Synchronization routine for real-time synchronization of robotic total stations

Large geodetic projects often require measurement configurations with multiple robotic total stations (RTSs). In order to combine the observables of the spatially separated RTSs, a common time frame has to be established. In this paper, we introduce a novel synchronization routine for relative, real-time synchronization of multiple RTSs. The proposed routine, consisting of two main steps, is independent of ambient conditions and requires no additional hardware. In the first part, we analyze the characteristics of the RTS's internal time at stable meteorological conditions by comparing it to the reference time established using a dedicated GNSS receiver. Referring to the findings, we propose a calibration procedure for the temperature calibration of the RTS's internal time. We determine the drift rate at different temperatures in dedicated experiments within a climate chamber and derive a calibration function from this data. In the second part, we apply the calibration function in practical measurements and show its applicability for selected RTSs at variable temperatures. As demonstrated in two experiments, the proposed drift compensation combined with a cross-correlation based initial time delay estimation (TDE), proves to be a reliable approach for the time synchronization of spatially separated RTSs. After the measurement duration of eight hours, the RTSs are synchronized within one sampling interval, i.e. better than 50 ms.