The Quality of the Prediction for the NVIS Propagation with ITS- HF Propagation

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1 The Quality of the Prediction for the NVIS Propagation with ITS- HF Propagation (Full text in English) Iulian BOULEANU 1, Marius GHEORGHEVICI 1, Robert HELBET 1 1 Department of Technical Sciences, Academy of land Forces Nicolae Bălcescu, Sibiu, România Abstract The article presents a study on the prediction quality of the ITS HF Propagation application suite: VOACAP (Voice of America Coverage Analysis Program), ICEPAC (Enhanced Profile Ionospheric Communications Circuit Analysis and Prediction Program) and REC533 (ITU Recommendation RS 533) for point to point connections of the NVIS (Near Vertical Incidence Sky-wave) type. The simulation results were compared to the measurements obtained on a NVIS direction carried out in Romania and with the measurement results obtained and published by Walden in [1] for a NVIS network established in the UK. Compared with the measurement data obtained in Romania, the results predicted by the three applications have achieved average values of the correlation factors. These low values regarding the quality of a predictive application are due to the short observation period of the ionospheric channel. In these circumstances, REC533 achieved the lowest mean value of the standard deviations compared to VOACAP and ICEPAC. Regarding the measurement results published by Walden, the three applications have achieved strong values. These results lead to high values of the coefficient of determination, ensuring a very good power of prediction. REC533 provided the most correlated values and the lowest average value of standard deviations. The results provided by ICEPAC and VOACAP are highly correlated for each month under review. Of the three applications analysed, the best predictions were obtained by REC533. Keywords: NVIS, propagation prediction, HF measurements, VOACAP, ICEPAC, REC533 Received: September, 01, Introduction The possibility of establishing HF connections depends on the Ionosphere properties and the parameters of the emitting and receiving systems [2]. When comparing the highest transfer rates obtainable in this band, of maximum 120 kbps [3] [4] [5] [6] with those obtainable in higher frequency bands, continuing research and development of communications in this band may seem daunting, but the advantage of transmitting information over a very long distance using a single-link is the reason it arouses the interest of the military, the sailors and the communications services for emergency interventions [7] [8]. When modern communication networks do not exist in a certain area or are deteriorated due to natural disasters or subversive actions, HF communications can pass the data traffic needed for coordinating the activities of the structures involved. A distinguishing feature of emergency communications and of a large part of military communications is that the necessary connection distances are tens or hundreds of kilometres. In the case of ionospheric links, 1 are the smallest possible distances and can only be obtained on NVIS channels. To achieve this type of connection it is necessary to use a subdomain of HF frequencies (2 MHz 10 MHz), the available frequencies being dependent on the time of connection. The waves are transmitted with very low incidence angles, up to 20 (and very high elevation angles, between 70 and 90 ) [9] [10] [11]. On NVIS channels, the propagation is usually done in a single hop by refraction in the F2 layer. There are a lot of software applications on the market that predict the ionospheric channel. A selection of these applications accompanied by a brief description can be accessed at 1. VOACAP, ICEPAC and REC333 are part of the ITS HF Propagation applications suite that can be free downloaded from The applications of this suite estimate the monthly average value of MOF, which is called MUF (Maximum Useable Frequency), which obviously leads to a temporal variability of the results obtained according to the days of the month for which the prediction is made. The predictions made for the other parameters are software.htm [Accessed: August 20, 2015]

2 98 ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ, vol. 64 (2016), nr. 1 also monthly average values. The comparative results between the estimates made by VOACAP and the measurement values for NVIS channel [1] [12] [13], support the fact that VOACAP generally provides accurate estimates of the ionospheric channel, although there are some discrepancies in the estimation of the SNR. This paper presents the results of a study comparing three forecasting applications of HF propagation (VOACAP, ICEPAC and REC 533) with the measurements results on ionospheric channels of the NVIS type. 2. Materials and Methods A first set of measurements were obtained from the analysis of a NVIS link established in Romania, on June 4th, 2015, in the midst of the solar cycle, with maximum solar activities. The second set of measurements was extracted from the results published by [1]. They