A series of measurements of the properties of equatorial ionospheric irregularities were made at Kwajalein, Marshall Islands (M.I.) in August 1977 and July–August 1978. These measurements, sponsored by the Defense Nuclear Agency (DNA), involved coordinated ground‐based and in situ sensors. The ARPA Long‐Range Tracking and Instrumentation Radar (ALTAIR), operated by Lincoln Laboratory, obtained backscatter and transmission data during five nights in August 1977 and eight nights in July–August 1978. This report describes the ALTAIR data from the night of August 11, 1978, which yield direct quantitative measurements of 1‐m and ⅜‐m irregularities and of plasma depleted regions. These plasma depleted regions, previously predicted on the basis of theoretical analysis and in situ data, were observed during the decay phase and not the generative phase of the field‐aligned