VOACAP Frequently Asked Questions (Revised 22-FEB-96)

VOACAP Quick Guide: Home

[This document is part of the help files integrated into the ITS HFBC software package.]

This FAQ was compiled by the Voice of America and was added here as a convenience to users.

Note: There is probably no one left at VOA who can answer questions about VOACAP (1/26/99).

1. What is VOACAP and why is it better than other models?

VOACAP is a modified version of IONCAP. It is a model that has been under development by the U.S. Government since 1942. The strength of the model is that it uses world maps of ionospheric parameters to construct the ionospheric path and uses path-specific statistics to evaluate the system performance factors. There is no claim that it is better than any other model. "Better" is usually in the eye of the user and is based on how well a model works for a particular application. IONCAP was selected by the VOA in 1985 because it provided the system performance analysis capability we needed for design specifications and it had a proven track record.

2. How does VOACAP differ from IONCAP?

Minor changes were made to the logic to correct for errors which could occur if only one mode was found, the mode was above-the-MUF and the path had 3 or more hops. Signal power values in the working bands predicted by VOACAP differ from IONCAP by more than 3 dB in only 1.5% of the cases. Very large differences can be found at frequencies above-the-MUF and below the LUF.

The LUF and the MUF are the limiting frequencies which define the coverage footprint. The changes made to VOACAP are believed to improve its ability to predict area coverage.

No changes were made to the subroutine in IONCAP that computes the actual value of the maximum usable frequency for the lowest order mode. The values of the system parameters (i.e. signal power, reliability, etc.) may be different.

A math error in the formula that is used to calculate Reliability and Required Power Gain was corrected. This change makes VOACAP less optimistic than IONCAP by about 2 to 4 dB.

A smoothing function was added as a transition between short path and long path predictions. It is activated when the user specifies Method 30 and when the path distance is greater than 7,000 km. The parameter which is smoothed is the signal power distribution. All other parameters which depend up the received signal power are also smoothed.

The most significant differences between VOACAP and IONCAP are in the input and output processors. This version of VOACAP includes the ability to create area coverage maps in B&W or color. Also it has an atlas of city locations, country boundaries and CIRAF zones for international broadcast applications.

In addition VOAAREA, the mapping program, will allow the user to overlay maps and display the sum, difference, greatest value, least value and the signal-to-interference ratio for the case of a wanted signal and one interfering signal.

3. Are the changes to VOACAP documented anywhere?

Yes. All of the recent changes since May 1993, have been documented in the HFMAP Newsletter. A summary of the changes with cross references to the newsletter are contained in PC-VOACAP NEWS which is also posted on this VOA Internet server. The original changes made from 1984 until April 1993 are documented in Broadcast Engineering Technical Report 01-93, "Voice of America Coverage Analysis Program (VOACAP), a Guide to VOACAP." It also provides flow charts for original IONCAP and VOACAP showing the order in which the subroutines are called and for what purpose. This document (PB93-155174) is for sale from the National Technical Information Service, 5285 Port Royal Road Springfield, VA 22161 USA (Tel: +1 703-487-4650) (FAX: +1 703-321-8547).

4. Is VOACAP completed now?

No. We are working on signal-to-interference point-to-point and mapping capability, and possibly providing NEC capability to VOACAP. This work should be completed in 1996.

5. How can I get updated version of VOACAP?

Updated binary versions of VOACAP will be available on FTP.VOA.GOV

Please periodically visit VOA Gopher to check for the updated VOACAP versions. Make sure to read the README file for news concerning with the updated version.

6. Can I obtain source code for VOACAP?

Yes. One can find the FORTRAN source code of VOACAP in the subdirectory named VOACAP. This is only the source code for the DOS propagation calculation model. The source code for the GUI interface (either the DOS or Windows version) cannot be obtained.

7. I loaded VOACAP on my computer and nothing happened. What should I do?

Your computer must contain a math co-processor and 500 free Kilobytes of RAM. You must update the PATH statement in your AUTOEXEC.BAT file, and have ANSI.SYS defined as a device in your CONFIG.SYS file. See the README file for details.

8. Will VOACAP run on any machine? Or just on a PC under DOS?

VOACAP DOS-Version runs only on DOS machines. One can get the FORTRAN source code from the subdirectory named VOACAP, and adapt it to run on any machine. Users attempting this feat are encouraged to thoroughly benchmark results with the PC code.

9. Who should use VOACAP? Is it better than some of the smaller and easier to use prediction programs?

VOACAP is intended for the user wishing to estimate the region of coverage for a frequency, hour and monthly smoothed sunspot number for the period of a month. It is not accurate for a prediction of a single day within the month. There are other models which are more accurate for frequency prediction on a daily basis. They generally take into account solar flares, magnetic disturbances and daily sunspot numbers.

