Excerpts from Original ICEPAC User's Manual


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


The Institute for Telecommunication Sciences and its predecessors in the U.S. Department of Commerce have been collecting ionospheric data and developing methods to use these data in the prediction of the expected performance of high-frequency (HF) sky-wave systems since the start of World War II.

Much of these data and the techniques for using the data are stored for use by computers. Several "standard" output formats have also emerged to assist in the planning and operation of high-frequency systems using sky-waves. This report describes the use of the latest developed method - The "Ionospheric Communications Enhanced Profile Analysis and Circuit Prediction Program" (ICEPAC). The input and output characteristics in this report relate to ICEPAC version .10. The version number was generated to historically document the ICEPAC program as it currently exists and to facilitate a means of identifying subsequent versions of the program.

For many years, numerous organizations have been employing the HF spectrum to communicate over long distances. It was recognized in the late thirties that these communication systems were subject to marked variations in performance. The effective operation of long-distance HF systems increased in proportion to the ability to predict variations in the ionosphere, since such an ability permitted the selection of optimum frequencies, antennas, and other circuit parameters. A worldwide network of ionosondes was established to measure ionospheric parameters. Worldwide noise measurement records were taken, and observed variations in signal and amplitudes were recorded over various HF paths.

The results of this research established that most variations in HF system performance were directly related to changes in the ionosphere, which in turn are affected in a complex manner by solar activity, seasonal and diurnal variations, as well as latitude and longitude. 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 (CRPL, l948) outlined the state of the art in HF propagation. Manual techniques were given for analyzing HF circuits of short, intermediate, and long distances. Because the manual methods were laborious and time consuming, various organizations developed computer programs to analyze HF circuit performance. All these programs were based on manual methods for short or intermediate distances and used various numeric representations of the ionospheric data. The IONCAP program was the latest program developed and used in HF propagation predictions. The IONCAP prediction program had poor performance in the polar region and used some of the older profile structures. The program described here is a direct descendant of the IONCAP program. Use of the Ionospheric Communications Enhanced Profile Analysis and Circuit Prediction Program (ICEPAC) is described in this report.

The Ionospheric Communications Enhanced Profile Analysis and Circuit Prediction Program (ICEPAC) is in modular form and coded in simple FORTRAN, following as much as possible the ANSI 77 standard. The modular form allows any subsection to be replaced without affecting the rest of the program. As much as possible, table look-up techniques are used to reduce computer run time, to facilitate the modular structure, and to assist in the detection of errors in any subsection. The program is divided into seven largely independent sections:

1    input subroutines,
2    path geometry subroutines,
3    antenna subroutines,
4    ionospheric parameter subroutines,
5    maximum usable frequency subroutines,
6    system performance subroutines, and
7    output subroutines.

The input subroutines handle the various input options. There are four inputs: Line command disk file, a long-term data disk file, corrected geomagnetic disk file, and an antenna disk file. The line command disk file contains the circuit parameters and control run options. The long-term data disk file contains numeric coefficients for ionospheric parameters and for atmospheric noise as well as tables of parameters needed for circuit performance. The corrected geomagnetic coordinate file provides geomagnetic latitude and longitude for the polar model. The antenna disk file contains optional antenna patterns which can either be generated by the ICEPAC program or obtained from some other source.

The path geometry subroutines determine the circuit geometry, select areas to sample the ionosphere, and evaluate the magnetic field at these sample areas.

The antenna subroutines process antenna data command lines, calculate antenna gains, and output antenna patterns. The program has the simple subroutine from ITSA-1 (Lucas and Haydon, 1966) for the basic antenna models. These assume the antennas are associated with existing systems that have been properly designed.

The ionospheric parameter subroutines evaluate the ionospheric parameters needed by the program. Previous programs assumed an implicit two parabola ionosphere. An explicit electron density profile is used in this program. Observation indicates that absorption equations using the secant law require modification when frequencies do not traverse the entire absorbing region, i.e., with reflection heights lower than 90 km. An empirical modification to the secant law is included in this program.

The maximum usable frequency (MUF) subroutine is a direct determination of the junction frequency based on an electron density profile derived from monthly median parameters of the ionosphere rather than an iterative search. A corrected form of Martyn's theorem (Martyn, 1969) is used. The E, Fl, and F2 layer MUFs are considered. There is also a separate sporadic-E MUF.

The system performance subroutines evaluate all the usual circuit performance parameters. There are two basic subroutines: one for shorter distances and one for long distances (greater than 10,000 km). The models for the shorter distances and the long distance models have previously been incorporated in the IONCAP computer program and are continued in the ICEPAC program. The short-distance models correspond to the manual method given by Haydon et al. (1969). A manual method somewhat like the long-distance models is given in NBS Report 462, (CRPL, 1948). The short-distance model evaluates all possible ray paths for the circuit, including high and low angle modes; E, Fl, and F2 modes; above the MUF modes; and sporadic-E modes. Losses include regular D-E absorption (CCIR-252 loss), deviative losses, and sporadic-E losses. The CCIR-262 loss is basically for F2 modes. For E-layer modes, an adjustment of the absorption is required, and for frequencies which have low reflection heights (less than 90 km) a further correction to the frequency dependence is added. The noise at the receiver site is evaluated and combined with signal statistics to estimate the signal-to-noise statistics.

