The Basic Operating Principles of Airborne Weather Radars

While an aircraft is designed to withstand a number of extreme weather conditions, some changes in weather can produce unprecedented turbulence, which in some cases, can lead to injuries to passengers and crew. For this reason, the airborne weather radar is a vital system that can be used to prevent the potential of these scenarios taking place. This system allows pilots to assess the intensity of convective weather, and makes adjustments to the trajectory of the flight. With this in mind, this blog will cover the advantages of the airborne weather radar as well as its basic operating principles.

If properly used, an airborne weather radar relieves pilots of having to manually assess changing flight conditions, but also reduces the risk of encountering unfavorable weather patterns. Put simply, a radar system uses high-speed electromagnetic waves to determine distance, velocity, direction being travelled, and altitude of both stationary and non-stationary objects. These objects include weather formations, motor vehicles, ships, aircraft, spacecraft, and even terrain. Essentially, the radar system transmits electromagnetic energy and analyzes the energy reflected back to it by the object. If these reflected waves are received again at the place of their origin, this lets the pilot know that there is an obstacle in the propagation direction.

Another important component of the airborne weather system is the radar antenna located in the nose of the aircraft. The energy-sensing signals emitted from the antenna are processed by a computer located inside the cockpit. The information is presented on a screen, or Navigation Display (ND), for the pilot to interpret and draft a plan for safe trajectory. Water droplet size is a good indicator of strong updrafts within cumulonimbus clouds, which are typically associated with turbulence. In fact, it is the reflectivity of these droplets that give the pilots information about changes in weather patterns while in flight. These droplets are color coded for intensity on the ND with red indicating high reflectivity as would be the case for wet hail and green indicating low reflectivity as is the case for drizzle.

Airborne weather radar systems can also detect wind shear, or the sudden change in wind velocity and/or direction. This is especially important because wind shear can cause turbulence, violent air movement, sudden increase or reduction of airspeed, as well as the sudden increase or decrease of ground speed and/or drift. In some cases, pilots are aided by airport based warning systems in the case that the aircraft is experiencing dangerous wind shear conditions. Weather radar systems have become so central to the safe operation of an aircraft that European Union regulations require them to be an integral part of most aircraft. 

There are four features that need to be assessed to operate the radar: antenna tilt, range control of the ND, gain control, and radar modes. The antenna tilt is defined as the angle between the center of the beam and the horizon. The range of control the pilot has of the ND influences the optimum tilt setting. Gain control adjusts the sensitivity of the receiver and radar modes account for weather and turbulence. Moreover, to use the weather radar system to the best of its abilities, the crew must have good meteorological knowledge of weather phenomena as well as an in-depth understanding of the many radar functions.

It is important to keep in mind that, like most technologically advanced systems, there is room for error and limitations. One such limitation is that the weather radar system only detects liquid water. At a higher altitude, the quantity of liquid water decreases. So in the case of a thunderstorm, the weather radar system may misinterpret the data and find that there is not any  danger. This does not mean that the convective cloud and its associated threats are reduced because little reflectivity was detected. In fact, the danger may surpass the ‘radar top,’ or upper detection limit of the weather radar. What this means is that reflectivity is not always an accurate indication of the level of risk a convective cloud may pose. This is especially the case for aircraft flying over equatorial overland regions where converging winds produce large uplifts of dry air. While these weather cells may have minimal reflectivity, the turbulence may be more severe than can be determined by the radar. Similarly, near the sea where the air is very humid, thermal convection produces clouds full of water that have high reflectivity, but don’t pose a high threat. For these reasons, it is paramount that the crew understands how to strategize in the event that the weather radar system cannot detect certain weather patterns.

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