were obtained in a ham network, composed of three transmitting beacons and five receiving stations which operated on the frequency of 5290 MHz between 2009 and 2011, for 23 months, in conditions of minimum solar activity. The corresponding situations for each set of measurements were simulated using three HF propagation prediction applications: VOACAP, ICEPAC and REC533. After processing the data and comparing the simulation results with the measurement results, a number of conclusions was drawn concerning the prediction quality of the ITS HF Propagation applications suite Measuring the signal and SNR levels for the NVIS channel Sibiu-Lupșa There has been developed a system which automatically emitted, received and registered the signals and the SNR for HF NVIS communications. The emission system comprised a Rohde & Schwarz SM 300 signal generator, an AR 50 W 1000 B power amplifier, an antenna coupler and a cross dipole antenna for NVIS links of the Harris RF-1938 type, lifted 8.6 m above the ground [14] [15]. Harris RF-1938 is an antenna designed for HF NVIS links. The vertical directivity pattern is shown in Figure 1. 2 Harris Corporation, RF Communications Division, Specifications for RF -1936/1938 antenna, Figure 1. Radiation pattern in vertical plane for RF 1938 antenna 2 Horizontally, the directivity pattern is omnidirective, which simplifies the orientation process towards the correspondent. The system was designed to emit with a power of 20 W for three minutes every half hour on 9 frequencies for 24 hours. The used frequencies were: MHz, MHz, MHz, MHz, MHz, MHz, MHz, MHz, and MHz These frequencies were licensed and protected against other users in Romania, but they were obviously susceptible to noise and interference from users of other countries. The point of emission was Sibiu, at the coordinates N, E on June the 4th, The point of reception was located in Lupșa, a village situated 92 km away from Sibiu, at the coordinates N, E. The reception hardware was a Rohde & Schwarz FSH3 spectrum analyser and a broadband dipole antenna of the Diamond W330 3 type, lifted 6 m above the ground. The antenna was oriented at 120 (towards Sibiu) and the plane formed by the two feeders of the antenna was oriented horizontally so that the maximum radiation direction was vertical, which is favourable for NVIS links. Because of the limitations due to the sensitivity of the receiving system, which is close to the noise level of a 3 khz operating channel (-120 dbm) [16] [17], the following settings were used for the Rohde & Schwarz FSH3 spectrum analyser: detector type Root Mean Square (RMS), channel bandwidth 3 khz, sweep time (SWT) 5 s, resolution bandwidth (RBW) 10 Hz [18]. The reception was software controlled and synchronized with the emission using a Lab Windows CVI 2012 application. The application allowed modifying the parameters of the receiving system (frequency, sweep time, 3

3 ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ, vol. 64 (2016), nr frequency resolution, sweep mode, detector type, and amplitude of the reference level) according to the parameters of the transmitting system (power, emission frequencies, emission time and start and stop time of each emission session) so that measurements on each channel could be performed only during transmission. The measurement results, consisting of the value of the emission frequency, the power level of the received signal, and the time of registration, were saved in *.txt files that could be further processed The NVIS propagation prediction using ITS HF Propagation ITS HF Propagation is a suite composed of 3 different sets of applications for forecasting the propagation in the HF band, allowing the analysis of point to point propagation of the signal-interference ratio, of the radio coverage of a certain emission source, and of the area where an emission source should be placed so that the emitted signal can be received in a given location. In the same package there also is an application (HF Ant) used to generate files with the antenna parameters other than the default ones used. This paper used only the set of applications for point to point propagation VOACAP (Voice of America Coverage Analysis Program), ICEPAC (Ionospheric Communications Enhanced Profile Analysis and Circuit Prediction Program) and REC533 (ITU-RS Recommendation 533) with the purpose of comparing simulation results and identifying the propagation model that approximates best the sets of measurements. 