Tropospheric Scatter Communication

long-distance radio communication based on the scattering of electromagnetic energy in the electrically inhomogeneous troposphere as radio waves propagate in it. Tropospheric scatter communication is carried out in the decimeter and centimeter wavelength ranges (seeRADIO-WAVE PROPAGATION).

Figure 1. Diagram of a radio-relay system for tropospheric scatter communication: (T) terminal receiving and transmitting stations, (I) intermediate receiving and transmitting stations, (R) distance between stations along an arc of the earth’s surface, (1) terminal receivers and transmitters, (2) terminal receiving and transmitting antennas, (3) intermediate receivers and transmitters, (4) intermediate receiving and transmitting antennas, (5) scattering regions of the troposphere

The electrical inhomogeneities of the troposphere, that is, the inhomogeneities of its dielectric constant e, are caused by random local variations in temperature, pressure, and humidity and by the regular decrease in these quantities with increasing height. The scattering of energy occurs in the regions where the directional patterns of the transmitting and receiving antennas intersect. The distance between the points of transmission and reception may be as large as 1,000 km. In practice, however, radio-relay communications systems are usually constructed in which tropospheric scatter communication is used either in all or only in some of the links of the system (see Figure 1). The length of such systems may be as large as several thousands of kilometers.

The mean signal strength in tropospheric scatter communication is very low owing to the low intensity of tropospheric inhomogeneities, that is, the small fluctuations of e. The mean signal strength varies with distance R as 1/Rⁿ, where n = 10–12. Random fluctuations in the level of the radio signal (fading) occur constantly; they are caused by spatial and temporal changes in e. For this reason, high-power transmitters (1–50 kilowatts), highly sensitive receivers, and large antennas (up to 40 × 40 m²) must be used in tropospheric scatter communication, along with special transmission methods that make it possible to reduce the effects of signal fading. These methods include the transmission and reception of the same message on several carrier frequencies and reception by spaced antennas.

The power parameters of modern receivers and transmitters make it possible to establish up to 120–240 telephone channels (seeMULTICHANNEL COMMUNICATION) in one high-frequency trunk line when R = 150–250 km and up to 12 channels when R = 800–1,000 km. The transmission of television signals is possible only if R < 150–200 km. The quality of such transmission, however, is low because many waves with different delay times arrive at the point of reception.

Tropospheric scatter communication systems are usually established in sparsely populated regions that are not easily accessible, when the construction and operation of the systems is economically and technically warranted.