For radio waves transmitted through the ionosphere with frequencies much larger (50 to 1300MHz) than the plasma frequency (up to 15MHz), very small-scale fluctuations of electron density will scatter a portion of the signal back toward the transmitter. These fluctuations are caused by the random thermal motions of the electrons. The height at which the scattering occurs can be determined by either a pulse technique or by the use of two intersecting cylindrical radar beams. The height resolutions of a backscattered pulse or beam can vary from 4 to 35 km at a height of 100 km to about 300 km at altitudes above 1000 km, depending on the equipment used and the geographical location of the station. From analysis of the backscattered signals, electron density profiles, electron and ion temperature profiles, and ion composition and ionospheric drift can be determined. The equipment used for the observations determines the vertical extent of the backscattered profiles and the resolution, as well as influencing the procedures used to convert the scattered return to measures of meaningful ionospheric parameters. Radar installations that can make these observations are complex and expensive to operate; consequently, these observations are scheduled on an "as needed" basis, and the equipment is shared for other uses. Although the NOAA National Centers for Environmental Information (formerly National Geophysical Data Center) has several data tapes, each one is an independent experiment with its own parameters and data format. The National Center for Atmospheric Research (NCAR) now archives these data. Their collection includes the NCEI data and newer Incoherent Scatter data that they receive. NCAR has done significant development work on these data sets. They have created a common archive data format and converted all data to this new format. For additional details about this data set, contact one of the people at the address listed below. NCAR HAO, P.O. Box 3000, Boulder, CO 80307-3000, USA. FAX: 303-497-1589. Barbara Emery: 303-497-1596, emery@ncar.ucar.edu. Roy Barnes: 303-497-1230; bozo@ncar.ucar.edu.