can my phone or non-cellular tablet (or even 10 year old Garmin GPS) get a consistent GPS signal indoors/under trees, but our drones can’t?  Different tech?

If you have enoug tree cover or a big metal roof, that can block anything.
It comes down to how good the GPS antenna is and how much its skyview is blocked.
Which Phantom are you using?
The more modern Phantoms are better in tricky conditions than the early models.

Another chip. There are several brands of GPS chips on the market. And the one is better than the other one.

In the new P3SE, the GPS / Glonass connection is excellent. This has shown the Russians their added value.

Mobile phones use what is known as assisted GPS (A-GPS).  These devices are augmented by signals from cell towers so they are more tolerant of 'shadowing'..  Along with cell tower signals many WiFi capable devices can determine their position from nearby Wifi hot spots, completely eliminating the need for GPS.  https://en.wikipedia.org/wiki/Wi-Fi_positioning_system

I have a Garmin Nuvi and a Magellan handheld GPS, neither of which can use cell tower or WiFi signals.   I find my Phantom performs just as well as they do in relation to indoor reception.

Doesn't a standard GPS as in one for cars or tablets only require four satellites?  I thought that was the case which might explain this since the drones require six minimum

Mark The Droner Posted at 2017-9-25 05:59
Doesn't a standard GPS as in one for cars or tablets only require four satellites?  I thought that was the case which might explain this since the drones require six minimum

A GPS unit (not talking about Glonass etc) requires a minimum of 4 satellites to obtain a 3D fix.
Even with 4 sats, the geometry of the sats may not be suitable and the fix could be unreliable.
DJI chose to set a min of 6 to ensure that all fixes are good fixes and prevent using possibly inaccurate GPS data.

Mobile phones using triangualtion from towers and/or wifi can get very poor fixes.
This only helps get a rough position but does nothing for the accuracy that a proper GPS fix gives.