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Oblique sounding of an auroral ionospheric HF channel
Journal of Atmospheric and Terrestrial Physics, Vol. 57, No. 1, pp. 51~i3, 1995
~ Pergamon
Copyright © 1994 Elsevier Science Ltd Printed in Great Britain. AU rights reserved
0021-9169(93)E0020-A
0021-9169/95 $9.50+0.00
Oblique sounding of an auroral ionospheric I-IF channel BENGT LUNDBORG,* MATS BROMS and HARALD DERBLOM National Defence Research Establishment, Department of Information Technology, P.O. Box 1165, S-581 11 Link6ping, Sweden
(Received infinal form 28 September 1993; accepted 22 November 1993) Abstraet-49bservations of the HF skywave ionospheric channel during one year over three paths within Sweden are reported. The major aim of the work was to gain insight into prevailing propagation effects on representative fixed circuit paths at auroral latitudes, with a view to being able to assess prediction accuracies, and to exploit abnormal propagation modes so improving circuit reliabilities. The main instruments in the study were chirpsounders, HF FM~CW sounders covering the frequency range from 2 to 30 MHz. A data base of about 250,000 recordings was built up during the campaign, which covers the maximum of solar cycle 22. Some measurements were made with transmissions on fixed frequencies, mostly > 20 MHz.
1. I N T R O D U C T I O N
The ionosphere has been used for many decades as a propagation medium for H F electromagnetic waves. Its great variability, with geographic location, time of the day, season and solar activity, is well known and studied. The severe: limitations this may impose on frequency selection in H F communication are also well known. Observations of the H F skywave ionospheric channel have been carried out during one year over several paths in Sweden as shown in Fig. 1. The campaign ran during 1989-19!)0 and thus covered the maximum of solar cycle 22. The major aim of the work was to gain insight into prevailing propagation effects on representative fixed circuit paths at auroral latitudes, with a view to being able to assess prediction accuracies, and to exploit abnormal propagation modes so improving circuit reliabilities. The main instruments in the study were chirpsounders: H F F M - C W sounders from BR Communications, U.S.A., covering the frequency range from 2 to 30 M H z with a sweep rate of 100 k H z s -l. The output power of the transmitters is 100 W and the signals are sampled during 1 s for each spectrum. Hence the chirpsounders are more sensitive and detect much weaker signals than traditional, pulsed sounders ; the difference may be as great as 15-20 dB. In general, one oblique ionogram (graph showing received signal power vs group delay and signal frequency) for each
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Fig. 1. The major paths used for the chirpsounders were: Kiruna to Lycksele (KI --, LY, 370 kln), Kiruna to Uppsala (KI ~ UP, 900 km) and t)stersund to Ursvik (OS--¢ UR, 460 km). The paths LinkSping to Lycksele (LI--* LY, 670 km) and Link0ping to Uppsala (LI ~ UP, 225 km) were used only occasionally.
* Also at: Swedisl~. Institute of Space Physics Uppsala Division, S-755 91 Uppsala, Sweden. 51
52
B. LUNDBORGet al.
path was generated every 5 min and stored on floppy disc. In this way, a huge data base of about 250,000 recordings was built up during the campaign. The transmitters were located in Kiruna (67.8°N, 20.4~'E geographic coordinates; 64.5°N, 103.4°E corrected geomagnetic coordinates [CGCS]), Ostersund (63.2°N, 14.7°E geogr. ; 60.1°N, 95.9°E CGCS) and Link6ping (58.3°N, 15.5°E geogr.; 54.8°N, 94.1°E CGCS). The reception points were Lycksele (64.6°N, 18.9°E geogr.; 61.3°N, 100.1°E CGCS), Uppsala (59.8°N, 17.6°E geogr. ; 56.3°N, 96.5°E CGCS) and Ursvik (Stockholm, 59.4°N, 18.0°E geogr.; 55.9°N, 96.6°E CGCS). The paths Kiruna ~ Lycksele, Kiruna ~ Uppsala and LinkOping ~ Lycksele all lie within the sector between geomagnetic NE SW and NNE-SSW. The path Ostersund ~ Ursvik is very near geomagnetic N - S and the path Link6ping Uppsala is directed along geomagnetic WSW-ENE. When the magnetic activity reaches Kp ~ 4, the midpoint of the path Kiruna ~ Lycksele lies within the auroral oval. With an activity as high as K o ~ 7, also the path Kiruna ~ Uppsala has its midpoint in the auroral oval. The minimum of the trough is generally located between Lycksele and Uppsala. This information concerning the trough and the auroral oval was extracted from GOODMAN (1992, Sects. 2.4 and 3.4). Some measurements were made over the path Ostersund ~ Uppsala with beacon transmissions on fixed frequencies, mostly above 20 MHz, but some recordings were made at ~ 5 MHz. In these experiments the radiated power was typically 200 W and the receiver was a Rohde & Schwarz EK 070. This paper highlights the significant results of the experiments and discusses their possible consequences for H F communication. The full presentation of our results can be found in a series of five reports in Swedish, published by the National Defence Research Establishment (FOA) (DERBLOM et al., 1991a-e); cf. also LUNDBORG et al. (1992). Theoretical analyses of the chirpsignal and its spectrum are given by POOLE (1985) and by LUNDBORG and LUNDGREN (1991, 1992).
2. REGULAR C O N D I T I O N S
Communication via the ionosphere is usually affected by various kinds ofmultipath propagation; signals arrive at the receiver over several paths with differing delays and phases. For normal F-layer heights and distances within Sweden (typically < 1000 km) the difference in group delay between single-hop and multihop paths is 1-2 ms, which may cause intersymbol interference or fading. The situation is very much the
same for multipath involving several ionospheric layers. Multipath also arises through the low and high rays which are both, in principle, present for each reflecting layer. In practice, however, the high ray is weak, and only significant close to the maximum usable frequency (MUF). Whilst fixed frequency systems are sensitive to time-delay spread, frequencyhopping radios with high enough hop rates are less sensitive to such multimodes. They lock on to one of the paths and have shifted to another frequency before the other mode or modes arrive (JOHANSSON, 1986; LINDBLAD, 1986, 1989). Another regular feature of ionospheric propagation is the magneto-ionic splitting of the radio wave into ordinary and extraordinary modes propagating over slightly different paths. These two modes may overlap completely or partly, or show up separated in time. Figure 2 shows examples of fixed-frequency recordings, where the effects of O/X-mode splitting can be seen. The O/X delay differences are clearly much smaller than the multihop delay differences, and frequency hopping is often insufficient to separate the two signals. A study of the fading patterns, that arise due to the O/X delay differences, can be found in LINDSTROM (1992). This effect can obviously be disastrous in digital communication, but may be cured by the use of antenna arrangements which restrict transmission and/or reception to one of the two modes. We illustrate these regular ionospheric phenomena by a number of oblique ionograms in Fig. 3. Our chirpsounder network lacked absolute synchronism between the transmitters and the receivers, and hence the signal delays along the vertical axes are only relative. In the figure, we see examples of multihop in A, B and D, multiple layers in C and magneto-ionic splitting as horizontally translated duplicate traces in all of these cases. The relative position of the O- and X-mode traces shifts with propagation distance and maximum observed frequency (MOF). For propagation over the path Kiruna ~ Uppsala during a particular winter day and night, the differencefx-fo was found to vary between 0.7 MHz when the M O F was low ( ~ 5 MHz) and 0.4 MHz when the M O F was high ( ~ 15 MHz) ; however, the spread around these values was large (DERBLOM et al., 1991a, Sect. 8.3). This seems to agree, at least qualitatively, with the geomagnetic-field dependence discussed in the QL and QT approximations by DAWES (1990, Sect. 6.4). Traces of the high ray are present in A and D as a continuation of the 1-hop traces beyond the limit of the M O F towards higher delays and lower frequencies. In C both the F1- and F2-1ayer traces show hints of the high ray. Wave propagation over Swedish territory is regu-
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Fig. 2. Two cases of transmission of a 1 ms pulse over the path Ostersund ~ Uppsala at ~ 5 MHz. The 12.5 kHz IF output of the receiver was recorded by a plotter connected to an oscilloscope; this frequency is seen in the regular oscillations of the curves. Both diagrams show the 1-hop (left) and 2-hop (right) signals separated by ~ 1.5 ms. For each of these paths the O- and X-mode signals can be seen partly overlapping and interfering with each other.
