interface, the "golden standard" in HF propagation prediction matter.
propagation analysis and prediction program (I)
Foreword. This review was written in 2004 (with updates until
today) at a time where NTIA/ITS provided (and continue to do) an executable version of the VOACAP
engine that you can downloaded from Greg Hand's
website. We will see at the end of last
page that since 2010 an online version is available with new
functionalities, preventing users the tedious task to select the appropriate
model and entering all data manually.
However, the web interface is based on the same original engine that
everybody can download if needed, and which review of menu items and paramaters
is always interesting to know to well understand what are its performances but
also its limitations. Therefore I keep this full review online.
who prefer to test the online version without background can connect
to VOACAP online.
VOACAP stands for the Voice of America Coverage Analysis
Program. It is an ionospheric model predicting the expected performance of HF transmissions. It takes
into account tens of parameters to support you in the planning and operation of long distance amateur
traffic or broadcast transmissions.
to the online version which is much more complete, the
original VOACAP (classic) that we are going to review is a "Point-to-Point"
analysis tool. Point-to-Point means that the propagation
is calculated not for a general coverage over a location or to provide a global
status of the ionosphere, but rather for a specified path or circuit between a transmitter and a receiver.
is based on the Ionospheric Communications Analysis and Prediction Program
(IONCAP model), a famous scientific product released in the '80s by the U.S. National
Telecommunication and Information Administration and free of rights.
history being associated to many researches and developments, if you discover
the product for the first time it is not always easy to understand who's who,
what are those institutions and acrynoms listed, and all these additional models
that complete the product. Therefore a review could not be complete without
introducing VOACAP in telling how all that began, because it's a long story, all
the more interesting that the program is accessible to radio amateurs for many
years, and is still considered by many experts as being the
"rolls" and the "golden standard" of propagation programs.
Back in time
the years 1920s and 1930s amateurs
and professionals discovered HF and DX communications, and SSB.
In the late '30s, many organizations were involved in the
study of HF communications. A considerable
effort was made in the U.S.A. and other countries to investigate
ionospheric parameters and determine their effect on radio waves and
the associated reliability of HF circuits.
Soon a worldwide network of vertical incidence sounders was
established to measure values of parameters such as critical
frequencies foE, foF1, foEs, foF2, and h'F. After the first
two-way commercial transatlantic transmissions, operators noted the
influence of the sun on daytime propagation conditions. In addition,
worldwide noise measurement records were started to understand what
phenomenon disturbed these communications, and
steps were taken to record observed variations in signal amplitudes
over various HF paths. These analysis and researches extended over half a century up to
recently include soundings and satellite measurements. Of course
these studies are always pending. Thanks to these data recorded at
all levels of the ionosphere (50 to 800 km or so) and under all
working conditions, scientists have got a better view of propagation and how the
different parameters interacted each others.
the tens of discoveries made during these experiences like Van Allen belts, the South Atlantic
Anomaly or sporadic plasma clouds, sientists have discovered that HF system performance is
related in a very complex manner to solar activity, time of the day, season, and last but not
least to the details of the shortwave path, like the ground properties and noise. Thanks to
the first mainframes and their famous punch cards, scientists translated these discoveries in
equations and developed the first ionospheric models.
IONCAP to VOACAP
the Institute for Telecommunication Sciences of the National Telecommunications and Information
Administration (NTIA/ITS) released
for the U.S. Army a program developed by George Haydon, John Lloyd
and Donald Lucas called the "Ionospheric Communications Analysis and Prediction"
program, IONCAP for short. Written in Fortran77 for mainframes, it was
designed in modular format to allow more flexibility for development
of models for the key parts of the program, as many extensions that we should call today
"subroutines" or "add-ons". Earlier much of the analysis
had to be handled manually, a very time consuming process. Thanks to
IONCAP scientists and engineers could get result in a few minutes
with a greater accuracy.
