In the frame of its research concerning real-time positioning and control of road construction equipment, the Laboratoire Central des Ponts et Chaussees, has carried out in 1996 a study to know more about the actual vertical accuracy that a Ž. Ž. real-time kinematic RTK global positioning system GPS sensor could reach, under work site conditions. This study has widely used the dedicated testing facility called SESSYL, built to perform high-accuracy and real-time evaluation tests on positioning systems. It has been performed in collaboration with the French road contractor COLAS and Ž. the Ecole Superieure des Geometres et Topographes ESGT. ´´F irst, there is the proposed adapted geodetic transformation procedure, compatible with the high accuracy requirements. Then, the main results of a special SESSYL tests program are presented, where the impacts of several influencing parameters on the vertical accuracy have been carefully examined. The core part of the paper is the analysis of the typical RTK GPS set of data, from which we have tried to extract two different components: a high-frequency noise, rather easy to filter, and a low-frequency bias. This bias, given its good repeatability, can be modelled and used in prediction to improve in real-time the raw accuracy of the data. As a full-scale validation of our study, a site experiment is finally described, carried out this time on a real piece of Ž. equipment an asphalt paver during real roadwork.