An extension of the single-hop model to long paths would lead to the expectation that failure of propagation at any of the reflection areas would cause propagation to fail altogether. Empirically, however,it has been found that propagation does not fail until the ionosphere either fails to launch a sky-wave or does not permit sky-wave reception; i.e., these are control areas about 2,000 km from each end of the path. The long-distance model evaluates a sky-wave launch capability at the transmitter and a sky-wave intercept capability at the receiver, using an antenna-gain-minus-ionosphere-loss function at each end of the path. Losses are the same as for the short paths at each end of the path, with a loss per kilometer function used to fill in the path. Noise and signal statistics are the same for the short-distance or the long-distance paths.

The output subroutines generate all the output options as line printer images which can be printed or saved on disk. The available output options and the corresponding input required to generate the output is described in this report.

Much of the work completed is an incorporation of the combined efforts of various laboratories, both government and private, domestic and foreign. Although this program is coded so that revision of any sub-part is relatively easy, it is difficult to join so many diverse sub-models while maintaining consistency and continuity of the entire program. The whole in this case is much more than a sum of the parts.

The use of the program with a description of input and output options is described in this report. The underlying assumptions and the mathematical-physical models are described in a companion report.

1. Introduction

Ionospheric Communications Enhanced Profile Analysis and Circuit Prediction Program User's Manual

This report describes the operation and use of the Ionospheric Communications Enhanced Profile Analysis and Circuit Prediction Program (ICEPAC). The computer program is an integrated system of subroutines designed to predict high-frequency (HF) sky-wave system performance and analyze ionospheric parameters. These computer-aided predictions may be used in the planning and operation of high-frequency communication systems using sky-waves.

This report contains instructions for the use of ICEPAC. A description of the input data requirements, including data definition, organization, and instructions for setup of the various analysis tasks, is presented. Procedures and formats are given for preparing the input data and executing the program. The various outputs are presented and described with an interpretation of the analysis results.

Key Words: communications; computer model; high frequency; ionosphere; LUF; MUF; sky-wave; user's manual

In the initial planning or in the modification of many communication systems, there may be an appreciable delay between the circuit planning and the actual circuit construction or modification. This is of particular importance for high frequency circuits which have marked time and geographic variations in optimum frequency, required power, and system performance. Predictions of ionospheric characteristics and techniques for using these characteristics are, however, available and may be used to anticipate the performance of HF communication circuits and thereby provide the lead time for necessary equipment selection, frequency selection, and frequency and time-sharing arrangements.

High-frequency radio communication depends upon the ability of the ionosphere to return the radio signals back to earth. Prediction of ionization levels in the various regions of the ionosphere is, therefore, essential to any prediction of HF sky-wave circuit performance. The maximum frequency returned from the ionosphere usually establishes the upper limit of the useful HF range. The degree of ionization in the various regions is useful in estimating probable modes, and the transmission loss for these modes is combined with the antenna performance and transmitter power available to estimate the expected HF sky-wave signal available anywhere at any time.

The expected sky-wave signal may be compared with the expected radio noise environment to predict the likelihood that the circuit will operate satisfactorily. This likelihood may be used to select optimum frequencies, proper antennas, required transmitter power, optimum time of operation, and broadcast coverage as a function of time and/or frequency.

The ICEPAC program performs four basic analysis tasks which are discussed in detail in the ICEPAC theoretical report and summarized below:

(1) Ionospheric Parameters. The ionosphere is predicted using parameters which describe four ionospheric regions: E, Fl, F2, and Es. For each sample area, the location, time of day, and all ionospheric parameters are derived. These may be used to find an electron density profile, which may be integrated to construct a predicted ionogram. These options are specified by methods 1 and 2.

(2) Antenna Patterns. The user may precalculate the antenna gain pattern needed for the system performance predictions. These options are specified by methods 13, 14, and 15. If the pattern is precalculated, then the antenna gain is computed for all frequencies (1-30 MHz) and elevation angles. If the pattern is not precalculated, then the gain value is determined for a particular frequency and elevation angle as needed.

(3) Maximum usable frequency (MUF). The maximum frequency at which a sky-wave mode exists can be predicted. The 10% (FOT), 50% (MUF), and 90% (HPF) levels are calculated for each of the four ionospheric regions predicted. These numbers are a description of the state of the ionosphere between two locations on the earth and not a statement on the actual performance of any operational communications circuit. These options are specified by methods 3 to 12.

