LF radio service planning

Pergamon www.elsevier.nlllocate/asr

Adv. Space Res. Vol. 27, No. 1, pp. 145-152.2001 0 2001COSPAR. Published by Elsevier Science Ltd. All rights reserved

Printed in Great Britain 0273- 1177/01 $20.00 + 0.00 PI1: SO273-1177(00)00150-2

IRI AND VLF/LF RADIO SERVICE PLANNING P A Bradley

Slough SL2 3ES, United Kingdom

ABSTRACT Existing propagation models of the International Telecommunication Union are reviewed and it is shown that use of improved D-region electron-density height profiles to be embodied within IRI-2000 should lead to more accurate results. Additionally, attention is drawn to limitations in the ‘equivalent frequency’ concept contained within the wave-hop method which equates ionospheric reflection coefficients at related wave frequencies and angles of incidence. 0 2001 COSPAR.Publishedby Elsevier Science Ltd. All rights reserved. INTRODUCTION Control of use of the radio spectrum is vested in the International Telecommunication Union (ITU), a United Nations Specialised Agency, and all countries party to its International Telecommunication Convention undertake to introduce national legislation consistent with the Radio Regulations of that body. The Radio Regulations are amended from time to time at periodic World Radio Conferences to take account of evolving requirements and improved understanding of spectrum utilisation and propagation characteristics. These radio conferences are advised by the Radiocommunication Sector of the ITU, abbreviated to ITU-R and formerly known as the CCIR, where technical matters are addressed in isolation of political considerations and where emphasis is directed to service-planning aspects and the needs of achieving reliability and compatibility of individual radio circuits. In particular, the ITU-R maintains a number of standing Recommendations for the estimation of propagation losses in different frequency bands. There are also obvious advantages to individual radio operators to use the same models as the ITUR, both for circuit design and for frequency-management strategies aimed at optimising available resources on a day-to-day basis. A number of different propagation models have been considered in arriving at agreed procedures. In principle in model development, two basically differing approaches are possible: (i) use of best ionospheric models and best estimates of propagation effects, where ‘best’ represents a compromise between complexity and accuracy, or (ii) from a representative data base of past measured signal characteristics associated with calibrated transmitters and antennas. Unfortunately, though, both techniques have difficulties: - ionospheric models over large regions of the world, notably over the oceans and in the S hemisphere, have to be produced by indirect techniques, and so tend to be less accurate in these cases, - there is great spatial and temporal ionospheric variability at high latitudes - some propagation phenomena, notably those associated with off-great-circle paths, the influence of horizontal ionospheric gradients and scattering from ionospheric irregularities are poorly modelled,

145

F! A. Bradley

146

- there are only limited numbers of signal measurements for restricted sample path lengths, geographic regions, times and frequencies, - considerable uncertainties are associated with knowledge of transmitter radiated powers, antenna performance at both transmitter and receiver, and with the calibration of the receiving systems.

In these circumstances it is understandable that after lengthy international examinations, compromises have been reached for the propagation models to adopt. In some cases, reference signal variability statistics about the monthly median estimates are also quoted. It is generally recognised that improved model development is an ongoing activity. Several of the present Recommendations would benefit from a reassessment in the light of the latest version of IRI, referred to as IRI-2000, and to be formally released at the June 2000 Warsaw COSPAR General Assembly (Bilitza, 1998; Bilitza and Papitashvila, 1999). Specifically, at present a wave-hop method of evaluation is recommended for short distances less than 1OOOkmat VLF and for all distance at LF (ITU-R, 1994). For longer-distance propagation at VLF, where the wave-hop approach would be inappropriate because of the number of separate modes involved, it is recommended to use instead fullwave calculations through representative model ionospheres (IT&R, 1994). Distinct from conditions at lower frequencies, at HF there is a cut-off frequency known as the maximum usable frequency (MUF) above which ionospheric support is not possible. So for HF, rather than applying ray tracing in each case, empirical formulations based on a series of such results are used to yield mean MUF-factors, which are the amounts by which vertical-incidence critical frequencies must be multiplied to give MIX’s, and equivalent mirror-reflection heights from which ray-path elevation angles and propagation losses are determined (ITU-R, 1999a). The IRI-2000 lower F2 and Fl-region models should be used to re-visit the current formulations. Many trans-ionospheric propagation effects are directly dependent upon.slant-path total electron content (TEC). The current recommended means of determining these, which should evidently be updated, is via integration through the earlier model IRI-90 (ITU-R, 1999b). This present paper is restricted, as an example, to an examination in more detail of the VLF/LF situation. VERY LOW TO MEDIUM BEYOND 1OOOkmRANGE

