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Review of HF propagation analysis & prediction programs

The VOACAP interface.

55 programs at a glance

While listening or planning to work a DX station, the radio amateur would like to know what are the current propagation conditions, know if the propagation is open toward a specific location at a specific time, or at what time the band will be open for a specific point-to-point circuit.

At other occasions, the amateur notes that the band is suddenly dead without apparent reason and open again after a few hours of true blackout. Knowing the whys hidden behind such behaviours can help the amateur in understanding how work the space weather, to help him to prepare his ham activities, specially his search for DX stations.

We will see that tens of parameters can be taken into account to calculate an accurate forecast. With a ham community counting more than one million active hams, these programs are numerous but their accuracy as their complexity are various. 

The most accurate programs are VOACAP-based applications that we are going to review below. They are on-the-air oriented and show an excellent accuracy if you know well the meaning of each parameter and limitations of algorithms. Most of these applications request about 30 input parameters, except for some light programs working with default values and giving an overview of propagation conditions.

Facing to so many parameters, if you have never worked with a VOACAP-based application, you really need to read at least once the documentation provided by the publisher before using the program. A simple error like using the current sunspot number instead of the SSN or setting the reliability or the S/N ratio too low or too high can false all your forecast, and for example predict a poor signal strength and a band closed although it is wide opened with a strong signal !

There are at least two kinds of VOACAP-based applications : the ones running VOACAP algorithms without correction and those, very few, having added new functions to improve its accuracy under some working conditions (e.g. geomagnetic effects at high latitudes, DX over 10,000 km, etc).

 But VOACAP displays some limitations. Currently there is no way to adequately model global irregular variations in the ionosphere on a time scale smaller than a month because random values cannot be predicted. But they can be described statistically or take into account using near-real-time data. 

It is why the VOACAP engine produces monthly medians, deciles, standard deviations, probabilities of service, etc. Such statistics are predictable and accurate although predictions for a particular date and hour are just speculations, using interpolations that are not supported by VOACAP. In addition  no application considers weather conditions or gyro-magnetic effects for top band propagation. Furthermore, VOACAP is still a "trial version" using not less than 30 calculation methods, some of them having never been cross-checked with on-the-air results, and are thus not validated by users and show some errors in some specific conditions of use. In spite of these limitations, the VOACAP engine remains the most accurate and flexible down-sized ionospheric model available for the radio operator.

Do not hesitate to give me your feedback with any additional information.

DOS Programs - Non-VOACAP - VOCAP-based 

Beacon monitors - Web & Research oriented

VOACAP Online - VOACAP - HFWin32 - ICEPAC - DXAtlas - Ham CAP - MultiProp

ACE-HF Pro - WinCAP Wizard - GeoAlert-Extreme Wizard - DXLab - PropMan-2000 - IONPAC - IONPRED

OH6BG,HZ1JW,OH8GLV

Online, freeware

2010-2013

VOACAP Online

VOACAP Online was released in 2010. It has been developed by Jari Perkiömäki (OH6BG), James Watson (HZ1JW) and Juho Juopperi (OH8GLV).

Available for years as engine for other propagation prediction programs listed below, with this version VOACAP it is now available free to all Internet users as an online service.

As explained in my review of VOACAP,  this engine is the most accurate propagation prediction system available on the market with over 50 years of radio propagation experience built in.

The use of the interface is very simple : you drag and drop the location (QTH) of the Transmitter and Receiver (or you encode them), select the day of the year, your antenna and power. Note that the system will automatically take into account the sunspot number (SSN).

In fact, VOACAP uses the smoothed SSN figures which are calculated monthly for many years ahead by NOAA/NGDC as explained in the short history of the Smoothed Sunspot Number. Currently VOACAP Online has included smoothed SSN until 2015 and is working to include the next years.

Based on these data, the program will immediately display a colored "pie-chart" showing the propagation probability (it is a average) calculated for all bands over 24 hours (second graph at left).

If you press "Run prediction", a second page will open to display the circuit reliability in a more standard way, in a time vs.frequency chart with all circuit details for all HF bands (first graph at left).

You can also experiment with different antennas, transmitter power and sunspot numbers.

The VOACAP website provides an illustrated Quick guide, many links and information about the VOACAP engine, including support.

A blog has also been created on which you will find since 2013 the User manual and other information.

See also my review of VOACAP for a better understanding of this engine.