(4) Systems Performance. A comprehensive prediction of radio system performance parameters (up to 22) is provided. Emphasis is upon the statistical performance over a period of a month. A search to find the lowest usable high frequency (LUF) is provided. These options are specified by methods 16 to 29.

4. Output options

There are two outputs, a list of ionospheric parameters (METHOD 1) and an ionogram (METHOD 2). The data for METHOD 1 are divided into a set of header lines and a body of output lines. The header lines are self-explanatory.

METHOD 1 body

Consists of the result of sampling the ionosphere at one, three, or five areas along the path. The output consists of 21 parameters for each hour. To display this output on an 80 column page, the output was folded with the first 11 parameters on line 1 and the last 10 parameters on line 2 for each hour.

Parameter	Description

YE	E layer semi-thickness (km)
HE	E layer height of maximum ionization (km)
HS	Es layer reflection height (km)
LAT	Latitude  at sample area location
LONG	Longitude at sample area location
LMT	Local Mean Time at the sample area location
UT	Universal Time at the sample area location
E	foE  (MHz) - median E critical frequency
F1	foF1 (MHz) - median F1 critical frequency
Y1	F1 semi-thickness, ymF1 (km)
H1	F1 height of maximum ionization, hmF1 (km)
FH/2	one-half the gyro-frequency, fH (MHz)

F2Z	F2 zero MUF (foF2 - 1/2 fH) (MHz)
Y2	F2 semi-thickness, ymF2 (km)
H2	F2 height of maximum ionization, hmF2 (km)
PB	Pole   -ward boundary of auroral trough (degrees North Latitude)
CEN	Center -ward boundary of auroral trough (degrees North Latitude)
EB	Equator-ward boundary of auroral trough (degrees North Latitude)
M3000	F2 M(3000) factor
TCGM	Local geomagnetic time (hours)
RAT	ratio of hmF2 and ymF2
ZEN	zenith angle (degrees)
FLAG	flag that shows the location of the reflection point

FLAG = 0 = low lat/mid lat nighttime point
	FLAG = 1 = auroral zone nighttime point
	FLAG = 2 = unused
	FLAG = 3 = unused
	FLAG = 4 = low lat/mid lat sunrise/sunset point
	FLAG = 5 = auroral zone sunrise/sunset point
	FLAG = 6 = low lat/mid lat daytime point
	FLAG = 7 = auroral zone daytime point
	FLAG = 8 = unused
	FLAG = 9 = polar cap point (all times)

MAGL	geomagnetic latitude

METHOD 2 body

Up to three ionograms are constructed from the up to five sample areas along the circuit path. The ionogram plots are printed on two pages. The first page contains the left half of the ionogram plot and the second page contains the right half of the ionogram plot. Lines 2, 3 and 4 are the same as for METHOD 1. Line 5 gives the universal time, GMT, the local time at the sample area, the sample area location, the distances in kilometers from the transmitter at which the E and F parameters were taken, the type of integration used, and the manner in which the Fl layer was added to the electron density profile. The graph is vertical sounding frequency (in megahertz) versus true and virtual heights (in kilometers) where true and virtual height are represented by "." and "x" respectively in the graph. The Es layer is given by a line at the Es reflection height. The character "U" is used until the 90 percent value of foEs is exceeded; "M" is used until the 50 percent value is exceeded; and "L" is used until the 10 percent value is exceeded. The parameters for each layer are given in the upper left corner. The table on the right gives the sounding frequencies and the true and virtual heights.

4.2 MUF Output Options, METHOD [3 - 12]

The maximum usable frequency (MUF) output options include all mode information as well as the distributions of the MUF for each layer. First note that METHODS [3,4,5,6] refer to the MUFs for the E(Fl) and F2 layers using the old nomogram method. This assumes a virtual height of about 300 km. The results of this model are not always valid in the 4000-10000 km range. METHODS [7-12] comes from a complete electron density profile.

METHOD [3-12] body

The body of the figure is composed of four lines for each hour, consisting of MUF information for each layer [E, F1, F2, Es].

Parameter	Description
UT		Universal Time at the transmitter
LT		Local Time at the transmitter
FOT		10% value of the MUF.  The MUF is expected to exceed the FOT 90% of the days of the month.
MUF		50% value of the MUF.
HPF		90% value of the MUF.
ANGLE		Radiation angle (degrees)
VIRTL		Virtual height of the reflection (km)
TRUE		True height of the reflection (km)

Note: if the Fl layer is missing, the E values are used.

