NVIS PROPAGATION THEORY AND PRACTICE

Transcription

1 NVIS PROPAGATION THEORY AND PRACTICE Introduction Near-Vertical Incident Skywave (NVIS) propagation is a mode of HF operation that utilizes a high angle reflection off the ionosphere to fill in the gap between line-of-sight ground wave and long-distance skip sky wave communications. This mode of operation requires careful frequency selection, special antennas and in some cases, significant operator skill. We are all operating NVIS when we check into a state MARS net and can communicate with all stations (for example, the 0800 local net on KCE). This same frequency, KCE, does not presently perform well for early morning and evening nets because a certain controlling parameter of NVIS propagation is different for these two nets. German Panzer forces utilized NVIS propagation during WWII to maintain tactical communications at ranges out to 400 miles. The photo below shows a Panzer command vehicle with an NVIS loop antenna (looks like a canvas cover support frame). NVIS was more fully documented and used by US forces in Vietnam. A properly generated NVIS signal has a limited ground wave signal and the vertically propagating signal is very difficult for an enemy operator to Direction-Find. Definition NVIS propagation is considered to be F-layer ionospheric reflection at angles from 50º to 90º. The height of the F-layer of the ionosphere varies, as can be seen at the web site: A typical plot of the ionosphere for both day and night is shown in Figure 1. The 300km or mile height of the F2 layer allows NVIS propagation coverage out to a range of some 444 miles in all directions at an incident angle of 50º. 1

2 Figure 1: Day/Night Ionospheric Structure NVIS operation is optimized by understanding and controlling two factors: (1) accurate selection of the operating frequency, and (2) effective antenna design and placement. This document will cover NVIS propagation including selection of the proper frequency and other propagation effects. A separate document will cover antenna design with realworld examples of effective NVIS antennas. Frequency Selection Optimum NVIS propagation is achieved by operating at or slightly below the local Critical Frequency (CF). The CF is defined as the highest frequency signal that will reflect directly back to its transmission location as shown in Figure 2. Typically the signal will be reflected from the higher F-layer if the operating frequency is near the fof2 (frequency of the Ordinary wave reflected from the F2 layer) critical frequency. The CF is dependent on the intensity of the ultra-violet (UV) radiation from the sun and so varies with the time of day and day of the sunspot cycle. Increased UV radiation will increase the CF of the F-layer. The CF is measured by ionosondes located all over the world. An ionosonde measures the structure of the ionosphere directly overhead by transmitting a sequence of varying frequency pulses and then analyzing the echoes. Data from this 2

3 world-wide series of ionosondes can be found at: We are fortunate to have an ionosonde centrally located in Texas at Dyess AFB near Abilene. Clicking on Dyess iono.txt will bring up the last 24 hours of 15 minute data. The first four columns, as shown in Figure 3, are the date/time group of the measurement and the fifth column, fof2, is the Critical Frequency. Figure 2: Critical Frequency Definition 3

4 Figure 3: Tabulated Ionosonde Data Figure 4 shows a 24-hour plot of the CF for Oct. 3, 2006 during our current low sunspot period. When the 1 hour, daylight saving time bias is removed, you can see the reason for the poor NVIS performance of the Texas MARS 7 PM (local) and 6 AM (local) nets. Depending on the amount of UV ionization, and the exact time of the year, the rapid fall of the CF can perfectly coincide with our 7 PM Net. The extra deep drop to 2.5 MHz has been occurring near the beginning of the 7 PM net, preventing even KBN from being an effective NVIS frequency. From Figure 4 the following frequencies would be best for our nets: 7 PM (local) Net KBN or KAH, but expect periodic net failures 6 AM (local) Net KBN 8 AM (local) Net - KCE 1 PM(local) Net KCO or KDG As the days lengthen and we move back to CDST, we will recover the functionality of the 7 PM Net. The 6 AM Net only will have effective NVIS performance on KBN. 4

5 Dyess AFB Ionosonde Data (Oct. 3, 2006) Critical Frequency - MHz Time - hhmm (CDST) Figure 4: Critical Frequency Plot, Low Sunspot Cycle Period In contrast, Figure 5 shows the CF about 1 year before Figure 4, when the ionosphere was more highly ionized by increased UV radiation from the sun. Dyess AFB Ionosonde Data (Nov. 13, 2005) Critical Frequency (MHz) Time, local (HHMM) Figure 5: Critical Frequency Plot, Higher Sunspot Activity 5