large scale ionospheric data available in IONCAP were collected
during the International Geophysical Year (IGY) between 1958 and
1963. Data were then reduced to look up tables
which were valid for 2 or 3 month periods at the even hour. These
maps were dedicated to the estimation of critical frequencies foE,
foF1, foF2, as well as to the F-days distribution, M-3000 (for the
MUF) and excess system loss tables. In addition, radio noise was
mapped in 3 month groups and 4 hours time interval blocks, the
original data of the an hour basis being lost, remain
way to create a model using averaged values was subject to many
comments until it was decided by George Lane from the Voice of America broadcasting company,
and the Signal Corps Radio Propagation Agency engineers to
force the program to give a smooth transition across the days of the
month. This is in this way, using statistical values, which accuracy
is no more to demonstrate, that the concept of reliability is born.
IONCAP could quite accurately calculate the distribution of the
day-to-day variation and forecast for example that such conditions
'd be met three days at a specific hour during the month. But it was
unable to define when will they occur during the month.
parallel John Wang at the FCC developed a medium wave prediction
model suited for long distance paths into darkness that was adopted
by the ITU-R. However, IONCAP and Wang's model using different
semi-empirical data set and functions, from a pure statistical point
of view, both models were not compatible and both theories were
never merged together.
was probably the best studied propagation program and,
until the late 90's was arguably considered by some engineers as the best basis for judging other programs. However the source code has
evolved but the concept is stayed the same.
1983, George Lane cleaned up and corrected IONCAP to retain
all of the theory as put forth by Lloyd, Haydon and Lucas >and
developed a new model for the account of VOA. Two years later VOA adopted
the new version of IONCAP as the approved engineering model to be used for
broadcast relay station design and antenna specification.
VOA funded the U.S. Naval Research Laboratory (NRL) to make specific changes
to the IONCAP methodology, and renamed it, as expected, to VOACAP so as to avoid
confusion. That version of VOACAP was completed in April 1993 and distributed to
participants at IES 93 (Ionospheric Effects Symposium May 1993, Alexandria, VA., USA).
Simultaneous to funding NRL to enhance the model, VOA also funded the NTIA/ITS to
enhance the user interface. A few months later NTIA/ITS had converted the program
in a DOS-based application.
Microsoft released Windows 95, on May 15,1996, VOA released the first
official Windows 16-bit version, also called VOACAP. All previous
released were Beta test versions. The next year the Windows NT 32-bit version
was released. Today OACAP is at release 04.0324W and runs on all Windows 32-bit plateforms.
years the development of this program was an endless project and new
versions were released from time to time. Unfortunately, today all future development
of this software will be limited to the Windows version and currently there
is however no plan to modify VOACAP further.
comes thus in several versions under different names : VOACAP, VOAWIN and
HFWIN32, this latter being dedicated to Windows 32-bit plateforms
although previous versions work fine in these environments as well.
Actually VOACAP and
alike (e.g. WinCAP Wizard, ACE-HF, Ham CAP, etc) are simple graphic user interfaces,
GUI or shells as say programmers, added to create a user-friendly layout between you and the
VOACAP engine that processes data.
the meantime, as expected, additional models have been added to the
VOACAP engine like ICEPAC, S_I_VOACAP, VOAAREA, REC533, HFANT, etc,
as many very accurate models that deserve your attention and that we
will review later.
parallel geophysicists have worked on a more complete ionospheric model
called the International Reference Ionosphere (IRI) from which have been
extracted several specialized models like the F2-peak model that we often
find in many amateur propagation programs.
the VOACAP engine is used by tens of programs that have tried to
extend the user interface. You will find it to HF broadcasters like
VOA, BBC, ITU, HFCC, military, defense contractors, airliners, users of
HF Email – Sailmail / Airmail, BPL and at least in the radio amateurs' ham shack.
close our short history of VOACAP. Now it's time to speak more
Download, support and installation
As of October 1998, NTIA/ITS
does no longer accept funds to register VOACAP. As they quote on their website,
that means that the program is only available to users thanks to an
executable to download via the Internet.