4.3 Antenna Output Options, METHOD [13, 14, 15]

The detailed description of each antenna pattern and the required input definitions are given in this section. The gain subroutines used in the ICEPAC are approximate models using the "one-term" theory and assume that the antenna parameters are within the design limits of each antenna and that the operating frequency is such that the antenna is close to resonance. These antenna models are appropriate when used in a propagation model where other uncertainties overshadow the uncertainties in the gain due to the antenna mode limitations. However, it is not appropriate to use these models to design antennas or to evaluate their performance outside of their design limits or far from resonance. Also, they will not typically produce accurate results for full-wavelength or multiple-full-wavelength antennas--again due to the limitations of one-term theory. Because of the 80 column limits on the outputs, the antenna patterns are divided and placed on two consecutive pages. Frequencies 2 MHz through 11 MHz on the first page and frequencies 12 MHz through 30 MHz on the second page.

4.4 System Performance Output Options, METHOD [16 - 29]

The prediction of the performance of a communication system operating between two points on the earth's surface is the main output of the ICEPAC program. There are two principle forms of output: first, a table of up to 22 performance variables, and, second, a table or graph of the lowest usable high frequency (LUF), METHODS [26-29]. The program has been written in such a way that by using METHOD 23, the user may specify the system performance lines and the header identification lines desired. See the Output Parameters under HELP/Contents for the Point-to-Point program for complete details of the definitions of the output variables.

5. Applications

The primary application of the ICEPAC program is to use the system performance options to select a frequency complement. While intended mainly for program test and evaluation procedures, the other output options, if used with some interpretation, may provide the analyst with enough information to solve a particular problem.

5.1	Ionospheric Parameters Applications
5.2	MUF Applications
5.3	System Performance Applications
5.3.1	Selecting an Optimum Frequency
5.3.2	Selecting a Frequency Complement for a Single Circuit
	in the Absence of Other Circuit Interference
5.3.3	Standard Frequency Complement
5.3.4	Two-Frequency Complements
5.3.5	Three-Frequency Complements
5.3.6	Four-Frequency Complements
5.3.7	Time Sharing on Circuits Separated Geographically
5.3.8	Time Sharing in the Same Geographic Area
5.3.9	Frequency Sharing
5.3.10	Broadcast Coverage
5.3.11	Optimum Times for Communication
5.3.12	Selection of Relay Locations
5.3.13	Determination of Lowest Effective Transmitter Power
5.4	Antenna Selection or Design

5.1 Ionospheric Parameters Applications

The ionospheric description output consists of a table of parameters (METHOD 1) and a graph and table of ionograms (METHOD 2). The parameters are the output of the long-term world maps of the ionospheric parameters. The accuracy of the maps were determined when they were generated. The ICEPAC theoretical report has details and references to these maps. The ionogram output includes the vertical sounding frequency and the virtual height, so that this ionogram may be compared to measured ionograms. Note, however, a monthly median may not be characteristic of that for a given day. The effect of the changes of critical frequencies from the maps may be studied using the FPROB control variables or by use of the EFVAR and ESVAR commands.

5.2 MUF Applications

In the absence of any other criteria for the planning of a system using HF sky-wave, the first, and simplest, criteria is an estimate of the frequency having efficient ionospheric reflections. Normally, an estimate of the frequencies expected to have efficient ionospheric support 90 percent of the time, FOT, and those having efficient ionospheric support 50 percent of the time, MUF, are adequate estimates of upper frequency limits for system planning. METHODS [3-12] allow the possible MUF outputs. Note that METHOD [7] gives all mode information and MUF distribution for each layer. These MUF calculations are only a description of the state of the ionosphere and do not include any system parameters. They should not be confused with the maximum operation frequency (MOF) for transmission between two points on an existing circuit. A full system performance calculation should be made to estimate the MOF or to compare with observed MOFs. If a full system performance computation is generated, the user should examine the frequency complement predictions rather than the MUF.

5.3.1 Selecting an Optimum Frequency

The complexities of propagation, the diversity of service requirements, and the fluctuation of spectrum congestion preclude any clear simple criteria for the selection of optimum frequencies. An adequate signal-to-noise ratio at the receiver for the specified type and quality of service is often a useful criterion. In general, within the HF spectrum, radio noise tends to decrease as frequency is increased. During the daylight hours when HF power requirements are highest, the propagation loss tends to decrease as frequency is increased. Since the noise normally decreases and signal normally increases with frequency, it is a general rule for HF sky-wave circuits that the higher the frequency the better the signal-to-noise ratio until frequency is increased to a point where reflection from the ionosphere becomes improbable. A first approximation to the optimum frequency in the absence of interference may, therefore, be made by estimating the highest frequency having an efficient ionospheric reflection consistent with the circuit reliability required; i.e., the MUF calculations described in Section 5.2 above.

Since there is normally limited flexibility in the selection of frequencies and since the optimum frequency based on the probable upper useful frequency limit has mixed diurnal, seasonal, and other variations, it is desirable to establish the probable useful range of frequencies. The FOT as shown is based on a 90 percent probability of efficient ionospheric support and may be used as an estimate of the probable upper frequency limit; a corresponding lower useful frequency limit may be estimated by considering the probability that the available signal-to-noise ratio will be adequate. Since noise normally increases as frequency decreases and signal normally decreases as frequency decreases, there is usually a frequency below which the probability of an adequate signal-to-noise ratio is unacceptable. This probability is often set at 90 percent and the corresponding frequency is known as the lowest useful frequency (LUF). These limits; i.e., FOT and LUF, are shown in METHOD [26 - 28].

