Moot, the error bars I assume you may mean the arrows? They are to run the menu of the sat guidance unit, adjusting the width of the swath you wish to plot, height of antenna, ground tilt adjustments, selecting degree/direction of swath, meaning we can set the guidance to go anywhere from 0 to 359 degrees on a compass heading and it will even do it to 4 places, ie 270.3478 degrees, and there is a setting for contour mode, which means it will plot a course parallel to the last one you drove freehanded. So if you had to go around a lil curve or whatever, it plots the next pass and will guide itself provided the turn is not overly sharp.
If you mean the circle of lights on the top and the line of lights on the bottom of the unit, they are in fact "error bars". The circle shows how far you would need to turn left or right to get back on center. The bar on the bottom tells you how you are off of the center line. The pic is not a good one, as when I took it all lights were centered. On a mile long pass, the bottom light never crosses over to left or right at all, ocasionally the circle light will go left or right one light before the guidance corrects it. We are talking only a few inches literally here. Sometimes the right or left side of the sweep will hit a hard or soft spot in the field, and the tractor/sweep will try to move accordingly, which is why maybe one light will come on to left or right of the center light once in a while. When you look back at a mile long pass you just did, it looks virtually arrow straight.
There is another electronic unit mounted on the floor behind the seat in the tractor. This contains some FM electronic stuff (F***ing Magic) like gyroscopes and accellerometers that sense how fast the tractor is either tilting or whatever and it in turn controls the hydraulic units mounted on the underneath side of the tractor, which does the steering on the tractor. The hydraulic unit is pretty simple as it is solenoids and valves and fluid flow adjustments that control the hydraulic cylinders used to steer the tractor. To disengage the auto steer while you are moving in field, you either turn the steering wheel slightly, poke the big black button on the bar I installed in the tractor, or poke the red stop guidance button on the head unit of the guidance system, and well you can poke the green button which is the enter button, to turn it on and off too. Its pretty simple to run, really.