6 Note that for this CF behavior, the following frequencies would be optimum for our net operations: 7 PM (local) Net KCE with a possible move to KBN near end of net. 6 AM (local) Net - KCE 8 AM (local) Net KDG 1 PM (local) Net KFF or KFH Another solar phenomenon, D-Layer absorption, limits our low frequency operation during the daylight hours. Most all the UV radiation from the sun is absorbed by the F- layer, producing the ionization we need for NVIS skip communications. The X-rays from the sun penetrate through the F and E-Layers and are absorbed by the D-layer. The ionization of the D-layer produces increased absorption of radio signals as they pass through on the way to and from reflections off the F-layer. D-layer absorption is inversely proportional to frequency squared (f 2 ), so operating at the highest useable frequency will minimize this loss. Figure 6 shows why longer distance stations will fade during the day. The lower slant angle path will travel a longer distance in the absorptive D-layer than a high angle wave, thus experiencing greater attenuation. Near the end of the 9 AM (local) nets, you may have noticed that some of the more distant net members begin to lose signal strength. This fading will grow worse as the sun rises, unless the net is moved up in frequency as we do during our extended /X and /E nets. Figure 6: D-Layer Absorption Note that unlike the CF phenomenon, D-Layer absorption can be partially compensated for by increasing transmit power or using modes that need less signal to noise ratio than SSB. Modes like CW, or certain digital modes (Olivia and MT-63), can be effective when SSB has faded. Fortunately, as night falls, the D-layer quickly dissipates (ions recombine), significantly increasing skip distances. The difference in re-combination rates between the D and F-layers at dusk is what produces Gray-Line enhanced propagation. For NVIS propagation users, we simply want to operate at the highest possible CF so as to minimize D-Layer absorption. In the low sunspot cycle period, we simply do not have much wiggle room and may have to accept limited NVIS ranges during the maximum sun period of the day. 6

7 The more general ionospheric-skip phenomena is shown in Figure 7. The Skip Zone is eliminated, as discussed above, by operating at or below the local CF of the ionosphere. If our operating frequency is above the local CF, then for higher incident angles, our signal will simply go straight through the ionosphere, never to return. The simple equation for this relationship is MUF = CF/cos Ө (Equation 1) where MUF Maximum Useable Frequency (between two locations) CF Critical Frequency Ө - Angle of the incident ray with a vertical line at the incident point Figure 7: Ionospheric Skip Propagation Note that if you know the CF and your operating frequency (above the CF), you can compute the skip zone dimension given the height (hmf2) of the ionosphere. For example, when the CF has drops to 2.5 MHz and I am attempting to operate on the 7 PM (local) Net at a frequency of KBN, my skip zone has a radius of 305 miles in every direction. From Austin, I will hear AAR6QE in far North Texas, but AAR6LN 90 miles away will not be detectable. The band goes-long, when the sun sets, because D- absorption and the CF both drop, allowing only longer distance stations to be received. To move traffic during this period of time it maybe necessary to go to another state s MARS net or to a distant MARS HF Winlink station. 7

8 Ionosonde Interpretation For Selection of Critical Frequency The actual ionosonde ionograms used to generate the data, shown in Figure 3, can be found at The two nearest Ionosondes are located Dyess AFB and Eglin AFB. The plots are stored by year, date and time. Typically a plot is available about 5 minutes after the actual measurement time. An idealized ionosonde plot is shown in Figure 8. Figure 8: Ionosonde Plot Definitions The Critical Frequency, fof2, is the frequency at which the Ordinary Wave reflection rises rapidly in height (Range). The Virtual height of the reflection is h F2. An actual ionogram from Dyess AFB is shown in Figure 9. 8

9 2 nd Echo Critical Frequency Electron Density (Height of F2 layer) MUF Chart Figure 9: Dyess AFB Ionogram Observe that the colors of the Ordinary and Extraordinary Wave plots are reversed from Figure 8. A typical second, ionosphere/ground/ionosphere reflection can be seen above the first reflection. If the controlling computer can interpret the data, the ionospheric parameters will be listed in upper left table. A very useful second table, the MUF Chart, can be seen in the lower left part of Figure 9. The parameter D is the skip (exclusion) distance in Km. For example, operating on a frequency of 5 MHz will result in a 600 Km skip zone around the transmitting station when the critical frequency is 3.8 MHz. Many times the computer is unable to interpret the date, but the plot is still available for your interpretation. Figure 10 shows an un-interpreted ionogram with a critical frequency of 3.8 MHz. 9

10 Computer failed to Compute critical frequency But you can do it! Critical Frequency Figure 10: Ionogram Without Computer Interpretation If the Dyess AFB Ionosonde is not reporting, a common occurrence, the ionosonde at Eglin AFB can be used. The longitude time between the Eglin AFB and Dyess AFB locations is 52 minutes, and the sun ionization moves East to West, so the data at Eglin one hour earlier will be approximately the same as present time at Dyess. If the sun and ionosphere are stable (no flares and the planetary K index is 1 or 2), the critical frequency for the previous 24 hour period can be used. Finally, if both ionosondes have not reported for several days and again the sun and ionosphere are stable, a Web software modeling prediction site, shown in Figure 11, can be used to get an approximate CF. 10

