ICEPAC Technical Description

1. Introduction

This report describes a propagation predictions model (ICEPAC) that is an extension of the IONCAP program. It differs in the polar region structure of the ionosphere and the low and mid latitude ionospheric structure. The ICED (ionospheric conductivity and electron density) profile model is a statistical model of the large-scale features of the northern hemisphere ionosphere. The model recognizes the different physical processes that exist in the different regions of the ionosphere. It contains distinct algorithms for the subauroral trough, the equator-ward portion of the auroral zone, the polward region of the auroral zone, and the polar cap. This report will be a complete description of the ICEPAC propagation prediction program.

The predictions are used primarily for long-term (month-to-month, year-to-year, etc.) frequency management and circuit planning, but are often used for hour-to-hour and day-to-day operations as well. Most important, propagation considerations are basic to studies of electromagnetic compatibility, and analytical computer prediction methods such as the one described in this report are essential to a practical solution.

It should be emphasized that a computer program is a tool for convenience in calculation; the user must exercise his own engineering judgment in determining the applicability and limitation of the results to specific problems.

1.1 HF radio propagation history

For many years, numerous organizations have been employing the High Frequency (HF) spectrum to communicate over long distances. It was recognized in the late 30's that these communication systems were subject to marked variations in performance, and it was hypothesized that most of these variations were directly related to changes in the ionosphere. Considerable effort was made in the United States, as well as in other countries, to investigate ionospheric parameters and determine their effect on radio waves and the associated reliability of HF circuits.

A worldwide network of vertical incidence sounders was established to measure values of parameters such as foE, foF1, foEs, foF2, and h'F. Worldwide noise measurement records were started and steps were taken to record observed variations in signal amplitudes over various HF paths. The results of this research established that ionized regions ranging from approximately 70 to 1000 km above the earth's surface provide the medium of transmission for electromagnetic energy in the HF spectrum (2 to 30 MHz) and that most variations in HF system performance are directly related to changes in these ionized regions. The ionization is produced in a complex manner by the photoionization of the earth's high altitude atmosphere by solar radiation. Within the ionosphere, the recombination of the ions and electrons proceeds slowly enough (due to low gas densities) so that some free electrons persist even throughout the night. In practice, the ionosphere has a lower limit of 50 to 70 km and no distinct upper limit, although 1000 km is somewhat arbitrarily set as the upper limit for most application purposes.

The vertical structure of the ionosphere is changing continuously. It varies from day to night, with the seasons of the year, and with latitude. Furthermore, it is sensitive to enhanced periods of short-wavelength solar radiation accompanying solar activity. In spite of all this, the essential features of the ionosphere are usually identifiable, except during periods of unusually intense geomagnetic disturbances.

The Radio Propagation Unit of the U.S. Army Signal Corps provided a great deal of information and guidance on the phenomena of HF propagation in 1945. By 1948, a treatise of ionospheric radio propagation was published by the Central Radio Propagation Laboratory (CRPL) of the national Bureau of Standards. This document (NBS, Circular 462, 1948) outlined the state of the art in HF propagation. Techniques were included for: predicting the maximum usable frequencies (MUF); determining the MUF for any path at any time taking into account the various possible modes of propagation by combining theory and operational experiences; and estimating skywave field strength.

Laitinen and Haydon (1962) of the U.S. Army signal Radio Propagation Agency furthered the science of predicting HF system performance by developing empirical ionospheric absorption equations and combining them with the theoretical ground loss, free-space loss, and antenna gain factors so that expected field strengths could be estimated for radio signals reflected from the E- and F2-regions, considering the effect of solar activity and seasonal and diurnal variations.

In the United States, the first automated HF path prediction computer program was developed in 1957, for the U.S. Army Signal Corps, Radio Propagation Agency (Contract DA 360039 SC-66438), now part of the U.S. Army Strategic Communications Command (see Stanford Research Institute (SRI), 1957). A later version was published in 1961 (Radio Corporation of America, 1961). The first fully automated program, in which the oblique transmission equations for parabolic layers were used, was developed in 1966 (Lucas and Haydon, 1966) by ESSA's Institute for Telecommunication Sciences and Aeronomy (ITSA), which preceded the Institute for Telecommunication Sciences (ITS).

This work was continued in two separate paths, one for communications analysis and predictions, reported in ITS-78 (Barghausen et al., 1969) and another for analysis and prediction of OTH radar systems reported in NRL Tech. Reports 2226 and 2500 (Headrick et al., 1971; Lucas et al., 1972).

The culmination of this work was the IONCAP program which uses the above described development for the shorter paths and other techniques for the long path predictions (Whale, 1969).

Fundamental to all efficient HF computer prediction programs are the synoptic numerical coefficient representations of the ionospheric characteristics. These were first developed by ITSA (formerly the Central Radio Propagation laboratory, National Bureau of Standards) and first published in 1960 (Jones and Gallet, 1960). Subsequent modification led to the technique now used (Jones et al., 1966), which will be discussed later. >

1.2 General description

The techniques used in the computer program described in this report are procedurally similar to the earlier ITS programs (ITSA-1, ITS-78, HFMUFES, IONCAP), but there have been sufficient significant changes to warrant further documentation.

The literature on the ionosphere and its role in HF sky-wave radio communications is very extensive. Theories concerning ionospheric propagation will not be repeated here in detail, but some background material will be given where necessary for an understanding of the prediction processes and the philosophy of the program.

In the basic model, it is assumed that the ionosphere can be represented by one or more Chapman layers (Dudney, 1983), given sufficient information concerning the height of maximum ionization, semi-thickness, and electron density. Sufficient data must be available to predict an average electron density distribution with height for any possible transmission path. The model retains the equivalent path theorem (Breit and Tuve, 1926; Martyn, 1935) and its transmission curve solution (Smith, 1939), since this is the method for scaling and predicting ionospheric characteristics.

The program predicts the long-term operational parameters, such as maximum usable frequency (MUF), optimum traffic frequency (FOT), and lowest useful frequency (LUF), in terms of the probability of successful transmission for a particular circuit. The probability of successful transmission depends on the probability that the transmission frequency is below the critical frequency (i.e., the maximum frequency for reflection) of the F2 layer and the probability that the available signal-to-noise ratio is above a specified level.

Throughout the report, attempts have been made to clarify duplication of nomenclature and symbols commonly accepted in wave propagation and antenna studies.

Word Perfect 5.1: Complete document may be obtained via anonymous FTP from: elbert.its.bldrdoc.gov/hf_prop/DOS/tecman.exe. It is a PKZIP self-exploding file.