Ionospheric factors affecting frequency sharing in the vhf and uhf bands




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Rec. ITU-R P.844-1

RECOMMENDATION ITU-R P.844-1*

IONOSPHERIC FACTORS AFFECTING FREQUENCY SHARING IN THE
VHF AND UHF BANDS (30 MHz-3 GHz)

(Question ITU-R 218/3)

(1992-1994)

Rec. ITU-R PI.844-1

The ITU Radiocommunication Assembly,



considering

a) that the ionosphere, while primarily responsible for reflection of radio waves below approximately 30 MHz, is capable of supporting propagation in the VHF (30-300 MHz) band under some conditions, during relatively short periods of time, and in certain areas of the world;

b) that propagation mechanisms which exist for relatively short periods of time should be taken into consideration when planning radio systems which share frequencies;

c) that frequency sharing is important in the efficient use of the radio spectrum,



recommends

that the following information be taken into account when planning radio systems which make use of frequency sharing in the VHF and UHF bands (30 MHz-3 GHz).


1. Terrestrial propagation

1.1 Introduction


Radio propagation at VHF is mainly controlled by physical objects, such as terrain and ground cover (clutter), and tropospheric factors, principally refraction. However, at VHF, ionospheric propagation over long distances can take place with relatively small losses. Such propagation events may be significant in causing interference at VHF, particularly for systems requiring high reliability.

1.2 Summary of potential ionospheric interference mechanisms


Table 1 provides a summary of ionospheric propagation mechanisms which may be significant causes of interference at VHF. Further information on the more important mechanisms is given in the following sections.

1.3 Normal F-region propagation


Near the peak of the solar cycle long-distance propagation via the F2 layer can occur for a significant fraction of the time at frequencies above 30 MHz. This effect extends to 70 MHz at low latitudes. Figures 1, 2 and 3 show, for a path length of 4 000 km, values of the MUF exceeded for 1% of hours for three seasons at sunspot maximum.

1.4 Trans-equatorial propagation (TEP)


Strong transmission can occur, particularly during high sunspot years, over long North-South paths spanning the geomagnetic equator.

There appear to be two types of trans-equatorial propagation characterized by the times of peak occurrence, fading characteristics and modes of propagation.


TABLE 1

Main causes of ionospheric interference to stations working at frequencies between 30 and 300 MHz





Cause of
interference



Latitude
zone



Period of severe


interference

Approximate
highest frequency
with severe
interference
(MHz)

Approximate frequency
above which
interference is
negligible
(MHz)

Approximate
range of
distances
affected
(km)

Principal distinguishing features

Regular F-layer
reflections

Mid

Day, equinox and winter, solar-cycle maximum

50

60

E-W paths
3 000-6 000
or

Occurrence broadly in accordance with regular-layer morphology




Low

Afternoon to late evening, solar-cycle maximum

60

70

N-S paths
3 000-10 000







High

Night

70

90

500-4 000

Principally during summer months in mid latitudes. Sudden onset and conclusion, beginning later and ending earlier with increase of operating frequency. Area concerned

Sporadic-E reflections

Mid

Day and evening, summer

60

83-135 (1)




relatively small and mobile. Duration minutes or hours. No associated signal enhancements at short range




Low

Day

60

90







Sporadic-E
scatter

Low

Evening to midnight

60

90

Up to 2 000




(1) For 0.1% of the time during the hours 0800-2300 LT for May to August (111 min total) the following frequencies may be derived from Annex 1 to Recommendation ITU-R P.534 for a distance of 1 800 km and   30 dB for temperate zone:

Region A (Europe and North Africa): 083 MHz

Region B (North America): 093 MHz

Region C (Asia): 135 MHz

Region D (Average for Northern Hemisphere): 115 MHz



TABLE 1 (continued)







Cause of
interference



Latitude
zone



Period of severe


interference

Approximate highest frequency
with severe
interference
(MHz)

Approximate frequency
above which
interference is
negligible
(MHz)

Approximate
range of
distances
affected
(km)

Principal distinguishing features

Reflections from meteoric ionization

All

Particularly during showers

May be important anywhere in the range

Up to 2 000

Signal bursts with durations from a fraction of a second to several minutes. Marked diurnal variation, maximum 0600 h local time, minimum 1800 h. Some activity present at all times, but considerable increases during predictable shower periods

Reflections from magnetic field aligned columns of auroral ionization

High

Late afternoon
and night







Associated with geomagnetic disturbances, typically when local K-index reaches 5 or more. Characteristic rasping note due to multiple Doppler shifting. Normal duration a few hours, often afternoon to midnight.

Scattering in the F region

Low

Evening to midnight, equinox

60

80

1 000-4 000




Special transequatorial effects

Low

Evening to midnight

60

80

4 000-9 000

Paths generally aligned symmetrically across the dip equator. Generally around equinoctial periods with regular occurrences. Strong signals. Refer to main text for further details.




FIGURE 1 PI.844-1 [D01]

The first type of TEP, which is called the afternoon type, has the characteristics:

– a peak occurrence around 1700-1900 h LMT, the time being measured at the point where the circuit cuts the magnetic equator;

– normally strong steady signals with a low fading rate and a small Doppler spread (about  2-4 Hz);

– path lengths of about 6 000-9 000 km and sometimes longer.

The second type of TEP, which is called the evening type, generally supports higher frequencies than the afternoon type and has very different characteristics:

– a peak occurrence around 2000-2300 h LMT;

– high signal strengths but with deep and rapid fading at rates up to about 15 Hz and a large Doppler spread which sometimes exceeds 40 Hz;

– path lengths usually shorter than for the afternoon-type mode, being about 3 000-6 000 km.



FIGURE 2 PI.844-1 [D02]


1.5 Sporadic-E propagation


Sporadic-E ionization appears as an intensification in ionization in the form of a horizontal sheet of about 1 km average thickness and a horizontal dimension of the order of 100 km. The height is commonly 100 to 120 km. Such sporadic-E layers can cause abnormal VHF propagation for periods lasting for several hours.

The occurrence of sporadic-E propagation decreases with increasing frequency, but can be a significant cause of interference at frequencies up to about 135 MHz.

Recommendation ITU-R P.534 provides a method for calculating sporadic-E field strengths and probability of occurrence.

1.6 Meteor-trail ionization


Scattering from ionization due to meteor-trails can produce VHF interference over ranges up to approximately 2 000 km. Although individual meteor trails remain effective for periods measured in seconds, meteor bursts can support continuous or near-continuous propagation for much longer periods of time.

The effect of meteor trails on VHF propagation shows spatial, diurnal and seasonal variations. Further information may be found in Recommendation ITU-R P.843.



FIGURE 3 PI.844-1 [D03]


1.7 Auroral ionization


Field-aligned irregularities in the auroral zones appear during magnetically disturbed periods. Such ionization can produce significant reflections causing propagation which is normally off the great-circle path. This may cause interference at VHF, mainly in mid to high geomagnetic latitudes.

2. Earth-space propagation


Radio propagation at VHF and UHF on Earth-space paths through the ionosphere is subject to attenuation, polarization rotation, amplitude and phase scintillation and ray-path bending. These effects may influence the extent of frequency sharing for satellite services and between satellite and terrestrial services.

For orbiting satellite systems the effect of time and location variations of irregularities in the ionosphere may be important, notably in equatorial and auroral regions.




* Radiocommunication Study Group 3 made editorial amendments to this Recommendation in 2000 in accordance with Resolution ITU-R 44.


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