System Performance Modeling

There are a number of HF system performance models (i.e., IONCAP, VOACAP, ICEPAC, ASAPS, and REC533) that can be used to assist in frequency management and related endeavors. There is an ITS-Boulder web site from which VOACAP, ICEPAC, and REC 533 may be downloaded. (Suggestion: Go to the ITS website using your browser, click on radio propagation software, and then on HF models.) A detailed discussion of these and other models may be found in Chapter 5 of Goodman [1991]. The internationally accepted HF system performance model is REC533 [1997], but a number of system engineers and HF practitioners in the United States prefer VOACAP. A valuable resource outlining many of the VOACAP features is now available [Lane, 2001],

Many workers in the Pacific Rim and Australasia use the ASAPS model [McNamara, 1991], and it has also developed a following in other regions as well. One interesting feature about ASAPS is that it uses a so-called T-index as a proxy for sunspot number. The T-index is based upon ionospheric measurements, and like the sunspot number, is readily available for use as a driver in the calculations.

The models cited above are based upon ionospheric submodels that may be either simple or complex. But it is important to recognize that the ionospheric submodels are largely based upon the same climatology (i.e., the same set of values for foE, foFl, foF2, M3000F2, etc.) for the same periods of time and circuit geometries. Hence, to first order, the system performance models yield similar, although certainly not analogous, results. Differences derive from how the parameters like signal-to-noise ratio and system reliability are handled, and the differences can be quite significant. Table 4-6 provides some information about selected full performance prediction models. Bare in mind that there are additional less capable models developed for special limited purposes, such as simply computing the maximum and minimum frequencies for a specified circuit (i.e., MUF and LUF). The U.S. Navy MINIMUF program is well-known model of this category. In general, the MUF and LUF information is not sufficient to run a circuit properly, nor understand its idiosyncrasies.

From a space weather perspective, we are mainly interested in the determination of storm-driven departures from climatological predictions. We can hope that we have conducted our analysis properly, and can run VOACAP or REC533 using the proper set of system parameters. Typically we specify a running average sunspot number, and possibly some representation of magnetic activity. With this information, we derive median ionospheric properties and system performance parameters such as the signal strength distribution function and the system reliability.

Table 4-6: HF Communication Performance Models (since 1970)

MODEL NAME

ORIGINATOR

REFERENCE

FTZ

Deutsche Bundespost

Ochs [1970]

CCIR-252-2

CCIR/ITU

CCIR Rpt.252-2 [CCIR, 1970]

CCIR-252-2 (Supplement)

CCIR/ITU

CCIR Rpt.252 Supp [CCIR, 1982]

CCIR-894-1

CCIR/ITU

CCIR Rpt.894 [CCIR, 1986]

HFBC84

WARC/ITU

[ITU-WARC, 1984]

REC533

ITU-R

[ITU-R REC533,1987]

HFMUFES

ITS-Boulder

[Haydonetal., 1976]

IONCAP

ITS-Boulder

Peters et al., 1983]

VOACAP

ITS-Boulder/NRL/VOA

ITS website (see text)

ICEPAC

ITS-Boulder/U.S. Air Force

ITS website (see text)

RADARC

ITS-Boulder/NRL

[Lucas et al„ 1972] [Headrick et al., 1971]

ASAPS

IPS-Australia

[McNamara, 1991]

It is understood that the ionosphere has a recognized level of anticipated variability, even when the ionosphere is undisturbed, and some of this is accounted for in the model. However, the "common" climatological data base (referred to earlier) does not include ionospheric storm data. In short, the manner in which the ionosphere behaves under magnetic storm conditions is not contained in the historical record. Hence, the ability to predict ionospheric response, and HF performance, under storm conditions is typically based upon ad-hoc algorithms or software patches to existing codes. Some methods use imbedded ray tracing, and estimate signal strength through an analysis of ray path density within the coverage area, but these methods can be problematic in a realistic ionosphere. Furthermore there is no guarantee of consistency. A practical solution to the evaluation of large disturbances is to modify certain parameters within the ionospheric submodel, such as the foF2 value. In a number of models this can be accomplished easily. A more pressing problem is how to make climatological models like VOACAP or ICEPAC useful for future operations. To do so, one must have confidence in space weather forecasting algorithms (e.g., the S TORM model discussed in Chapter 3), and a system that is blessed with a degree of resilience (i.e., organic adaptability). For practical reasons, it is more important to have a forecast of ionospheric properties for HF system utilization than a current assessment or nowcast. This is because it takes time for system resources to be reorganized and managed. The good news is that upstream space weather data tends to be correlated with lagged values of pertinent ionospheric factors. Lane [2004] has suggested that VOACAP, with suitable modification, can be used in connection with ALE radios, even in the absence of space weather data.

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