  The techniques of precision agriculture include the possibility to execute
crop monitoring tasks through the application of Unmanned Aerial Vehicles
(UAVs). These platforms are flexible, easy to use and low-cost, and they are
the best candidates for improving the farm efficiency and productivity. In this
research, a guidance algorithm and a robust control system are combined to
guarantee the robustness of the system to additive noise (i.e. wind
disturbance) and uncertainties (i.e. model parameter variations). A small
fixed-wing UAV with an autonomy of about $1$ hour is proposed as case study, to
reduce the cost of monitoring and increasing the stability performance of the
system. A waypoint-grid on a paddy field is verified by hardware-in-the loop
tests. The control scheme provides good results with a low computational
effort, guaranteeing the repeatability of the monitoring and reduction of the
costs.
