IMUs not used in position calculation? (Relevant to gpshealth and magZ issues)

[BorisTheSpider]

Just a technical point of discussion after reading all the threads about gpshealth dropping and the aircraft reverting to atti mode.

It seems, if a simple loss of GPS can lead to the aircraft referring to quite a basic, attitude only, form of stabilization and positioning that the inertial reference units can only be being used to provide pitch/roll/yaw information, and possibly short term stabilisation using the accelerometers.

Any ideas about why DJI haven't used the inertial units along with the GPS and compass data to compute a "best mix" type position and velocity as it's done in traditional manned aircraft?

It seems pretty crude that just losing GPS leaves you with no map type aircraft position, no ability to RTH and not even the capability to hold a position in wind. If the IMUs are what I think they are, ie. a fairly conventional set of 3 orthogonal gyros and accelerometers, it's not complicated nor does it require significant processing power to just integrate the accelerations and thereby maintain an inertial model of the groundspeed and position in 3 dimensions.

It should be possible with that data, assuming the IMUs are reasonably accurate, to maintain "GPS like" positioning and navigation capability for quite some time following a complete loss of external (satellite) position references. Certainly I'd expect long enough to fly home and land following a loss of GPS a few minutes away from "home".

Not trying to talk down the pretty cool stuff being done with the mavic which is rather sophisticated for a remarkably low cost, but it does seem worth discussing why the thing can't fall back to an inertially determined position when external guidance is absent.

 

[Robbyg]

I thought about that and figured that it's probably another liability factor. If you end up in a NFZ and the map showed you were half a mile away, it might be problematic for DJI.

Rob

 

[harfordhawk]

I speculate, and it is only that, that dji is using some sort of algorithm to determine if GPS is correct and when they see these other factors that may indicate it is not, they failsafe the craft to ATTI mode. In my opinion, they didn't work this out just right, and need to tweak this algorithm to rely more on the gps. It is not hard to have a really good gps fix these days with even a cheaper style version, ( which I would not expect the mavic gps to be). So I can only guess that gpshealth is the measure they are using to switch mode back to ATTI. I suggest they could tweak it and I hope they will in the future.

 

[BorisTheSpider]

Rob: certainly a possibility, who knows the thought process. More likely I actually think it's that they haven't developed the system rather than that it's a conscious choice not to, but your suggestion could well be correct.

HH: Yeah I agree, however with the caveat that a fallback to a pretty basic mode when you have 2 IRSs on board seems like throwing away a lot of capability.