NTIA/ITS provides also information and the program (engine) at respectively
these both addresses : input
screens and outputs and the Windows
If you require a software
change, in the past there was a way to fund ITS to develop your
project but nobody took advantage of this opportunity.
stated in a popup menu that opens just after installation and in the
readme.txt, Greg Hand who handled the officious support retired :
"In February 2005, I have retired from the US Government. Questions, comments, suggestions may be addressed
to Greg Hand E-Mail:email@example.com. Responses will depend on how much fun I am
short, if since 1998 there was no more warranty of results and accuracy,
today the sole support to expect for the Windows version is the user assistance
given by Christopher Behm from ITS through email, and
it can only be done on a
if you want a change, knowing that Jari
Perkiömäki, OH6BG/OG6G, and his team have developed the web
interface, before any action contact him first.
The latest Windows version
of VOACAP "classic" is
named "HFWIN32" and the final DOS version should only be used
if you are really desperate and can't find another more recent version, HI!
The latest fix for the Windows 32-bit version was incorporated in
main files packaged with the VOACAP engine. VOACAP,
ICEPAC and REC533 are the main input/ouput for the
Point-to-Point models. In addition HFANT permits to
explore radiation patterns of any predefined or custom
installation process is intuitive and very fast. After you downloaded the
4.4 MB EXE file, run it to install it in a specific directory. The
program setup will automatically start. During the process it calls the Tarma Installer to
prevent problems with creating the desktop icons on Windows systems.
The program requires also to be installed in a NON long file name
directory and without blank space, thus not under "\Program Files" or any
other folder using more than 8 characters otherwise some modules will not
start (thus use e.g. \itshfbc or \voacap).
the application is saved under a folder name \itshfbc. If you open this small folder you
will find 17 subdirectories gathering 12 MB of data among them
VOACAP and additional models ICEPAC, REC533, VOAAREA, HFANT, etc. If you need
free space and only use the VOACAP module, you can delete some
data set files (e.g. additional antennas, extra locations, etc),
including many samples and output files that you will never use. But let
them there for the moment.
some approximations that have been corrected,
VOACAP does not differ much from the IONCAP interface developed by John
Lloyd in 1978, and inherited thus of most of its drawbacks but also of its
flexibility and power. Like in IONCAP, there are various models to estimate the absorption, noise level and
reliability of a service (mode) at some frequencies.
not only with statistics of F-layer ionization (e.g. through the MUF
and the like), but also down lower where absorption and noise
have their origin. So, like IONCAP, VOACAP has F-region methods which
give not only the availability of a path, the fraction of days in a
month it is open on a given frequency, but also D-region methods
which give the reliability of a mode, the fraction of time the
signal/noise ratio exceeds the minimum required for the mode.
interesting heritage is for example the antenna analysis program,
HFANT. This external module is used by VOACAP each time that the path, the gain of the antenna
and the takeoff angle of the specified signal need to be
considered. With time, from a simple module able to process simple isotropic
vertical or various dipole designs, it became a complete program
able to take into account many antenna geometries from the longwire
to the beam and it can compete against other programs offering more complex
functions. In addition HFANT allows you to create your own antenna
design and to check its properties in injecting its parameters into
Where IONCAP program is
still more powerful is when you call the ionospheric parameters
subroutine (the first calculation method). Explicit electron density profiles
are not calculated by mathematical approximations but rather in the form of look
up tables, thus using predefined data. These tables gives excellent results and,
better, increase the computation speed for very complex systems,
allowing the use of alternate ionospheric models in VOACAP.
