Compact active transponders (CATs) – also termed electronic corner reflectors – are compact electronic devices designed to receive, actively amplify and re-transmit a radar signal, e.g. a C-band radar signal received from a Sentinel-1 satellite. CATs can potentially be useful for a number of purposes, e.g. if co-located with geodetic infrastructure. However, CATs have only recently become commercially available, and therefore, the usability and long-term performance of CATs are not well known. In this study, two CATs are tested under realistic operating conditions for a period of 14 months, from July 2020 to September 2021. The displacement time series of the CATs are determined from a persistent scatterers interferometric synthetic aperture radar processing of four tracks of Sentinel-1A/-1B data with a passive corner reflector (CR) as the spatial reference. The displacement time series of the CATs are evaluated against a ground truth established from repeated levellings between the CR and the CATs. Based on the results of this study, it is found that a sudden vertical displacement of a CAT can be determined with an accuracy better than 1 cm, possibly a few millimetres. Furthermore, it is found that the mean vertical velocity of a CAT, calculated from 14 months of interferometric synthetic aperture radar displacement time series, can be determined with an accuracy of a few mm/year. Finally, the line of sight (LoS) phase error is generally found to be moderately correlated with temperature, with an instrument-specific linear relationship between LoS error and temperature ranging between approx. 0.1 and 0.2 mm/°C. This correlation between LoS phase error and temperature can in principle be used for instrument-specific calibrations, which is a topic that should be addressed in future studies.