Running VOACAP Predictions

Running VOACAP predictions

After you have set all the circuit parameters in the VOACAP main screen, it is time to run the analysis.

Textual circuit predictions

Choose Circuit from the Run menu (abbreviated later in these web pages as Run > Circuit). A new window will open, containing a lot of data. The data example below is the start of the prediction for the circuit of Pori (Finland) - Johannesburg (South Africa).

IONOSPHERIC COMMUNICATIONS ANALYSIS AND PREDICTION PROGRAM
                    VOACAP   VERSION 01.1210W


         1         2         3         4         5         6         7
123456789012345678901234567890123456789012345678901234567890123456789012345

COMMENT    Any VOACAP default cards may be placed in the file: VOACAP.DEF
LINEMAX      55       number of lines-per-page
COEFFS    CCIR
TIME          1   24    1    1
MONTH      200112.00
SUNSPOT    110.
LABEL     YLE PORI            JOHANNESBURG
CIRCUIT   61.47N    21.58E    26.25S    28.00E  S     0
SYSTEM       1. 145. 3.00  90. 67.010.00 0.05
FPROB      1.00 1.00 1.00 0.00
ANTENNA       1    1    2   30     0.000[hfcc\HFBC_218.P15    ]160.0  350.0000
ANTENNA       2    2    2   30     0.000[default\SWWHIP.VOA   ]  0.0    0.0000
FREQUENCY  6.07 7.20 9.7011.8513.7015.3517.7321.6525.89 0.00 0.00
METHOD       30    0
EXECUTE
QUIT

    CCIR Coefficients        ~METHOD 30   VOACAP 01.1210W  PAGE   1

Dec    2001          SSN = 110.                Minimum Angle= 3.000 degrees
YLE PORI            JOHANNESBURG          AZIMUTHS          N. MI.      KM
61.47 N   21.58 E - 26.25 S   28.00 E    174.24  356.94    5275.6   9769.6
XMTR  2-30 REC705 #01[hfcc\HFBC_218.P15    ] Az=160.0 OFFaz= 14.2 350.000kW
RCVR  2-30 2-D Table [default\SWWHIP.VOA   ] Az=  0.0 OFFaz=356.9
3 MHz NOISE = -145.0 dBW     REQ. REL = 90%    REQ. SNR = 67.0 dB
MULTIPATH POWER TOLERANCE = 10.0 dB   MULTIPATH DELAY TOLERANCE =  0.050 ms

 1.0 13.1  6.1  7.2  9.7 11.9 13.7 15.4 17.7 21.6 25.9  0.0  0.0 FREQ
     F2F2 F2F2 F2F2 F2F2 F2F2 F2F2 F2F2 F2F2 F2F2 F2F2   -    -  MODE
      4.0  7.6  7.8  3.3  4.0  4.0  7.8  7.8  7.8  7.8   -    -  TANGLE
     12.0  7.6  7.8  3.3 12.0 12.0 22.0  4.0  8.0  7.8   -    -  RANGLE
     34.4 34.2 34.2 33.9 34.3 34.6 35.2 34.6 35.5 34.9   -    -  DELAY
      360  290  297  314  332  387  466  402  574  458   -    -  V HITE
     0.50 1.00 0.99 0.95 0.73 0.38 0.12 0.01 0.00 0.00   -    -  MUFday
      142  146  143  142  138  145  157  198  268  289   -    -  LOSS
       44   35   39   48   47   41   29   -5  -77  -96   -    -  DBU
      -87  -90  -87  -84  -83  -90 -102 -143 -212 -233   -    -  S DBW
     -167 -153 -156 -162 -165 -168 -169 -171 -173 -176   -    -  N DBW
       80   63   69   78   83   78   68   28  -39  -58   -    -  SNR
       13   16   12   10   11   16   26   66  133  138   -    -  RPWRG
     0.74 0.24 0.57 0.74 0.78 0.70 0.51 0.03 0.00 0.00   -    -  REL
     0.00 0.00 0.00 0.64 0.00 0.00 0.00 0.00 0.00 0.00   -    -  MPROB
     0.35 0.19 0.28 0.34 0.38 0.33 0.23 0.03 0.00 0.00   -    -  S PRB
     25.0  9.7 11.2 19.2 25.0 25.0 25.0 25.0 25.0  9.1   -    -  SIG LW
     17.7  4.9  4.9  6.2 11.6 21.5 25.0 25.0 25.0  4.9   -    -  SIG UP
     26.7 12.4 13.8 21.2 26.7 26.8 26.8 26.8 26.8 13.3   -    -  SNR LW
     18.5  7.6  7.1  7.8 12.7 22.2 25.7 25.7 25.7  7.6   -    -  SNR UP
     19.7 21.9 21.9 18.3 19.7 19.7 21.8 21.8 21.8 21.7   -    -  TGAIN
     -0.9 -2.1 -1.9 -7.0 -0.9 -0.9  0.0 -5.4 -1.8 -1.9   -    -  RGAIN
       54   51   55   57   56   51   41    1  -66  -71   -    -  SNRxx

