The Medium Resolution Imaging Spectrometer (MERIS) is one of 10 sensors deployed in March of 2002 on board the polar-orbiting Envisat-1 environmental research satellite by the European Space Agency (ESA). The MERIS instrument is a moderate-resolution wide field-of-view push-broom imaging spectroradiometer capable of sensing in the 390 nm to 1040 nm spectral range. Being a programmable instrument, it had the unique capability of selectively adjusting the width and location of its 15 bands through ground command. The instrument has a 68.5-degree field of view and a swath width of 1150 meters, providing a global coverage every 3 days at 300 m resolution. Communication with the Envisat-1 satellite was lost suddenly on the 8th of April, 2012, just weeks after celebrating its 10th year in orbit. All attempts to re-establish contact were unsuccessful, and the end of the mission was declared on May 9th, 2012.

The 4th reprocessing cycle, in 2020, has produced both the full-resolution and reduced-resolution L1 and L2 MERIS products. EN1_MDSI_MER_RR__2P is the short-name for the MERIS Level-2 reduced resolution, geophysical product for ocean, land, and atmosphere. This Level-2 product comes in a netCDF4 package that contains both instrument and science measurements, and a Manifest file that provides metadata information describing the product. Each Level-2 product contains 64 measurement files that break down thus: 13 files containing water-leaving reflectance, 13 files containing land surface reflectance and 13 files containing the TOA reflectance (for all bands except those dedicated to measuring atmospheric gas - M11 and M15), and several files containing additional measurements on ocean, land, and atmospheric parameters and annotation.