NWRA Ionospheric Research Papers

Scintillation Papers

Fremouw, E. J. and A. Ishimaru, Intensity Scintillation Index and Mean Apparent Radar Cross Section on Monostatic and Bistatic Paths, Radio Science, 27(4), 539-543, 1992.

Abstract:

We develop an expression herein for the intensity scintillation index on a two-way path, in terms of the one-way index and the correlation between scintillations produced on the uplink and downlink. The expression is apporoximate for monostatic (fully correlated) and bistatic (totally or partially uncorrelated, or anticorrelated) paths whose links are statistically similar and obey Nakagami m statistics. A companion expression for the mean apparent radar cross section (RCS) in the presence of scintillation describes enhancement on monostatic paths and energy-conserving depletion of mean apparent RCS on small-angle bistatic paths. The companion expression, which does not depend on Nakagami m statistics nor require statistical similarity, is consistent with more detailed calculations by previous authors. Special cases of both expressions are consistent with recent monostatic measurements.

Secan, J. A., R. M. Bussey, E .J. Fremouw and Sa. Basu, An Improved Model of Equatorial Scintillation, Radio Science, 30(3), 607-617, May-June 1995

Abstract:

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 Over the past year this section of the WBMOD model has been replaced by a model developed using data from three additional stations (Ascension Island, in the South Atlantic Ocean, Huancayo, Peru, and Manila, Phillipines; data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.

Secan, J. A., R. M. Bussey, E. J. Fremouw, and Sa. Basu, High-Latitude Upgrade to the WBMOD Ionospheric Scintillation Model, Radio Science, 32(4), 1567-1574, July-August 1997.

Abstract:

The high-latitude sections of the WBMOD ionospheric scintillation model have been upgraded extensively, based on analysis of scintillation data from the Defense Nuclear Agency Wideband, HiLat, and Polar BEAR satellite-beacon experiments. Data collected at Sondre Stromfjord, Greenland; Tromso, Norway; Ft. Churchill, Canada; and Bellevue, Washington (USA); over a four-year period were analyzed, and the results of these analyses were used to construct a completely new model for the behavior of the height-integrated irregularity-strength parameter (CkL) at high latitudes. The new high-latitude CkL model includes variations with SSN, Kp, latitude, local time, longitude, and season. The new WBMOD CkL models (equatorial and high-latitude) have been implemented in a more versatile code, SCINTMOD, which has the capability to generate a wide range of user-controlled maps of scintillation effects over large spatial areas. Examples of the types of graphical output that SCINTMOD can generate are presented.

Ionospheric Tomography Papers

Fremouw, E. J., J. A. Secan, and R. M. Bussey, A Status Report on Applying Discrete Inverse Theory to Ionospheric Tomography, Intl. Journal of Imaging Systems and Technology, 5, 97, 1994.

Abstract:

Discrete inverse theory (DIT) provides an orderly framework in which to combine measurements of total electron content (TEC) with a priori information to image the ionoshpere tomographically. We have developed a DIT-based tomographic processor for use with relative TEC data. The processor's a priori information comprises the global mean of over 17,000 profiles generated from an ionoshperic model, for use as a "generic background"; empirical orthogonal functions (EOFs) spanning the same model profiles, for use as vertical basis functions; and a red power-law horizontal spectrum. Relative TEC data are used to evaluate coefficients multiplying the EOFs and harmonics, thus quantifying a perturbation electron-density field. The perturbation field, which need not be small, is added to the a priori background to produce the image. We present here several images produced by employing the processor with simulated TEC data based on in situ ionoshperic measurements and incoherent- scatter radar observations.

Zhou, Chucai, E .J. Fremouw, and J. D. Sahr, Optimal Truncation Criterion for Application of Singular Value Decompression to Ionospheric Tomography, Radio Science, 34(1), 155-166, 1999.

Abstract:

In this paper we present a generic method to solve the subspace- oriented estimation problem. We have optimized our approach by taking account of the a priori and posteriori covariances of both the data and the model parameters in general linear inverse notations. In our work, singular value decomposition (SVD) was employed to provide a robust optimal solution. In computation of the generalized matrix inverse, a very simple truncation criterion on the singular value (SV) spectrum was set up which guarantees the minimal variance of the estimate. This algorithm based on SVD produces an optimal estimate independent of computing resources. Specifically, we applied this method to studies of ionospheric tomography by inverting total electron content (TEC), which may be measured by means of satellite beacons. We processed two simulate cases. The residual variances of the a posteriori covariances of the model parameters were used as the measure to evaluate the uncertainties of the estimates. Our examples indicate that this algorithm can resolve about 60% of the a priori variance while achieving a significant decrease of the computation time by truncation of the SV spectrum.

Acknowledgement. This research was supported by the National Science Foundation under grant ATM-9528146.

Other Ionospheric Papers

Secan, J. A., and P. J. Wilkinson, Statistical Studies of an Effective Sunspot Number, Radio Science, 32(4), 1717-1724, 1997.

Abstract:

Two decades ago, the USAF Air Weather Service space forecasting group began generating what was termed an effective sunspot number (SSNe) by fitting an foF2 model to observed foF2 values. Initially a preprocessing step in a larger analysis package, this parameter has taken on a life of its own and is now used in various applications for both forecasts and specification of the global foF2 field. This paper describes the various ways in which this parameter is calculated, investigates the behavior of this parameter over solar cycle 21 (1976 through 1986), and compares it to other solar-ionospheric indices, including R12, IF2, IG, and the IPS T-index.

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