7.1 The Mobile User

Air-Ground Communications

7.1.1

Comparison with Point-to-Point Circuits

High frequency circuits that propagate via the ionosphere are used extensively for long-range point-to-point communications and broadcasting. Characteristics of such links have consequently been studied extensively; commercial services are available for prediction of optimum working frequencies and the quality of communications at these frequencies. Most point-to-point land-fixed HF communications circuits use high-gain rhombic or log-periodic antennas, whilst arrays of horizontal dipoles, also with significant directivity, are popular for broadcasting using the sky wave mode. In principle, therefore, the capability of these links may be optimised by good engineering design and practice in respect of the equipment and antenna systems, whilst high transmitter power is often available.

Much more difficult problems, however, are presented by HF communications to mobiles. Communications are often required at ranges from a few kilometres to several thousand kilometres over a wide variety of terrain; this implies different modes of propagation according to range. Physical constraints are placed upon the antenna so that its efficiency may be degraded; radiation patterns are obtained that may not be suited to the propagation mode. The primary transmitter power may be constrained severely for the mobile user, whilst serious excess noise, both acoustic and electrical, may be present at the mobile terminal. Additionally for an aircraft, its height may give rise to further multipath propagation mechanisms, whilst its speed may cause Doppler frequency shifts.

To achieve satisfactory results over an HF link of this kind, careful consideration must be given to the terminal radio equipment, the planning of operational links and the management of the frequencies to be used over those links. An understanding of the overall system considerations is essential to the satisfactory design and operation of HF mobile radio links.

7.1.2

Parameters Critical to the Mobile User

From equation (6.9) it was shown that the received signal-to-noise is given, in logarithmic units, by

(7.1) where E_p is the effective radiated power

L is the propagation path loss

D_r is the receiving antenna directivity factor against far field noise N_i is the noise power incident at the antenna.

The comments made above, in section 7.1.1, can be related to the components of equation (7.1) in terms of three attributes of the mobile terminal:

Physical Constraints

The physical size and shape of the mobile may limit E_p in two respects. The primary transmitter power may well be much less than for a static installation whilst the constraints upon size and siting of a suitable antenna may severely limit both its

efficiency and polar diagram. The net effect will be that E_p may be many decibels below that for a typical fixed ground station. The parameter D_r is also affected by the limitations placed upon the mobile’s antenna. It may be impossible to provide any directional properties for the antenna. Thus discrimination against far field noise, often provided at a fixed site, is extremely difficult to achieve for the mobile.

Environmental Constraints

The local interference environment of a mobile user is likely to be much more severe that for a fixed terminal, thus N_i is much larger. Many electrical equipments may be housed within a relatively small volume, causing high levels of noise to the HF receiving antenna. In contrast, at fixed ground stations the receiving antennas can often be sited at a considerable distance from potential interfering equipments.

Mobility Constraints

Propagation path loss L can be minimised by selection of an appropriate frequency but this choice depends upon many factors including communications range. Large changes in communications range over relatively short time intervals, as would be experienced by rapidly moving mobiles particularly aircraft, imply the need to change frequency very regularly. Because of the need to continually optimise the communications frequency it is very likely that the path loss L for the mobile user will be greater than that for an equivalent point-to-point circuit.

The overall effects of the constraints imposed upon the mobile user are that E_p and D’_r will be less than, and L and N_i will be greater than, for a corresponding point-to-point link. Thus from equation (7.1) the signal-to-noise ratio may be reduced dramatically for the mobile user. The problems that need to be addressed are many and varied. It is convenient to consider a particular example to highlight the fundamental principles involved. An excellent illustration of all of the above limitations is provided by a study of air-ground communications, the subject of the present Chapter.

In order to understand the impact upon communications reliability and performance it is necessary to examine first the limitations imposed by the aircraft terminal itself. These limitations, which stem from 1, 2 and 3 listed above are now to be discussed in terms of: Antenna related effects; Noise environment; Frequency selection.

7.2

Characteristics of the Airborne Terminal 7.2.1

Antenna Radiation Efficiency

HF antenna systems on aircraft comprise the radiating structure, ancilliary devices such as RF switches and reactive loading components, the antenna tuning and matching unit (ATU) and the feeder cable. The importance of antenna siting is crucial.

