This part of the ISO 12188 specifies the process for evaluating and reporting the performance of agricultural vehicles equipped with automated guidance systems (AGS) based on a global navigation satellite system (GNSS) when operating in an automatic steering mode.The main performance criterion is the lateral deviation of a representative point on the vehicle from a desired trajectory for that point. This performance criterion integrates the uncertainties associated with the performance of all components of the vehicle guidance system including the positioning device(s), automated steering components, and vehicle mechanisms and dynamics.This part of ISO 12188 focuses on steady-state tracking performance of the automated guidance system while travelling on straight paths over a level surface.