11 Figure 11: Web Critical Frequency Prediction Site Deployed MARS stations without Internet connectivity should rely on home based MARS stations with Internet connectivity to provide real-time critical frequency and other propagation information. If Internet connectivity is lost to all stations, then an approximate CF can be determined by HF stations received. You are operating below the local CF if you can hear stations outside your ground wave range,25-50 miles, but less than 100 miles. A pair of MARS stations at a range of about 100 miles can be directed by NCS to try assigned state MARS frequencies until connectivity is lost. Dropping down to the frequency below the connectivity loss frequency will be as close to the CF as the net can operate. Another option is to solve equation 1 for the CF using the highest receivable WWV signal. Use an F2 layer height of 186 miles and the range between your station and Fort Collins, CO to compute Ө. Note that the CF computed will be the CF of the F2 layer halfway between the two stations. Solar Effects on Ionospheric Propagation It is possible to obtain information about the factors that influence the ionosphere and to forecast what conditions might be like without using actual ionosonde soundings of the ionosphere. There are a number of indicators that are used and these include the solar flux index (SFI), a measure of the level of radiation from the sun, and the geomagnetic indices (Ap and Kp), a measure of the stability of the earth s magnetic field. The ultraviolet (UV) radiation from the sun makes ionospheric propagation possible but direct measurement of the level of UV radiation is difficult. Both the solar flux index and the sunspot number are indirect indicators of the level of UV radiation from the sun. The solar flux index is a measure of the level of radio noise from the sun at 2800 MHz. The 11

12 solar flux index has been found to be statistically related to the level of ionizing UV radiation reaching the ionosphere. Solar flux can vary from a low of about 65 to a high of 200. The smoothed sunspot number can vary from 0 to greater than 200. While UV radiation from the sun is beneficial, x-ray radiation, coronal mass ejections (CME) and high solar wind (coronal holes) can disturb or interrupt ionospheric propagation. The three types of disturbances the sun can cause are Geomagnetic and Ionospheric Storms, Solar Radiation Storms and Radio Blackouts. Geomagnetic and Ionospheric Storms CME s and to a lesser extent, coronal holes throw high-velocity charged particles into space. Figure 12 shows a CME striking the earth s geomagnetic field. Figure 12: CME Striking the Earth s Geomagnetic Field If this material impacts the earth s geomagnetic field, it can cause turbulence and disruptions in the ionosphere. The travel time for this material is between 1 and 2 days and its effects include depression of the Critical Frequency and increased D-absorption. Several satellites either located between the earth and the Sun (SOHO Solar and Heliospheric Observatory) or in a sun orbit on either side of the earth (STEREO - Solar TErrestrial RElations Observatory) watch for CME s and measure the solar wind. For solar wind velocity and other solar data See: There are two indices used to report the severity of the magnetic fluctuations in the earth s magnetic field. The K-index is a three hour measurement of the logarithmic variation of the magnetic field compared to that of a quiet day. Individual K-indices are measured around the world and a planetary or Kp index is reported. The Kp varies from 0 (very quiet) to 9 (major geomagnetic storm). The A-index is a linear number, derived from the K-index averaged over one day. The planetary A-index will vary from 0 (very quiet) to 400 (major storm). Solar Radiation Storms Solar disturbances may also eject vast quantities of high-energy protons. These typically take about four hours to reach the earth and will be directed by the earth s magnetic field into the polar regions. This will result in a Polar Cap Absorption event, a dramatic increase in D-absorption only in the polar regions. Unless very severe, only ionospheric propagation that passes through the polar regions will be affected. 12

13 Radio Blackouts A large M or X class solar flare will generate high levels of x-rays that will increase the D-layer absorption producing a radio blackout called a Sudden Ionospheric Disturbance (SID). A picture of a large Solar Flare can be seen in Figure 13. Figure 13: Solar Flare Since this radiation travels at the speed of light, there is no warning. This is a daylight affect and D-layer will quickly return to normal as soon the flare ends or the earth rotates away from the sun. The daily solar flux index (SFI), Kp and Ap and any other solar disturbances are reported on WWV at 18 minutes past each hour. The NOAA Space Weather Prediction Center at ay.html and other agencies maintain web sites that reports solar and geomagnetic conditions. Figure 14 and 15, from the NOAA site, show solar and geomagnetic conditions. Figure 14: Solar and Geomagnetic Conditions 13

14 Figure 15: Solar X-Ray Flux From Figure 14, a sudden increase in solar wind from a corona hole can be seen at time 0400Z SEP 28 first as a change in the GOES Hp (magnetic field) followed 4 hours later by a significant increase in the planetary Kp index to a value of 4. Any X-ray or proton emissions would have occurred several days before this event, but typically, a coronal hole event does not have a solar flare associated with it, so this event only caused minor changes in the ionosphere. Conclusions NVIS is a special form of ionospheric propagation that allows the use of the traditionally excluded or skipped zone of propagation to provide over-the-horizon communications out to ranges of approximately 400 miles. The operating frequency must be below the local Critical Frequency but high enough to minimize D-layer absorption during the daylight hours. The Critical Frequency is a function of the time of day and the day in the solar cycle and is best determined by real-time measurements using a local ionosonde. Disruptions of NVIS and normal long range propagation can occur during solar storm events like Coronal Mass Ejections and Solar Flares. 14