  One of the most critical tasks in tractor operation is the accurate steering
during field operations, e.g., accurate trajectory following during mechanical
weeding or spraying, to avoid damaging the crop or planting when there is no
crop yet. To automate the trajectory following problem of an autonomous
tractor-trailer system and also increase its steering accuracy, a nonlinear
model predictive control approach has been proposed in this paper. For the
state and parameter estimation, moving horizon estimation has been chosen since
it considers the state and the parameter estimation within the same problem and
also constraints both on inputs and states can be incorporated. The
experimental results show the accuracy and the efficiency of the proposed
control scheme in which the mean values of the Euclidean error for the tractor
and the trailer, respectively, are 6.44 and 3.61 cm for a straight line
trajectory and 49.78 and 41.52 cm for a curved line trajectory.
