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The WBMOD Ionospheric Scintillation Model

Introduction

The WBMOD (for WideBand MODel) ionospheric scintillation model was developed by researchers at Northwest Research Associates, Inc. (NWRA). This document is designed to provide an introduction to this model.

Background

Ionospheric scintillation, which manifests itself as increased noise on a radio signal intensity and phase, is caused by small-scale variations in the ionospheric electron density along a transionospheric propagation path between the transmitter and receiver. Phenomenologically, intensity scintillation is very similar to the twinkling of stars due to variations in atmospheric density caused by turbulence. Scintillation effects are most severe at high latitudes (within and poleward of the auroral region) and in the equatorial region in the post-sunset to midnight local-time sector. Effects on systems range from loss of signal in deep fades, which can cause loss of information or even loss of lock on the signal, to loss of coherence in phase-sensitive systems (such as deep-space radars) due to phase scintillation.

The WBMOD computer model, developed over the past two decades by NWRA with support from the US Government, can be used to calculate estimates of the severity of scintillation effects on a user-specified system and scenario (location, date, time, geophysical conditions). WBMOD consists of an ionosphere model, which provides the global distribution and synoptic behavior of the electron-density irregularities that cause scintillation, and a propagation model that calculates the effects these irregularities will have on a given system.

Detailed Description

The electron-density irregularities model, EDIM, is a collection of empirical models which describe the geometry, orientation, strength, and motion of the irregularities as a function of location (latitude, longitude), date, time of day, solar activity level (sunspot number, SSN), and geomagnetic activity level (planetary K-index, Kp). These models were developed from analysis of large databases of scintillation measurements collected during the Wideband, HiLat, and Polar BEAR satellite experiments and from the USAF Phillips Laboratory equatorial scintillation monitoring network.

One of the basic parameters generated by this model is the height-integrated electron-density irregularity strength, denoted CkL, which is a measure of the total "power" in the electron-density irregularities along a vertical path passing through the entire ionosphere. Of the eight parameters used within the model to characterize the electron-density irregularities, CkL is the most dynamic. The two figures accompanying this description illustrate some of the variations found in the model for this parameter.

WBMOD Figure 1

The first example of WBMOD output, is a contour map of log(CkL) over the equatorial region of the earth from the EDIM model valid for 2300 GMT on March 21, 1994, for a fairly high level of solar activity (SSN = 150). The red areas show where scintillation is expected to be the most intense for the given conditions, which occur just after local sunset along two bands spaced roughly 15 degrees north and south of the geomagnetic equator (indicated by the long-dash curve).

WBMOD Figure 2

The second example, is a similar figure showing a comparison of observed (left plot) and modeled log(CkL) (right plot) at high latitudes. These figures show the variation of log(CkL) as a function of magnetic local time (counter-clockwise from midnight at the bottom of the plot) and distance (latitudinal) from the auroral electron-precipitation boundary (located along the dotted circle in each plot). The observations and model values were averaged over all geomagnetic conditions and are valid for SSN values in the range 50 to 75.

The propagation model, SCNPROP, takes the description of the electron-density irregularities from EDIM and uses it to calculate the expected scintillation effects on a user-defined system. This model is based on a phase-screen theory developed by Dr. C. Rino. As presently implemented, SCNPROP calculates three parameters which specify the expected effects on the signal's phase: two characterize the phase-scintillation power-density spectrum and one characterizes the total RMS phase variance due to scintillation. The model also calculates two parameters which specify the effects on intensity: the RMS variance of intensity and the 95th-precentile fade depth. The output can be either the value of these parameters at a user-specified occurrence percentile, or the percent of time that a user-specified threshold (of either phase or intensity scintillation) would be exceeded.

NWRA has implemented EDIM and SCNPROP into WBMOD Version 13.04, which is available for release. If you are interested in obtaining the model, contact NWRA.

We have also developed several tailored versions of WBMOD for various users. A modified version of this code is the heart of the Ionospheric Scintillation Prediction and Specification System (ISSPS) developed by NWRA for operational use at the USAF Space Forecast Center at Falcon AFB, CO. This system permits use of near real-time observations of scintillation and electron-density irregularities to update the EDIM CkL model within WBMOD in order to provide a more accurate specification of current conditions.

Please see our scintillation services pages for examples of the type of tailored products which can be generated from the SCINTMOD code, an NWRA-proprietary implementation of WBMOD.

References:

Fremouw, E. J., and J. A. Secan, Modeling and scientific application of scintillation results, Radio Sci., 19, 687-0694, 1984.

Fremouw, E. J., and A. Ishimaru, Intensity scintillation index and mean apparent radar cross section on monostatic and bistatic paths, Radio Sci., 27, 539-543, 1992.

Rino, C. L., A power law phase screen model for ionospheric scintillation, 1, Weak scatter, Radio Sci., 6, 1135-1145, 1979.

Secan, J. A., R. M. Bussey, E. J. Fremouw, and Sa. Basu, An improved model of equatorial scintillation, Radio Sci., 30, 607-617, 1995.

Secan, J. A., R. M. Bussey, E. J. Fremouw, and Sa. Basu, High-latitude upgrade to the Wideband ionospheric scintillation model, Radio Sci., 32, 1567-1574, 1997.

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