The airframe tends to be the dominant radiator at frequencies near its natural electrical resonances. For example, a half-wave longitudinal resonance occurs in a small fixed-wing aircraft at about 9 MHz; if the aircraft is fitted with a notch antenna the airframe radiation is dominant between about 5 to 15 MHz. It so happens that this frequency band is also suitable for long-range sky wave communications, for which the aircraft may be regarded as a horizontal half-wave dipole.

Although the radiation efficiency of an aircraft antenna may be very small in some circumstances, receiver noise at the aircraft terminal is generally dominated by the external noise field. Thus, provided that radiation efficiency exceeds a threshold value, further increase in radiation efficiency provides no advantage for reception only and the relevant antenna parameter is its directivity D. This is obtained as a function of angular coordinates and polarisation from the normalised radiation patterns.

At the low end of the HF band poor radiation efficiencies[1] are exhibited by small and medium sized aircraft for the following reasons:

a) Many aircraft HF antennas are electrically extremely small.

b) The whole airframe (regarded as a radiator) is itself electrically small.

c) There are constraints upon antenna type and siting.

AIR-GROUND COMMUNICATIONS 97

A very adverse combination of factors occurs in small, fixed-wing, high-performance aircraft, for which radiation efficiencies approaching −50 dB at 2 MHz have been measured. Thus for 100 W transmitter power only some 10 mW is radiated, the rest being dissipated in loss resistance. Typical antenna radiation efficiencies for small, medium and large aircraft are shown in Figure 7.1.

At most frequencies the antenna exhibits a highly reactive impedance so that an ATU is required to tune it to the operating frequency and to present a good impedance match to the transmitter. For transmission bandwidth calculations the antenna system may be treated as a tuned circuit and sometimes very high circuit quality factors (Q values) are encountered implying very small transmission bandwidths. Usually the bandwidth of a tuned circuit is defined at the −3 dB points, giving

(7.2)

However, in an aircraft HF radio system this definition is inappropriate for both transmission and reception. On transmission there is an engineering limitation on the maximum VSWR which can be tolerated by the power amplifier stage. The −3 dB point of a tuned circuit corresponds to a VSWR of 5.8:1, which is much higher than current transmitter specifications allow. Since the receiving system performance should be externally noise limited, the relevant antenna system criterion on receive is that the radiation efficiency (taking account of impedance mis-match) should exceed a threshold value determined by the values of the external noise field and the receiver noise figure. Generally the useful antenna bandwidth determined in this way is much greater than the −3 dB bandwidth.

7.2.2

Antenna Radiation Patterns

As the radio frequency is increased from 2 MHz, radiation efficiency increases, whilst radiation patterns may vary considerably. These characteristics depend on the antenna/airframe combination.

Ideally the radiation patterns should be matched to the propagation mode that needs to be used. It is convenient in the first instance to treat the aircraft antenna as an elevated electric dipole as follows:

a) Ground wave. A vertical dipole gives the required vertical polarisation and omni-azimuth coverage.

b) High angle sky wave. A horizontal dipole gives the required high angle coverage, azimuth orientation being immaterial.

c) Low angle sky wave. Either a vertical or a horizontal dipole may be employed, but the latter exhibits reduced directivity for angles near the dipole axis.

Fig. 7.1 Frequency dependence of aircraft antenna efficiencies 98 HF COMMUNICATIONS: A SYSTEMS APPROACH

Operation in the low frequency end of the HF band is necessary for both ground wave and high angle sky wave links, so that from a and b above there are conflicting antenna requirements. Long-range sky wave links employ higher operating frequencies and such conflicts do not necessarily occur in this case.

Some types of aircraft may be fitted with tail-fin notch or vertical loop antennas which radiate as magnetic dipoles. Such antennas give good high angle coverage together with vertical polarisation in the azimuth plane, but the radiation patterns exhibit broadside nulls. Thus they offer reasonable compromise radiation patterns for both ground wave and high angle sky wave links. Aircraft-installed antennas are compound radiators and in some cases it is possible to configure the antenna to give vertically polarised omni-azimuth coverage whilst at the same time the airframe gives high angle coverage.