5.3.2 Selecting a Frequency Complement for a Single Circuit in the Absence of Other Circuit Interference

The range of useful frequencies (METHOD [26-29]) is basic to the selection of frequency complements and should be obtained for representative months over the time period the circuit under consideration will be required to operate. For a semi-permanent operation, diurnal variation of the useful frequency range for seasonal extremes (e.g., June and December) and solar activity extremes (e.g., sunspot number 10 and 110) are normally adequate.

5.3.3 Standard Frequency Complement

Absolute continuity of any radio service, however desirable, is improbable even with an unlimited choice of operation frequencies, when high frequency sky-wave propagation must be relied upon. Moreover, the return in improved continuity for an enlarged frequency complement beyond a certain size, depending upon the service, diminishes so rapidly that it can rarely be justified in the congested spectrum. Frequency complements can, however, be based on a concept of maximum feasible continuity; i.e., the theoretical increase in circuit continuity may be negligible if additional frequencies are added, but a significant decrease is possible if fewer frequencies are available. Since the required frequency complement depends upon the circuit parameters or usage, the following classification of circuits is introduced:

a. Circuits requiring maximum feasible continuity. These are the usually heavily loaded telegraph and telephone circuits, which must be available with good traffic capacity at all times. Because of their loading, they employ relatively elaborate terminal equipment. Telegraph circuits of this category are operated at machine speeds, while the telephone circuits generally employ several channels of a single-side-band system and are extended to line networks. Such circuits are characterized by the use of large directional antenna systems, diversity reception in telegraphy. and high-powered transmitters. Some other circuits, notably those immediately concerned with safety of life, may have an urgent need for continuous availability, although not necessarily carrying continuous traffic. The standard frequency complement for these circuits provides at least one frequency between the LUF and FOT at all times, plus one or two additional frequencies to permit flexible operations in the event of interference and during ionospheric disturbances. The maximum complement for these circuits should rarely exceed four. If more than four frequencies are considered necessary, re-engineering of the circuit should be investigated.

b. Circuits requiring moderate continuity. Distinct from circuits requiring maximum continuity, there exists a larger group of circuits which, by nature of their operation, require only moderate continuity. These circuits generally provide communication under circumstances where the needs are insufficiently critical to warrant the extension of wire, cable, or VHF facilities. Many such circuits are operated to provide occasional service to remote installations. There are also many circuits which may be designated nominally as continuous in operation, but on which the nature of the traffic is such as to allow occasional delays, reduction in transmitting speed, or rerouting. Judicious scheduling of traffic contributes significantly to the satisfactory operation of these circuits. Most circuits of an administrative nature belong in this category. For frequency-complement considerations, circuits (except safety services mentioned earlier) should generally be constrained to this category if they employ manual telegraphy or telephony not extended to line networks, or are equipped with simpler transmitting and receiving installations than are capable of providing maximum feasible continuity. Radio circuits of this kind have been operating on one or two frequencies in many parts of the world for many years, providing a quality of service consistent with particular needs. The standard frequency complement for these circuits is two, one day frequency and one night frequency.

5.3.4 Two-Frequency Complements

Two-frequency complements are recommended with the clear understanding that the services receiving such complements are of the kind which do not have sufficient traffic to justify attempts to operate them on a twenty-four hour basis. These circuits must nevertheless be assured of as many hours of communication per day as possible within the two-frequency limitation. Actually, in a very large number of cases during intermediate conditions of solar activity, two-frequency complements will give very nearly twenty-four hour service, except in the auroral regions where experience has shown that no complement of high frequencies can ever give a highly reliable service.

a. General considerations. There is available, at the present state of our knowledge, enough information on propagation, if the other factors such as terminal equipment, type of service, and actual operating conditions are known, to permit useful estimates of the total number of hours of satisfactory service, assuming no interference, to be expected from a given frequency over an average solar cycle. It is, however, impossible to estimate for any particular circuit the frequency that would give the maximum number of hours of useful service over the solar cycle. However, this frequency might not necessarily be one of the two frequencies assigned, since the cumulative number of hours of usefulness is a less urgent consideration than that of assuring operation during the normal peak traffic periods.

b. The day frequency. A simple method for selecting the day frequency consists of choosing a frequency just below the lowest daytime FOT curve. In the north temperate zone, this would be the FOT curve for June for the period of minimum solar activity. Simply choosing one frequency, or some small range of frequencies on this basis, ignores the situation with regard to LUF, and could lead, for an actual circuit, to failure to provide communication during the middle of the day in months of maximum absorption at the period of maximum solar activity. An improved method is derived from the opinion that, if possible, the day frequency should not fail to give service during the midday hours even at the condition of maximum normal absorption which occurs at the maximum levels of solar activity.

