Troposcatter Overview

  • Troposcatter Introduction
  • Troposcatter Performance Factors
  • Troposcatter Benefits
  • Example Troposcatter Applications

What is a Troposcatter System?

  • Advanced Troposcatter Systems provide high capacity, low-latency data links for over-the-horizon distances, without the need to rely on satellites or multiple line-of-sight repeaters.
  • They also provide a level of resistance to intercept or detection that exceeds most line-of-sight microwave and satellite communications.
  • These systems have little in common with the troposcatter (tropo) equipment developed for long-haul communications from the 1950s to the 1980s, much like computers today bear little resemblance to the computers of those eras.
  • The common element is that all troposcatter systems function by using the non-homogenous elements present in the lowest level of the atmosphere— such as water vapor, dust, and atmospheric variations— to scatter a small portion of the transmitted energy forward in a predictable manner.

A troposcatter system is a point-to-point link that requires a terminal on each end, with each terminal both transmitting and receiving. Terminals can range in size from a portable transit case system to a vehicle-mounted system or large fixed installation. The antenna(s) at each terminal are aimed at a fixed point in the troposphere, generally slightly above the horizon. The region where the antenna beams intersect is known as the “Common Volume.” The Common Volume is where this predictable scattering phenomena occurs.

Due to the nature of troposcatter propagation, only a tiny portion of the energy is scattered forward in a usable manner. The vast majority of energy passes on into space and is lost, and a small amount of non-useable scatter energy is lost as it disperses in other directions. A troposcatter system uses the remaining energy to create a stable and secure data link.

While the frequency selection and the diversity configurations required for a stable tropo link make troposcatter communications largely immune to short-term fading from rain and other weather patterns, they are affected by the long-term seasonal variations in the atmosphere. For example, more moisture is present in the atmosphere to scatter energy in humid summer months than in dry winter months, resulting in more efficient propagation.

With long-term seasonal variations in mind, troposcatter paths are designed to meet the required data rate and availability at the worst possible time of the year, and typically have excess power available during the remainder of the year. Troposcatter systems incorporating features such as automatic code rate, Forward Error Correction (FEC), and implicit diversity within the modem itself are able to make the most efficient use of the transmitted energy, thereby ensuring that the lowest possible amount of energy is required to support the given data rate and availability.

TCS2It is important that a tropo system has the capability to constantly monitor the link and adjust transmit power. This ensures that the system only uses the minimum transmit energy required to maintain the user-defined requirements at any given point in time, avoiding the interference problems that troposcatter equipment has experienced in the past.

In order to maximize the effectiveness of a troposcatter link, modern tropo systems are configured in dual or quadruple diversity configurations. These diversity configurations may be achieved using a combination of antenna spacing, multiple transmit frequencies, polarization, or specialized angle diversity antennas. The performance gains associated with implementing diversity configurations is much more efficient than simply scaling up power or antenna size requirements.

Troposcatter System performance is also impacted by terrain. The amount of usable forward scatter is directly related to the angle of the transmission, with shallower angles resulting in greater usable forward scatter. While troposcatter systems can function over uneven or obstructed terrain, the closer they are to an obstruction, or the taller the obstruction, the greater the transmission angle needed to clear it, reducing the amount of usable energy scattered forward. Increased gain from amplifiers, antenna size, or diversity configurations is then needed in order to compensate for the increased path loss in order to avoid reduced range, lower data rates, or decreased availability.

  • Troposcatter systems provide data rates exceeding that of typical satellite communications (up to 50 Mbps per system) without incurring any ongoing costs for usage.
  • These systems have minimal latency, typically less than 15ms, alleviating packet loss concerns and communications issues common to VSAT systems.
  • A properly designed troposcatter system is extremely reliable, requiring only basic maintenance, with many systems still performing after 15 or more years of operation.
  • Additionally, due to the unique nature of troposcatter propagation, tropo communications are extremely difficult to intercept covertly.
  • Connecting a forward operating base to an HQ when line of sight is obstructed by a mountain range and hostile activity make unprotected repeater sites impractical
  • Increasing the coverage area of air defense missile batteries that were previously restricted to line of sight due to sensitivity to latency
  • Secure communication paths between installations requiring lower probability of intercept than is possible with satellites
  • Quick-deploy communications that do not require coordination for transponder space
  • Long-range inter-island or shore to offshore communications over water
  • Replacement of limited capacity remote satellite links in a communications backbone with high-capacity, long-haul troposcatter links, resulting in cost-savings and more efficient allocation of the space segment
  • Robust, redundant communications backup for VSAT, Wireless, or Fiber