3. Results From the measurement results obtained for the Sibiu-Lupșa direction, all the results for which the level detected was close to the noise level (SNR <3 db) were removed. The remaining results were averaged so as to obtain the signal level value for each time and each operating frequency. From the results obtained from the simulations only those were selected where the wave refracts in the F2 layer (to make comparisons in the same propagation conditions) and where the output parameters MUFday and Probability were higher than 0.03 (those for which the link is predicted to be possible for at least one day in the reviewed month). For the comparative analysis performed in this paper from the data published by [1], the results were obtained for transmissions made between the emitters and receivers identified in Table 1. Table 1. Network parameters [1] Month March 2010 Nov August 2009 January 2010 SSN Distance [km] Call sign GB3RAL GB3ORK GB3RAL GB3RAL Emission Coordinates N, 1.29 W N, 3.16 W N, 1.29 W N, 1.29 W Antenna gain 0 dbi 3 dbi 0 dbi 0 dbi Power 10 W 10 W 10 W 10 W Call sign G3WKL GM4SLV G3SET G4ZFQ Reception Coordinates N, 0.71 W N, 1.43 W N, 0.57 W N, 1.29 W Antenna gain 0 dbi -14 dbi 3 dbi 3 dbi The ionospheric channel parameters and the common parameters of the communications systems for the three applications are shown in Table 2. Table 2. Common parameters of the propagation analysis applications Parameter Value Coefficients CCIR Time UT Propagation path Short Reception noise -150 dbw Minimum antenna angle 30 Link reliability 90% Necessary SNR 38 Multipath tolerance 3 db Multipath delay 0.1 ms Absorption model Normal Antenna type Sample.00 The other input parameters have been modified according to the situations in which the measurements were made. To generate results, VOACAP and ICEPAC used Method 20, and REC533 used Method 6. The gain of each antenna was deducted from the directivity characteristics pattern corresponding to the angles of NVIS links forecasted by the applications. For example, for the Sibiu- Lupșa connection, according to the radiation pattern of Figure 1., at 83 (radiation angle forecasted by each of the three applications) the gain of the RF-1938 antenna for frequencies in the 3 MHz... 8 MHz band is between 0 dbi and 5 dbi. Similarly, for the antenna used at the receive side (of the Diamond W330 type)

4 100 ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ, vol. 64 (2016), nr. 1 the gain is between -2 dbi and -6 db. For link directions corresponding to experiment 1, the gain values obtained are recorded in Table 1. The losses on the radio frequency cables of 4 db for each antenna were also taken into account. The antenna used in all applications, both for emitting and receiving, was Sample.00 with isotropic pattern. The gain of each antenna has been set so as to take into account the attenuation induced by the antenna's power cord. To obtain valid prediction results with the VOACAP application, [19] insist on paying attention to the correct setting of the input parameters. Of these, the most important is the average number of sunspots (SSN - Smoothed Sunspot Number). The source providing the type of data that were used in the statistical validation of the application is 1.For the same reason, the set of coefficients to be used is CCIR, not URSI. In obtaining accurate predictions regarding the quality of the received signal, the SNR value is also very important. It is recommended to use values which are appropriate for the technologies used, according to [20]. For other situations it has to be known that the applications of the ITS HF Propagation suite can determine the level of external noise from the point of reception by setting the value corresponding to the type of environment, but for VOACAP and ICEPAC the user must consider the noise due to the bandwidth of the transmission because all analysis are made for a bandwidth of 1 Hz. In contrast, in the System menu, REC533 allows setting the radio frequency bandwidth of the transmitted signal. In order to reduce the number of variables, the input parameters for ICEPAC and REC533 were set as for VOACAP. The three applications provide a variety of output parameters in both graphics and text formats. For analysing the data, we preferred the text format (Circuit) in order to obtain data packets that allow the simultaneous analysis of multiple parameters. By using Method 20 for VOACAP and ICEPAC and Method 6 for REC533, the parameters targeted for further processing which occur in the output data packet provided for each hour and each frequency are: the signal level at the point of reception (dbw and dbu), the number of hops and the name of the layer where the wave is refracted, the height at which the refraction occurs, the angle of the radiation, the probability of link establishment, and the signal to noise ratio. 4. Discussion 4.1. Sibiu Lupșa measurements results versus the simulations results The connections made on frequencies above 7 MHz were strongly affected by propagation conditions and provided very few valid results (with SNR >3 db) during the measurement process, all occurring around noon. No valid results were obtained on the MHz frequency (Figure 2.). Figure 2. Values and differences between measurements and predictions on the Sibiu-Lupșa circuit on the MHz frequency Figure 2a presents comparative results obtained by measurements (green line) and by simulation (VOACAP - blue, ICEPAC - brown, REC533 - red) for the circuit established on the MHz frequency. Because one of the above mentioned conditions was not met, the results predicted or measured between 8 13 UT are not shown. Figure 2b illustrates a comparison of the standard deviations obtained by simulations 1 and by measurements. In Figure 2a, it can be noticed that the simulated results tend to follow the trend of the measured results for each of the three applications. All three applications provide higher power levels than those measured. The standard deviations shown in Figure 2b reveal that REC533 provides values closer to the measured ones than the other two applications. In general, the same tendency 2020.gif

