Abstract: The modulation frequency and additive constant of an electronic tacheometer Wild Tachymat TC 1600 were tested versus temperature and supply voltage.The differences between measured and predicted frequencies did vary by only ±1.4 Hz(or±0.1ppm) from the average difference across the entire temperature range from-10℃ to +50℃.Thevariation of the additive constant over the same temperature range was very small(±0.23mm) and statistically insignificant.No significant effects of supply voltage on frequency and additive constant were found.In addition,the additive constant was tested versus signal strength.Changes of 1 mm maximum were found at very low signal strengths creating scale errors of less than 0.5ppm under field conditions.When establishing the frequency-versus-temperature characteristic,full use was made of the accessibility of internally measured temperatures and internally computed predicted frequencies.Internal temperatures differ between 0 and +11.5℃ from ambient temperatures,depending on length of operation,previous temperature exposure and present temperature.The warm-up effects on the difference frequency and the additive constant were also insignificant with 0.2ppm per 30 min.and 0.24mm per 100 min.,respectively.A standard deviation of a single TC 1600 distance observation("DIST" mode) of±0.5mm was found during laboratory tests.On an EDM calibration baseline,average standard deviations of(±0.4mm+0.4ppm) were realized for means of 4 to 8 distance measurements.Short periodic errors were found to change with distance but did not exceed 1 mm amplitude.