There are 24 blocks by hour in this example prediction. One hour block, as seen above out of 24, contains 14 columns. The first column is the time ("1.0") in UT (Universal Coordinated Time). Hours are centered on the hour. Thus, "1.0" represents the time from 0030 to 0130 UT centered at 0100 UT.

The second column (the data under "13.1", ie. 13.1 MHz) is the predicted median maximum usable frequency (MUF) at that hour. Then follows the 11 user-specified frequency columns (the frequencies are in MHz). In our example, only 9 frequencies have been specified, so two last columns out of 11 are empty. The final column contains 22 parameters that have been calculated for these frequencies in the hour block. The first parameter FREQ indicates the frequency (in MHz) used in calculation.

A note about the antenna used in the example

For this general study, a Curtain Array AHR(S) 4/4/1 has been defined but with no design frequency. This means that for every frequency calculated, the design frequency would be set equal to the operating frequency (an idealised antenna). In reality, this situation does not occur. Rarely is one transmitting antenna valid for all frequencies from 6 to 26 MHz. Therefore, you have the ability to define different antennas for different frequency ranges.

VOACAP output parameters

Below you will find brief definitions for the 22 parameters in the final column:

Parameter	Definition
MODE	Ionospheric path for the most reliable mode (ie. the mode with the highest reliability of meeting the REQ.SNR). For the Short Path Model, the number of hops and mode type [E, F1, F2, Es layers] for the MRM are given, eg. 2F2. For the Long Path Model (not to be confused with the long path circuit!), the mode at the transmitter end and the mode at the receiver end are given, eg. F2F2 in our example above.
TANGLE	Radiation angle for the MRM (degrees).
RANGLE	The angle at the receive end (for long path model only).
DELAY	Time delay for the MRM (milliseconds).
V HITE	Virtual height of the MRM (kilometers).
MUFday	Fraction of days in the month at that hour that the operating frequency is below the MUF for the MRM.
LOSS	Median system loss for the MRM (dB).
DBU	Median field strength expected at the receiver location (dBu).
S DBW	Median signal power expected at the receiver input terminals (dBW).
N DBW	Median noise power expected at the receiver (dBW).
SNR	Median Signal-to-Noise ratio for power summation of all modes (dB-Hz).
RPWRG	Required combination of transmitter power and antenna gains (in dB) needed (+) or excess (-) to achieve the required reliability.
REL	Circuit reliability. Fraction of days per month for which the SNR equals to or exceeds the required SNR (REQ.SNR) at the given hour. In our example above, the REL of 0.78 at 11.9 MHz means that a 67-dB SNR or better can be maintained on 78% (ie. 23 days, see Z Tables) of days per month on 11.9 MHz at 0100 UT.
MPROB	Probability of an additional mode within the multi-path tolerances (short path only).
S PRB	Service probability. The probability that the required reliability will be met. Not completed, do not use.
SIG LW	Lower decile range for the signal power (dB).
SIG UP	Upper decile range for the signal power (dB).
SNR LW	Lower decile range for the SNR (dB).
SNR UP	Upper decile range for the SNR (dB).
TGAIN	Transmitter antenna gain at TANGLE (dBi).
RGAIN	Receiver antenna gain at TANGLE or RANGLE (dBi).
SNRxx	Signal-to-Noise ratio at the Required Reliability (REQ.REL.). Also known as SNR90, if the REQ.REL. is 90% (dB-Hz).