5 ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ, vol. 64 (2016), nr was maintained for the results obtained on the other frequencies, too. For the statistical analysis of the prediction quality in relation with the measurement data, we used the mean values of standard deviation obtained by each application and the correlation factor between each predicted data set and measured data set. Only valid data were taken into account (if one result in one of the data sets was invalidated due to the conditions described above, the results obtained for the other data sets were also removed from the analysis). The means of standard deviation values obtained by each application for the valid data are: M VOACAP = 9.21 db, M IONCAP = 8.73 db and M REC533 = 5.94 db. After the correlation analysis, the following coefficients were obtained: r VOACAP = 0.34, r IONCAP = 0.35, r REC533 = The values obtained are within the lower part of the average domain of the correlation results, a fact which confers insufficient trust in the simulation applications. Because the measurements made for analysing the Sibiu-Lupșa link are conducted over a too short period of time (only one day) compared to the specific predictions by the three applications that provide average monthly output parameters, we consider that the results of this comparison are not representative. In order to overcome this drawback, this study takes into consideration the monthly mean values of the results obtained and published by [1] for the months of August 2009, November 2009, January 2010 and March Measurement results obtained by [1] versus the simulation results Figure 3 illustrates a comparison of the average values of the received signal power level obtained by measurements and by simulation with the three applications for August 2009 in the case of the GB3RAL-G3WKL link. Figure 3. Values and differences for the signal power measured and predicted for August 2009 Figure 3b presents the standard deviations of the forecasted data sets compared to the ones obtained by measurements for each of the 24 hours of the month. VOACAP and ICEPAC show a similar pattern, which is also confirmed by the correlation index between the forecasted data sets (r VOA_ICE = 0.99). Standard deviations vary with up to 18.3dB (ICEPAC at 7 o'clock) at a monthly average of 7.04dB for VOACAP and 7.89dB for ICEPAC. The set of results obtained by simulation correlates strongly with that obtained by measurements (r VOACAP = 0.84 and r IONCAP = 0.81). Significantly better results are obtained by REC533 for which, with an even higher correlation factor (r REC533 = 0.88), there are obtained standard deviations less than half of the ones obtained for VOACAP and ICEPAC. The same trend is observed for the results of January 2010, shown graphically in Figure 4. Figure 4. Values and differences for the signal power measured and predicted for January 2010

6 102 ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ, vol. 64 (2016), nr. 1 The results of the statistical analyses are presented in Table 3. Table 3. Standard deviations and correlation factors for predicted and measured results Month Parameter VOACAP ICEPAC REC533 August Std. dev. (db) Correlation Nov. Std. dev. (db) Correlation January Std. dev. (db) Correlation March Std. dev. (db) Correlation TOTAL Std. dev. (db) Correlation The correlation factors are lower than those obtained for August 2010, but are still in the area of strong correlation values. The standard deviations are higher than those for January. VOACAP and ICEPAC provide results with coefficients of determination (r 2 ) which indicate powers of prediction of over 50% for average standard deviations of db for VOACAP and db for ICEPAC. For January 2010, REC533 provides the most reliable results (r REC533 = 0.86), for standard deviation values half the values of the other two applications. Figure 4b indicates better standard deviation values for almost all hours of the day (an exception is 3 a.m. UT for VOACAP). The comparative results for November are shown in Figure 5. Figure 5. Values and differences for the signal power measured and predicted for November 2009 The correlation factors are still high. The monthly average values of standard deviations are between those for January 2010 and August 2009 and are almost equal to each other (see Table 3). The correlation factor obtained by REC533 is still the highest (r REC533 = 0.90 versus 0.79 for VOACAP and 0.78 for ICEPAC). The best results for the months studied are the results for March 2010 (Figure 6.). Figure 6. Values and differences for the signal power measured and predicted for March 2010 The correlation factors have very strong values (r >0.9) for each application (r VOACAP = 0.92, r ICEPAC = 0.91, r REC533 = 0.92) and the monthly averages of standard deviations have the lowest values reported to the other months for VOACAP and ICEPAC (see Table 1). The results analyzed separately for each month provide coefficients of determination (r 2 ) indicating powers of prediction of at least 50 %, thus confirming the quality of the applications in relation to the analysed data. Taken as a whole, for all the four month studied there are obtained means of standard deviation values of 7.41 db for VOACAP, 7.99 db for ICEPAC and 5.27 db for REC533, with values of the correlation factor in the range