larly affected by a phenomenon called the ionospheric trough, a wide region of markedly reduced electron density. The trough has a mainly magnetic east-west orientation and can often be seen in the vertical ionograms from Lycksele and Uppsala, where it shows up as a 'double ionosphere' with differing critical frequencies and virtual heights. Signals travelling obliquely through such a region with horizontal gradients also show these features ; see Fig. 4 for an example. All ionograms throughout the campaign (one recording every 5 min for each path) have been scaled with a semi-autornatic program in order to obtain
statistics about the various propagation modes; Fig. 5 shows an example of this condensation of data for one day. The regular MOF (the + data in Fig. 5) has been subsequently extracted from the data files and presented in the form of monthly statistics for each of the three paths in DERBLOM et al. (1991a). As a comparison, the prediction program IONCAP (TETERS et al., 1983) has been used to produce median M U F curves for the same paths and time periods. A typical example for 0stersund ~ Ursvik in November 1989 is seen in Fig. 6. These statistics verify that the coarse features of ionospheric propagation and their variations are well understood. The comparison
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Fig. 3. Oblique ionograms for regular ionospheric conditions. The horizontal axes show frequency in MHz and the vertical axes relative group delay with 1 ms between divisions. The small graph above each flame reflects the integrated signal power as a function of frequency. The situations shown are for Ostersund --* Ursvik A winter day and B winter night, and for Kiruna --*Uppsala C summer day and D summer night. In frame B a short horizontal trace is seen at ~ 14 MHz ; this is caused by meteoric reflection.The horizontal trace covering the interval 9-28 MHz in C is due to normal sporadic E, showed that the median frequency prognoses agree roughly with the median of the measured data, with the best fit found during the summer months. In wintertime, the daily excursions from the median are great, however, and span several MHz; on some days the MOF falls totally outside the traditional pattern• This is well illustrated by Fig. 6. In frequency prognoses, FOT (defined as the lower decile of the MUF) is considered a more realistic measure of practical band availability. Hence, DERBLOMe t a l . (1991a) also give diagrams comparing FOT, as predicted by IONCAP, to the lower decile of the MOF data. The agreement was fairly good for the more mid-latitude path 0stersund ~ Ursvik and less accurate for the other two paths. During quiet conditions, the maximum bandwidth If'"
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for communication purposes is not set by the ionospheric medium but by external interference and equipment limitations. Theoretically, the dispersion of the quiet ionosphere (group-delay slope with frequency) would permit bandwidths ranging from a few tens of kHz up to about 1 MHz for single-Path communication, except near M U F where the bandwidth tends to zero (LUNDBORG, 1990). Experimental data for mid-latitude paths give values in reasonable agreement with this (LYNCH e t a l . , 1972; SALOUS, 1989). The pulse broadening found by ENDERSBEEe t a l . (1989) for a path between the U.K. and Australia would give much lower bandwidths than this; the reason for this is probably multipath mixing rather than dispersion. Realistic bandwidths for propagation through the auroral region are as low as a few kHz, even under rather favourable conditions (JODALEN and TI-mANE,1991).
3. SPORADIC E
Fig. 4. Propagation Kiruna ~ Uppsala through the trough region. Horizontal gradients in the reflectionregion give rise to additional multipath.
Sporadic E (Es) refers to ionization, caused by nonregular mechanisms, at E-layer heights. In spite of its name, the phenomenon has many non-sporadic properties; three types, with distinct features, occur fairly regularly in the data. These are the normal, temperate latitude Es, meteoric ionization at E heights and auroral sporadic E. New, adaptive systems will
55
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Fig. 5. Example of condensed data (288 ionograms) for 11 January 1989 over the path ()stersund --* Ursvik, LOF (Lowe,;t Observed Frequency) is given by dots and MOF (Maximum Observed Frequency) for the F layer by + signs. At each time the vertical straight line shows the range of frequencies over which sporadic E was observed. At each time the short horizontal bar ( - sign) shows the highest frequency for traces of spread in the ionogram.
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Fig. 5. The dots in the figure show the regular MOF values over the path Ostersund ~ Ursvik for the entire month of November 1989. The thick solid curve is the median values of these data. As a comparison, the thin curve shows the predicted median MUF for the same path and time period.
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probably be designed to make automatic use of sporadic E openings. The normal, temperate latitude Es is formed by action of the neutral wind pattern which collects longlived metallic ions into thin layers at about 100 km height. The horizontal extent of such an Es patch is of the order 100-200 km. The formation is favoured during the summer, peaking during June-July and during daytime with two maxima commencing at prenoon and late afternoon hours. The lifetime varies with the season and is up to 5 h during July and <0.5 h in January. The lifetime is frequency-dependent and decreases rapidly with increased frequency. An example of this kind of Es is the straight distinct trace in Fig. 3C. Communication via normal Es is characterized by low dispersion, which permits high bandwidths. Multipath with sporadic E requires several Es patches along the path between transmitter and receiver. In such cases the delay differences will be very small, only fractions of a millisecond, since the reflecting layers occur at low heights. Mixed paths, involving combinations of Es and normal F-layer reflection also occur with the same typical small delay differences. This may cause problems with fading due to interference between the multipath signals. Because of the small group delay differences, such problems may also be expected with the frequency hopper discussed earlier. Because of the high sensitivity of the chirpsounders, a lot of the traces recorded as Es are rather due to meteor ionization, that is, they are signals reflected by the ionized trails created by meteoric impact at Eheights. Meteor trails have a short lifetime and the signals show up as horizontal traces at E-layer heights covering a few MHz stochastically in the frequency range of the sweep. Due to the short lifetime, each meteor trace is only seen in one ionogram in a sequence ; the short horizontal trace at ~ 14 MHz in Fig. 3B is an example. In our fixed-frequency measurements at 24.5 MHz, using a 0.2 kW transmitter and a total antenna gain of 10 dB, on average 1.8 meteors were recorded per min with a mean lifetime of 2.6 s ; the resulting mean duty cycle is 8 per cent. Hence, rather few meteors have the lifetime of 10 s needed to produce the trace pointed out in Fig. 3B. It is quite possible to use the concept of meteor burst communication with high bandwidths on HF, but the standard equipment has to be revised and adapted. The possibility of using this mode on H F has also been pointed out by WILLIAMS (1981) and CANNON (1989). Figure 7 is representative of a third possibility of communication via sporadic ionization at E-layer
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Fig. 7. Example of auroral sporadic E for the path Link6ping ~ Lycksele. heights, which occurs predominantly during nighttime in connection with aurora. Auroral activity is infamous for its detrimental effects on communication through absorption and spread. However, certain phases of the auroral evolution, visually characterized by smooth stable arcs, show low absorption and high ionization in a layer some 10 km thick at E-heights, a layer which is well suited for communication. The mode is called auroral sporadic E and is in general recognized in the ionograms by weak regular traces and an Es-trace which is more fuzzy than the temperate latitude Es-trace, often showing some dispersive slope and running up to rather high frequencies. Signals transmitted via auroral Es frequently show spectral broadening, interpreted as multiple scattering of the radio signal within the layer. Hence this propagation mode does not allow as high data rates as the normal Es. Statistics of the experimentally recorded E s - M O F and the Es occurrence frequency, based on the scaling ofionograms illustrated by Fig. 5, have been produced and presented in the form of diagrams by DERBLOM et aL (1991b). These show the variation of the probability of Es with time of the day, season, frequency and propagation path ; see Fig. 8. No distinction has been made between various types of Es in this presentation. Thus, during the winter nights the meteoric ionization and auroral Es dominate the statistics. Below we give some conclusions that could be drawn from these graphs. The short northernmost path Kiruna ~ Lycksele (370 km) has high Es occurrence during winter nights, > 80 per cent during some hours at low frequencies, whereas the winter daytime values hardly exceed I0 per cent even at low frequencies. The summer level lies at some 30-40 per cent more evenly distributed in frequency and time. The long path Kiruna ---,Uppsala (900 km) differs somewhat from this picture. The winter daytime Es occurrence is rather similar ,~ 10 per cent but the night-time values reach only 20-30 per cent. Daytime Es frequency is considerably higher for the rest of the year, even exceeding 80 per cent during
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Fig. 8, Monthly average of Es incidence as a function of frequency and hour of the day. The Case shown is for the path Kiruna --*Uppsala, September 1989.
some hours in July; the mechanism involved here is the normal, wind-driven sporadic E. The more southern path 0stersund ~ Ursvik (460 km) is rather similar to the Kiruna ~ Uppsala path. In all three paths an unexplained minimum can be seen, centred on 18 UT. Another common feature in the data, when the Es covers a large frequency interval, is a certain periodicity in the fi'equency dependence. This is possibly due to HF interference, since the occurrence dips show some correlation with the broadcast bands. Another explanation for this could be efficiency resonances in the broadband dipoles ; they were of the same type at all the sites. Statistics on Es occurrence, over a 10-year period 1966-1976 and obtained with the three Swedish vertical ionosondes, have also been summarized by DERBLOM et al. (1991b). These ionosondes are of the pulsed type, and were not able to detect meteorreflected signals. The average occurrence frequency of the other two type,; of Es (normal + auroral) was ~ 57 per cent. The normal, temperate latitude Es dominated over auroral Es in Uppsala and Lycksele, whereas the opposite was true for Kiruna.
