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 . A related system is meteor burst communications , which uses the ionized trails of meteors to improve the strength of the scattering . 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 ratio s . High gain dish or billboard antenna e are required for tropospheric scatter systems as the propagation loss es are very high ; only about one billion - billionth ( 1 x 10 ) 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 . Modern military systems are " Wideband " as they operate 4 - 16 Mbit / s digital data 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 horn s – 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 difference s 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 . Tropospheric Scatter Communications Networks . The Tropospheric scatter phenomenon has been used to build both civilian and military communication links in a number of parts of the world , including : - Communications to British Forces Germany , running from Swingate in Kent to Lammersdorf in Germany . A system linking the Philippines with Taiwan . Tactical Troposcatter Communication systems . As well as the permanent networks detailed above , there have been many tactical transportable systems produced by several countries : - The U . S . Army and Air Force use tactical tropospheric scatter systems developed by Raytheon for long haul communications . The systems come in two configurations , the original " heavy tropo " , and a newer " light tropo " configuration exist . The systems provide four multiplexed group channels and trunk encryption , and 16 or 32 local analog phone extensions . The U . S . Marine Corps also uses the same device , albeit an older version .