1、Rep. ITU-R P.2011-1 REPORT ITU-R P.2011-1 PROPAGATION AT FREQUENCIES ABOVE THE BASIC MUF 1 (1997-1999) 1 Introduction Recommendation ITU-R P.373 defines the basic MUF as “the highest frequency by which a radio wave can propagate between given terminals, on a specified occasion, by ionospheric refrac
2、tion alone”. The Recommendation also recognizes that this does not necessarily define the maximum transmission frequency for those circumstances, since a definition is also given for the operational MUF. The operational MUF is taken to be the highest frequency that would permit acceptable performanc
3、e at a given time under specified working conditions. The various mechanisms which may contribute to propagation above the basic MUF (here for convenience referred to as ABM propagation) are described in Q 2. However the situation is complicated for two main reasons: - the definition of the basic MU
4、F implies that this is determined by extraordinary mode propagation, taking no account of any differences in signal amplitude from that of the ordinary mode; for ITU-R purposes it is necessary to predict signal intensities; such predictions have their basis in monthly median maps of ionospheric char
5、acteristics and the instantaneous values of basic MUF will not be known. Moreover the day-to-day variability in ionospheric characteristics may result in the frequency used for communication being above the basic MUF on some days and below on others. - It may be noted that instantaneous basic MUFs m
6、ay only be determined from the examination of oblique-incidence ionograms measured along the propagation path. Approximations to such values may be obtained from well-placed vertical-incidence sounders along the great-circle path, assuming ionospheric homogeneity or specific horizontal gradients. Re
7、al-time channel-evaluation systems will not generally give a clear indication of the basic MUF. 2 Propagation mechanisms responsible for propagation at frequencies above the basic MUF Propagation mechanisms and ionospheric characteristics which may give rise to ABM propagation are as follows: 2.1 Io
8、nospheric roughness The ionosphere is always likely to contain spatial inhomogeneities over the volume responsible for reflection of the bulk of the components contributing to the received signal power. These inhomogeneities can occur at all heights, and all those below the height of ray reflection
9、may be significant. Under benign conditions, spatially random electron-density fluctuations will have intensities that vary with both position and time. Large changes arise for example during the presence of travelling ionospheric disturbances. Theoretical studies have been conducted using model ion
10、ospheres representative of average mid-latitude conditions. In these studies the ionospheric fluctuations were taken to be of a turbulent type with spatial correlation function of Kolmogorovian form and a diffraction analysis in terms of stochastic (fluctuational) and coherent components lead to the
11、 conclusion that significant signal intensities due to this mechanism arise only over frequencies up to some 60 kHz above the basic MUF, which is a smaller frequency extension (see below) than that typically observed in practice. Under other conditions, intense but spatially contained ionospheric ir
12、regularities may be present, such as those responsible for the phenomenon of spread-F observed on vertical-incidence ionograms. Spreading in either range or frequency may permit propagation by refraction or scatter at frequencies above the basic MUF for the average region of the ionosphere. This phe
13、nomenon is observed on some oblique-incidence ionograms where the maximum observed frequency is determined by a “nose extension” beyond the junction frequency of the low- and high-angle rays. Ionospheric irregularities may be expected to have electron densities proportionate to the bulk ionosphere,
14、so that such frequency extensions would be enhanced the greater the basic MUF. 2 Rep. ITU-R P.2011-1 2.2 Ground back- and side-scatter Off-great-circle propagation, involving two ionospheric hops and intermediate scattering at the Earths surface, may permit propagation at frequencies beyond the grea
15、t-circle basic MUF. Such scatter signals often have fading rates greather than l/s and are received with variable azimuth angles of arrival. Signal strengths gradually decrease to 25-40 dB less than that of the great-circle mode as the propagation paths progressively deviate from the great circle. S
16、cattering coefficients of the Earths surface may be quite variable, depending upon the nature of the surface and the elevation angle. The ground-scatter coefficient is a function of azimuth and azimuth difference between the down- coming and upgoing rays, the presence of land or sea, the ground roug
17、hness, elevation angles and also upon focusing due to ionospheric curvature for grazing-incidence angles. There have been conflicting observations concerning the intensities of signals scattered from the sea as compared with those from land. The intensities of back- and side-scatter signals at frequ