This approach suggests immediately that a band of particularly useful frequencies exists between the frequency which is just above the maximum midday LUF and the highest frequency that will provide essentially skip-free service during the middle period of the daylight hours in the months of minimum daytime FOT. The lowest frequency in such a band will provide the maximum hours of service, and such service may well exceed that obtained from a frequency selected only on the basis of minimum daytime FOTs. Fortunately, a useful band of frequencies does exist in nearly all cases, and the day frequency should be chosen in the lower part of this band.

The possibility of skip on frequencies chosen in this way is unlikely when consideration is given not only to normal FOTs, but also to the well-established role of sporadic-E reflections.

c. The night frequency. The choice of the night frequency is complicated, depending not only on the type of service, terminal equipment, and a number of propagational considerations, but also on the day frequency selected. As in the case of the selection of the best day frequency, it is not sufficient to select a frequency that will give the maximum number of hours of service throughout a complete solar cycle. The solution is to make the best choice for those hours that the day frequency is not suitable.

At night ionospheric absorption is minimal, except in the high latitudes, and the LUFs are rather critically dependent on the type of service and equipment used, including the antennas and the required signal levels. Too high a night frequency will be subject to skip too much of the time. However, a very low and, therefore, virtually skip-free night frequency may make it impossible to deliver adequate signal-to-noise ratios to the receiving location for substantial periods of time. The selection problem for this single night frequency is one of deciding what constitutes a suitable balance of these conflicting considerations. For circuits of 4000 km or less in length, the difference in local time between the terminals can never be large in the regions of the world where the bulk of the HF circuits operate. It is reasonable that traffic handling during the predawn hours be avoided, since, apart from the usual erratic behavior of the ionosphere during these hours. normal activities at both terminals are at their diurnal minimum during this period. In view of this sort of consideration, it would seem incorrect to choose a night frequency so low that skip-free operation was safely assured during the predawn period of the months and solar activity conditions having the lowest FOTs at the expense of rendering communications impossible during other time periods because of high noise levels normally experienced at the lower frequencies.

It is recommended, therefore, that the night frequency be chosen as the highest frequency of which less than four hours of skip is indicated on the lowest of the FOT curves for the required months of operation. This is usually during the winter months; e.g., December in the northern hemisphere and June in the southern hemisphere.

5.3.5 Three-Frequency Complements

Many radio circuits require a greater continuity of service than can generally be assured with two frequencies. This is especially true of telephone circuits intended for extension to line networks. The following suggestions are applicable to determining frequency complements for circuits less than 4000 km in length requiring maximum feasible continuity and employing telephony for extension to line networks, and also for speed of telegraphy for services not seriously troubled by multi-path effects. Multi-path protection required for certain types of high-speed machine telegraphy and facsimile services is not necessarily provided by these three-frequency complement standards.

There is some justification for enlarging the complement to four frequencies in the case of telephony and manual telegraph services subject to the severe magnetic disturbances characteristic of the high latitude regions. In other regions, however, it is possible by a suitable choice of three frequencies to maintain the signal-to-noise ratio high enough to provide an entirely adequate service. This is possible because higher frequencies are generally usable in the lower latitudes as a consequence of the observed higher complement-limiting FOTs, daytime levels of which survive in many cases far into the night. This situation is in marked contrast with the high latitudes where circuits are in all cases significantly more difficult to operate.

a. The high frequency. The highest frequency of a three-frequency complement is purely a day frequency. Since a middle frequency is available, it is no longer necessary for the highest frequency of the complement to be constrained to give useful service at the minimum phases of the solar cycle. This frequency should give many hours per day of useful service at all seasons during the maximum phases of the solar cycle. The high frequency is the highest frequency which will be below the FOT at least four hours during all months during the period the circuit is to be operated.

b. The low frequency. The lowest frequency of a three-frequency complement is entirely a night frequency. It must be selected on a basis related to the minimum FOT during any month the circuit is required to operate. Select the frequency which indicates less than two hours of skip on the lowest of the FOT curves for the required months of circuit operation.

c. The middle frequency. The middle frequency is selected so as to maximize the number of hours during which at least one frequency is between the LUF and FOT during the required period of circuit operation.

