GPS Offset - but why?

[Nicodema]

Hi guys

I'm pretty new here, but not new to multicopters. I built my first MK over 5 years ago, recently put together a CarbonCore 500mm quad and recently built up my first hex, a Vulcan 900mm Pro witha the Naza-M.

I took it for its short maiden flight this evening and I'm very pleased.

I've now mounted up the GPS unit and I'll be setting that up tomorrow, but it raises a question I can't think of an answer to, namely:

If the IMU is in the main Naza unit, why would you ever need to tell the flight control system the offset of the GPS from the CofG? What possible difference would that make to the interpretation of the GPS or magnetometer readings?

IMU offset I totally get, as rotations induce linear accelerations due to offset from the roll centre, but GPS and compass headings, not so much.

Please explain

Thanks

Martyn

 

[Electro 2]

I'll be anxious to see what you get for an answer on this as I have wondered too. The only reason I can figure is for the declination error setting on mag-compass which is also in the same physical package. It's not like GPS is accurate enough to resolve single digit centimeter distances, or anything.

 

[Nicodema]

So, I did a bit of research and it would appear that the Naza actually uses the GPS/Mag unit to run a GPS/INS strategy. This uses the additional data from GPS to increase the accuracy of the Inertial Navigation System, i.e. the flight controller stabilisation algorithm. That usually means throwing it into a Kalman Filter so that GPS glitches are thrown out of the stream and it only uses data that makes sense within the scope of the IMU data.

GPS is low accuracy but high precision, so when you have a Kalman filter just using good data it makes sense to setup the offsets so that the fairly small displacements you see from the GPS unit actually make sense in the context of the IMU/INS calculations.

Wow, I'm pretty impressed

 

[PeterLester]

IMHO - I do believe a few centimeters difference between the compass heading and the Naza controller IMU placement makes a considerable difference with MR compass headings which are used in conjunction with GPS headings.

Kalman filtering applies to IMU functions only which means it applies to:
magnetometers
accelerometers
gyros