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A model for estimating the cosmic ray induced bit error rate in dynamic MOS RAMs is developed and used to calculate the bit error rate in NMDS dynamic RAMs used in an operating satellite system. The calculated error rate agrees sufficiently well with the observed error rate to conclude that cosmic ray ionization is a likely cause of observed satellite bit errors. The susceptibility of M1S RAMs to cosmic ray induced error is a result of the small charge (sub-picocoulamb) typically stored on a MDS gate to represent a data bit. Relatively small energy deposition (a few MeV) can discharge a storage node, resulting in a bit error. The heavy ion particles present in galactic cosmic rays can provide this energy, resulting in a significant bit error rate for large nmeory systems in satellites. The dynamic RAM operational factors and design factors affecting ionization-induced bit error rates are discussed.