d. A special case for modifying the low frequency. Certain special considerations may affect the choice of a low frequency for circuits between about 3600 and 4000 km in length. For such circuits, the vertical angles of departure and arrival for single-reflection F2-layer transmission at night become very low. With many antennas, very little energy can be transmitted or received at these low angles. For these antennas, transmission, when possible, by two reflections from the F2 layer provides a superior service. If highly-directional broadside arrays, which are relatively efficient at low angles, are used at sites permitting use of low angles of departure and arrival, the single-reflection transmission remains useful. The possibility of site conditions which make impossible the use of low angles of arrival or departure should not be over- looked. During the conditions of lowest MUFs, unobstructed sites, and efficient low-angle antenna systems, the low frequencies for paths 3000-5000 km will remain useful, but there will usually be a skip on the higher vertical-angle two-reflection mode. With low-angle radiation or reception limited by inadequate antenna systems, terrain, etc., in the ways suggested, this skipping in the two- reflection mode may well interrupt the service. In these instances, where it appears that reliance must be placed on two-reflection transmission during the period of minimum MUFs, it is preferable to assign a frequency appropriate to two- reflection transmission. This frequency will be automatically displayed in METHOD [7] or on graphs displayed in METHOD [8 - 11] if the minimum vertical angle is set at the lower limit of adequate antenna performance; e.g., 3 to 6 degrees.

5.3.6 Four-Frequency Complements

Standard three-frequency complements are confined to a number of services, operated over paths less than 4000 km in length where maximum feasible continuity of operation is required, but when the effects of multi-path propagation are not serious. Services which use high-speed digital transmission techniques are seriously affected by multi-path distortion, and a three-frequency complement may be insufficient. Many such services can, however, be adequately satisfied by a three-frequency complement with respect to both continuity and multi-path protection. This is particularly true for circuits in the 2000 to 4000 km range of lengths, where the probability of multi-path is low, and to a lesser extent for shorter circuits. Whether or not the three-frequency complement for a particular circuit provides the requisite multi-path protection for high-speed service may be determined by a system performance prediction for the frequencies selected (e.g., METHOD=23, using a multi-path tolerance of two milliseconds and a power tolerance of 10 db) and noting the multi-path probability for frequencies having an acceptable reliability.

It was suggested in the discussion on the applicability of three-frequency complements that there is some justification, apart from multi-path considerations, for the assignment of a four-frequency complement to a circuit operating in high latitudes which has a critical need for maximum feasible continuity. If one or both terminals of a circuit lie above 60 degrees north geomagnetic latitude, the circuit may be regarded as sufficiently high latitude to merit consideration for a four-frequency complement.

a. The highest frequency. The highest frequency of a four-frequency complement is purely a day and evening frequency. Since three frequencies are available below it in a given complement, there is no longer any need that it give any important service at the minimum phases of the solar cycle, nor need it be the only frequency of the complement for service during maximum LUF periods. The main purpose of this frequency is to permit the reception of high-speed digital information free of destructive multi-path distortion. It is usually required in afternoon and evening periods when the second highest frequency--while giving a perfectly adequate signal-to-noise ratio--would be subject to multi-path distortion. This frequency must, therefore, be chosen with the maximum FOT in mind; it should nevertheless be as low as possible to give as much service as possible. In some of the complements, the highest frequency will give few cumulative hours of service over a solar cycle, though remaining indispensable to avoid multi-path. In other complements, it will give considerable cumulative service and prove to be particularly useful in obtaining a good signal-to-noise ratio.

In general, the highest frequency is chosen to exceed 65 percent of the maximum FOT during the required period of circuit operation. This procedure is intended to provide substantial protection against multi-path distortion on the highest frequency of the complement during hours and seasons of occurrence of maximum MUFs. At shorter distances, because of the impracticability of providing complete multi-path protection, the factor provides as much protection as can reasonably be afforded by a four-frequency complement, while at the same time providing a highest frequency that will have significant usefulness at the maximum phases of the solar cycle.

It is necessary to invoke a LUF-determined lower limit or floor value below which the highest frequency of the complement is not selected regardless of the results of the above procedure. This limit is intended to provide a minimum additional margin of signal-to-noise ratio over that provided by the second highest frequency for high-grade service during periods of maximum LUF. The lower limit is set at 1.4 times the maximum LUF during the required period of circuit operation.

b. The second-highest frequency. The second-highest frequency of a four-frequency complement is the most important frequency of the complement in many ways. It is certainly the frequency likely to receive the greatest cumulative use over a solar cycle. It is low enough to provide reliable daytime operation at sunspot minimum during periods of minimum daytime MUF. While the frequency may receive considerable daytime use at sunspot maximum, it will also be needed during the evening and night transition periods at sunspot maximum to permit continuation of high-speed digital operation. Since there exists a frequency still higher in the complement and two lower, this frequency is chosen to remain just above the maximum LUF, as was the daytime frequency of the two-frequency complements. On some circuits, it can be expected that this frequency will provide service far into the night at sunspot maximum during much of the spring, summer, and autumn seasons. The principal use of this frequency is, nevertheless, as a day or evening frequency.

c. The third-highest frequency. The third-highest frequency of a four-frequency complement is probably the second most important frequency of the complement in terms of cumulative hours of use over a solar cycle. While this frequency may receive some use in the early morning under certain conditions, its main use is as an evening or night frequency. During sunspot maximum conditions, it will, in a large number of cases, cover the late night period even in winter; it will certainly be sufficiently low for summer night use, even down to sunspot minimum in many temperate regions. In the high-noise regions, this frequency will nearly always be sufficiently low to cover the predawn period at the noisiest seasons, and for this reason it has not been necessary to give special consideration to high-noise-region floors for night frequencies in this report.

