The increasing number of GNSS applications with reliability and integrity demands calls for the implementation of monitoring systems that can be used to improve the employed error and threat models. For signal power monitoring systems, a characterization of satellite antennas plays an important role. In previous studies, a methodology for reconstructing gain patterns of Galileo satellite antennas using signal power observations from a single monitoring station has been proposed. Such a scheme has been recently extended to a multi-station approach. The present contribution explores the application of single- and multistation approaches for the case of GLONASS satellites in the context of antenna performance monitoring. It has been motivated by the observation during a routine data exploration of a sudden power decrease of the signal transmitted by the GLONASS satellite R720 in the G1 band on April 4, 2019. In order to evaluate the influence of the satellite antenna on such an event, gain patterns have been reconstructed using data from selected stations of the International GNSS Service (IGS) in the period 2018-2019. Comparisons of gain patterns before and after April 4, 2019 exhibit differences in one of the regions of the antenna at the 1–2 dB level (95%). A further analysis of data also revealed the likely occurrence of a previous event in 2018. By using the multi-station approach, it was possible to spot the exact date of such an event, which resulted in changes on another antenna region at the 1 dB level (95%). A final examination using the multi-station approach reveals changes in the reconstructed gain pattern at the 1.4 dB level (95%) after the two mentioned events. These findings exhibit the importance of signal power monitoring systems that may be able to spot the presence of changes of a satellite antenna characteristic as well as to provide the ability of warning GNSS users of receive power-sensitive applications. Similarly, the analysis carried out in this study suggests that the employed multi-station approach for satellite antenna gain pattern reconstruction may be suitable for setting up a (global) permanent multi-constellation GNSS signal power monitoring system.