NSIDC provides a suite of value-added data sets to aid in investigations of the variability and trends of sea ice cover. These data sets provide users with information about sea ice extent, total ice-covered area, ice persistence, monthly climatologies of sea ice concentrations, and ocean masks. The maps are in ArcInfo export file format. All other data sets are in flat binary or ASCII format with accompanying Portable Network Graphics (PNG) browse images. Products are derived from Nimbus-7 SMMR; DMSP-F8, -F11, and -F13 SSM/I; and from DMSP-F17 SSMIS sea ice concentrations at a grid cell size of 25 x 25 km. Data begin in October 1978 and processing is ongoing.