STBY Compass and Gyro compass going haywire

[dcoded]

Dear All,

I currently fly the Challenger 850 and some days ago we made a fuel stop in Yekaterinburg.
Some minutes after shutdown we got an Efis Miscomp (because CMD and FO headings were split) Its normal sometimes if we stand close to hangars etc etc.
During the taxi out it continued, and we tried to manually align the gyros with stb compass.
On line up we did the final manual inputs to align with runway heading.
We departed and airborne we got the Efis Miscomp message again. We were wings level. And the STB compass was turning by itself for almost 50 degrees,
FMS calculated winds of 70kts at 5000' (which seems rather unlikely due that on landing the wind was around 15kt)
Then later after 15000' all slowely went back to Normal.

Now my question is. What can cause the magnetic systems so act so erratic?
I noted on ground that there were som Mil activity of taxing AN12 aircraft.
Also the Primary and SSR radar were located in the middle of the airfield between the two runways.
Can there be some relation between powerful Primary radars and compas misbehaving?

[dcoded]

No one with any inputs or similar experience on other AC types?

[Amadis of Gaul]

In six years of flying all kinds of CRJs, the EFIS COMP MON message was very common, some places more than others. When you realigned it to runway heading, did you switch to DG mode from MAG mode? That's what we used to do when taking off from, for instance, Rwy 13 at LGA where the mismatch would be particularly awful. We'd realign, switch to DG mode, take off, and switch back to MAG mode somewhere towards 10,000'. God only knows what kind of magnetic anomalies they have in Sverdl...err...Ekaterinburg.

[clunckdriver]

At KLGA the runway extensions are in fact steel plate on top of a girder structure, thus one gets the mag compass wandering when in position, this was noted on our charts in the past, have managed to avoid going there for the last few years and havnt looked at the charts.

[dcoded]

Thanks very much for both of your replies!
I've heard it is a common problem to have that EFIS MISCOMP
Although, I never tried putting it in DG mode.
Next time I encounter it I will def do it, thanks for the tip!! :D