FREQUENCY

PROPAGATION

(16~1OOOkHz)- EXCEPT

AT VLF

The wave-hop approach supposes there to be geometrical reflection from fixed heights of 70 and 90 km respectively for day and night conditions. Atmospheric refraction is taken into account in determining the elevation angles of very low-angle ray-paths, but ionospheric refraction is ignored. Multiple-hop modes of equal length are considered as necessary depending on great-circle propagation distance, with vectorial addition of the contributions from the different modes. The field strength Et of the downcoming sky wave, before reflection at the ground in the vicinity of a receiving antenna for a vertical component of electric field Eu radiated from a short vertical electric dipole at unit distance, is given as: Et = Eu cos w ,,R,,F, (D/L)

(1)

where L is the sky-wave path length, w is the angle of departure and arrival of the sky wave at the ground relative to the horizontal, and F, is a transmitting antenna gain factor which allows for the type of ground over which this is situated. D is a focussing factor determined from geometrical and difI?action considerations (Wait, 1962), expressed as a length in the range l-3 km when also L is given in kilometres. It increases with hop-ground range and frequency and is slightly greater at the shorter distances by day (Figure 1) than at night (Figure 2). ,,R,,is an ionospheric reflection coefficient, equal to the ratio of the

IRI and VLF-Service

147

electric field components parallel to the plane of incidence before and after reflection. Empirical values of ,,R,,derived from many different VLF and LF signal measurements obtained by workers in various countries for a number of frequencies and path obliquities have been grouped and smoothed best fit values produced as a function of f.cos i, where f is the transmitted frequency and i is the ionospheric angle of incidence. Figure 3 shows reference values of ,,R,,determined in this way for solar minimum conditions. It is to be seen that different mean curves are taken as applying for the separate seasons.

E \ 4:

__

3 8

L

1 1000

500

1500

2000

2500

Ground range / km Fig. 1. VLFiLF ionospheric focussing factor D by day for frequencies f of 20-2OOkHz adopted in the wave-hop propagation evaluation method of ITU-R (1994).

t

i

i

i

i

i

1000

i

i

i

1500

2000

2500

Ground range / km Fig. 2. VLF/LF ionospheric focussing factor D by night for frequencies f of 20-200 kHz adoptedin the wave-hop propagation evaluation method of ITU-R ( 1994). It is recognised that there will be a solar-cycle dependence of reflection coefficients, but the precise form of variation remains to be properly quantified. Prior to 1982, the CCIR produced two sets of values of reflection coefficients of the form of those of Figure 3, but for low and high solar activity respectively (CCIR, 1978). However, a reanalysis of all the observational data, applying for each the same allowances

P. A. Bradley

148

10

10)

102

Effective frequency Ecos i I kHz

Fig. 3. Reference values of ionospheric reflection coeffkient ,,R,,at solar minimum adopted in the wavehop propagation evaluation method of ITU-R (1994). Smoothed curves fitted to measurement values as indicated are given as a function of ‘effective frequency’ f.cos i (fi-om ITU-R, 1994). Key: A Night; 0 Winter day; 0 Summer day for ionospheric focussing and antenna factors, led to the changes at different frequencies noted above in the mean relationships for solar minimum; at the same time the solar maximum curves have been withdrawn. During solar maximum years the base of the ionosphere is lower and the electron-density gradient is steeper than during solar minimum years. Thus VLF waves which are reflected from this lower layer are less absorbed in sunspot maximum years, whereas MF waves, that are reflected above this lower layer, are more absorbed. Clearly, the transition between greater and smaller reflection coefficients \?rould be expected to be a function of. frequency, time of day, season and epoch of the solar cycle, so that a discontinuity in the reflection coefficient versus frequency curve at some particular frequency and time might be expected. Figure 4 gives the latest ITU-R reference values of the change in reflection coeffkient from solar minimum to maximum as a function of f.cos i for different times.

d

10

10’

102 ECOS

i I k&

Fig. 4. Change in ionospheric reflection coeffkient ,,R,,from solar minimum to solar maximum function of ‘effective frequency’ f.cos i (from ITU-R, 1994). Key: A Night; 0 Winter day; 0 Summer day