Graphical circuit predictions

Our data example is again from the circuit of Pori (Finland) - Johannesburg (South Africa). Choose Graph from the Run menu (abbreviated later in these web pages as Run > Graph). A new window will open, containing all the available output parameters:

The graphical charts provide a nice overview of the parameters on the frequency/time scale. To view the exact details of the values, click anywhere on the chart, the details appear below the menu bar:

While in a parameter output window, another parameter can be chosen by clicking the Parameters menu. The frequency scale (on the left side) can be changed from the Scale menu. If you wish the window to be pasted into another program, click the to Clipboard menu. To print the window, choose from the Print menu.

Some of the most prominent parameters to look at are SNR, REL and SNRxx. Below are those parameters from our circuit. The thick black line that runs through the chart is the median MUF.

SNR (aka SNR50, or median SNR)

The SNR values (in dB-Hz) that can be maintained 50% of days (15 days) in the month.

REL (Circuit Reliability)

The percentage of days in the month when the SNR (SNR50) value will equal to or exceed the REQ.SNR.

SNRxx (aka SNR90)

The SNR values (in dB-Hz) that can be maintained 90% of days (27 days) in the month.

Area coverage predictions (VOAAREA)

From a point-to-point circuit analysis we can proceed to an area coverage analysis. It sometimes happens that a frequency that is most suitable for a certain point-to-point circuit, is not the best possible for providing good service to a specific area.

Let us take our example circuit of Pori (Finland) to Johannesburg (South Africa) at 1 UTC in December 2001. We have had an analysis of this circuit in the article Understanding Above-the-MUF Predictions. It shows that the best frequency would be 11.9 MHz. Now let us see how that frequency would work for the African continent.

The following screen reflects the basic settings of VOAAREA - a companion program to VOACAP - needed for running the coverage analysis:

We are interested in the circuit reliability in Africa, therefore we choose REL from Parameters. You may choose several parameters at a time. As some locations in our target area are 10,000 km away, we choose the Method 30, Short/Long Path Model Smoothing. Be sure to select CCIR (Oslo) coefficients (not URSI88). Also, keep the Fprob values as shown above. The following screen shots explain how to set some other input values shown above:

Plot Center

Groups

System

Running the Maps

Choose Run > Calculate > Save{temp.VOA}/Calculate:

When the calculations are done, choose Run > Plot results. A window will appear, choose File > Open TEMP.VG1:

The following window will appear:

To plot the map, just choose Plot to > Window. You can also plot or print the map to the printer. To view the remaining maps, choose File > Open. The file names are of format TEMP.VG? where the question mark is a number between 1 to 7.

Furthermore, in this window you can choose several output parameters for viewing if you did not do so in the main screen. Just select them by clicking the Parameters button.

If you wish to change the colours or values to be plotted, click the Contours button. The following window will appear:

Then click the REL button. The following window will appear:

You can change the values and colours here. Note that a value of -999 is used to fill unused lines. Be sure to select a proper colour for that value too! If you change any of the default values, click the Values assigned are: button. The text AUTO scaled will change to USER defined. Otherwise, your settings will be ignored.

Area Coverage Analysis

Using the output parameter of REL, the Circuit Reliability - the percentage of days in the month when the SNR (SNR50) value will equal to or exceed the REQ.SNR. (67 dB-Hz) - the following coverage maps can be produced for the frequencies of 9.7, 11.9, 13.7 and 15.4 MHz at 1 UTC in December 2001. We see that 9.7 MHz can serve a larger portion of Africa than 11.9 MHz. It also provides better service to Europe. The frequencies of 13.7 and 15.4 MHz can provide service to Southern Africa but their overall performance is not adequate.

9.7 MHz	11.9 MHz	Legend

13.7 MHz	15.4 MHz