Pii: S0273-1177(01)00144-2

A radio holographic approach, developed by Pavelyev (1998), Hocke et al. (1999), Igarashi et al. (2000), is applied to observation of wave phenomena in the upper atmosphere using Global Positioning System "Microlab-l" satellite (GPS/MET) radio occultation data. In the current state the radio holography approach uses the radar focused synthetic aperture principle to obtain high spatial resolution, and to remove the interference part corresponding to scattering from the upper ionosphere. High spatial resolution and accuracy of the radio holographic method is validated by means of revealing the weak signal reflected from the sea in the GPS/MET radio occultation data. The radio holographic method gives a new possibility to measure directly the vertical gradient of the electron density altitude profile in the D-layer using the radio occultation signal. The results of the application of radio holographic analysis to two GPS/MET occultation events (07 February 1997, No. 0447, 0158), in the D-region of the ionosphere, are discussed. Wave structures in the electron density concentration with a vertical spatial period of 1.4-6 km, and variations in the electron density gradient from +5.10 9 to +8' 109 [l/(m3km)], have been retrieved from the D-layer data. The features observed in the vertical electron density profiles may be connected with breaking of gravity waves in the D-layer of the ionosphere. © 2001 COSPAR. Published by Elsevier Science Ltd. All rights reserved.