[](https://www.youtube.com/watch?v=oBn7nAoBcE4) A corner reflector was installed in the Montsec Observatory (Sant Esteve de la Sarga, Catalonia) in 2021. This facility, managed by the Institute of Space Studies of Catalonia (IEEC), has many advantages for the operation of the reflector, and it also includes a GNSS station, functioning since June 2023. Placed on a prominent location, the observatory is far from the possible interferences caused by urban centres, while close to Sentinel-3 and Sentinel-6 ground tracks. It is equipped with a meteorological station that offers a large amount of data needed for the calculation of small additional geophysical corrections. Furthermore, the fact that it is located inside the enclosed area of the observatory facilitates maintenance tasks by IEEC technical staff. After almost four years of operations, the facility has demonstrated its capability to produce long and steady series of measurements for Sentinel-6 satellites. It has also showed good performances in collecting other radar altimeters time series, such as CryoSat-2 (since 2022) and Sentinel-3B (since 2023). [Learn more](https://www.isardsat.cat/news/corner2).

_View of the corner reflector in the Montsec Observatory. © isardSAT. _ The first measurement campaign of the corner reflector at the Montsec Observatory, which took place between September 2021 and April 2022 shows promsing results ([F. Gibert et al., 2023](https://ieeexplore.ieee.org/document/10026283)). After being processed with a FF-SAR algorihtm, the values obtained are comparable to what is currently achieved by means of active transponders, and therefore, it is demonstrated that passive reflectors may be of interest to support radar altimeter regular calibration. _Impulse response functions for the Sentinel-6A passes over the corner reflector (blue) and the Crete transponder (red) measured on 22 and 29 March 2022, respectively. Overall agreement is found in both facilities, except for some asymmetries in the secondary lobes. _ The success of this campaign confirms their utility in some radar altimeter key calibration aspects that need to be regularly monitored. The results highlighted the need to equip the facility with a permanent GNSS monitoring system, as the analysis presented was done assuming only the ETRS89 drift with respect to the ITRS system, therefore neglecting eventual residual displacement due to local tectonic activity. [Learn more](https://www.isardsat.cat/news/corner2) _Centered radargram with the 2-D impulse response function of the corner from the pass on October 25, 2021. © isardSAT. _

_ View of Montsec Observatory © isardSAT _ After nearly four years in operation, the calibration site at Montsec Observatory has proven its ability to deliver long-term calibration measurements for the Sentinel-6 satellites with excellent results ([F. Gibert et al., 2024](https://ieeexplore.ieee.org/document/10642903)). Furthermore, it has incorporated data from other radar altimeters, including CryoSat-2 since 2022, and Sentinel-3B since 2023, both achieving similar results to Sentinel-6. As the prominence of the site ensures low background signals (or clutter) to the corner reflector’s measurements, the site has become a crucial benchmark to test new calibration procedures and to investigate the residual noise sources that contribute to the actual radar altimeter’s sensitivity. For instance, the current data series allows to monitor the power decay of the satellite due to system aging, or to set upper limits to the overall drifts observed, among others; topics being of high interest for the design of new generation of altimeters, and the geodesic and climate change communities.