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What Does GPS Mode Really Mean?

Shedi

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Recently I had a scare flying my Mavic Pro. I took it to the village of Kakkopetia in the Trodos Mountains in Cyprus. Once my RC display showed I was in GPS Mode I took off, then flew around the village for about 12 minutes. Normally I will fly back to the landing manually, but in this case the drone was into sun and difficult to see, so I used the Come Home button.

Unusually, I did not hear my Mavic approaching overhead, despite the height display showing it was descending to land. At first I spotted what I thought to be a bird, but it was in fact my drone about to descend into a built up area at least 200 yards away. I abandoned the Come Home and managed to fly it back to a successful landing.

Analysis of the flight recorder showed that the home position had not been updated for a full 30 seconds after take off, thus recording the home position 200 yards away from where I took off. So what does GPS Mode on the RC really mean? It would seem that it does not imply that an accurate fix has been obtained. In this case, the terrain would have made less satellites visible, and yes I did not hear 'The Home Point Has Been Updated' message, but if that message is spoken through my mobile phone and the volume is down, then maybe not surprising.

GPS Mode certainly creates stability in the post take off hover, but what is the drone really using at this stage? Is it keeping station by ground image comparison or by a GPS fix? If the latter were true, I would expect the Home Position to respect that fix and not move dramatically as in this case. My only explanation is that a poor fix was obtained initially that was lost after take off then regained 30 seconds later. Would it not be better if the RC showed GPS Mode only after the fix was good enough to nail the home position? Subsequent tests have shown a return to normal operation.

Any thought please on how GPS Mode really works?
 
Analysis of the flight recorder showed that the home position had not been updated for a full 30 seconds after take off, thus recording the home position 200 yards away from where I took off
From what you're describing, it sounds like you took off before DJI GO showed the green GPS banner at the top. Can you post your flight log here so we can check it out?
 
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GPS mode simply means that it has acquired some satellites, not necessarily enough to establish a home point.

You really need to wait until “The homepoint has been recorded “ message befor taking off.

Also remember that if you’re having trouble with sun triggered obstacle avoidance, RTH could be at risk if smart RTH is enabled, and should be relied on only as a last resort.

Better to disable the forward sensors (takes about 10 seconds) and fly it home.
 
From what you're describing, it sounds like you took off before DJI GO showed the green GPS banner at the top. Can you post your flight log here so we can check it out?
Give me a day or two as I am very busy this weekend. Cheers
 
GPS mode simply means that it has acquired some satellites, not necessarily enough to establish a home point.

You really need to wait until “The homepoint has been recorded “ message befor taking off.

Also remember that if you’re having trouble with sun triggered obstacle avoidance, RTH could be at risk if smart RTH is enabled, and should be relied on only as a last resort.

Better to disable the forward sensors (takes about 10 seconds) and fly it home.
Thanks Robert. I have got into the habit of hearing that message as the Mavic climbs away. I am not sure if that message is broadcast via the RC handset or my phone, which may have been set to low volume?
 
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I have got into the habit of hearing that message as the Mavic climbs away. I am not sure if that message is broadcast via the RC handset or my phone, which may have been set to low volume?
You can also look for the "H" marker on the map in DJI GO. If you don't see the marker, the home point hasn't been marked yet.

67615
 
The United States government currently claims 4 meter RMS (7.8 meter 95% Confidence Interval) horizontal accuracy for civilian (SPS) GPS. Vertical accuracy is worse. Mind you, that's the minimum. Some devices/locations reliably (95% of the time or better) can get 3 meter accuracy
 
Analysis of the flight recorder showed that the home position had not been updated for a full 30 seconds after take off, thus recording the home position 200 yards away from where I took off.
It sounds like you were not in GPS mode at all when you launched.
You mentioned terrain - you need a clear view of most of the sky to get good satellite reception.
If you want clarification, rather than guesses, post flight data and someone will clear up what actually happened.

Go to DJI Flight Log Viewer | Phantom Help
Follow the instructions there to upload your flight record from your phone or tablet.
That will give you a detailed report on the flight data.
Come back and post a link to the report it gives you.
 
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Ye
Thanks Robert. I have got into the habit of hearing that message as the Mavic climbs away. I am not sure if that message is broadcast via the RC handset or my phone, which may have been set to low volume?
Yes, it comes from your phone and most likely your volume was down. I make it part of my preflight to check volume, and of course homepoint notification. Lately I’ve started wearing a small bluetooth headset so I dont miss anything.
 
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