Satellite data have now been proven to be a viable, complementary source of information for climate change monitoring besides ground measurements and climate model results. Satellite data enable regular, global retrieval of essential climate variables (ECVs), defined by the Global Climate Observing System (GCOS). Many ECVs can be monitored with satellite sensors - and to some extent - with an impressive degree of accuracy. However, this is only valid for the latest missions with sufficient stability and sensor calibration (e.g., Copernicus’ Sentinel-3). These data sets are a valuable contribution to monitoring changes on Earth’s surface and atmosphere. One challenge for continuous measurements is that most systems in orbit have a short lifetime of approximately 5-10 years. The World Meteorological Organization (WMO) reported that a time series of at least 30 years is needed to retrieve statistically significant changes to ECVs. Considering that these are extended periods, only a limited selection of satellites and sensors can be used for reliable and statistically significant global monitoring. One of these is the Advanced Very High-Resolution Sensor (AVHRR) onboard NOAA satellites since 1978 and on the EUMETSAT platform MetOp since 2006. The long time series of AVHRR data can be extended with Copernicus Sentinel-3 A/B data, which have a similar spatial resolution but improved spectral and radiometric quality.