THE RESURGENCE OF TROPOSCATTER COMMUNICATIONS IN THE MILITARY
By Mark Yamamoto and David Stephenson, Comtech Systems, Inc.
Early tropospheric scatter communications systems provided militaries much needed high capacity wireless links supporting voice and data, over long distances without the need for intermediate repeaters. These systems were characterized by the use of very high power amplifiers, large to very large antennas which were cumbersome to deploy in their transportable configuration. Due to the required high transmitted power the tropospheric scatter systems of the time caused a concern about interference with other communications networks so detailed frequency planning was necessary if it was to be used in larger networks.
Today, a new breed of troposcatter equipment is reaching the battlefield. The new equipment is smaller, lighter, easier to deploy and operate and is capable of providing high data rate digital traffic in IP or E1 formats. Automatic power adjustment ensures the system transmits only what is needed in a real time environment for the given communications link, reducing the possibility of interference and intercept.
As a result of recent experience, military forces are re-discovering the benefits of troposcatter communications for long range beyond line of sight links. Satellite bandwidth for tactical use or home nation reachback is very limited and is becoming cost prohibitive. Troposcatter communications are able to provide networked multimedia services with attractive data rates that are quick to deploy and easy to operate. Innovations in equipment design are enabling troposcatter systems to be used in new applications that are further enabling tactical networked communications across the battlefield.
Following a brief summary of the operation of troposcatter communications , recent experience with the operational deployments of troposcatter communications and a look at the future of troposcatter in a military environment are described to highlight the renewed interest in troposcatter for military use.
Troposcatter radio systems, also known as beyond line of sight microwave, or simply tropo, provide communications over the horizon by using a diffraction or scattering radio path through the troposphere. The troposphere is the lowest layer of the atmosphere extending to and altitude of 8 to 15 km above the earth’s surface. Because of its proximity to the earth, the troposphere has clouds, precipitation, humidity and active convection currents that lead to variations in refraction index. These refractive index gradients enable forward scattering of transmitted signals, which in turn make beyond line of sight troposcatter communications possible. Using scattering to achieve beyond line of sight communications means that the path loss is higher than normal line of sight microwave transmission. This in turn requires high gain antennas, high power amplifiers and modem designs to account for the additional energy loss in both fast and slow fading conditions. Other paths can be set up under appropriate conditions using diffraction over high terrain features to close the link.
Diversity operation is required by all troposcatter systems to overcome multi-path and rapid fading conditions, and it is the diversity need that has primarily dictated the size, weight and mobility of the tactical troposcatter systems. Diversity operation is achieved by combining the radio energy from two or more independent paths over the radio link using one or more of following techniques:
• Space diversity (two antennas separated by space)
• Frequency diversity (transmitting the same information on two different frequencies that are approximately 50 MHz apart).
• Angle diversity (one antenna receiving at two different elevation angles).
These approaches may be used separately or in combination to gain additional orders of diversity. The mos