Tropospheric Scatter

Tropospheric scatter (known as "troposcatter" among practitioners) is a method of transmitting and receiving microwave radio signals over considerable distances – often up to 300 km. This method of propagation uses the tropospheric scatter phenomenon, where radio waves at particular frequencies are randomly scattered as they pass through the upper layers of the troposphere (hence troposcatter). Radio signals are transmitted in a tight beam aimed at the tropopause, midway between the transmitter and receiver sites; as the signals pass through the troposphere they are scattered, allowing the receiver station to pick up the signal.

Normally, microwave signals, transmitted at various frequencies, usually around 12 Gigahertz (GHz) or 19 GHz, are only used for ‘line of sight’ applications, where the receiver can be ‘seen’ from the transmitter. However, tropospheric scatter signals use a frequency of around 2 GHz.

Because the troposphere is turbulent and has a high proportion of moisture the tropospheric scatter radio signals are refracted and consequently only a proportion of the radio energy is collected by the receiving antennae. Frequencies of transmission around 2 GHz are best suited for tropospheric scatter systems as at this frequency the wavelength of the signal interacts well with the moist, turbulent areas of the troposphere, improving signal to noise ratios.

High gain dish or billboard antennae are required for tropospheric scatter systems as the propagation losses are very high; only about one billion-billionth (1 x 10−12) of the transmit power is available at the receiver. Typically, dish antennae with isotropic gains of between 40 decibels(dB) and 60dB are used with transmitter powers of 1 Kilowatt(kW) to 10 kW.

Tropospheric scatter is a fairly secure method of propagation as dish alignment is critical, making it extremely difficult to intercept the signals, especially if transmitted across open water, making them highly attractive to military users. Military systems have tended to be ‘thin-line’ tropo – so called because only a narrow bandwidth ‘information’ channel was carried on the tropo system; generally up to 32 analogue (4 kHz bandwidth) channels.

Civilian troposcatter systems, such as the British Telecom(BT) North Sea oil communications network required higher capacity ‘information’ channels than were available using HF (high frequency – 3 to 30 MHz) radio signals, before satellite technology was available. The BT systems, based at Scousburgh in the Shetland Islands, Mormond Hill in Aberdeenshire and Row Brow near Scarborough, were capable of transmitting and receiving 156 analogue (4 kHz bandwidth) channels of data and telephony to / from North Sea oil production platforms, using frequency division multiplexing (FDMX) to combine the channels.

Because of the nature of the turbulence in the troposphere, quadruple diversity propagation paths were used to ensure 99.98% reliability of the service, equating to about 3 minutes of downtime due to propagation drop out per month. The quadruple space and polarisation diversity systems needed two separate dish antenna (spaced several metres apart) and two differently polarised feed horns – one using vertical polarisation, the other using horizontal polarisation. This ensured that at least one signal path was open at any one time. The signals from the four different paths were recombined in the receiver where a phase corrector removed the phase differences of each signal. Phase differences were caused by the different path lengths of each signal from transmitter to receiver. Once phase corrected, the four signals could be combined additively.