4. S P R E A D F
Irregularities, electron-density fluctuations of almost any size embedded in the large scale ionospheric structure, influence the propagating radio signal and cause spreading. This phenomenon has long been known and named as spread F ; see, for example, DAVIES (1990, Sect. 5.6) for a discussion of this. It is perhaps the most important source of problems in the high-latitude HF traffic, especially during the winter nights. Moderate spread F is seen in the ionograms as a broadening of a varying degree of the regular traces and is interpreted as due to irregularities along the normal ray path, which split a single ray into a multitude of close-lying rays; see Fig. 9A and B. When the irregular ionization becomes stronger, signal power may be reflected for frequencies much higher than the traditional MUF, even if the ionization is located far away from the regular reflection region. Spread associated with such strong aurora extends beyond the normal ionogram traces with significant signal levels up to much higher frequencies ; the delays
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may also become longer than for the normal paths. Figure 9C and D shows examples of this type of spread. In the latter cases the concept of spread F may not be quite adequate, but in spite of this all these situations are here treated together, since they only represent different levels of irregular ionization. Note that the spread F phenomenon is highly dynamic, so that considerable temporal evolution occurs during the 280 s of the sweep of an oblique ionogram. Also note that, as a result of the particular chirp technique, range and Doppler spread occur in a mixed way in the recordings. The presence of spread, as exemplified by Fig. 9 A D, was noted in the routine scaling of the oblique ionograms. For each ionogram, spread was hence described by the frequency interval within which the fuzzy traces were seen. From these data, mean values of the incidence of spread for each month, as a function of sounding frequency and time of the day, were derived and presented as diagrams by DERBLOM et al. (1991c); see Fig. 10. Daytime solar radiation is a strong homogeneous ionization factor, which has an inhibiting effect on the irregularity formation. Precipitation of auroral particles, which is a cause of irregular ionization, has its maximum during the late evening hours. Thus the occurrence frequency of spread F is high during winter nights, > 70 per cent at low frequencies, but exceedingly small during summer daytime. On the Kiruna --* Lyeksele path, the spread reaches even above the limit 30 MHz of the chirpsounder for a significant part of the winter night hours. Spread F signals fluctuate rapidly in time, due to the rapid motion and turbulence of the irregularity
structures. FFT-spectra of the received signal during such conditions show that the signal is broadened in synchronism with enhanced spreading activity in the ionosphere. A CW signal can become spectrally broadened by more than + 50 Hz. In general, this broadening is symmetric; see Fig. 11 and DAVIES (1990, Fig. 9.31). During a strong disturbance one would expect phase variations up to 1000 radians, fading periods of 0.2 s, fading correlation distances of the order of metres and correlation bandwidths of only 50 Hz; see BOOKERand TAO (1987) and BOOKER et al. (1987). If pulsed (digital) transmission is used, mixing of individual data dements is unavoidable under such conditions. If the disturbance is moderate, the major part of the spread signal components may be removed by clever processing of the signal. An uncontaminated signal component having an amplitude some 10-15 dB above the spread components, as in Fig. 11 C, is frequently observed. One way to remove the spread components might, therefore, be simply to reduce the transmitter power or introduce discrimination at a certain amplitude level in the receiving system. 5. SCATI'ERING
Some roughness is present in the ionospheric structure even under smooth conditions. This gives rise to forward scattering (ionoscatter) on frequencies higher than the traditional M U F , with contributions to the signal from the ionospheric volume common to the transmitter and receiver antenna beams. The signal level is low, according to estimates 50-60 dB below the normal ionospheric wave. Our measurements confirm
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Fig. 10. Mo:athly average of the incidence of spread as a function of frequency and hour of the day, i.e. the percentage of time during a particular month for which spread was present at a particular hour and a particular frequency. The case shown is for the path Kiruna ~ Lycksele, November 1989. that ionoscatter is hardly an attractive alternative, if based on traditional HF equipment. A more promising mode for communication above the traditional M U F is ground backscatter or sidescatter. The mechanism involved is low-angle radiation being transmitted to large distances via the regular ionospheric layers and then scattered back by structures on the ground or sea surface to the receiver at closer range (DAVIES, 1990, Sect. 6.8). Measurements and a theoretical analysis concerning this type of propagation mechanism are reported in a recent work by GIBSON and BRADLEY (1991). Our beacon transmissions show that ground scatter may be an alternative for co~amunication when there are severe disturbances on the normal path. Figure 12 shows two recordings, each covering 24 h, of the received amplitude with our 24.5 MHz beacon over the path Ostersund -~ Uppsala. The signals were F F T analysed anti the peak level of each spectrum monitored and subsequently plotted by a strip chart recorder. With this technique, the practical bandwidth is the resolution ~',0.5 Hz of the FFT, and hence very weak monochromatic signals, undetectable with tra-
ditional receiver bandwidths, could be recorded. Figure 12A shows a day with undisturbed conditions. The night-time signal is caused by the fairly regular inflow of meteors. At dawn the ionization sets in and the long ranges open up for the groundscatter mechanism; the signal then rises to a rather steady level some 10-20 dB above the meteor-reflected signals. The recorded levels were highest when the Yagi antennae were beaming in the same direction (e.g. both southwards or both westwards) and considerably lower when they were directed toward each other on the direct path. Figure 12B shows a disturbed day; when the groundscattered signals drop due to absorption (of the long-range rays), the meteor-reflected signals (from closer ranges) can also be seen in the daytime. The groundscatter signals are weak, but always detectable in the daytime even at the high frequency used here. Sometimes the signals are spectrally broadened, but not to the extent found for auroral spread. Occasionally the peak is Doppler-shifted, indicating ionospheric bulk motion. Backscattering of HF signals by solid structures has found applications in over-the-horizon radars. To our
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0 25 50 -50 -25 0 25 50 Hz Hz Fig. 11. Broadening of a CW signal for various degrees of ionospheric disturbance, increasing through the frames A-D. The examples shown are for the path Ostersund -~ Uppsala, recorded on different occasions in September 1989. Propagation is via the F-layer with ~ 5 MHz signalfrequency.The span of the frequency axis is 125 Hz and the vertical axis spans 80 dB. The peaks at +50 Hz are probably due to a.c. hum from the transmitter power supply.
knowledge, HF ground backscatter/sidescatter has not been used for communication before, although our measurements have shown that this is clearly feasible. With usual HF transmitter powers of the order of 1 kW and simple antenna configurations narrow filtering is required in the receiver, so only very low data rates can be used. The backscattered signal energy (analysed with 0.5 Hz resolution in the fixed-frequency recordings) is spread out over some milliseconds (i.e. some 100 Hz) with the chirpsounder technique. Hence, one may expect some 25 dB lower S/N ratio with this technique than with our fixed-frequency recordings, so the backscatter/sidescatter mode is perhaps more difficult to detect with chirpsounders. Furthermore, it is often difficult to distinguish groundscatter from auroral spread in the ionograms. Analysing the geometry of the backscatter/sidescatter mechanism, one finds that scattering can be received from regions located at distances greater than the skip distance from the receiver as well as the transmitter; see DAVIES (1990, Sect. 6.8). Hence the signal shows more delay than the traditional ionospheric signal. The echoes appear in our ionograms somewhere at the end of the 2-hop F-
trace and extend to higher frequencies with increasing delays ; see Fig. 13. 6. CONCLUSIONS Below, we summarize the results and emphasize points relevant for communication within the HF band. Several of these points are well known, but are substantiated by an abundance of new data in the background reports (DERBLOMet al., 1991a-e) ; other points are new. (1) The coarse features of ionospheric propagation and their variations are well known for the paths studied. The usual median frequency prognoses agree roughly with the median of the measured data. The daily excursions from the median are, however, great, and span several MHz. It seems therefore that priority should be given to the development of methods which predict the deviation from the median rather than the median itself. (2) During quiet conditions the maximum bandwidth is not set by the ionospheric medium but by interference and equipment limitations. Theoretical bandwidths for the smooth ionosphere seem to be of the order of a few tens of kHz up to 1 MHz.
