One of the greatest dangers to aviation are thunderstorms. When evaluating accidents related with thunderstorms it became apparent that the dangers associated were not always recognized by the aviators and possible evasive action was not carried out leading to disastrous results.

Airborne weather radar is not very common in homebuilt experimental aircraft but it can be used effectively to avoid thunderstorms, hence this page.

Airborne weather radar

Radar has been developed in the last century as an early detection device for enemy aircraft. Soon it became apparent that weather had its influence on the radar returns. From this the weather radar was developed followed by the installation of radar in the aircraft for detecting and avoiding adverse weather.

Reflection of energy
Some part of the emitted energy of the Airborne Weather Radar (AWR) is reflected by precipitation. The amount reflected depends of the type, size and concentration of the droplets. Good reflection comes from big rain droplets and melting hailstones or snowflakes if covered by a water layer. Reflection is poor from cloud vapor, drizzle. This depends on the radar frequency used. Water vapor, turbulence (just dry air) give very little to no reflection at all.

AWR Description
The AWR transmits a pulse with a duration of approx. 5-10 microseconds in the X (3cm) or C (5,5 cm) bands, with a beam width of 7 degrees. When this pulse is reflected and received by the antenna, time speed and distance calculations can be made to convert this data in intensity and range information for display on the radar scope.
The transmitted radar pulse will be attenuated due to absorption and scattering in the atmosphere caused by air molecules, dust, ice crystals, water vapor and or rain. Distance attenuation is inversely proportional to the fourth power of distance: at 200 NM the reflected pulse will be 16 times weaker than at 100 NM. To compensate gain control by STC (Sensitivity Timing Control) is used.

Airborne Weather Radar display

STC limits sensitivity during the first microseconds of receiving. The result is that targets within a certain distance do not grow or seem larger than targets further away. These targets keep their same relative size on the radar scope.

On the radar scope precipitation is displayed in a number of intensity levels or colors.

AWR Restrictions

Water attenuation
When the radar beam hits a dense water concentration, most of the energy is absorbed by it on the edge and the most severe part of the Cb might be shown as a clear area. What looks like a clear area could be the worst part of the storm.

Beam width
As the beam of the radar is of a certain width and height, it must be tilted upwards and moved to the left and right to spot the top of the Cb's.

Lightning sensitivity
As the radome is one of the most sensitive part of the airplane, a lightning strike can destroy the antenna when it is needed most.

Stormscope

Features
A stormscope displays information regarding location and intensity of the electrical discharges of thunderstorm. It is a passive device and does not emit radar pulses. Turbulence and precipitation can also not be detected, but one can assume that these will be present in the vicinity of electrical activity.

Application
As the price of this device is lower than the Airborne Weather Radar it is of particular interest for General Aviation. It is also a lightweight and has a low power usage. Ideal for general aviation aircraft. It is also possible to display the information of a stormscope and AWR on one display.