Magnet Error

Since the firmware version 0.86 the value of the magnetic compass sensor is carefully checked.

These are the different cases:

Error during calibration
Error before the start
Errors in flight

Reason for the error message

The GPS needs needs the exact compass direction for using GPS. Otherwise, the MK flies simply not in the right direction.
If the "flying circles" is mentured in conjunction with GPS, the reason is a rwong compass value in almost 99% of the cases.
With a compass error of >30°, the position hold with GPS is simply impossible.
Changing the MK-Parameters in the settings will not really help regarding this issue
Typical is that there are directions where GPS works wonderful and the directions 90° to that the GPS works bad - then you might think: "funny, it worked one minute ago"

The NC simply needs needs a compass value with significantly less than 30° error.

Calibration

You have to use the new method used for calibration. Only then the FC can messure the magnetic field carefully.
If the minimum - and maximum values do not have a minimum distance, the compass was either not properly calibrated, or the magnetic field is disturbed
The user then could get "invalid value compass" after calibration

Then you should take a look to this error message:

Here the MK was not not correct turned around in step 4 (Z-Axis calibration)
Then the signal between min and max (amplitude) is not big enough. The minimum amplitude is shown in the graph.
Reason for failure may be also magnetic iron or something magnetic (Buzzers, etc.)

If the calibarion fails

(compass still reports "invalid compass value)

make sure there is no magnetic material close the compass - like iron
do the calibration outside
use the new calibration method - the old would not work in the 0.86
check in the terminal if you have a message like above
The NC (since V0.26e) calulates the earth magnetic field by the GPS coordinate. That is nessecary in regions with <30uT (Brasil and south Africa). There you need a GPS-Fix for compass-calibration.

When you rotate the MK in different directions (like you would do in the new calibration), you should have magnetic curves like this. The min and max-values of the magnet sensor must be bigger than +/-300 6 we had this case in the Betatest - there an iron plate (as heatsink) was the problem. He was not able to calibrate either. After removing the plate - everything was fine

FC installed rotated

If the FC is installed 45° rotated (X-formation), you must rotate over the arrow of the FC and not over one rigger
If you would twist ith the FC installed 45 degrees and not turned over the corners of the FC, in calibrating the amplitudes of the X and Y axes are only 71% of the normal value.
Thus it is possible that the calibration does not go through.
Furthermore, you get 130% as magnetic value and obtain the magnetic error. You can not even fly properly - even if you ignore the error message.

Error right at the start

If, you get the message even before the start (or in the moment when the motors get started), probably iron or magnetic material are involved. Maybe iron is in the ground or close to the MK.

These are the two values, which are reviewed by the NC:

Magnetic value -> must be between 80 and 120% otherwise you will get the error message.
Inclination shall not deviate more than 10 ° from the theoretical value (depending on geographical location, e.g. 60-70°, link).
The theoretical value comes when the GPS has a fix
if the calibration procedure was not correct or done by the old method, you will also get this message

$/!\$ The error message would also prevented from starting.

Important: Before starting always calirate the gyros (like usual). The MK must not move then.
The (Inclination error) Error message also comes when the ACC is not calibrated carefully. (MK must stand 100% even when calibrationg the ACC)
And at the FC1.x versions may also, if the temperature of the gyro drift is too big.

Error in flight

The magnetic value may not differ more than 20% in flight. So it must be between 80% and 120%.

In flight, the incorrect inclination is not reported. Only the 20% deviation of the magnetic field.

If the error occurs only in the air, then it is probably due to power cables. The MK only reports the Error, but it changes nothing on flight behavior.
Typical is that there are directions where GPS works wonderful and the directions 90° to that the GPS works bad - then you might think: "funny, it worked one minute ago"
The Magnet values can be viewed in the anlog values and in the GPX-Log
Typically is also when the values runs away with the increase of the current.

In the GPX data log on the SD card, these values are recorded:

(Tool for viewing the log data: MK_GPX

resolve the current errors

One should first look at the plus and minus cables from the BL-Ctrl and the LiPo cables
The Power cables must be parallel - and make no loops of single wires
If necessary, slightly twist the pairs of wires
Enlarge the distance between the compass sensor and the cables
Cause may be the internal structure of the Lipos - then you probably have to enlarge the distance between lipo and Compass

checking magnet values

In the transmitter telemetry (Jeti + HoTT) and in the Koptertool, the values can also be monitored. (since FC0.86c)

Press (SET) in that menu

Earth field[%] + Compassangle unfiltered[°]
Measured Inclination[°] + theoretical Inclination[°]

before starting the Earth field must be around 95-105% and the differences of the inclination below 3° - otherwise you must recalibrate Gyros, ACC and Compass (or there is iron in the ground)
when hoovering the Earth field must be around 85-115% and the differences of the inclination below 5°
Maybe you can also attach the MK to the grund, start the motors and apply almost hoover gas. Then you can see the change of the values.
In strong flights and strong turns the values may have a bigger Error - but when hoovering they must go into that range within 10seconds

Disable the error message

Under certain circumstances, the MK will not start.
If the MK must me launched from an iron pedestal (or iron is close), it might be nessecary to disable this message.
If you fly in buildings, the error message should be disabled. (also don't to use GPS and Compass there)

This only deactivates the error message. If the compass value is wrong because of magnetic disturbances, the MK will not fly good in GPS-Mode - no matter of this setting

Videos

How magnetic fields arise and what implications these have for, you can see in these videos:

(Here insert a link to a category)