Rapid Green flashes may not indicate a GPS Fix

[soler]

I was flying this weekend at a new location and it was taking a little longer to get a GPS fix on my Naza than normal. on my iOSD i was seeing 6 satellites and then I see the rapid green flashes showing a GPS lock. However in the iOSD it was still showing 9.8KM away home point. I unplugged the batteries and tried again, exactly the same happened again.

This continued 3 or 4 times until i was able to get 8+ satellites at which point it reset the home position (this happened 10- or 20 seconds after the green flashes)


What concerns me is if people have a low sat count and based on the rapid green flashes think that they have a GPS lock then they may be in for a shock. Can anyone else with IOSD confirm this?

 

[wokwon]

There are two sets of rapid green flashes. The first is "IOC forward direction recorded", the second one is "GPS home location recorded". That sounds like exactly what you're seeing.

 

[soler]

Ok, I checked the manual and it does mention that the IOC is automatically recorded after 30seconds of power on and that the home position is recorded after 6 sats are found.

Not a very good idea to use the same LED flashing procedure for two different setting in my view. At least I can verify the home position is set correctly though the iOSD.

Next flight I will check to see if i get two sets of green flashes.

 

[W5HYN]

Home lock setting

I was flying this weekend at a new location and it was taking a little longer to get a GPS fix on my Naza than normal. on my iOSD i was seeing 6 satellites and then I see the rapid green flashes showing a GPS lock. However in the iOSD it was still showing 9.8KM away home point. I unplugged the batteries and tried again, exactly the same happened again.

This continued 3 or 4 times until i was able to get 8+ satellites at which point it reset the home position (this happened 10- or 20 seconds after the green flashes)


What concerns me is if people have a low sat count and based on the rapid green flashes think that they have a GPS lock then they may be in for a shock. Can anyone else with IOSD confirm this?

Wow, this is very good information I have not seen before - 9.8 KM! I just checked the manual again and GPS satellite acquisition is indicated ONLY by the red flashes. No red flashes > 6 satellites, 1 red flash = 6 satellites, 2 red = 5 satellites, 3 red < 5 satellites. Home lock position is set when the motors are powered up IF 6 or more satellites have been acquired. 20 rapid green flashes indicates automatic course lock heading recorded after 30 seconds. 20 rapid green flashes CAN indicate course lock OR home lock if either is set MANUALLY.

So according to the manual if you had lifted off when you saw 6 satellites your home lock position would have been recorded 9.8 KM away! This could explain a lot of the reported fly-aways. The bottom line is that if you want to be sure your home lock position is recorded correctly you need the DJI iOSD. Unfortunately not an option for me - flying Naza lite. I use a third party OSD and I have noticed that 8 satellites are required to show an accurate home position. But I have never seen anything as far away as 9.8 KM.

 

[soler]

First of all I thought that the 9.8Km could have been coming from the previous days flying in a different location, but I have now measured this on google maps and it was only 4.5km from where I was the day earlier. I had a concern that it could be saving the location to memory which would be a bad thing but I do not think this is the case.

I would prefer a flashing indicator for a home location lock rather than a course lock, a failed course lock is much less likely to cause problems than a incorrect GPS home position. From a software point of view it would be easy to build in failsafes to prevent this type of fly away. If home location > 15m then do not allow motors to be armed, or something similar. Have it as a setting in the software to keep everybody happy.

Could this be the reason of some of the fly aways? I hear people talking about their quad drifting away without control which is much the speed of a return to home type action.

 

[W5HYN]

I would be very interested to see what you experience in the future. The iOSD is a very valuable method of checking.

 

[i3dm]

i dont think this explains any fly-aways.
the typical fly-away behaviour is full power into some heading, or sometimes going completely crazy - not typical "RTH to a far away home" behaviour.

 

[PeteDee]

I have recently tested home lock and failsafe by manually switching flight mode to failsafe and the position has been recorded fine and I have landed within inches of takeoff point.

Pete

 

[soler]

Yes it should if you have enought sat's in view. What I am referring to is that I was seeing the green flashes yet due to a low sat count I had not recorded my home point. As it turns out the green flashes indicate recording the IOC direction and the home point may or may not be stored at this point. I have only seen this once when I had a low day count and could only find this as I have IOSD. For non IOSD users it could be easy to mistake the green flashes to indicate there home point is set.

Basically never fly if you have any single or double red flashes before take off.