The article defines necessity to use ship squat when computing underkeel clearance. Review of full-scale trials methods for squat with the use of global navigational sattelite systems was made. Sources of method’s errors were analyzed and its values were evaluated. Justification of necessity to ajust trial methods has been given, based on inland waterways emvironment condtions of Gorodets – Kstovo route and performance of GNSS in this area. The authors defined parameters to be measured during full-scale trials for the purpose of numerical methods improvement.