This online version will surely please to all fans of HF propagation, listeners, and licensed amateur radios.

VOACAP Online requires a web browser and an active Internet connexion. It runs on all platforms.

Parameters than can be displayed in charts.

NTIA/ITS

Freeware

VOACAP

It is an enhanced IONCAP model which methodology was modified by USIA/VOA to be used for broadcast relay station design and antenna specification. Most ionospheric functions were devised and corrected by George Lane of VOA. The model was then completed and freely distributed at the Ionospheric Effects Symposium in May 1993. To enhance the user interface VOA funded the NTIA/ITS after Frank Rhodes  from the U.S. Naval Research Laboratory enhanced the model. The last implementations including the VOAAREA module where made by Gregory R.Hand from ITS who maintains currently the product until 2005. There is however no plans to modify VOACAP further.

A recently published U.S. Department of Defense ALE guidebook lists VOACAP as the prediction model of choice and it remains today the worldwide gold standard of propagation models.

Indeed, VOACAP is the most powerful, flexible and complete product predicting the ionospheric propagation (only HF) to date. Forecasts are established only after have considered all a communication circuit from the transmitter power and antenna gain to the QRM at the receive location, what represents not less than 30 parameters to input. Complex to master at first sight, it provides however many functions taking into account the signal quality like the circuit required reliability (SNRxx), the S/N reliability (SNR) and tens of other parameters.

VOACAP shows however some limitations. First, it works only with median values like SSN and provide no real-time updates. It doesn't use A or K index, and thus predictions do not take into account some important effects like short-term disturbances or the aurora oval on short-wave bending at high latitudes. It uses approximations for the F2-layer and calculating the MUF. Its predictions must be interpreted with care at short-terms (e.g. set up the reliability is mandatory) and are also almost useless for the top band because it ignores weather influences, the sunrise/sunset effect and the gyro-frequency of ionospheric electron (important near 1.8 MHz). In fact VOACAP is designed for automatic modeling under normal, undisturbed conditions. Hopefully some competitors using this engine are working on these improvements.

On another side we must say that using median values like SSN can be considered as an advantage as well. I explain. The SNR Distribution tables and Excess Gain tables that are internal to the model were conceived and calibrated from many user 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.  

VOACAP has been released for all Windows plateforms, 16 or 32-bits. The latest version is "HFWIN32" (see above). The VOA FTP website where the program could be downloaded is no more available for years. VOACAP can also be downloaded from OH6BG's website who provides the complete engine as well as a quick online guide. A more complete user guide written by George Lane when is was under contract with Rockwell Collins is also available but it is very expensive ($60).

VOACAP comes with ICEPAC and REC533 the two main models supported, completed with VOAAREA, HFANT, S_I_VOACAP, and some additional tools. Read my review.

For Windows 95/98/NT/2000/XP/ME.

Freeware. The last version is always supported on an email basis.

NTIA/ITS

Freeware

HFWin32

It is the 32-bit version of the original VOACAP model. It offers thus the same interface, same functionalities,  and show the same accuracy, and same limitations. It is also provided with the same additional models (ICEPAC, REC533, VOAAREA, HFANT, S_I_VOACAP, etc). 

You must use this version if you work on a Windows 32-bit platform and select one of the previous version only if you are desperating to find the last version.

For Windows 95/98/NT/2000/XP/ME. Read my review of VOACAP.

Freeware. The last version is always supported by NTIA/ITS on an email basis.

NTIA/ITS

Freeware

ICEPAC

It is an enhanced IONCAP model developed by NTIA/ITS. It shows thus the same functionalities, accuracies and limitations as VOACAP and shares the same data as well. Like VOACAP it is almost useless to predict the propagation for the top band, or for a specific day of the month as it works with median values only. However, using interpolations, it can be used for hour-to-hour or day-to-day operations at the condition as states the manual that "the user exercises his own engineering judgment in determining the applicability and limitation of the results to specific problems".

Conversely to VOACAP, ICEPAC includes the ICED (ionospheric conductivity and electron density) profile model. It is a statistical model that recognize the different physical processes that exist in the large-scale features of the northern hemisphere ionosphere. It contains for example 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.

ICEPAC was no as extensively validated as VOACAP and it doesn't take into account a smoothing function between the hop and forward-scatter regions, a feature used in VOACAP. Therefore, in some circumstances, ICEPAC shows discontinuity of nearly 10 dB at mid-path, causing artificial SNR losses exceeding 20 dB. On another side, ICEPAC provides some additional iso-contours maps not available in VOACAP like the SRNxx (Method 20).  Results are more accurate and the graph also more complete.