Few radiation patterns of large aircraft have been measured, but at frequencies when their length is much greater than half a wavelength they exhibit structured radiation patterns, giving degraded link performance near directions of the minima. As a simple working rule the number of lobes (and minima) at wavelength for a cylindrical dipole of length L is given by

(7.3)

The lobes exhibit cylindrical symmetry. For a cylinder of length 50 m (roughly representing a large aircraft), a 4-lobe radiation pattern is exhibited at 6 MHz, with nulls broadside to the axis. At frequencies of 9,12 and 15 MHz, 6–, 8– and 10-lobe radiation patterns respectively are exhibited. Due to the non-cylindrical shape of the aircraft the radiation patterns exhibit departures from the cylinder case, but remain highly structured with consequent communications link reliability implications.

7.2.3

Aircraft-generated Noise

Probably the most important feature of the ground-to-air link is the noise environment of the aircraft. All aircraft systems which use electrical energy are likely to generate unwanted electromagnetic energy, and this may couple into the aircraft radio systems and degrade their performance. Conversely, almost every aircraft radio transmitter generates intense electromagnetic fields which may effect other avionic systems, including installed radio systems operating at the same time. Coupling mechanisms between an interference source and the rest of the avionic installation may be complicated; interference levels are affected by factors such as design, practice and workmanship of the avionic installation, imperfect shielding of braided coaxial cables and the RF attenuation offered by the aircraft skin. A detailed discussion of these effects has already been given in section 5.3. From the estimates of aircraft-generated noise given in section 5.3.5 it can be seen that aircraft noise levels in the HF band may be 15 dB or more above those at a ground station in a rural location.

7.2.4

Flight Paths and Frequency Selection

In considering the use and performances of HF communications systems for aeronautical purposes it is convenient to categorise the usage into en-route and off-route applications. Civil aircraft flying the North Atlantic route between Europe and North America provide a good example of en-route usage. The routes are laid down geographically and most flights are scheduled. The aircraft need to communicate over long ranges by HF. This is undertaken in order to give positional information at certain times and such information as estimated time-of-arrival, fuel state, airfield diversion, etc. These messages from the aircraft are relatively short and usually require a short acknowledgement from the ground. If the information does not get through first time but takes a few minutes or so, this is usually acceptable.

The communications frequencies to be used, both primary and secondary, are provided at crew briefing. Because the tracks flown are well charted, past experience helps a great deal in achieving reliable communications. Communications are generally relatively satisfactory, but of course problems occur during ionospheric disturbances. For long-haul transport routes in temperate and lower latitudes, high availability and reliability is usually achieved.

A different picture emerges, however, for off-route usage. Military aircraft fly on routes that are not regularly used. They include large aircraft with crews which may or may not include a specialist radio operator, and high performance aircraft with only a one or two man crew who use the radio as just one more item of airborne equipment. In these cases it could be that the aircraft does not wish to transmit unless it is essential, but that when it does, the response of the communications system must be fast and highly reliable. The choice of the correct frequency is in this case important. As an example take an aircraft operating to the north of the UK in summer 1977 and wishing to communicate with a base in southern England. Table 7.1 indicates the frequency[2] that should be used depending on time of day and distance from the base. Up to eight frequencies could easily be required for a typical sortie.

Table 7.1 Optimum frequencies (in MHz) for aircraft communication to a site in southern England in summer 1977

AIR-GROUND COMMUNICATIONS 99

7.3

Impact upon System Performance 7.3.1

Effect of Antenna Efficiency

Consider now how the properties of the aircraft antenna would change the shape of the reliability curve already given in Figure 6.7. A typical graph of aircraft antenna efficiency against frequency[3] is shown schematically in Figure 7.2a. The frequencies f₁₀, f₂₀…designate antenna efficiencies of −10 dB, −20 dB…and are different for large and small aircraft as already seen from Figure 7.1. The modified reliability curve for R₀=20 dB in Figure 7.2b is then given by the chain curve, which passes through the R₀=30 dB curve at f=f10, R₀=40 dB curve at f=f₂₀ and so on. The effect of the frequency variation of antenna efficiency is therefore to shift the original R₀=20 dB curve towards the higher frequencies and this restricts the frequency range capable of providing a given reliability from (f₂ −f₁,) to (f₂ − f₁), see Figure 7.2b. The width of the frequency window is reduced. The centre of this window moves along the frequency axis according to the prevailing conditions as discussed in section 6.3.5.