This frequency is the geometric mean of the second-highest and the lowest frequency of the complement. This procedure results in frequency intervals which provide the maximum possible multi-path protection between these complement members. It provides, at the same time, a good order of frequencies for intended major usage during evening and night periods.

d. The lowest frequency. The lowest frequency of a four-frequency complement is entirely a night frequency. It must be relied upon at all times when the third-highest frequency is too high to carry the service. It is selected just below the minimum FOT during the period the circuit is required to operate.

5.3.7 Time Sharing on Circuits Separated Geographically

Since there are marked diurnal and geographical variations in the useful frequency range, it is often possible to use this variation to develop time sharing plans when circuits are separated geographically.

Tabulations or graphs of useful frequency range (METHOD [26-29]) should be obtained for the time periods of interest. Whenever a frequency complement is such that it contains a frequency within the useful frequency range for one circuit, while outside the useful frequency range for the other circuit, this information may be used to develop time-sharing schedules. The time period may be extended whenever other frequencies in the complement are useful on the second circuit.

5.3.8 Time Sharing in the Same Geographic Location

When long and short paths are involved in the same geographic area, the useful frequency ranges for each may differ sufficiently that sharing plans may be developed in a manner similar to that described above (see Section 5.3.7).

5.3.9 Frequency Sharing

The development of frequency-sharing plans requires the prediction of the available signal along the unwanted as well as the wanted radio path, taking particular account of the expected antenna performance for the unwanted radio path. The circuit reliability estimates (e.g., METHOD=23) can be used to develop frequency-sharing plans (i.e., share whenever the reliability is high for the wanted paths but low for the unwanted paths).

A quick determination of a frequency sharing opportunity may sometimes be made by MUF-FOT computations for the wanted paths (e.g., METHOD 8) and the HPF (frequency having efficient ionospheric support only 10 percent of the days) computation for the unwanted paths (e.g., METHOD 9). Sharing should be possible if a frequency is below the FOT on the wanted path but above the HPF on the unwanted path.

5.3.10 Broadcast Coverage

Circuit reliability is probably the most valuable single output from the prediction program. As various parameters are fixed, a computation of circuit reliability as a function of a remaining variable will often assist in decision making. The question of broadcast coverage is a good example. With time, frequency, antenna, transmitter location, transmitter power, etc. fixed, circuit reliability to sample points within a geographic area of interest will describe the coverage of the area in terms of the percentage of days within the month that satisfactory reception may be expected. This process may be performed automatically by the computer to provide reliability tabulations over selected areas (e.g., one hemisphere). Although this output is not available as a "standard output", tabulations of this type can be made using a special input command line processor and output variable formatter.

Use the Area Coverage version of the program to obtain contours of output parameters over an area of the world.

5.3.11 Optimum Times for Communication

For given transmitter location, receiver location, antenna types, etc., the diurnal variation in circuit reliability may be used to choose optimum communication time (e.g., METHOD=23).

5.3.12 Selection of Relay Locations

Careful consideration should be given to increasing the frequency complement, increasing power, antenna redesign, etc. before relay stations are used. If there is no other solution, consider the use of relay stations. Normally, if possible, these relay stations should be separated by at least 3000 km and preferably not more than 7000 km. The relays should also assure the propagation path does not go to high latitudes; i.e., temperate or equivalent routes avoiding high noise regions are preferred. In the final selection, it is a question of computing circuit reliability for the direct path and making a comparison with potential relay sites.

5.3.13 Determination of Lowest Effective Transmitter Power

Compute circuit reliability as a function of transmitter power with other variables fixed and plot reliability versus transmitter power. The lowest effective power is the lowest power providing the required reliability. An alternative is to use the required power plus antenna gain output, RPWRG, METHOD [16, 20-23].

5.4 Antenna Selection or Design

For the frequency or frequencies under consideration, make predictions covering the required time period of operation using a constant gain antenna with a typical gain; e.g., 12 db. Determine the time or times the circuit reliability is the lowest. Using these times, repeat the computation for the antennas under consideration to select an antenna. Caution: the calculated vertical angle for the most reliable mode provides some guidance in determining the antennas to be considered, but this angle alone should never be used as the sole criterion for antenna selection. To select an antenna, repeat the computations for available antennas. To design an antenna, repeat computations for variables in the antenna design; e.g., antenna height, rhombic leg length, etc. Note, however, that these parameters must be part of a "well designed" antenna. See the comments in Section 4.4 above.

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