ICEPAC is installed in the same time as VOACAP and other VOAAREA.

For Windows 95/98/NT/2000/XP/ME. Freeware.

Menuing superimposed on the global ionospheric conditions map (MUF)

The current auroral oval displayed is cartesian projection. HamCAP shows the propagation on 20m at the same time.

The current auroral oval displayed is spherical projection. HamCAP shows the propagation conditions on 40m at the same time.

Propagation conditions on 20m in cartesian (HamCAP) and spherical (DXAtlas) projections.

VE3NEA

Shareware, $29.95

DXAtlas

DXAtlas is often known for what it was in its first version, a superb geographical atlas showing DX prefixes assigned to each entity and territory completed with additional information (list of cities, islands, gray line map, topographical map, pins, grid locator, etc).

From version 2.24 released in 2004, Alex Shovkoplyas, VE3NEA, interfaced this fine atlas with Ham CAP (see below) to use the power of the VOACAP engine and estime propagation conditions at a global scale. In option it is also interfaced with IonoProbe, a small tool monitoring in real-time space weather conditions. Each of these products can run in stand-alone if necessary, but it is obviously by far preferable to use DXAtlas in conjunction with both "add-ons"; it becomes then a very powerful tool.

Today, DXAtlas with Ham CAP and IonoProbe is one of the very seldom package to take advantage of the IRI-2001 ionospheric model, a long awaiting solution that is known to provide accurate forecasts, of course in the limitations of the VOACAP engine to handle global irregular variations in the ionosphere.

This accuracy appears not only at long but also at short-terms as well. In this configuration DXAtlas takes into account additional parameters and models most of the time ignored by the other applications. DXAtlas does not use Kp (or Q or the Storm model) but it does much better ! In tandem with IonoProbe, DXAtlas uses direct, real time ionosonde measurements of the F layer critical frequency to produce real-time ionospheric maps ! That means that instead of trying to guess how Kp influences foF2 (Storm model), Alex just uses the foF2 itself, and therefore DXAtlas is able to model ionospheric disturbances more accurately.

DXAtlas can estimate the foF2 distribution and thus display the foF2 but also the F2-layer height in km, the MUF, auroral oval (with the power flux P in W/m2/sec), geomagnetic storms, D-layer peak density, geomagnetic latitude (normal and corrected), and magnetic dip (normal, modified and latitude), all this at four resolutions. In addition, these ionospheric maps can be displayed in cartesian, equi-distant (azimuthal) or spherical projection.

Parallel to the "ionospheric mode", the standard operating mode of DXAtlas is always available clicking on the menu icons; maps can be displayed with or without prefixes or grid, you can zoom in the map (without interpolation) or switch to the topographical map.

At last, between us, know that Alex counts among the few experts who have "put the finger" on some bugs or errors in the IRI-2001 model and incorporated the fixes in DXAtlas... So, if you need a quasi bug-free down-sized "IRI-2001compliant" program, DXAtlas must be yours, Hi ! But of course it will never help you to predict propagation conditions for a complete circuit. As it does not permit to access to all output parameters of VOACAP, I had to remove its 5th star, but I gave it another one for its accuracy in limitations of its capabilities. Really, a must you if are serious in amateur radio !

Requires Ham CAP and IonoProbe in option. For all Windows 32-bit platforms.

DXAtlas and IonoProbe are shareware to register, respectively at $29.95 and $20 with a 30-day-free-trial period.

Ham CAP forecast (without DXAtlas or Ionoprobe).

Ham CAP antenna radiation pattern.

VE3NEA

Freeware

Ham CAP

This program uses the VOACAP engine and works optionally with DX Atlas version 2.24 or higher and IonoProbe, both shareware also developed by Alex Shovkoplyas, VE3NEA  (see above).

When we say that "small is beautiful", this expression might apply to this point-to-point prediction tool that displays its results in a small windows of 383x288 pixels. 

The main purpose of Ham Cap is to let the ham operator see at a glance the trends of propagation while he or she is on the air, with a minimal distraction from the transceiver.