Figure 7.3 shows a schematic example to illustrate these effects. The result of the poorer antenna efficiency on the small aircraft is twofold relative to the large aircraft:

a) The usable frequency range is decreased; from 4.4–11.8 MHz to 6.3– 11.8 MHz for =60%.

b) The reliability at the lower frequencies is decreased, from 60% to 20% at 4.4 MHz.

7.3.2

Effect of Aircraft Noise

The effects discussed above regarding the antenna efficiency relate to the air-to-ground link, but as already mentioned they are usually unimportant for reception at the aircraft. Of much greater concern on the ground-to-air link is the effect of aircraft noise. Suppose that the ambient noise levels at the aircraft are 20 dB above those at a given ground station. The reliability of the ground-to-air link compared to a point-to-point link with the same transmitting station over the same range is then degraded from, for example, the R₀=20 dB curve to the R₀=40 dB curve.

Fig. 7.2 Effects of degraded aircraft antenna efficiency on circuit reliability 100 HF COMMUNICATIONS: A SYSTEMS APPROACH

7.3.3

Effect of Frequency Choice

Experience shows that it is the presence of interference on an HF air-to-ground link, rather than propagation conditions, that normally limits system performance. The interference is found to be least during the daytime at frequencies well below the MUF; it may sometimes be advantageous to operate below the optimum working frequency accepting some loss in propagation quality in exchange for less interference from other spectrum users.

The dependence of reliability upon frequency is shown in Figure 7.4, which gives predicted values[4] for a small aircraft operating over two short-range links at noon in January 1976. Note that at the lower frequencies there is a rapid decrease in Fig. 7.3 Example of reliability degradation

Fig. 7.4 Variation of reliability of an air-ground voice link

AIR-GROUND COMMUNICATIONS 101

reliability due to the decrease in antenna efficiencies (see Figure 7.1 ). The curves X to Z show examples of progressive improvements which might be made. Curve Y shows the effect of a 10 dB ground directivity factor whilst curve Z shows how, in addition, increasing the aircraft antenna efficiency could increase reliability. January 1976 was a period of low sunspot activity, implying that operating frequencies were well down in the region where aircraft antennas are least efficient.

Progressive improvements in link reliability may be achieved by improving aircraft antenna efficiency, ground antenna directivity and by working over longer-range links. Ground antenna directivity gives improved link performance when the system is limited by external noise, but is of little help where reliability of the propagation path is itself a major constraint.

The curves in Figure 7.4 demonstrate the need for good frequency management, but even with some favourable assumptions about the communications terminals the link reliability often falls short of what would be regarded as desirable, particularly at night. Propagation modes can fail very rapidly, so that if frequency management is poor the effects on communications can be very serious.

7.3.4 Effect of Flight Path

Conditions are likely to change as a result of aircraft flight path and these changes can affect the overall reliability for a typical mission profile[4].

Consider an example of air-ground voice communications utilising 10 dB ground receiving antenna directivity and assume that the mission is of 10 hours duration. The ground station is in northern Britain and the aircraft flies northward at 450 knots for 3 hours, remains at this range for 4 hours and then returns, again at 450 knots, during the last 3 hours of the mission. The period is one of low sunspot activity.

In Figure 7.5a predictions are shown for the above mission during January and April 1976, a low sunspot number year.

Curves are given for the MUF and for the frequency of optimum reliability (FOR), given by the frequency which provides the best reliability (this is not necessarily the optimum working frequency). Figure 7.5b shows the reliability factor for this best frequency (FOR) and the reliability factor for a given fixed frequency (3 MHz). The curves take account of a 10 dB ground directivity assuming that it was maintained throughout the flight.

Figure 7.5 shows a mission starting at 0200 hours. The optimum frequency is a compromise between sky wave availability and signal absorption. Note that the optimum reliability (FOR curves in Figure 7.5b) attainable is approximately constant over the whole mission. For a constant (3 MHz) frequency the January results (continuous curves) show that this would be a reasonable frequency for most of the mission, but when the MUF increases (around 1000 hours) the reliability of 3 MHz decreases and the FOR rises to about 8 MHz. Results for April (broken curves) show that the FOR is greater than for January

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