What is Troposcatter Communications?

Troposcatter Communications are used for beyond-line-of-sight (over the horizon) communications between remote geographic areas where cable links are not feasible. They transmit and receive radio signals by reflecting them off the Earth’s troposphere. This method uses particles that make up the troposphere as a reflector for microwave signals. It enables long-range communication, up to 250 km, making it particularly useful for military, civilian and remote communication applications. Most of the Troposcatter systems work in the frequency band of 4400 to 5500 MHz. A little demand exists for 1800 to 2400 MHz, and 7100 to 7499 MHz frequency bands as well, but the licensing is difficult due to competing demands from cellular, mobile, satellite, and related applications.

How does it work?

Troposcatter communication utilizes the phenomenon of tropospheric scatter, which is the scattering of radio waves as they interact with irregularities such as water droplets, dust particles, and gases in the atmosphere. The antenna at the transmitting end generates and transmits a signal in the UHF or super UHF frequency range. The signal is directed upward at a specific angle just above the horizon. This angle ensures that the signal interacts with the troposphere over a long distance.  A troposcatter system has an antenna at both ends, that is usually capable of both transmitting and receiving signals, aimed at a fixed point in the troposphere slightly above the horizon. The common region where the antenna beams intersect is the area where the forward scattering phenomenon takes place.  

As the signal travels through the troposphere, it encounters the various irregularities in the atmosphere which causes the signal to scatter in multiple directions instead of a straight path. As a result, a portion of the signal is scattered back towards the Earth’s surface. This scattering occurs over a wide area, allowing the signal to be received by a distant receiver station location within the area of coverage. The signal is received by the receiving antenna where it is processed and demodulated to extract the original information.

To ensure reliable communication, the signal must be transmitted at a high power and in a specific frequency range ( 1- 10 GHz ). Also, the signal must be transmitted at a specific angle, typically between 2 and 5 degrees above the horizon, to ensure effective scattering. 

Advantages of Troposcatter Communications

Troposcatter communication can span distances up to several hundred kilometres, which makes it ideal for applications where traditional line-of-sight communication is impractical or impossible. It is a reliable communication method in rugged environments as it is not susceptible to a signal blockage from obstacles such as building or terrain features, and due to its low probability of interception and detection, it offers inherent security and hence can be used for military and battlefield communication systems.

US Army TRC-170 Troposcatter Microwave System

Challenges 

Troposcatter communication faces some obstacles despite its capability to transmit data over long distances. The irregularities present in the atmosphere, such as moisture, gases, dust, etc., result in signal fading, which results in fading of the overall signal. Also, bandwidth limitations constrain the data transmission capacity. Therefore, specialized equipment like high-power transmitters and sensitive receivers, along with precise alignment instruments, are used for tropospheric communication systems.