Ham CAP takes into account the smoothed sunspots number (SSN), and, like VOACAP, doesn't use the geomagnetic indices by default. However the K-index can be taken into account in option. Ham CAP requests also the date and month, home and target locations, transmitter power, and antenna gain (15 models from the isotropic to Yagi). A short engine permits to display the radiation pattern of the selected antenna and see how change its radiation pattern with the frequency, a very instructive feature using an user-friendly GUI. 

Results are displayed either in a chart showing the S/N variation as a function of time over the specified path or on a global ionospheric world map, with the gray line and the MUF in option. The signal strength can be displayed in a gray scale, black and white or using pseudo-colors at three resolution levels. Working with monthly values, that means that its algorithms use median values only and its forecasts are thus more optimistic than the ones provided by more complete software taking into account real-time data or the circuit required reliability. However, it deserves a try for its global ionospheric map, its antenna charts and its small size.

Freeware. Requires VOACAP. For all Windows 32-bit platforms.

AA6YQ

Freeware

DXLab

Written by David Bernstein, AA6YQ, DXLab is a suite of interoperating applications constituted of DXView, PropView, SpotCollector, DXKeeper, PathFinder, and Commander.

The PropView module uses a reduced set of the IONCAP engine (see below) to predict MUF and LUF along a single circuit. It uses SFI, A- and K-indices from up to 6 DX clusters thanks to SpotCollector. The target location can be specified in several ways : by simply entering the DX station's callsign, by clicking on an incoming DX spot, as well as by entering lat/long., entering a grid square, an IOTA tag, or clicking on a location on the world map tat calls DXView. This latter module is specialized in DXCC info, beam heading, sun position calculations and is able to control antenna rotators. It also supports "DX Atlas" by VE3NEA but does not share information with it.

PropView displays solar and geomagnetic curves over the last 30 days and shows the bands open over a 24-hour period chart, with the option to display critical frequencies. In addition the auroral oval is displayed on a world map as a function of the K-index, along with the strength of your signal what permits to estimate the likely degree of auroral interaction. It displays also the solar terminator and calculates sunrise/sunset times for any location over any 30-day interval. 

At last the beacon functionality is not a simulation like do many other programs. PropView controls your transceiver and QSY each time as required to monitor each beacon in sequence, providing a very accurate assessment of current propagation conditions.

Last but not least DXLab can establish an arbitrary monitoring schedule and rotate an antenna to follow that schedule.

Version 1.4.4 now offers the option of using the ICEPAC or VOACAP programs as "forecasting engines" as well as IONCAP.

Freeware. For All Windows 32-bit platforms but ME.

 

AC6LA

Shareware, $39

MultiProp

This is an impressive package that includes an very interesting propagation prediction program using together the flexibility and power of MultiNEC, an antenna modeling program and the VOACAP engine. 

Taking at best advantage of both programs, its accuracy is similar to the one of these applications. However, using Excel worksheets and macros, the encoding of all parameters of an antenna is complex and not easy to master in a few keystrokes. To help you a free illustrated user manual is available in PDF format (62 pages very well written that deserve a reading) of the author's website.

The accuracy of radiation patterns calculated by the VOACAP engine are limited to a grid size of 61x61 (vs. 361x361 for VOAAREA). But even compared to results calculated at the same scale by VOAAREA, MultiProp results show a lower resolution (by step of 12 dBW vs. 5 dBW in the iso-contour map, not important, a little more than 1 S-unit, but visible on the map).

Requests MS-Excel, MultiNEC and VOACAP.

Shareware. For all Windows 32-bit platforms.

ACE-HF

License, $99

ACE-HF Pro

ACE stands for "Animated Communications Effectiveness", a coverage display technique originally developed for U.S. Navy submarine communications.

Written for Windows 32-bit by Richard Buckner, this program uses GeoClock maps, a format that has been choosen in order to match the displays with ACE-VLF used by military clients.

ACE-HF Pro uses the VOACAP engine without modification of algorithms (thus with the same accuracy and limitations), and can be interfaced with GeoClock and a logging software. It can simultaneously calculate several circuits, each using up to five antennas at each end of each circuit, an option asked by contesters using "antenna farms" on as many as five bands. Some data are extracted from external files like antennas and locations that list 35000 cities world-wide and calls several other programs. Representing about 50 MB on disk, 500 files and 50 directories, it is thus not available for demo purposes due to its size. 

