Measuring canopy height using satellite-based X-band interferometric SAR (InSAR) is promising for accurate monitoring of forest biomass. A prerequisite for applying this at large scale is that the penetration of the radar microwaves into the forest canopy is stable over time, i.e. not influenced by weather conditions. We investigated this in a tropical rainforest in Indonesia using 10 TanDEM-X InSAR data sets acquired during a 2-year period. We found that mean InSAR-derived canopy height varied with a standard deviation of about 0.5 m between acquisitions. The standard variation was 0.8 m; however, about 0.3 m could be attributed to errors stemming from technical properties of the acquisitions. In conclusion, this further supports the use of X-band InSAR from satellite missions for forest monitoring.