Oblique sounding of ionospheric HF channel
-
-F.~.~IIJ~"I:
........
, .
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6
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15
18
21
24
12 15 18 21 24 Hours (UT) Fig. 12. Two ~:ecordingsof the received amplitude over the path 0stersund ~ Uppsala at 24.5 MHz. Each panel covers 24 hr (midnight-midnight) and the amplitude scale is l0 dB per division. Panel A shows a day with quiet conditions, whereas B shows a disturbed day.
(3) Multipath propagation is a regular feature of ionospheric communication. For normal F-layer heights and distances within Sweden, the differences in group path-delay are of the order of 1-2 ms, which may cause problems with the mixing of information in a data stream. For sporadic E propagation typical differences between multihops are only fractions of a ms.
(4) Another regular feature of the ionosphere is the magnetoionic splitting of the radio wave into O- and X-mode rays. These two modes may overlap completely or partly or show up at the receiver separated in time. The result raay be disastrous for digital communication, but may be cured rather easily on fixed
s r~-.,~!-:~ljf~.-:--~: 4t
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Fig. 13. Example of backscatter/sidescatter signals in an oblique ionogram. This case is a night-time recording over the path Kiruna -, Uppsala.
links by the use of antenna arrangements which restrict transmission and/or reception to one of the two modes. (5) New, adaptive systems will probably be designed to make automatic use of sporadic E openings. The evaluation of these experiments has resulted in extensive statistics over the regular features of sporadic E. As expected, the occurrence of temperate latitude sporadic E is high during the summer. (6) Meteor-reflected signals are frequent within the HF-band. According to our measurements at 24.5 MHz, using a 0.2 kW transmitter and a total antenna gain of l0 dB, the mean duty cycle seems to be 8 per cent. It is quite possible to use meteor burst communication on HF, but the standard equipment has to be revised and adapted. (7) Auroral Es is a predominantly wintertime, night-time phenomenon which shows correlation with auroral activity. It is well suited as a reflector for HF communication up to rather high frequencies, but with lower data rates than for the other two kinds of Es. (8) Spread F is, perhaps, the most important source of problems in the high-latitude H F traffic, especially during winter nights. Its degrading influence increases with auroral activity. The ionograms show that there is signal present under these conditions, however, with
62
B. LUNDBORGet al.
severe spreading in time as well as in the frequency domain. Because o f the high occurrence o f spread F, priority should be given to finding ways o f m a k i n g use o f spread F signals. (9) G r o u n d backscatter/sidescatter m a y be a n alternative for c o m m u n i c a t i o n on frequencies higher t h a n the M U F when there are severe disturbances o n the n o r m a l path. The signal is weak but r a t h e r stable during daytime on the frequency 24.5 M H z of our tests. This p r o p a g a t i o n m o d e is clearly quite feasible with n o r m a l t r a n s m i t t e r powers a n d a n t e n n a configurations, provided t h a t very low data rates are used. Acknowledgements--This study has been performed with financial support from the National Defence Material Administration (FMV) and with personnel, localities and
equipment from the National Defence Research Estabhshment (FOA) in Link6ping and the Uppsala, Lycksele and Kiruna divisions of the Swedish Institute of Space Physics (IRF). In particular we wish to thank Lennart Elfberg and his colleagues of the Technical School of the Swedish Army (ATS) in Ostersund for their enthusiastic support to the project in the form of transmissions on various frequencies and for running the chirpsounder. The chirpsounder transmitter in Kiruna has been operated by Inge Marttala, IRF. Georg Lejemo, Telub, has been responsible for the recordings in Ursvik. Thanks are due to Stig Boberg and Bj6rn Johansson at FOA, Inger Arlefj~ird IRF and/~ke Hedberg, formerly at IRF, for their work with computer programs for data collection, scaling and presentation. The huge data volume has been scaled by Ove Klang, Robert L~,ngstr6m and Kurt Lundgren at Lycksele Ionospheric Observatory, Eva Arlefj~ird and Inger Arlefj~ird at 1RF, Uppsala, Lars Knutsson, Ing-Marie Mildh at FOA and Rune Ullander, Teleplan.
REFERENCES
BOOKER H.G. andJING-WEITAo
1987
BOOKER H. G., JING-WEI TAO and BEHROOZI-TOOsIA.
1987
B.
CANNON P.S.
1989
DAVIES K. ENDERSBEEB. A., FARMER F. T. and O'SHANNASSYJ.
1990 1989
GIBSON A. J. and BRADLEYP. A.
1991
GOODMANJ. M.
1992
JODALEN V. and THRANE E. V.
1991
JOHANSSONB.
1986
LINDBLAD A.
1986
LINDBLADA.
1989
LINDSTROM R.
1992
LUNDBORG B.
1990
LUNDBORG B., BROMsM. and DERBLOMH.
1992
LUNDBORG R a n d LUNDGRENM.
1991
A scintillation theory of the ~ding of HF waves returned from the F-region: receiver near transmitter. J. atmos, terr. Phys. 49, 915-938. A scintillation theory of fading in long distance HF ionospheric communications. J. atmos, terr. Phys. 49, 939-958. Morphology of the high latitude ionosphere and its implications for HF communication systems, l E E Proc. I 136, 1-10. Ionospheric Radio. Peter Peregrinus, London. Pulse transit times and waveform observations for ionospheric propagation between England and Australia. J. atmos, terr. Phys. 51, 291-295. A new formulation for above-the-MUF loss. Fifth International Conference on HF Radio Systems and Techniques, l E E Conf. Publ. No. 339, pp. 122-125. H F Communication, Science and Technology. Van Nostrand-Reinhold, New York. A study of the high latitude ionosphere using a digital spread spectrum ionospheric sounder. Fifth International Conference on HF Radio Systems and Techniques, l E E Conf. Publ. No. 339, pp. 136-140. Introduction of a HF frequency hopping system and its basic parameters. Proc. Nordic Shortwave Conference, HF86 (WALDE C.-H., Ed.), pp. 171-172. An experimental HF frequency-hopping system. Proc. Nordic Shortwave Conference, HF86 (WALDEC.-H., Ed.), pp. 173-178. A DSP implementation of a FSK correlation detector. Proc. Nordic Shortwave Conference, HF89 (WALDE C.-H., Ed.), pp. 205-213. Wideband modelling of the ionospheric channel. Proc. Nordic Shortwave Conference, HF92 (CARLSSONO., Ed.), pp. 21-32. Pulse propagation through a plane stratified ionosphere. J. atmos, terr. Phys. 52, 759-770. Oblique sounding of the ionospheric channel. Proc. Nordic Shortwave Conference, HF92 (CARLSSON0., Ed.), pp. 9 20. Analysis of the width of chirpsounder spectra. Fifth International Conference on HF Radio Systems and Techniques, IEE Conf. Publ. No. 339, pp. 146-151.
Obfique sounding ofionospheric HF channel LUNDBORG B. and LUNDGRENM.
1992
LYNCH J.T., FENWICKR. B. and VILLARDO. G.
1972
POOLE A. W.V.
1985
SALOUSS.
1989
WILLIAMSH.P.
1981
Reference is also made to the following unpublished material : DERBLOM H., BROMSM. and LUNDBORGB. 1991a DERBLOM H., BROM,';M. and LUNDBORGB.
1991b
DERBLOM H., BROMSM. and LUNDBORGB.
1991c
DERBLOM H., BROMSM. and LUNDBORGB.
1991d
DERBLOM H., BROM:~M. and LUNDBORGB.
1991e
TETERS L. R., LLOYDJ. L., HAYDON G. W. and LUCASD. L.
1983
63
On the spectral width ofchirpsounder signals. J. atmos. terr. Phys. 54, 311-321. Measurement of best time-delay resolution obtainable along east-west and north-south ionospheric paths. Radio Sci. 7, 925-929. Advanced sounding. 1. The FMCW alternative. Radio Sci. 20, 160%1616. Measurement of narrow pulse distortion over a short HF skywave link: Es and F2 summer results. Radio Sci. 24, 585-597. HF communications at frequencies above the predicted MUF. Second International Conference on Antennas and Propagation, Part 2: Propagation, l E E Conf. Publ. No. 195, pp. 245-248. Snedsondering av HF-kanalen. Del 1 (av 5). Jonosffirkanalens grovstruktur. FOA Report C 30623-3.5. Snedsondering av HF-kanalen. Del 2 (av 5). Sporadiska E-skikt och meteorsp~r. FOA Report C 30624-3.5. Snedsondering av HF-kanalen. Del 3 (av 5). Spridning mot irregulariteter i jonosf'~iren (spread-F). FOA Report C 30625-3.5. Snedsondering av HF-kanalen. Del 4 (av 5). Frekvenser 6ver MUF, framgttspridning och returspridning. FOA Report C 30626-3.5. Snedsondering av HF-kanalen. Del 5 (av 5). Kommunikationsaspekter. FOA Report C 30627-3.5. Estimating the performance of telecommunication systems using the ionospheric transmission channel (IONCAP). NTIA Report 83-127.