Its main originality over all other VOACAP-based programs is to display animated circuit graphs of SNR, reliability, required power gain, takeoff angle and signal strength. To achieve this, ACE-HF Pro employs cubic spline interpolation to produce chart values at five-minute intervals in order to better describe what happens in the real world. Of course these intermediate values show less precision that hourly predictions. Several charts can also be displayed, to name the MUF (with HPF and FOT), Best Frequency, and an SNR or reliability summary chart.

The input screens have been redesigned since version 1.0 and look great even if they show some "adaptations" due to the underlying Delphi engine that scales differently according to resolutions and operating systems. The world map is fully graphical, using EGA or BMP image which colors are a little too bright due to the use of GeoClock, ACE' standard interface.

Among additional tools, ACE-HF provides HFANT to model antennas, a beacon monitoring showing the strength of their signal and reliability, and an interface for NEC-Win Plus from Arraysolutions. 

The product in without any doubt as powerful as its competitors and maybe more flexible. Indeed, on request, right-clicking on the mouse most parameters can be displayed in a pop-up window and changed without having to enter in each submenu of the circuit or only partially.

The program comes with an extended help file, very complete and sorted by feature, but it is non contextual (you cannot press F1 and get help on a field) and you need to browse the document to find a specific item. It should be converted in Microsoft HLP format and fully indexed to be really useful as not all users will take the time to read its 46 pages, even if they are very educative and their reading highly recommended. In this context the narrative mode can help the casual amateur.

ACE-HF Pro runs on all Windows 32-bit plateforms and is now at version 2.04. It is provided on CD-ROM (with VOACAP) and a 3.5" floppy disk containing the transmitter location file (your license) that will be copied onto disk during the install procedure.

Compared to equivalent functions available to its competitors and its GUI, in my humble opinion the high price of ACE-HF Pro is no more justified, but exceptionally it keeps its four stars for its excellent animation using GeoClock map, and the great number of users, most professionals that trust in this product and the other prediction programs released by the publisher.

No demo available due to its size. However, an extensive "Take the tour" section is provided on the website as well as free and very instructive propagation tutorials written by Dick Buckner and George Lane.

License. Comes with VOACAP. For all Windows 32-bit platforms.

Kangaroo Tabor Soft.

License, $50

WinCAP Wizard

It is a enhanced graphic user interface (windows) of the famous CAPMAN which is no more supported.

WinCAP Wizard doesn't use the IRI model but rather the VOACAP engine, an improved IONCAP model developed by VOA. Like most VOACAP-based applications, it displays thus some approximations (F2, MUF, etc) and ignores some parameters that affect propagation (no Kp-index thus neither the PCA or the auroral oval is considered at high latitudes, nor magneto-ionic effect, sunrise/sunset or weather conditions for the top band, etc).

This is however a powerful and flexible tool but it requests some habits to be mastered (or to read my introduction to such models) due to its numerous settings. WinCAP provides HFpropagation forecasts taking into account a complete circuit, and even multiple circuits if necessary like all serious VOACAP-based application.

Like VOACAP there are not less than 30 inputs to set to get an accurate forecast for a single point-to-point circuit. Hopefully, like in VOACAP these inputs are grouped in several windows or groups (System, User, Receive system, Month, SSN, Xmtr Antennas, and Frequencies). 

WinCAP Wizard displays up to 7 charts partly customizable displaying MUF, Best Frequency, TX antenna takeoff angle, SNR, Reliability (SNRxx), and other signal strength at receive location. It can generate propagation charts or text reports for up to 18 locations anywhere in the world simultaneously (circuits). Its user batch manager permits also to replicate in one pass a modification common to various circuits. All windows are static excepting the world map on which you can clic to get a point-to-point prediction This map is also used to display NCDXF/IARU beacons and their relative power. 

The new version 5 includes "smart charts" and other quick chart to display propagation of beacons.

Today WinCAP Wizard comes free with GeoAlert-Extreme Wizard (see above) and Beacom-Time Wizard that complete the program with very interesting dynamic features.

Requires VOACAP (provided with the product if necessary) and an active Internet connexion if GeoAlert-Extreme Wizard is installed.

For all Windows 32-bit platforms. Read my review.

License or QSL-ware. Today at version 4.1. 60-day-fully-functional free trial.

With the passing of Jim Tabor in 2010, Taborsoft products are no longer supported.

Kangaroo Tabor Soft.

