I can't imagine how this could work since the Earth's field is NEVER horizontal.

It's neither necessary nor practical.  

The currently accepted means of measuring magnetic field for this type of application is a modification of a fluxgate compass, which has a basic accuracy no better than 0.1 deg.

The inertial nav system on a UAV such as Predator would be accurate to better than 0.001 deg.  

Since the Earth's field can vary drastically over short distances, a fluxgate compass would need to be compensated, while the inertial nav system is immune to this.

TTFN

A tenth of a degree should be atisfactory for leveling the wings since the system probably measured the differential of gauss values at at the wing tips. Keeping the wings level as opposed to letting the plane slowly roll into a graveyard spiral was probably their intent, not precise navigation.

Thanks,

WDA
 

The fluxgate is only about 1" across.  The fluxgate measures absolute magnetic orientation, but localized mass concentrations can throw it for a loop

The inertial attitude gyro is a mandatory requirement, and it tells you absolute roll relative to earth, so there is no need for any other roll indicator.

TTFN

A similar system was tried in the roll control of anti-tank missiles in the early 80's.  The missile had a shaped charge that required to overfly the tank in the right roll orientation due to the effects of a spinning tail stability system.  To do this, the missile emerged from the launch tube, took a magnetic "sounding" and used this as its roll datum.  Any deviation (due to roll) caused an emf to be generated in a transverse mounted coil and core - the output of which was conditioned and used to operate roll control fins.  This was, I believe, a purely comparative system.

wether it works on not, what do you plan to do when the poles flip or when you fly south over the poles?

http://geomag.usgs.gov/MagCharts/wmm-gif/WMM-00I.gif

Shows you the angle of dip of the earth's magnetic field. In the USA your plane will be flying at around 70 degrees to the horizontal when flying East West!

Cheers

Greg Locock

Gauss measurement devices were placed in each wing tip and in the nose and tail of the fuselage of a model airplane. Differential measurements were taken to determine bank and pitch respectively to stabilize the attitude. This approach was used in an R/C powered airplane to set some kind of flight endurance record. Unfortunately that was a couple of decades ago and no reference data are readily available today.

A similar attitude control approach should work for today's military and civilian UAVs, precluding the need for attitude gyros.

WDA

end
      

And what would tehn tell you that you were upside down?

TTFN