as a

IRI and VLF-Service

149

More is evidently needed to be done to better quantify reflection coefficients, since the present reference data, unchanged since 1982, are clearly derived from varied measurement results. Although to some extent account was taken in ,their formulation of theoretical calculations for then available ionospheric models (Piggott et al., 1965), a re-assessment using the latest IRI D-region model seems now long overdue. Furthermore, the supposition, based on classical theory in the absence of a magnetic field, that reflection coefficients are identical for different frequency and angle combinations giving the same f.cos i, should too be studied to see if some other more accurate empirical equivalence relationship may be formulated, perhaps comparable to that found for HF by George and Bradley (1973); ray-tracing calculations of Jones and Wand (1969) have demonstrated that, as at HF, the change in reflection coefficient with frequency and path obliquity also depends on direction of propagation relative to the magnetic field. The above reference reflection coefficients were derived in the main from measurements at steep incidence with path lengths less than about 200 km. It is suggested that the f.cos i law will hold approximately at more oblique incidence with path lengths exceeding about 500 km, but that at intermediate distances substantial errors may arise, because in such circumstances, the reflection coefficient and polarisation of the wave change rapidly with distance. Additional comparisons with calculations using IRI-2000 would too serve to determine if there are any systematic trends with latitude and time-of-day, as noted in some observational data (Belrose, 1968; Starick and Taurner, 1968). VLF LONG-DISTANCE

PROPAGATION

BEYOND

1OOOkm RANGE

In general at VLF for the longer ranges, the wave-hop method is not suitable because the number of modes that would need to be considered rapidly increases with propagation distance. A full-wave assessment can in principle be carried out. This requires an ionosphere and Earth’s magnetic field specification over the whole propagation path. Full-wave calculations can allow for arbitrary electron density and electron collision-frequency variations with height and position, and for Earth curvature, as well as the effects of anisotropy created by the Earth’s magnetic field. The lower wave-guide boundary may be regarded as a smooth homogeneous Earth, characterised by an adjustable surface conductivity and permittivity, so that the effects of propagation over terrain discontinuities such as land/sea boundaries may also be taken into account. Calculations proceed by determining the complex angles of incidence supporting the different modes between given transmitting and receiving terminals. An iterative solution to the desired range incorporating Runge-Kutta numerical integration of the appropriate differential equations is applied. Waves are pursued through successive slabs of constant plasma parameters and at each interface their separate components are determined. In every case the contributions are considered from both an upgoing wave emanating from ground level and a downcoming wave originating from the height of the upper waveguide wall. The four complex ionospheric reflection coefficients of the penetrating and mode-converted components ,,R,,,,,R,, I R ,, and IR, are calculated at each slab interface for both the upgoing and downcoming waves and their resultant 11and I terms evaluated. Equations may be solved for as many separate modes as is wanted to give for all of these equivalent excitation and attenuation factors, with the separate modes being summed at the receiver with appropriate allowances for their respective phases. Where inhomogeneities in the medium are gradual a WKB solution may be applied, but in order to more accurately allow for such features as day/night transitions a full modeconversion analysis must be carried out. In reality though, this way of proceeding is prohibitively complex for operational purposes and unjustified in terms of ionosphere specification capability. If instead of constant slabs of ionisation a continuous smoothed variation is assumed, simpler analyses are possible. Approximate waveguide mode solutions have been developed (MO&~ et al, 1981) and adopted by ITU-R (1994), in which the excitation and mode conversion coefficients apply for an exponentially varying electron-density height distribution that is separately defined at different positions, with contributions from individual path segments given by

150

F! A. Bradley

summation of pre-calculated and stored components. The. calculations require evaluation of the ionospheric conductivity w which strictly changes with height h as a function of electron density, positive . and negative ion densities, electron-neutral particle collision frequency and positive and negative ionneutral particle collision frequencies. However, ignoring the lesser ion effects the relationship simplifies, for an exponential ionosphere, to:

dh) = 0, expPO1 -4)

(2)

where o, is the conductivity at reference height h, and p is a scale;height parameter. As noted above, reference values of,these two terms are available for different conditions that have been derived from the fit of the model to oblique-path signal measurements (ITU-R, 1994). For daytime it is recommended to take p=O.3ikm and b=74 km for all latitudes and all seasons. The night-time ionosphere is more complicated with p assumed to vary linearly with frequency from 0.31 km at 10 kHz to 0.8/ km at 60 kHz. The low- and mid-geomagnetic latitude night-time ionosphere is characterised by h,,=87 km, while the polar ionosphere has h,,=80 km. Values of p and h,, for 30 kHz are as reproduced in Table 1. Table 1. Values of p and b given from ITU-R (1994) at 3OkHz for Twilight and Night-time. (x = solar-zenith angle; I = magnetic dip angle)

xfdeg

I/deg

Wm-’

hdkm

<90 90.0-91.8 91.8-93.6 93.6-95.4 95.4-97.2 97.2-99.0 99.0-night

<70 70-72 72-74 74-90 72-74 70-72 <70

0.30 0.33 0.37 0.40 0.43 0.47 0.50

74.0 76.2 78.3 80.5 82.7 84.4 87.0

A single exponential variation of collision frequency with height is assumed, though since collision frequency ‘is directly related to atmospheric pressure, changes with time and latitude should too be introduced (Friedrich and Torkar, 1983). A good review of the various methods of field evaluation that have been developed over the years is presented in CCIR (1990). It is evident that none of the available models can give fully accurate estimates because of the various assumptions and approximations on which they are based and because of lack of ionospheric knowledge. Assuming retention of the idealised exponential lower ionosphere distribution form, it would be appropriate to make comparisons between IRI-derived values of h,, and p and to investigate how these change under different conditions, including with location, season and frequency. CONCLUSIONS