18、encies above the basic MUF for the great-circle path will depend on their respective path lengths, and will vary in proportion to their respective basic MUFs. The lowest path loss should normally occur for the beam of intersection of the skip distances around the transmitter and receiver because of
19、skip-distance focusing. However, in practice the directivities of the transmitting and receiving antennas can influence the bearing of the maximum received signal strength. It has been suggested this is the dominant mechanism responsible for ABM propagation for single-hop shorter paths of up to 4 O0
20、0 km length. 2.3 Higher-order mode back-scatter As an extension of the two-hop ground back-scatter case, there may be back-scatter from longer ranges, involving multiple-hop propagation. This phenomenon is likely to be more important where there are significant ionospheric horizontal gradients in el
21、ectron density. 2.4 Ducted modes In some instances low-angle radiation may be able to enter into ducts formed by particular electron-density height profiles. In such cases, propagation may be possible to long ranges and at higher frequencies than given by the great-circle basic MUF. This mechanism,
22、combined for instance with off-great-circle ground back- and side-scatter, may contribute to ABM propagation. 2.5 Chordal-hop propagation Tilts in the ionosphere, most notably on either side of the magnetic equator, but also in the sub-auroral troughs, may permit ray paths which proceed from refract
23、ion in one region to another without an intermediate ground reflection. Trans-equatorial propagation of this kind has been observed to extend to frequencies well into the VHF range, often accompanied by a scatter-mode signal associated with the presence of ionospheric irregularities. Such signals, e
24、specially when combined with ground back- and side-scatter, may give rise to significant propagation above the basic MUF. 2.6 Direct ionospheric scatter Signal energy may be scattered from any of the ionospheric regions, both along the great-circle path and at other orientations. In high, auroral an
25、d equatorial latitudes strong ionisation density gradients may permit significant F-region scatter, but where there are no such strong gradients, scatter from the E-region is likely to be more important, and in this case would be limited to ranges of about 2000 km. The ionospheric scatter mode is di
26、scussed in detail in ex-CCIR Report 260-2 (1974) (now formally deleted). That Report indicates, at least for frequencies above 30 MHz, that the signal intensity varies as the inverse 7.5 power of the frequency. Ionospheric side-scatter from F-region irregularities located in the auroral regions has
27、been reported by several workers. In some cases the non great-circle signals are attributed to direct scatter from ionospheric irregularities in the absence of an ordinary type of ionospheric reflection. In other cases they are considered to result from multi-hop modes with the strongest scatter sig
28、nals being generated by ionospheric irregularities and involving a focusing mechanism. On trans-equatorial paths azimuthal deviations of up to I50“ have been observed that are confirmed as due to direct scattering from F-layer inhomogeneities near the Earths magnetic equator. The dominant scatter si
29、gnals have been attributed to weak field-aligned irregularities, or to specular reflection at horizontal gradients of electron density. Rep. ITU-R P.2011-1 3 2.7 Sporadic-E propagation The occurrence of sporadic-E ionisation may permit propagation, either by partial reflection or scatter, to signifi
30、cantly high frequencies. This mode may not be recognized, and is in any case not included in most prediction procedures. Thus it may be considered as a further contributor to propagation above the expected basic MUF, on path lengths up to 2 O00 km. 2.8 Auroral scatter Field-aligned irregularities in
31、 the auroral regions, associated with geomagnetic disturbances, give rise to a special kind of sporadic-E. Over propagation paths near the auroral regions such E-region irregularities can give direct back-scatter of signals at frequencies up to 100 MHz or higher. These signal components have been mo
32、delled in terms of both weak scattering and critical reflection. Reflections from such irregularities have to obey particular conditions of specularity. The volume scattering function has been approximated by an exponential frequency function. Although multi-hop propagation has been observed, the ef
33、fect will generally be limited to path lengths less than 2 O00 km. 2.9 Meteor scatter Propagation utilizing reflection or scatter from transient meteor ionization is addressed in Recommen- dation ITU-R P.843. With the appropriate geometrical circumstances, short duration propagation events may occur
