  Precipitable water vapour (PWV) strongly affects the quality of data obtained
from millimetre- and submillimetre-wave astronomical observations, such as
those for cosmic microwave background measurements. Some of these observatories
have used radiometers to monitor PWV. In this study, PWV was measured from 2021
April to 2022 April using Global Navigation Satellite System (GNSS) instruments
in the Atacama Desert, Chile, where several millimetre- and submillimetre-wave
telescopes are located. We evaluated the accuracy of these measurements by
comparing them to radiometer measurements. We calculated the PWV from GNSS data
using CSRS-PPP (Canadian Spatial Reference System Precise Point Positioning),
an online software package. When using GNSS data alone, the estimated PWV
showed a systematic offset of +1.08 mm. When combining GNSS data with data from
a barometer, which was co-located with the GNSS receiver, the estimated PWV
showed a lower systematic offset of -0.05 mm. The GNSS PWV showed a statistical
uncertainty of 0.52 mm with an averaging time of an hour. Compared to other PWV
measurement methods, GNSS instruments are robust in bad weather conditions,
have sufficient time resolution, and are less expensive. By demonstrating good
accuracy and precision in low-PWV conditions, this paper shows that GNSS
instruments are valuable tools for PWV measurements for observing site
evaluation and data analysis for ground-based telescopes.
