If you have a wi-fi connection, such as a hotspot connection on your phone, doesn't the map still show where you are? While this may not help RTH, as long as you have a connection with the quad, you should still be able to fly it back to your location based on the map.
Yes. And no.
WiFi location is very very inaccurate to say the least. IF the WiFi has location data in the first place.
The "Home" location, in therms of the Phantom, is initially set using the GPS coordinates received by the Phantom's own GPS receiver. It is not the GPS received by the tablet (assuming tablet has GPS).
RTH will attempt to return to that "home" position unless the "home" position is updated in some way.
The "home" position is NOT
your position. Its where the Phantom was powered up and took off from. While this is typically the same place as "your" position, the two are not the same in the overall scheme of things. Your position can move and differ from the "home" position depending on what transpires during the flight. If you stay at the takeoff location, all is good. Its when you move from there during flight that things go wrong really quick.
Take this scenario: You are on a boat in a lake. You fire up and take off from the foredeck. With the phantom in the air your boat begins towing a skier behind and you do some nice follow shots of the action. Battery gets low and the phantom goes into RTH mode. Where is it going to go?
1. Back to the foredeck of your boat in front of you?
2. Back to where the foredeck was when it took off?
Answer: 2. And the Phantom takes a swim unless you stop it and manually land or get your boat back to where you started this adventure before the Phantom gets there.
That happens unless you update the home location with your current location (dynamic home point in terms of Litchi) as the boat moves about the lake. This requires GPS coordinates be available to your tablet. This means it needs GPS hardware or access to it via BT for example. The GPS position is read, sent to the RC and is relayed to the Phantom to store as its new "home" position. No GPS on your tablet, then this can't happen.
The critical thing to consider is that the "home" point is just a point in space (longitude, latitude and altitude or x,y,z) to the Phantom. The phantom doesn't know
what it took off from, be that hard ground, the bed of your pickup, the front of a swamp buggy or the deck of a boat. It simply knows
where it took off from. If the home point stops being a viable surface to land on, the Phantom has no cognitive way to discern that. It simply knows it needs to return to (x,y,z) before the battery goes dead. If that (x,y,z) is no longer a boat deck but is open ocean now because the boat moved, the phantom is still going to attempt to land there.
Now, consider scenario #2: You are on a boat offshore but this time you have a device with no GPS. You again take off from the foredeck. This time you are out shooting video of a boat race (sail or power, doesn't matter). Your boat is drifting along in the current but you don't really notice that as you are focused on keeping the phantom aimed at the action of the other boats. You have also lost sight of your phantom by using goggles or being too focused on the FPV or simply flying too far to actually see it. Phantom battery gets low and goes into RTH mode. Now what is going to happen?
1. Phantom comes back to the foredeck of your boat in front of you?
2. Phantom goes back to where the foredeck was when it took off?
Answer: 2. Again. But this time you are pretty screwed. You only know on the map where home
was and where the phantom
is. You have no clue where
you are because you have no GPS and as such have no way to guide the phantom to you, which is where its only hope of landing on anything solid is. If it can reach shore on low battery being manually flown and guided by the cached maps you have, then maybe there is hope. If shore is too far from home or where phantom is, well, its getting wet.
If you never move from the "home" point, then there is little need for GPS in your device because your position and the home position are, for all practical purposes, the same place. RTH will bring it back to you (or precisely, the home point). You can guide it back to you using the map and bearing indicators because you and home are the same.
If you ever wish to move from where the phantom took off from, I very seriously recommend you get a device with GPS or get a GPS addon, otherwise you will have very diminished chances of getting your phantom to you rather than its home..