34、 well into the VHF range for path lengths less than 2 O00 km. 3 Measurement data already collected and needed Much of the limited measurement data which exist comprise examples of variations of signal intensity with frequency, or with time as the basic MUF changes, over specific paths. Measurements
35、over a range of paths with lengths between 400-2580 km are described by Hagn et al. 1993. Existing measurement data alone, together with present information, are insufficient for the establishment of comprehensive models which would include the dependence on path length, location, time, etc. More, a
36、nd new types of measurement information are urgently needed. Particularly there is the requirement to establish the dependencies and pattern of changes in propagation mechanisms in the different geographical regions with time-of-day, season, solar epoch and degree of ionospheric disturbance. 4 Stati
37、stical models of signal intensity at frequencies above the basic MUF The ITU-R data banks (Recommendation ITU-R P.845) include observations of signal intensity at frequencies above the basic MUF, although the values of basic MUF corresponding to each measurement are unknown and could not have been d
38、etermined without special additional measurements. Ideally, simultaneous oblique soundings should be carried out over the propagation paths involved, but generally that has not been possible. Accordingly, results have to be considered on a statistical basis. It may be noted that the day-to-day varia
39、tion within a month of the basic MUF at a given hour will have an inter-decile range of 30% to 40% of the median basic MUF. Frequencies used for communication within this range will on some days be below the basic MUF and on other days above. However, long-term prediction models must take as a start
40、ing point the monthly median basic MUF. Thus the modelling of these circumstances will seek to combine the signal intensity for a refracted path at a frequency just below the monthly median basic MUF with the contributions to ABM propagation described above, and then to include the statistics of day
41、-to-day variability. Supposing that on a given occasion a propagation mechanism exists yielding weak signals above the basic MUF, then day-to-day changes in basic MUF mean that the various formulae discussed below for above-the-MUF loss in terms of monthly median basic MUF must include an unspecifie
42、d element to allow for the fact that some days the basic MUF is below the monthly median value. 4 Rep. ITU-R P.2011-1 Besides day-to-day variability, the ionosphere also experiences within-an-hour changes. These changes can result in propagation being maintained by conventional means for part of an
43、hour and in ABM propagation taking place for other parts of the hour. This situation is particularly the case when travelling ionospheric disturbances are present. Skip-distance focusing and fading associated with the interference between low- and high-angle rays can arise, but all such effects have
44、 to be treated statistically using hourly median basic MUF estimates. The approaches adopted will give a probability estimate that signal intensity will occur, and this may be appropriate for the assessment of compatibility. However it may be inadequate for the prediction of circuit performance wher
45、e the channel-transfer function in terms of fading rate, the time-delay spread, and the frequency spread and shift may be different for frequencies below and above the basic MUF. 5 The definition of ABM loss The reduction in signal intensity at frequencies above the monthly median path basic MUF, as
46、 compared with the intensity for a refracted path at a frequency just below the basic MUF, is referred to as the ABM loss. 6 Existing loss formulae The various formulae which have been proposed for ABM loss are described by Hagn et al. 1993. 6.1 The Phillips-Abel model This model Phillips, 19631, ba
47、sed on measurements made in the United States of America, is the only currently available model which can be considered to relate to instantaneous values of the basic MUF, although in its implementation it is usually assumed to apply in terms of the monthly median basic MUF. The model assumes that t
48、he ionosphere is composed of a number of ionisation patches within the region of reflection, each of which would yield a different path basic MUF. These MUFs are assumed to be spatially normally distributed with standard deviation O. Phillips indicated for a path length of about 3 O00 km that O rang
49、ed between 1 MHz and 4 MHz dependent on the degree of ionospheric disturbance. Subsequent measurements by Wheeler and Hagn yielded values of O between 0.9 MHz and 3 MHz. Received signal power is taken to be given in terms of the spatial probability of wave reflection. In this model, ABM loss L, (dB) is given as: where: 1 p=l- J exp(-x/20) d(x/a) andx = f - fb -m Note in particular that this formulation indicates that Lm depends on the difference between the working frequency,f, and the basic MUF, fb. Modified versions of the Phillips-Abel formulation are implemented in current US HF pr
