A multifrequency (ten spectral lines between VHF and S band) coherent radio beacon is presently transmitting continuously from a 1000‐km, high‐inclination orbit for the purpose of characterizing the transionospheric communication channel. Its high phase‐reference frequency (2891 MHz) permits direct observation of complex‐signal scintillation, and its very stable, sun‐synchronous orbit allows repeated pre‐midnight observations at low latitudes and near‐midnight observations at auroral latitudes. We present here early results of the observations; salient points include the following. First, most of the data are consistent with phase‐screen modeling of the production of ionospheric scintillation, including an ƒ−2 frequency dependence for phase variance. Second, propagation theories invoking weak, single scatter seldom are adequate, because even moderate intensity scintillation usually is accompanied by phase fluctuations comparable to or greater than a radian. Third, under conditions producing GHz scintillation (near the geomagnetic equator), lower frequencies show marked diffraction effects, including breakdown of the simple ƒ−2 behavior of phase variance and loss of signal coherenceacross a band as narrow as 11.5 MHz at UHF.

One of the main limitations of the modeling work that went into the equatorial section of the Wideband ionospheric scintillation model (WBMOD) was that the data set used in the modeling was limited to two stations near the dip equator (Ancon, Peru, and Kwajalein Island, in the North Pacific Ocean) at two fixed local times (nominally 1000 and 2200). Over the past year this section of the WBMOD model has been replaced by a model developed using data from three additional stations (Ascension Island, in the South Atlantic Ocean, Huancayo, Peru, and Manila, Phillipines; data collected under the auspices of the USAF Phillips Laboratory Geophysics Directorate) which provide a greater diversity in both latitude and longitude, as well as cover the entire day. The new model includes variations with latitude, local time, longitude, season, solar epoch, and geomagnetic activity levels. The way in which the irregularity strength parameter CkL is modeled has also been changed. The new model provides the variation of the full probability distribution function (PDF) of log (CkL) rather than simply the average of log (CkL). This permits the user to specify a threshold on scintillation level, and the model will calculate the percent of the time that scintillation will exceed that level in the user‐specified scenario. It will also permit calculation of scintillation levels at a user‐specified percentile. A final improvement to the WBMOD model is the implementation of a new theory for calculating S4 on a two‐way channel.

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Tomographic processing of path integral electron density records is emerging as a viable tool for ionospheric research. Tomographic processors fall into at least two major classes: those applying the Radon transform and those employing linear algebraic matrix inversion. In this paper we apply one of the latter, the "weighted, damped, least squares" technique of stochastic inversion, to two simulated but realistic data sets. This method, which repeatedly has been applied successfully to ocean acoustic tomography, is particularly suited to solving inverse problems in geophysics because it provides an orderly mechanism for judicious use of a priori or external information to complement sparse or nonuniform path integral data. The limited range of angles through which the ionosphere may be viewed on satellite-to-ground paths represents such a nonuniformity m ionospheric tomography. The method also provides means for estimating uncertainty m the image field, uncertainty which itself is nonuniform. FREMOUW ET AL .-STOCHASTIC INVERSE THEORY IN IONOSPHERIC TOMOGRAPHY 723

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