12 The VISIR instrument has been designed as a lightweight facility to be embarked on future low Earth orbit operational satellites whose mission requirements include monitoring of open oceans, coastal zones and land surface areas. The instrument consists of an imaging spectrometer working in the visible range, dual-band thermal infrared radiometer and a single-band imager in the short-wave infrared. The instrument provides two operational modes, namely the global mode (ground pixel 1200 m, swath 1200 km) and the regional mode (ground pixel 300 m, swath 100 km) with steering capability. The high radiometric performances achieved in both operational modes show the capability to detect very small radiometric changes such as those induced by typical variations of the water quality parameters (chlorophyll, suspended sediments, yellow substance) and surface temperature anomalies. VISIR can achieve many of the MERIS and ATSR/AVHRR performance characteristics within a considerable smaller envelope that can lead to a more affordable instrument. The performances are achieved thanks to a newly designed high-performance (f/2.0 optical spectrometer based on a double-Schmidt configuration, the use of hybrid CMOS detectors for the VNIR and uncooled microbolometers for the TIR, and a hybrid whiskbroom scanning technique. This solution is an excellent compromise as regards the cost/benefit ratio, achieving many of the performances required for a coastal zone earth watching mission, with considerable simplifications with respect to equivalent existing instruments that can lead to much more affordable budgets in terms of mass, power and cost. The spectrometer has the potential to match other type of applications in different spectral bands and/or with different spatial and spectral resolution.