~ Pergamon
Copyright © 1994 Elsevier Science Ltd Printed in Great Britain. AU rights reserved
0021-9169(93)E0020-A
0021-9169/95 $9.50+0.00
Oblique sounding of an auroral ionospheric I-IF channel BENGT LUNDBORG,* MATS BROMS and HARALD DERBLOM National Defence Research Establishment, Department of Information Technology, P.O. Box 1165, S-581 11 Link6ping, Sweden
(Received infinal form 28 September 1993; accepted 22 November 1993) Abstraet-49bservations of the HF skywave ionospheric channel during one year over three paths within Sweden are reported. The major aim of the work was to gain insight into prevailing propagation effects on representative fixed circuit paths at auroral latitudes, with a view to being able to assess prediction accuracies, and to exploit abnormal propagation modes so improving circuit reliabilities. The main instruments in the study were chirpsounders, HF FM~CW sounders covering the frequency range from 2 to 30 MHz. A data base of about 250,000 recordings was built up during the campaign, which covers the maximum of solar cycle 22. Some measurements were made with transmissions on fixed frequencies, mostly > 20 MHz.
1. I N T R O D U C T I O N
The ionosphere has been used for many decades as a propagation medium for H F electromagnetic waves. Its great variability, with geographic location, time of the day, season and solar activity, is well known and studied. The severe: limitations this may impose on frequency selection in H F communication are also well known. Observations of the H F skywave ionospheric channel have been carried out during one year over several paths in Sweden as shown in Fig. 1. The campaign ran during 1989-19!)0 and thus covered the maximum of solar cycle 22. The major aim of the work was to gain insight into prevailing propagation effects on representative fixed circuit paths at auroral latitudes, with a view to being able to assess prediction accuracies, and to exploit abnormal propagation modes so improving circuit reliabilities. The main instruments in the study were chirpsounders: H F F M - C W sounders from BR Communications, U.S.A., covering the frequency range from 2 to 30 M H z with a sweep rate of 100 k H z s -l. The output power of the transmitters is 100 W and the signals are sampled during 1 s for each spectrum. Hence the chirpsounders are more sensitive and detect much weaker signals than traditional, pulsed sounders ; the difference may be as great as 15-20 dB. In general, one oblique ionogram (graph showing received signal power vs group delay and signal frequency) for each
68
!-
64
8
12
16
20
E
Fig. 1. The major paths used for the chirpsounders were: Kiruna to Lycksele (KI --, LY, 370 kln), Kiruna to Uppsala (KI ~ UP, 900 km) and t)stersund to Ursvik (OS--¢ UR, 460 km). The paths LinkSping to Lycksele (LI--* LY, 670 km) and Link0ping to Uppsala (LI ~ UP, 225 km) were used only occasionally.
* Also at: Swedisl~. Institute of Space Physics Uppsala Division, S-755 91 Uppsala, Sweden. 51
52
B. LUNDBORGet al.
path was generated every 5 min and stored on floppy disc. In this way, a huge data base of about 250,000 recordings was built up during the campaign. The transmitters were located in Kiruna (67.8°N, 20.4~'E geographic coordinates; 64.5°N, 103.4°E corrected geomagnetic coordinates [CGCS]), Ostersund (63.2°N, 14.7°E geogr. ; 60.1°N, 95.9°E CGCS) and Link6ping (58.3°N, 15.5°E geogr.; 54.8°N, 94.1°E CGCS). The reception points were Lycksele (64.6°N, 18.9°E geogr.; 61.3°N, 100.1°E CGCS), Uppsala (59.8°N, 17.6°E geogr. ; 56.3°N, 96.5°E CGCS) and Ursvik (Stockholm, 59.4°N, 18.0°E geogr.; 55.9°N, 96.6°E CGCS). The paths Kiruna ~ Lycksele, Kiruna ~ Uppsala and LinkOping ~ Lycksele all lie within the sector between geomagnetic NE SW and NNE-SSW. The path Ostersund ~ Ursvik is very near geomagnetic N - S and the path Link6ping Uppsala is directed along geomagnetic WSW-ENE. When the magnetic activity reaches Kp ~ 4, the midpoint of the path Kiruna ~ Lycksele lies within the auroral oval. With an activity as high as K o ~ 7, also the path Kiruna ~ Uppsala has its midpoint in the auroral oval. The minimum of the trough is generally located between Lycksele and Uppsala. This information concerning the trough and the auroral oval was extracted from GOODMAN (1992, Sects. 2.4 and 3.4). Some measurements were made over the path Ostersund ~ Uppsala with beacon transmissions on fixed frequencies, mostly above 20 MHz, but some recordings were made at ~ 5 MHz. In these experiments the radiated power was typically 200 W and the receiver was a Rohde & Schwarz EK 070. This paper highlights the significant results of the experiments and discusses their possible consequences for H F communication. The full presentation of our results can be found in a series of five reports in Swedish, published by the National Defence Research Establishment (FOA) (DERBLOM et al., 1991a-e); cf. also LUNDBORG et al. (1992). Theoretical analyses of the chirpsignal and its spectrum are given by POOLE (1985) and by LUNDBORG and LUNDGREN (1991, 1992).
2. REGULAR C O N D I T I O N S
Communication via the ionosphere is usually affected by various kinds ofmultipath propagation; signals arrive at the receiver over several paths with differing delays and phases. For normal F-layer heights and distances within Sweden (typically < 1000 km) the difference in group delay between single-hop and multihop paths is 1-2 ms, which may cause intersymbol interference or fading. The situation is very much the
same for multipath involving several ionospheric layers. Multipath also arises through the low and high rays which are both, in principle, present for each reflecting layer. In practice, however, the high ray is weak, and only significant close to the maximum usable frequency (MUF). Whilst fixed frequency systems are sensitive to time-delay spread, frequencyhopping radios with high enough hop rates are less sensitive to such multimodes. They lock on to one of the paths and have shifted to another frequency before the other mode or modes arrive (JOHANSSON, 1986; LINDBLAD, 1986, 1989). Another regular feature of ionospheric propagation is the magneto-ionic splitting of the radio wave into ordinary and extraordinary modes propagating over slightly different paths. These two modes may overlap completely or partly, or show up separated in time. Figure 2 shows examples of fixed-frequency recordings, where the effects of O/X-mode splitting can be seen. The O/X delay differences are clearly much smaller than the multihop delay differences, and frequency hopping is often insufficient to separate the two signals. A study of the fading patterns, that arise due to the O/X delay differences, can be found in LINDSTROM (1992). This effect can obviously be disastrous in digital communication, but may be cured by the use of antenna arrangements which restrict transmission and/or reception to one of the two modes. We illustrate these regular ionospheric phenomena by a number of oblique ionograms in Fig. 3. Our chirpsounder network lacked absolute synchronism between the transmitters and the receivers, and hence the signal delays along the vertical axes are only relative. In the figure, we see examples of multihop in A, B and D, multiple layers in C and magneto-ionic splitting as horizontally translated duplicate traces in all of these cases. The relative position of the O- and X-mode traces shifts with propagation distance and maximum observed frequency (MOF). For propagation over the path Kiruna ~ Uppsala during a particular winter day and night, the differencefx-fo was found to vary between 0.7 MHz when the M O F was low ( ~ 5 MHz) and 0.4 MHz when the M O F was high ( ~ 15 MHz) ; however, the spread around these values was large (DERBLOM et al., 1991a, Sect. 8.3). This seems to agree, at least qualitatively, with the geomagnetic-field dependence discussed in the QL and QT approximations by DAWES (1990, Sect. 6.4). Traces of the high ray are present in A and D as a continuation of the 1-hop traces beyond the limit of the M O F towards higher delays and lower frequencies. In C both the F1- and F2-1ayer traces show hints of the high ray. Wave propagation over Swedish territory is regu-
Oblique sounding of ionospheric HF channel I
I
I
I
l
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Fig. 2. Two cases of transmission of a 1 ms pulse over the path Ostersund ~ Uppsala at ~ 5 MHz. The 12.5 kHz IF output of the receiver was recorded by a plotter connected to an oscilloscope; this frequency is seen in the regular oscillations of the curves. Both diagrams show the 1-hop (left) and 2-hop (right) signals separated by ~ 1.5 ms. For each of these paths the O- and X-mode signals can be seen partly overlapping and interfering with each other.
