I have the M600 Pro+A3 Pro+RTK. I was working a job this past weekend and was within 20' or so from the building I was going to look at. The building was about 160' or so tall at this location. Everything fires up ok but it tells me I cannot take off, weak RTK signal. I double checked everything, restarted the sequence but still no GPS. I finally move the craft away from the building another 10' or so and I get the green light. Once in the air I could fly close to the building and GPS was anywhere from 11 - 15 sattelites.
I won't always have the luxury of moving away from a structure to get a green light GPS takeoff. What do you guys do in that situation? Do you take off in ATTI mode then switch to GPS once in the air? What do you suggest?

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)

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.

R.P. Posted at 2017-2-4 14:47
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.

Hi RP

This is not possible at the moment. Information provided on the DJI website (http://www.dji.com/d-rtk/info#faq) seems to contradict this:
{
14. Can a drone take off without the D-RTK having an adequately strong satellite signal?
Yes, but in this scenario the system will fall back to GPS positioning only. If signal strength increases during flight, D-RTK will take over.
}


A possible workaround for now might be to raise the aircraft by hand until it gets a fix. Shouldn't take more than 10-25 seconds. Sometimes this is all that is needed to get the first fix and the green light.
What you can also try is to roll back from 1.0.0.19 to the version prior to 1.0.0.18 (This one does not work with the M600). I could get the initial fix much quicker with this one. The problem is that DJI has tried to (and dare I say) dumb down the settings in exchange for ease of use which works perfectly for most users but the concept that satellite count is the only criteria for a fix creates a wrong perception because SNR, elevation masks and PDOP all plays their part in achieving good accuracy. Would be nice to have access to these settings in Assistant or Go App.

Rgds, LT

lt Posted at 2017-2-6 11:59
Hi RP

This is not possible at the moment. Information provided on the DJI website (http://www.dji.com/d-rtk/info#faq) seems to contradict this:

Thanks LT, I'll give that option a try.