The ionosphere is that part of the Earth's atmosphere that results mainly from the ionizing effect of Solar electromagnetic radiation. For poleware latitudes, the ionizing effect of incident energetic particles is also significant. Traditionally, the following ionospheric regions and their approximate height ranges have been designated: D region (60-90 km); E region (90-150 km); F1 region (150-250 km); and F2 region (above 250 km). Ionosondes utilize the radio wave-reflecting properties of the ionosphere. The product of the speed of light in a vacuum and half the elapsed time between vertical transmission of a single frequency electromagnetic wave and reception of the reflected wave at the transmitting location is defined as the virtual height of that frequency. A sweep-frequency ionogram is a plot of virtual height versus frequency and is recorded as nearly instantaneously as possible. These ionospheric data consists mainly of hourly values for at least one to a maximum of all of the following ionospheric characteristics (parameters). They are foF2, M(3000)F2, hF2, foF1, M(3000)F1, hF, foE, hE, foE2, hE2, foEs, fbEs, hEs, fmI, and fxI. The values are five byte (character) fields. The first three bytes of the field are reserved for a numeric value; the last two bytes are reserved for the qualifying and descriptive letter, if present. There are no decimals encoded in these data. Documentation is supplied with each data order. Data are archived on CDROM, optical disk, and magnetic tape. There are two CD-ROM's sold as a set. One contains 1957 - 1975 data; the other contains 1976-1990 data. The CDROM contains ASCII files and runs on DOS-compatible machines. A catalog, which lists the contents of the CDROM data set, is available via anonymous FTP.ngdc.noaa.gov and GOPHER.ngdc.noaa.gov in the /STP/IONOSPHERE/CATALOG directory. Additional data are available, but have not yet been put on CDROM. Data are also available on magnetic tape and IBM compatible diskette.