7 ELECTROTEHNICĂ, ELECTRONICĂ, AUTOMATICĂ, vol. 64 (2016), nr Conclusions The study comparing the predictions for the power level of the signal received on ionospheric NVIS channels made by three HF propagation prediction applications (VOACAP, ICEPAC and REC533) leads to the conclusion that each application shows coefficients of determination (r 2 ) indicating powers of prediction of at least 50%, confirming the good quality of the applications for data analysis. The correlation coefficients obtained by the set of results provided by the three applications in relation to the set of results extracted from the measurements published by [1] are: r VOACAP = 0.83, r ICEPAC = 0.81, r REC533 = The means of standard deviations values obtained from the sets of simulated results compared to the measured ones are: M VOACAP = 7.41 db, M ICEPAC = 7.99 db and M REC533 = 5.27 db. The highest value of the correlation coefficient for REC533, associated with the lowest value of the general average standard deviations, support the idea that, out of the three applications analysed, this is the application that offers the best predictions. The results provided by the other two applications (VOACAP and ICEPAC) are similar. Statistically, this is confirmed by the very strong correlation coefficient (r VOA_ICE = 0.99) between their sets of results and by the very small difference between the average values of standard deviations (0.58 db). Looking at the details offered in Table 3 it appears that VOACAP offers predictions of a slightly better quality than ICEPAC. By comparing the measurement results obtained for the Sibiu-Lupșa direction with the simulated results there were obtained coefficients of determination indicating powers of prediction of less than 12.5 %, which is much below the acceptable quality level. We believe that this is due to the short period of time taken for making measurements in the Sibiu- Lupșa direction (only one day) compared with the predictions of the three applications, offering average monthly output parameters. To check this hypothesis we plan to conduct in the future a similar study in Romania, but on a much bigger scale, with a minimum duration of one year and taking into account NVIS channels with different connection distances. 6. Acknowledgment The present research was funded by the Ministry of Education and Research of Romania by UEFISCDI, project code PN-II-PT-PCCA , contract no. 292/2014 awarded to Nicolae Balcescu Land Forces Academy in Sibiu, Romania, for the period Refferences [1] 1 WALDEN MC, Comparition of propagation predictions and measurements for mildaltitude HF near-vertical incidence sky wave links at 5 MHz, Radio Sci., 47, RS0L09, doi: /2011RS [2] BRUNO Z, LJILJANA RC, Ionospheric Prediction And Forecasting, ISBN , , 2014 Springer, doi: / [3] ANTONIOU S, CHRISTOFI L, GREEN PR, GOTT GF, High rate data transmission in the mid-latitude NVIS HF channel, IEE Proceedings on Communications, Volume 153, Issue 2, pp , [4] ICART I, et al. Design and demonstration of a very high data rate multimedia HF communication system, IRST MAY 2012 [5] ANTONIOU S, CHRISTOFI L, GREEN PR, GOTT GF, "High rate data transmission in the mid-latitude NVIS HF channel," in Communications, IEE Proceedings-, vol.153, no.2, pp , 1 April 2006 doi: /ip-com: [6] SCHEIBLE, MP, TEIG LJ, FITE JD, CUOMO KM, WERTH JL, MEURER GW, FERREIRA NC, FRANZINI CR, High Data Rate, Reliable Wideband HF Communications Demonstration, Technical Paper, MITRE Corporation, 2014 [7] AUSTIN BA, "Evolution of near vertical incidence skywave communications and the Battle of Arnhem," in Science, Measurement and Technology, IEE Proceedings -, vol.149, no.2, pp , Mar 2002, doi: /ipsmt: [8] BURGESS SJ, EVANS NE, "Short-haul communications using NVIS HF radio," in Electronics & Communication Engineering Journal, vol.11, no.2, pp , Apr 1999 doi: /ecej: [9] BENTUM MJ, SLUMP CH, SCHIPHORST R, Near Vertical Incidence Skywave Propagation: Elevation Angles and Optimum Antenna Height for Horizontal Dipole Antennas, IEEE Antennas and Propagation Magazine, Vol. 57, No. 1, February [10] WALDEN MC, "Analysis of chilton ionosonde critical frequency measurements during solar cycle 23 in the context of mild atitude HF NVIS frequency predictions," in Ionospheric Radio Systems and Techniques (IRST 2012), 12th IET

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