# Questionable Mission Planner Auto Analysis--Compass FAIL

Is the report's warning on Compass misleading?

I click on Auto Analysis in Mission Planner (MP).

In the report, look at the last line shown.

Compass = FAIL

Having no real documentation to help me understand this FAIL, I tried for years to get my ships to not FAIL ... to no aVAIL.

Then I got curious.  What are these numbers? Are they meaningful?

Answer:  They are quite meaningful when taken in context but not how Auto Analysis does it.

1) They appear to be the readings from the magnometer during calibration in mGauss in x, y, and z.

... unknown, but x might be N/S and y might be E/W if it follows earth standards (but it might not)

... z is directly down

2) Field length is the length of all three vectors combined = sqrt (x^2 + y^2 + z^2)

... +/- points to the direction of the field

... with the units of measure typically uT or mGauss or ...

... APM shows unitis in mGauss

3) The magnetic field length varies greatly (by more than 2.6X) in common flying areas

- If flying in Asuncion, Paraguay with no magnetic interference, the field length per NOAA is

... 185 N/S, -5 E/W, -114 N/S mGauss

... 218 mGauss Total

- If flying in Irkutsk (similar to N USA, Canada, Scandinavia, S Australia, N China)

... 182 N/S, -12 E/W, 577 N/S mGauss

... 605 mGauss Total

4) The most common field length (weighted by population of flyers) is 500 mGuass

MP Context:

- "length of the mag_field can be anywhere from 120 ~ 550"  ... not true

- "depending upon where in the world the vehicle is but it is normally around 330" ... not true

- "compass is the offsets which should set between -150 ~ 150"   ... from what, 330???

- field length > 550 is FAIL

- max mag deviation > 16% (from memory) is a caution

- max mag deviation > 35% (from memory) is a FAIL (read next line)

- "a short spike at the beginning of the graph but this can be ignored because it’s before the throttle is raised" ... but does the max deviation take that into account (it didn't appear to)???

I also don't know how the offsets are calculated.  Is it the

... the negative of the calibration reading (making 0 normal)

... or something else?

Given the

- incorrect assumptions on earth magnetic field strength levels

- high variance in magnetic field and a common magnetic field length without interference being over 550

- the impact of startup and EKF filtering (???) on the mag readings

- the fact that there isn't an acceptable average

shouldn't this part of the analysis be changed so it isn't misleading?

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