larly affected by a phenomenon called the ionospheric trough, a wide region of markedly reduced electron density. The trough has a mainly magnetic east-west orientation and can often be seen in the vertical ionograms from Lycksele and Uppsala, where it shows up as a 'double ionosphere' with differing critical frequencies and virtual heights. Signals travelling obliquely through such a region with horizontal gradients also show these features ; see Fig. 4 for an example. All ionograms throughout the campaign (one recording every 5 min for each path) have been scaled with a semi-autornatic program in order to obtain
statistics about the various propagation modes; Fig. 5 shows an example of this condensation of data for one day. The regular MOF (the + data in Fig. 5) has been subsequently extracted from the data files and presented in the form of monthly statistics for each of the three paths in DERBLOM et al. (1991a). As a comparison, the prediction program IONCAP (TETERS et al., 1983) has been used to produce median M U F curves for the same paths and time periods. A typical example for 0stersund ~ Ursvik in November 1989 is seen in Fig. 6. These statistics verify that the coarse features of ionospheric propagation and their variations are well understood. The comparison
54
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Fig. 3. Oblique ionograms for regular ionospheric conditions. The horizontal axes show frequency in MHz and the vertical axes relative group delay with 1 ms between divisions. The small graph above each flame reflects the integrated signal power as a function of frequency. The situations shown are for Ostersund --* Ursvik A winter day and B winter night, and for Kiruna --*Uppsala C summer day and D summer night. In frame B a short horizontal trace is seen at ~ 14 MHz ; this is caused by meteoric reflection.The horizontal trace covering the interval 9-28 MHz in C is due to normal sporadic E, showed that the median frequency prognoses agree roughly with the median of the measured data, with the best fit found during the summer months. In wintertime, the daily excursions from the median are great, however, and span several MHz; on some days the MOF falls totally outside the traditional pattern• This is well illustrated by Fig. 6. In frequency prognoses, FOT (defined as the lower decile of the MUF) is considered a more realistic measure of practical band availability. Hence, DERBLOMe t a l . (1991a) also give diagrams comparing FOT, as predicted by IONCAP, to the lower decile of the MOF data. The agreement was fairly good for the more mid-latitude path 0stersund ~ Ursvik and less accurate for the other two paths. During quiet conditions, the maximum bandwidth If'"
Jkade a l k k l ~ A
for communication purposes is not set by the ionospheric medium but by external interference and equipment limitations. Theoretically, the dispersion of the quiet ionosphere (group-delay slope with frequency) would permit bandwidths ranging from a few tens of kHz up to about 1 MHz for single-Path communication, except near M U F where the bandwidth tends to zero (LUNDBORG, 1990). Experimental data for mid-latitude paths give values in reasonable agreement with this (LYNCH e t a l . , 1972; SALOUS, 1989). The pulse broadening found by ENDERSBEEe t a l . (1989) for a path between the U.K. and Australia would give much lower bandwidths than this; the reason for this is probably multipath mixing rather than dispersion. Realistic bandwidths for propagation through the auroral region are as low as a few kHz, even under rather favourable conditions (JODALEN and TI-mANE,1991).
3. SPORADIC E
Fig. 4. Propagation Kiruna ~ Uppsala through the trough region. Horizontal gradients in the reflectionregion give rise to additional multipath.
Sporadic E (Es) refers to ionization, caused by nonregular mechanisms, at E-layer heights. In spite of its name, the phenomenon has many non-sporadic properties; three types, with distinct features, occur fairly regularly in the data. These are the normal, temperate latitude Es, meteoric ionization at E heights and auroral sporadic E. New, adaptive systems will
55
Oblique sounding of ionospheric HF channel 30
:
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Fig. 5. Example of condensed data (288 ionograms) for 11 January 1989 over the path ()stersund --* Ursvik, LOF (Lowe,;t Observed Frequency) is given by dots and MOF (Maximum Observed Frequency) for the F layer by + signs. At each time the vertical straight line shows the range of frequencies over which sporadic E was observed. At each time the short horizontal bar ( - sign) shows the highest frequency for traces of spread in the ionogram.
30
25
20 ....-'-...'"....._.
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Fig. 5. The dots in the figure show the regular MOF values over the path Ostersund ~ Ursvik for the entire month of November 1989. The thick solid curve is the median values of these data. As a comparison, the thin curve shows the predicted median MUF for the same path and time period.
56
B.
LUNDBORG
probably be designed to make automatic use of sporadic E openings. The normal, temperate latitude Es is formed by action of the neutral wind pattern which collects longlived metallic ions into thin layers at about 100 km height. The horizontal extent of such an Es patch is of the order 100-200 km. The formation is favoured during the summer, peaking during June-July and during daytime with two maxima commencing at prenoon and late afternoon hours. The lifetime varies with the season and is up to 5 h during July and <0.5 h in January. The lifetime is frequency-dependent and decreases rapidly with increased frequency. An example of this kind of Es is the straight distinct trace in Fig. 3C. Communication via normal Es is characterized by low dispersion, which permits high bandwidths. Multipath with sporadic E requires several Es patches along the path between transmitter and receiver. In such cases the delay differences will be very small, only fractions of a millisecond, since the reflecting layers occur at low heights. Mixed paths, involving combinations of Es and normal F-layer reflection also occur with the same typical small delay differences. This may cause problems with fading due to interference between the multipath signals. Because of the small group delay differences, such problems may also be expected with the frequency hopper discussed earlier. Because of the high sensitivity of the chirpsounders, a lot of the traces recorded as Es are rather due to meteor ionization, that is, they are signals reflected by the ionized trails created by meteoric impact at Eheights. Meteor trails have a short lifetime and the signals show up as horizontal traces at E-layer heights covering a few MHz stochastically in the frequency range of the sweep. Due to the short lifetime, each meteor trace is only seen in one ionogram in a sequence ; the short horizontal trace at ~ 14 MHz in Fig. 3B is an example. In our fixed-frequency measurements at 24.5 MHz, using a 0.2 kW transmitter and a total antenna gain of 10 dB, on average 1.8 meteors were recorded per min with a mean lifetime of 2.6 s ; the resulting mean duty cycle is 8 per cent. Hence, rather few meteors have the lifetime of 10 s needed to produce the trace pointed out in Fig. 3B. It is quite possible to use the concept of meteor burst communication with high bandwidths on HF, but the standard equipment has to be revised and adapted. The possibility of using this mode on H F has also been pointed out by WILLIAMS (1981) and CANNON (1989). Figure 7 is representative of a third possibility of communication via sporadic ionization at E-layer
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Fig. 7. Example of auroral sporadic E for the path Link6ping ~ Lycksele. heights, which occurs predominantly during nighttime in connection with aurora. Auroral activity is infamous for its detrimental effects on communication through absorption and spread. However, certain phases of the auroral evolution, visually characterized by smooth stable arcs, show low absorption and high ionization in a layer some 10 km thick at E-heights, a layer which is well suited for communication. The mode is called auroral sporadic E and is in general recognized in the ionograms by weak regular traces and an Es-trace which is more fuzzy than the temperate latitude Es-trace, often showing some dispersive slope and running up to rather high frequencies. Signals transmitted via auroral Es frequently show spectral broadening, interpreted as multiple scattering of the radio signal within the layer. Hence this propagation mode does not allow as high data rates as the normal Es. Statistics of the experimentally recorded E s - M O F and the Es occurrence frequency, based on the scaling ofionograms illustrated by Fig. 5, have been produced and presented in the form of diagrams by DERBLOM et aL (1991b). These show the variation of the probability of Es with time of the day, season, frequency and propagation path ; see Fig. 8. No distinction has been made between various types of Es in this presentation. Thus, during the winter nights the meteoric ionization and auroral Es dominate the statistics. Below we give some conclusions that could be drawn from these graphs. The short northernmost path Kiruna ~ Lycksele (370 km) has high Es occurrence during winter nights, > 80 per cent during some hours at low frequencies, whereas the winter daytime values hardly exceed I0 per cent even at low frequencies. The summer level lies at some 30-40 per cent more evenly distributed in frequency and time. The long path Kiruna ---,Uppsala (900 km) differs somewhat from this picture. The winter daytime Es occurrence is rather similar ,~ 10 per cent but the night-time values reach only 20-30 per cent. Daytime Es frequency is considerably higher for the rest of the year, even exceeding 80 per cent during
Oblique sounding of ionospheric HF channel
57
S
q
Fig. 8, Monthly average of Es incidence as a function of frequency and hour of the day. The Case shown is for the path Kiruna --*Uppsala, September 1989.
some hours in July; the mechanism involved here is the normal, wind-driven sporadic E. The more southern path 0stersund ~ Ursvik (460 km) is rather similar to the Kiruna ~ Uppsala path. In all three paths an unexplained minimum can be seen, centred on 18 UT. Another common feature in the data, when the Es covers a large frequency interval, is a certain periodicity in the fi'equency dependence. This is possibly due to HF interference, since the occurrence dips show some correlation with the broadcast bands. Another explanation for this could be efficiency resonances in the broadband dipoles ; they were of the same type at all the sites. Statistics on Es occurrence, over a 10-year period 1966-1976 and obtained with the three Swedish vertical ionosondes, have also been summarized by DERBLOM et al. (1991b). These ionosondes are of the pulsed type, and were not able to detect meteorreflected signals. The average occurrence frequency of the other two type,; of Es (normal + auroral) was ~ 57 per cent. The normal, temperate latitude Es dominated over auroral Es in Uppsala and Lycksele, whereas the opposite was true for Kiruna.
