Despite the enormous improvements in the accuracy of real-time GPS in recent years, there are still problems in reliably delivering the most demanding accuracies required in certain construction applications (e.g. asphalt paving). The limiting error source in such situations is phase multipath, especially that from highly reflective sources close to mobile antennas, such as that from the engineering plant itself. This paper reviews the role of precise real-time GPS on engineering sites and introduces the relevant parts of the mathematical and physical theory needed to understand fully the phase multipath problem. It also describes a testing procedure developed at LCPC for assessing multipath (and other) GPS errors in controlled static and kinematic environments. Details are given of tests with a new multipath mitigation technique invented by Leica Geosystems and based on a window correlator. It is shown that the technique is effective for multipath mitigation when the reflected signal has an additional path length of greater than about 7.5 m. Finally initial results from a proposed strategy to overcome this limitation are given.