A hybrid phase measuring system is described which is an adaptive development of traditional phase locked loops. It is based around a digital delay line. The target application is a slope-measuring continuous wave lidar designed to determine surface profiles at shallow angles of incidence. The lidar unit, of which the phase shifter is a key element, is a pioneering system designed to obtain a vector field representation of propagating deep water ocean gravity waves. The application of the system is in the new discipline of deterministic sea wave prediction (DWSP) and is intended to operate in fixed time mode. In the normal tracking mode, the phase difference to be estimated is much less than pi/2, but however, due to the nature of the physical situation, sudden large jumps of several times pi are possible. The system time constants adapt to such jumps, producing an evolving compromise between phase error and the time taken to recover the tracking mode.