4. S P R E A D F
Irregularities, electron-density fluctuations of almost any size embedded in the large scale ionospheric structure, influence the propagating radio signal and cause spreading. This phenomenon has long been known and named as spread F ; see, for example, DAVIES (1990, Sect. 5.6) for a discussion of this. It is perhaps the most important source of problems in the high-latitude HF traffic, especially during the winter nights. Moderate spread F is seen in the ionograms as a broadening of a varying degree of the regular traces and is interpreted as due to irregularities along the normal ray path, which split a single ray into a multitude of close-lying rays; see Fig. 9A and B. When the irregular ionization becomes stronger, signal power may be reflected for frequencies much higher than the traditional MUF, even if the ionization is located far away from the regular reflection region. Spread associated with such strong aurora extends beyond the normal ionogram traces with significant signal levels up to much higher frequencies ; the delays
58
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may also become longer than for the normal paths. Figure 9C and D shows examples of this type of spread. In the latter cases the concept of spread F may not be quite adequate, but in spite of this all these situations are here treated together, since they only represent different levels of irregular ionization. Note that the spread F phenomenon is highly dynamic, so that considerable temporal evolution occurs during the 280 s of the sweep of an oblique ionogram. Also note that, as a result of the particular chirp technique, range and Doppler spread occur in a mixed way in the recordings. The presence of spread, as exemplified by Fig. 9 A D, was noted in the routine scaling of the oblique ionograms. For each ionogram, spread was hence described by the frequency interval within which the fuzzy traces were seen. From these data, mean values of the incidence of spread for each month, as a function of sounding frequency and time of the day, were derived and presented as diagrams by DERBLOM et al. (1991c); see Fig. 10. Daytime solar radiation is a strong homogeneous ionization factor, which has an inhibiting effect on the irregularity formation. Precipitation of auroral particles, which is a cause of irregular ionization, has its maximum during the late evening hours. Thus the occurrence frequency of spread F is high during winter nights, > 70 per cent at low frequencies, but exceedingly small during summer daytime. On the Kiruna --* Lyeksele path, the spread reaches even above the limit 30 MHz of the chirpsounder for a significant part of the winter night hours. Spread F signals fluctuate rapidly in time, due to the rapid motion and turbulence of the irregularity
structures. FFT-spectra of the received signal during such conditions show that the signal is broadened in synchronism with enhanced spreading activity in the ionosphere. A CW signal can become spectrally broadened by more than + 50 Hz. In general, this broadening is symmetric; see Fig. 11 and DAVIES (1990, Fig. 9.31). During a strong disturbance one would expect phase variations up to 1000 radians, fading periods of 0.2 s, fading correlation distances of the order of metres and correlation bandwidths of only 50 Hz; see BOOKERand TAO (1987) and BOOKER et al. (1987). If pulsed (digital) transmission is used, mixing of individual data dements is unavoidable under such conditions. If the disturbance is moderate, the major part of the spread signal components may be removed by clever processing of the signal. An uncontaminated signal component having an amplitude some 10-15 dB above the spread components, as in Fig. 11 C, is frequently observed. One way to remove the spread components might, therefore, be simply to reduce the transmitter power or introduce discrimination at a certain amplitude level in the receiving system. 5. SCATI'ERING
Some roughness is present in the ionospheric structure even under smooth conditions. This gives rise to forward scattering (ionoscatter) on frequencies higher than the traditional M U F , with contributions to the signal from the ionospheric volume common to the transmitter and receiver antenna beams. The signal level is low, according to estimates 50-60 dB below the normal ionospheric wave. Our measurements confirm
Oblique sounding of ionospheric HF channel
59
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Fig. 10. Mo:athly average of the incidence of spread as a function of frequency and hour of the day, i.e. the percentage of time during a particular month for which spread was present at a particular hour and a particular frequency. The case shown is for the path Kiruna ~ Lycksele, November 1989. that ionoscatter is hardly an attractive alternative, if based on traditional HF equipment. A more promising mode for communication above the traditional M U F is ground backscatter or sidescatter. The mechanism involved is low-angle radiation being transmitted to large distances via the regular ionospheric layers and then scattered back by structures on the ground or sea surface to the receiver at closer range (DAVIES, 1990, Sect. 6.8). Measurements and a theoretical analysis concerning this type of propagation mechanism are reported in a recent work by GIBSON and BRADLEY (1991). Our beacon transmissions show that ground scatter may be an alternative for co~amunication when there are severe disturbances on the normal path. Figure 12 shows two recordings, each covering 24 h, of the received amplitude with our 24.5 MHz beacon over the path Ostersund -~ Uppsala. The signals were F F T analysed anti the peak level of each spectrum monitored and subsequently plotted by a strip chart recorder. With this technique, the practical bandwidth is the resolution ~',0.5 Hz of the FFT, and hence very weak monochromatic signals, undetectable with tra-
ditional receiver bandwidths, could be recorded. Figure 12A shows a day with undisturbed conditions. The night-time signal is caused by the fairly regular inflow of meteors. At dawn the ionization sets in and the long ranges open up for the groundscatter mechanism; the signal then rises to a rather steady level some 10-20 dB above the meteor-reflected signals. The recorded levels were highest when the Yagi antennae were beaming in the same direction (e.g. both southwards or both westwards) and considerably lower when they were directed toward each other on the direct path. Figure 12B shows a disturbed day; when the groundscattered signals drop due to absorption (of the long-range rays), the meteor-reflected signals (from closer ranges) can also be seen in the daytime. The groundscatter signals are weak, but always detectable in the daytime even at the high frequency used here. Sometimes the signals are spectrally broadened, but not to the extent found for auroral spread. Occasionally the peak is Doppler-shifted, indicating ionospheric bulk motion. Backscattering of HF signals by solid structures has found applications in over-the-horizon radars. To our
60
B. LUNDBORGet al.
A
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6
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0 25 50 -50 -25 0 25 50 Hz Hz Fig. 11. Broadening of a CW signal for various degrees of ionospheric disturbance, increasing through the frames A-D. The examples shown are for the path Ostersund -~ Uppsala, recorded on different occasions in September 1989. Propagation is via the F-layer with ~ 5 MHz signalfrequency.The span of the frequency axis is 125 Hz and the vertical axis spans 80 dB. The peaks at +50 Hz are probably due to a.c. hum from the transmitter power supply.
knowledge, HF ground backscatter/sidescatter has not been used for communication before, although our measurements have shown that this is clearly feasible. With usual HF transmitter powers of the order of 1 kW and simple antenna configurations narrow filtering is required in the receiver, so only very low data rates can be used. The backscattered signal energy (analysed with 0.5 Hz resolution in the fixed-frequency recordings) is spread out over some milliseconds (i.e. some 100 Hz) with the chirpsounder technique. Hence, one may expect some 25 dB lower S/N ratio with this technique than with our fixed-frequency recordings, so the backscatter/sidescatter mode is perhaps more difficult to detect with chirpsounders. Furthermore, it is often difficult to distinguish groundscatter from auroral spread in the ionograms. Analysing the geometry of the backscatter/sidescatter mechanism, one finds that scattering can be received from regions located at distances greater than the skip distance from the receiver as well as the transmitter; see DAVIES (1990, Sect. 6.8). Hence the signal shows more delay than the traditional ionospheric signal. The echoes appear in our ionograms somewhere at the end of the 2-hop F-
trace and extend to higher frequencies with increasing delays ; see Fig. 13. 6. CONCLUSIONS Below, we summarize the results and emphasize points relevant for communication within the HF band. Several of these points are well known, but are substantiated by an abundance of new data in the background reports (DERBLOMet al., 1991a-e) ; other points are new. (1) The coarse features of ionospheric propagation and their variations are well known for the paths studied. The usual median frequency prognoses agree roughly with the median of the measured data. The daily excursions from the median are, however, great, and span several MHz. It seems therefore that priority should be given to the development of methods which predict the deviation from the median rather than the median itself. (2) During quiet conditions the maximum bandwidth is not set by the ionospheric medium but by interference and equipment limitations. Theoretical bandwidths for the smooth ionosphere seem to be of the order of a few tens of kHz up to 1 MHz.
