These ionograms were digitized from the original ISIS-2 7-track analog telemetry tapes using the facilities of the former Data Evaluation Laboratory at the NASA/GSFC. This data restoration project is headed by Dr. R.F. Benson (NASA/GSFC). Ionograms were digitized at the rate of 40,000 16-bit samples/sec. This sample rate is higher than the Nyquist frequency of 30 kHz. The sample frequency of 40 kHz provides a measurement every 25 microseconds corresponding to an apparent range (c*t/2) interval of 3.75 km. Ionograms with this sample rate are designated as "full" ionograms because they have the full 3.75 km apparent-range resolution. The ionograms used for most analysis, and those available from CDAWeb, were produced by averaging every four samples of the sounder-receiver video amplitude output to yield an average value every 100 microseconds corresponding to an apparent-range resolution of 15 km. These ionogram files are referred to as "average" files with standard resolution. Each ionogram consists of a fixed-frequency and and a swept-frequency portion. The time resolution between ionograms is typically 14 or 22 seconds depending on the frequency sweep range.