10. I regularly make plots of parameter SNR90 from the area coverage program. What is this new parameter SNRxx, and is it better to use it?

SNR90 was specifically defined as the signal-to-noise ratio at 90% reliability. SNRxx is the SNR at the user's specified required reliability. Now you can make maps at any reliability value you wish. The user is warned that VOACAP behaves strangely when attempting to calculate maps of SNR90. Be sure to use SNRxx.

11. Are the methods that predict HPF, FOT or LUF in VOACAP reliable to use?

You have hit one of the problems of software development. We only benchmarked Methods 13, 14, 15, 20, 21, 22,and 25. When we first started debugging IONCAP we found that changing the Method could result in different predictions. So we concentrated on the above methods and did extensive testing to assure consistency in the predictions.

There are some real logic problems when using terms such as HPF, MUF, FOT and LUF and the IONCAP/VOACAP prediction program. Because IONCAP has an above-the-MUF model, it is possible to have sufficient system power gain to have reliabilities of 90% or higher at frequencies above the MUF. Thus, the FOT may be above the MUF.

Also since IONCAP is a quasi-ray trace model, there may be frequencies that just plain won't propagate between points A and B. For example, one can have a frequency gap between the LUF of the 1F2 and the MUF of the 2F2. Higher frequencies work just fine via the 1F2 mode and lower frequencies are good via the 2F2 mode. But in that frequency gap nothing will propagate. You can see that on oblique ionograms or when using ray trace programs. If one is predicting the LUF, IONCAP may have two or more LUFs.

Because of the confusion with these terms and the way IONCAP makes predictions, we stayed away from any methods that predicted HPF, FOT or LUF. In the process of cleaning up various subroutines and making corrections to a few logic errors in IONCAP, the Methods we ignored began to deviate greatly.

We had wanted to eliminate those methods but others felt we should leave them in just in case some one later wanted to fix them. The Users Manual has a warning statement that we never tested the other methods for correctness.

Although HPF, FOT, MUF and LUF have intuitive meaning to many old time HF users, there is no general definition for these terms that has a mathematical definition. Last time I looked, I found at least 6 different definitions of MUF. For example, in IONCAP the MUF is the estimate of the monthly median junction frequency for the ordinary ray. It is amazingly accurate, I think, based on some experiments we reported in the HFMAP Newsletter. But it certainly is not accurate to the exact hour. There is at least 30 min to an hour of uncertainty in the predictions, especially at the pre-dawn dip.

If you really want to find what VOACAP thinks the HPF, MUF, FOT and LUF are, run Method 20 with lots of frequencies.

Greg Hand at NTIA/ITS has found an error in Subroutine LUFFY which resulted in very strange values of the LUF found in Methods 16 and 28. He has corrected the program error but the predictions of LUF may still be suspect for the reasons mentioned above.

12. What is Service Probability and how can I use it?

The Service Probability calculation in IONCAP and included in VOACAP is a place holder for a future model. It was a first attempt at computing a confidence factor for the reliability value. In other words it is the probability that the hourly reliability value will be achieved over the days of the month. For example, if the reliability for a given circuit at a specified frequency, hour, month and sunspot number is 90 percent and the service probability is 70 percent, then one would expect that the required signal-to-noise ratio will be equalled or exceeded on 27 days of the month on 70 percent of the possible monthly attempts over numerous sunspot cycles.

Although the concept is well founded and useful in life-cycle management of a system, it was based on the best guesses as to what the standard error of estimate is on all of the variables used in calculating reliability. Since the estimates of error are SWAGS, one really can't put much faith in the SERVICE PROBABILITY values. At this point in time some two decades later, we are no closer to knowing what the errors of estimates are on such things as MUF variation, deviation from the predicted absorption, ad infinitum.

A very general rule of thumb for a VOACAP user is to be wary of any predicted reliabilities that fall below 70 percent or whose service probabilities are less than 50 percent. In these cases, the user can assume that the reliability is the "best guess" given the available data as to what will happen on that circuit hour over an undisturbed month. Some questions are better off not being asked.

13. So how good is the reliability value from VOACAP?

It all depends on how carefully you modelled the system and how closely the ionospheric and geomagnetic conditions fall to those assumed for the month. If the antenna patterns reasonably represent the actual antenna and if your assumed required signal-to-noise ratio is really acceptable, then the predicted reliabilities can be achieved for those cases where the general rule of thumb, described above, is applied. Testing at VOA using professional shortwave monitors have shown that the VOACAP predictions are very accurate. Basically, if you design a system and operate it on the correct frequencies, you will achieve or exceed a predicted reliability of 90 percent. That is about the highest reliability you can realistically achieve in the typical HF radio system over a long period of time. Your chances of success drop off rapidly as the predicted reliability falls below the 90 percent range. A reliability of 50 percent might result in a situation where no success is achieved over the month or one in which great success is achieved or anything in between.