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12 15 18 21 24 Hours (UT) Fig. 12. Two ~:ecordingsof the received amplitude over the path 0stersund ~ Uppsala at 24.5 MHz. Each panel covers 24 hr (midnight-midnight) and the amplitude scale is l0 dB per division. Panel A shows a day with quiet conditions, whereas B shows a disturbed day.
(3) Multipath propagation is a regular feature of ionospheric communication. For normal F-layer heights and distances within Sweden, the differences in group path-delay are of the order of 1-2 ms, which may cause problems with the mixing of information in a data stream. For sporadic E propagation typical differences between multihops are only fractions of a ms.
(4) Another regular feature of the ionosphere is the magnetoionic splitting of the radio wave into O- and X-mode rays. These two modes may overlap completely or partly or show up at the receiver separated in time. The result raay be disastrous for digital communication, but may be cured rather easily on fixed
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links by the use of antenna arrangements which restrict transmission and/or reception to one of the two modes. (5) New, adaptive systems will probably be designed to make automatic use of sporadic E openings. The evaluation of these experiments has resulted in extensive statistics over the regular features of sporadic E. As expected, the occurrence of temperate latitude sporadic E is high during the summer. (6) Meteor-reflected signals are frequent within the HF-band. According to our measurements at 24.5 MHz, using a 0.2 kW transmitter and a total antenna gain of l0 dB, the mean duty cycle seems to be 8 per cent. It is quite possible to use meteor burst communication on HF, but the standard equipment has to be revised and adapted. (7) Auroral Es is a predominantly wintertime, night-time phenomenon which shows correlation with auroral activity. It is well suited as a reflector for HF communication up to rather high frequencies, but with lower data rates than for the other two kinds of Es. (8) Spread F is, perhaps, the most important source of problems in the high-latitude H F traffic, especially during winter nights. Its degrading influence increases with auroral activity. The ionograms show that there is signal present under these conditions, however, with
62
B. LUNDBORGet al.
severe spreading in time as well as in the frequency domain. Because o f the high occurrence o f spread F, priority should be given to finding ways o f m a k i n g use o f spread F signals. (9) G r o u n d backscatter/sidescatter m a y be a n alternative for c o m m u n i c a t i o n on frequencies higher t h a n the M U F when there are severe disturbances o n the n o r m a l path. The signal is weak but r a t h e r stable during daytime on the frequency 24.5 M H z of our tests. This p r o p a g a t i o n m o d e is clearly quite feasible with n o r m a l t r a n s m i t t e r powers a n d a n t e n n a configurations, provided t h a t very low data rates are used. Acknowledgements--This study has been performed with financial support from the National Defence Material Administration (FMV) and with personnel, localities and
equipment from the National Defence Research Estabhshment (FOA) in Link6ping and the Uppsala, Lycksele and Kiruna divisions of the Swedish Institute of Space Physics (IRF). In particular we wish to thank Lennart Elfberg and his colleagues of the Technical School of the Swedish Army (ATS) in Ostersund for their enthusiastic support to the project in the form of transmissions on various frequencies and for running the chirpsounder. The chirpsounder transmitter in Kiruna has been operated by Inge Marttala, IRF. Georg Lejemo, Telub, has been responsible for the recordings in Ursvik. Thanks are due to Stig Boberg and Bj6rn Johansson at FOA, Inger Arlefj~ird IRF and/~ke Hedberg, formerly at IRF, for their work with computer programs for data collection, scaling and presentation. The huge data volume has been scaled by Ove Klang, Robert L~,ngstr6m and Kurt Lundgren at Lycksele Ionospheric Observatory, Eva Arlefj~ird and Inger Arlefj~ird at 1RF, Uppsala, Lars Knutsson, Ing-Marie Mildh at FOA and Rune Ullander, Teleplan.
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1991
A scintillation theory of the ~ding of HF waves returned from the F-region: receiver near transmitter. J. atmos, terr. Phys. 49, 915-938. A scintillation theory of fading in long distance HF ionospheric communications. J. atmos, terr. Phys. 49, 939-958. Morphology of the high latitude ionosphere and its implications for HF communication systems, l E E Proc. I 136, 1-10. Ionospheric Radio. Peter Peregrinus, London. Pulse transit times and waveform observations for ionospheric propagation between England and Australia. J. atmos, terr. Phys. 51, 291-295. A new formulation for above-the-MUF loss. Fifth International Conference on HF Radio Systems and Techniques, l E E Conf. Publ. No. 339, pp. 122-125. H F Communication, Science and Technology. Van Nostrand-Reinhold, New York. A study of the high latitude ionosphere using a digital spread spectrum ionospheric sounder. Fifth International Conference on HF Radio Systems and Techniques, l E E Conf. Publ. No. 339, pp. 136-140. Introduction of a HF frequency hopping system and its basic parameters. Proc. Nordic Shortwave Conference, HF86 (WALDE C.-H., Ed.), pp. 171-172. An experimental HF frequency-hopping system. Proc. Nordic Shortwave Conference, HF86 (WALDEC.-H., Ed.), pp. 173-178. A DSP implementation of a FSK correlation detector. Proc. Nordic Shortwave Conference, HF89 (WALDE C.-H., Ed.), pp. 205-213. Wideband modelling of the ionospheric channel. Proc. Nordic Shortwave Conference, HF92 (CARLSSONO., Ed.), pp. 21-32. Pulse propagation through a plane stratified ionosphere. J. atmos, terr. Phys. 52, 759-770. Oblique sounding of the ionospheric channel. Proc. Nordic Shortwave Conference, HF92 (CARLSSON0., Ed.), pp. 9 20. Analysis of the width of chirpsounder spectra. Fifth International Conference on HF Radio Systems and Techniques, IEE Conf. Publ. No. 339, pp. 146-151.
Obfique sounding ofionospheric HF channel LUNDBORG B. and LUNDGRENM.
1992
LYNCH J.T., FENWICKR. B. and VILLARDO. G.
1972
POOLE A. W.V.
1985
SALOUSS.
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WILLIAMSH.P.
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Reference is also made to the following unpublished material : DERBLOM H., BROMSM. and LUNDBORGB. 1991a DERBLOM H., BROM,';M. and LUNDBORGB.
1991b
DERBLOM H., BROMSM. and LUNDBORGB.
1991c
DERBLOM H., BROMSM. and LUNDBORGB.
1991d
DERBLOM H., BROM:~M. and LUNDBORGB.
1991e
TETERS L. R., LLOYDJ. L., HAYDON G. W. and LUCASD. L.
1983
63
On the spectral width ofchirpsounder signals. J. atmos. terr. Phys. 54, 311-321. Measurement of best time-delay resolution obtainable along east-west and north-south ionospheric paths. Radio Sci. 7, 925-929. Advanced sounding. 1. The FMCW alternative. Radio Sci. 20, 160%1616. Measurement of narrow pulse distortion over a short HF skywave link: Es and F2 summer results. Radio Sci. 24, 585-597. HF communications at frequencies above the predicted MUF. Second International Conference on Antennas and Propagation, Part 2: Propagation, l E E Conf. Publ. No. 195, pp. 245-248. Snedsondering av HF-kanalen. Del 1 (av 5). Jonosffirkanalens grovstruktur. FOA Report C 30623-3.5. Snedsondering av HF-kanalen. Del 2 (av 5). Sporadiska E-skikt och meteorsp~r. FOA Report C 30624-3.5. Snedsondering av HF-kanalen. Del 3 (av 5). Spridning mot irregulariteter i jonosf'~iren (spread-F). FOA Report C 30625-3.5. Snedsondering av HF-kanalen. Del 4 (av 5). Frekvenser 6ver MUF, framgttspridning och returspridning. FOA Report C 30626-3.5. Snedsondering av HF-kanalen. Del 5 (av 5). Kommunikationsaspekter. FOA Report C 30627-3.5. Estimating the performance of telecommunication systems using the ionospheric transmission channel (IONCAP). NTIA Report 83-127.