R.P.
lvl.2
Flight distance : 172375 ft
United States
Offline
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lt Posted at 2017-1-23 08:59
Hi RP. This is the downside of GPS tech. By being close to the building, you created a higher than normal elevation mask in the direction of the building. Getting the first "fix" is always the problem. Once this has been achieved, the GPS "should" keep the position even with lower satellites. We have designed quite a few devices with GPS and they all have this problem. It is all down to SNR of the signal received from the satellites and the calculated PDOP. Because of the RTK, and this is not proven at the moment because DJI does not make the information available in any flight logs or apps, I think they have quite stringent criteria for the acquisition. Would really love to be able to adjust the PDOP threshold somewhere in the app if at all possible. On the other side, being able to do this might open a can of worms because the accuracy "might" go down the drain in this case if the user doesn't understand what he is adjusting and nullify the efforts of the RTK system to get a good and accurate position hold.
Hope this helps and as far as I know, you won't be able to take off without the RTK on the M600 having a fix unless you disable it completely. I have had the same problem with ours (M600+RTK) when near buildings because of the higher elevation masks.
Now that the explanation of the effect is done, here are some advise: GPS satellites moves quite quickly in the sky and if you remove yourself (more than 20m away because the human body "might" also affect the acquisition. Some of these satellite signals are basically just above the RF noise floor) from the environment and leave it switched on, it should be able to acquire a fix as more satellites come into view. You get some GPS planning apps in which you can add an artificial elevation mask to plan the availability of satellites. You can also check on UAV forecast where you can set the minimum required satellites (its not a foolproof tool but will give you an indication per hour on how many satellites are available - we normally go 2 higher than absolute minimum required)
Thanks for the information. Is it possible to turn off the RTK ground unit for takeoff then switch it on after takeoff? It's my understanding that RTK requires more satellites for takeoff. My problem is, I won't always have the luxury of being away from surrounding structures. I need to find a way of taking off around tall structures. |
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