License, $20

GeoAlert-Extreme Wizard

The first version of this program was released in 2004 and replaced GeoAlert Wizard. In the line of its predecessor, it displays updated geophysical indices in a dashboard (showing SSN, SFI, Kp, Ap, solar wind Bz component, pressure, speed, etc) and various user-friendly charts related to solar and geomagnetic indices extending up to 24 years.

This program is stand-alone but requests the VOACAP engine and a Internet connexion to get online updates. It can be interfaced and comes free with WinCAP Wizard 3 or higher (see below).

Unlike the previous version, this new version takes advantage of the power and accuracy of VOACAP to display a world map showing the ionosphere status at earth scale, the MUF, rather than a point-to-point prediction. Its "Squares" option is a major improvement over the previous release. It is very powerful but at first run (and only once) it requests over 30 minutes to calculate all predictions on a 1 GHz computer. Why so long ? Because it generates a world map valid for all a month and all hours and take into account a possible zoom in the map. You must see it to believe it !

This map can be displayed in various forms (overview in blue scale at various resolution, colored per amateur band or for contest purposes). Its resolution changes according your amplification (zoom) factor, placing W6ELPro, Ham CAP, HFProp and other applications showing the same map now far behind its performances.
This application is first designed to get an overview of propagation. You cannot for example get the SNR or dBW on a target location clicking on the map yet. Therefore I don't give it four stars. But Jim is working on other products that will probably support this feature and many others.

Requires VOACAP and an active Internet connexion. For all Windows 32-bit platforms.

License or QSL-ware. Today at version 3.3.25. 60-day-fully-functional trial. Comes free with WinCAP Wizard. 

With the passing of Jim Tabor in 2010, Taborsoft products are no longer supported.

Rockwell Collins

License, $99

PropMan-2000

Ionospheric prediction program based on the VOACAP engine. It has been developed for the U.S. Government and can be interfaced with various external communication devices, most property of the Army. 

However, this application doesn't provide better predictions than the original VOACAP and it is much more expensive and less flexible than its competitors. Therefore I removed it one star for its untenable marketing position.

Requires VOACAP. For Windows 95/98/NT. Demo available

VOACAP Origins

George Lane and al.

No longer supported

IONCAP

This is the first professional ionospheric signal model down-sized for personal computers. It is based of HF ionospheric studies made between 1975 and 1978 by John Lloyd, George Haydon and Donald Lucas who developed an interface for the Army called the "Ionospheric Communications Analysis and Prediction" program, IONCAP for short. 

In 1983, George Lane then Teters and al. from the Voice of America modified the code for the broadcasting needs of VOA (see VOACAP above).

IONCAP uses a F-layer algorithm, has methods for calculating MUFs, and it deals with the range of values of critical frequencies resulting from the statistical variations in the sounding CCIR data.

IONCAP program has many other methods beside FOT-MUF-HPF and some give long-term availability figures, the fraction of a month the path would be open, as well as the S/N ratio and reliability. Thus, in contrast to Fricker's method which is based only on F-region considerations, IONCAP deals with fluctuations of signal strength, a D-region factor, as well as man-made noise.

IONCAP is no more supported as it "mutated" in VOACAP and ICEPAC.

Raymond Fricker

No longer supported

IONPRED

In the '80s Raymond Fricker from the BBC Worldservice created this complex propagation program that included a new scheme of hop-testing. Essentially, the program looked at each hop in detail, at the points where the E-layer is crossed and at the highest point where the critical frequency of the F-region is important. So the hop-testing involves determining whether the mode is reliable by seeing if operating frequency is above or below the E-cutoff frequency by 5% and less than the critical frequency for F-region propagation by 5%.

With an initial choice of radiation angle, the path structure can be sorted according to E- and F-hops, depending on the outcome of the tests along the way. Fricker also adjusted the height of the F-region according to local time so hop lengths are not constant along a path. As a result, the path could over- or under-shoot the target QTH. If the error is more than 25 km, another radiation angle is chosen and the process starts again.

IONPRED also calculates the ionospheric absorption expressed in dB, and adds that to the signal loss due to spatial spreading or attenuation and ground reflections. At last IONPRED uses the availability of the path, the number of days of the month that it would be open for reliable communication (a kind of FOT-MUF-HPF). The number of days is treated as a continuous variable in contrast to the upper or lower decile approach with the FOT-MUF-HPF method. However, in the '80s Fricker's method was time-consuming, to say the least.

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