Bloomberg.com: News Feb. 20 (Bloomberg) -- A glitch in satellite sensors caused scientists to underestimate the extent of Arctic sea ice by 500,000 square kilometers (193,000 square miles), a California- size area, the U.S. National Snow and Ice Data Center said. The error, due to a problem called sensor drift, began in early January and caused a slowly growing underestimation of sea ice extent until mid-February. Thats when puzzled readers alerted the NSIDC about data showing ice-covered areas as stretches of open ocean, the Boulder, Colorado-based group said on its Web site. Sensor drift, although infrequent, does occasionally occur and it is one of the things that we account for during quality- control measures prior to archiving the data, the center said. Although we believe that data prior to early January are reliable, we will conduct a full quality check.