The latest IRI electron-density model should be used to refine ITU-R propagation prediction methods at all frequencies, with in particular the D-region component yielding a more precise specification of ionospheric reflection coefficients for VLF short-distances and all LF paths. The dependencies on frequency, path obliquity, latitude, time-of-day, season and solar epoch all need to be further investigated. At VLF long distances, more representative reference conductivity and scale-height parameters following an exponentially varying electron-density height profile are wanted for use with full-wave calculations.

1.51

IRI and VLF-Service

REFERENCES Belrose, J. S., Low and very low frequency radio wave propagation, AGARD Lecture Series XXIX, NATO, Paris (1968). Bilitza, D., COSPAR ‘98 Session C4.1 - Lower ionosphere measurements 21 (1998).

and models, IRI News, 5, #3,

Bilitza, D. and Papitashvili, N., New parameters and options for the IRI web interface, IRI News, 6, #4, 9 (1999). CCIR, Sky-wave propagation at frequencies below 150 kHz with particular emphasis on ionospheric effects, Report 265-4, International Telecommunication Union, Geneva (1978). CCIR, Radio propagation and circuit performance at frequencies below about 30 kHz, Report 895-2, International Telecommunication Union, Geneva (1990). Friedrich, M. and Torkar, K. M., Collision frequencies in the high-latitude D-region, J. Atmosph. Terr.

Phys., 45, #4,267 (1983). George, P. L. and Bradley, P. A., Relationship between HF absorption at vertical and oblique incidence, Proc. IEE, 120, #ll, 1355 (1973). ITU-R, Prediction of field strength at frequencies below about 500 kHz, Recommendation 684- 1, International Telecommunication Union, Geneva (1994).

ITU-R P.

ITU-R, ITU-R methods of basic MUF, operational MUF and ray-path prediction, Recommendation R P. 1240, International Telecommunication Union, Geneva (1999a).

ITU-

ITU-R, Ionospheric propagation data and prediction methods required for the design of satellite services and systems, Recommendation ITU-R P.53 l-5, International Telecommunication Union, Geneva (1999b). Jones, T. B. and Wand I. C., Validity of the f.cos i theorem for the absorption of very low frequency radio waves in the ionosphere, Nature, 222,462 (1969). Morfitt, D. G., Ferguson, J. A. and Snyder, F. P., Numerical modeling of the propagation medium at ELFNLFILF in Medium, long and very long wave propagation (atfrequencies less than 3000 k-Hz)), AGARD ConJ Proc. 420,32-l, NATO, Paris (1981). Piggott, W. R., Pitteway, M. L. V. and Thrane, E. V., The numerical calculation of wave fields, reflection coefficients and polarizations for long radio waves in the lower ionosphere II, Phil. Trans. Roy. Sot.,

A-257,243 (1965). Starick, E. and T;iiumer, F., LF daytime propagation in the D-region of the ionosphere, 12, #2,89 (1968). Wait, J. R., Introduction to the theory of VLF propagation, Proc. IRE, 50, 1624 (1962).

Tech Mitt. RFZ,

152

R A. Bradley

ACKNOWLEDGMENT Acknowlegment is made to the International Telecommunication Union for permission to reproduce Figure, Table and equation material taken from Recommendation ITU-R P. 684-l. In accordance with the requirements of their Legal Affairs Unit the following authorization, disclaimer and full document source details are reproduced below: ‘The texts extracted from the ITU material have been reproduced with the prior authorization of the Union as copyright holder. The sole responsibility for selecting extracts for reproduction lies with the beneficiary of this authorization alone and can in no way be attributed to the ITU. The complete volume(s) of the ITU material, Tom which the texts reproduced are extracted, can be obtained from: International Telecommunication Union, Sales and Marketing Service, Place des Nations - CH-1211 GENEVA 20 (Switzerland), Telephone: +4122 730 6141 (English)/ + 4122 730 61 42 (French)/ +4122 730 6143 (Spanish), Telex: 421 000 uit ch/ Fax: +41 22 730 51 94, X400 : S=sales; P=itu; A=400net; C=ch, E-mail: [email protected] / http:/www.itu.int/publications.’