IONCAP which models involve both paths longer than 10000 km and a
"single hop" method for distance less than 3000 km, VOACAP
uses the same functions to consider all possible ray paths for the specified
circuit. However, in the specific case of DX where extension of paths require three or
more hops (over 5 to 6000 km away depending on solar conditions), the system
concludes that a path is available, which is far to be the case if
we take into account the various reflection modes and degree of
ionization of each layer. Therefore VOACAP uses a correction for these multi-hop paths longer
than 10000 km, often important for DXing. It achieves this using Method 21.
another side, if a propagation path does exist in a circuit, it has been shown
empirically that usually these circuits are dominated by what we call "control areas"
which are the regions within 2000 km around each end of the circuit (transmitter, receiver). At
the transmitter location the control area allows the sky wave to do its first
hop up to about 2000 km away, while the control area at the receive location allows the
remaining signal to be returned after the last hop. In between, the path is simply characterized by
a simple loss per distance statistical function, the noise and signal statistics
being the same for all paths. If this approximation gives good results, we must recognize that the
VOACAP model is more complete and thus more accurate for shorter
paths, say below 5000 km or 3 hops.
IONCAP that divided the year into twelve months without considering days, VOACAP
uses a decimal integer (e.g. 25 December is 12.25 instead of simply
12 in the previous model). The improvement avoids rough average and
possible inaccuracies using data of the current and the next month together. But IONCAP used
also two additonal coefficient tables for the ionosphere, CCIR/Oslo
and URSI/88 to get more accurate predictions.
developers corrected the predicted values using the CCIR database by
using Transmission Loss tables and determined the variability of the
MUF over the month using the F-tables. These corrections were
important because they are based on predictions calculated with CCIR
data and substracting them from the performance recorded between
1958-63. Differences were collected on these two sets of databases.
The changes apply to output parameters calculating the signal power,
SNR, reliability and the required power gain. But currently, if you
use a model considering only the F-region, and do not recalibrate
the predictions against measurement, nobody call tell if the
correlation is maintained or not. You can made an improvement or
ruin your prediction. VOACAP inheritated of both coefficients tables
but they don't work in a similar way. CCIR was calibrated to work
with the month only while URSI/88 requests the month and the day to
establish a prediction. URSI provides also a linear fit between the SSN calculated during the
minimum and the maximum or the solar cycle, and consider
data over the oceans as well. But VOACAP doesn't support the day-of-the-month
entry because URSI coefficients have not been validated during the
development phase. This is one of the major error that might do
users when they request a forecast. We will come back of
smoothed sunspot number (SSN) is also used but, under specific conditions, it can
be replaced by updated daily value instead of using monthly smoothed
average. However, one must say and underline twice that when the sun activity is
quiet with a low SSN, its value doesn't matter much, while near
the paroxysm of its activity, when the sun is very active, differences can be large and
conduct to false predictions of signal strength and circuit
In fact using median values like SSN must be considered as an advantage
rather than a drawback. If we take the SNR Distribution tables or
Excess Gain tables for example that are internal to the model, they
were conceived and calibrated by VOA engineers for years from many
listeners reports against a wide range of solar and geomagnetic
conditions and at the higher statistical levels (higher required
reliabilities, SNRxx). This is for these reasons that any attempt to
enter daily sunspot numbers for example instead of the SSN can cause
inaccuracies in VOACAP predictions. Therefore ITS recommend to use SSN
- and specially the one from NGDC - and no other "equivalent"
solar indice. We will come back on this important subject later too.
of the past, VOACAP does not
either include input for planetary indices (A- and K-indices). This
lack gives rise to inaccuracies in disturbed conditions, mainly at
high latitudes as well as for top band predictions where geomagnetic
and gyro-frequencies effects are not taken into account. However some newer
interface do include corrections for the geomagnetic effect when high latitude paths are
involved (e.g. ICED model used in ICEPAC) but indices are not
explicitely defined and predictions are sometimes less conform to
the reality than using VOACAP. Only some VOACAP-based competitors ask
explicitely for geomagnetic indices.
last, heritage of the first Fortran program using punch cards, like
in IONCAP, when you request a report, VOACAP as well as ICEPAC and
some VOACAP-based competitors output on screen an austere text file
as long as an old listing. But as long as charts are available and can
be printed, these reports become almost obsolete. Hopefully,
conversely to IONCAP, the input text file that mimicked the punch
card format has been replaced by a user-friendly interface that we
are going to detail in the next pages.
that we know VOACAP's origins and heritage, come back to the
present, and launch the program, Enter.
Main menu and settings