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. Stormscopes are more commonly found in these aircraft, 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 these radar energy returns. From these experiences weather radar was developed followed by the installation of radar in the aircraft for detecting and avoiding adverse weather.
Reflection of radio energy
A small part of the emitted energy of the Airborne Weather Radar (AWR) is reflected by precipitation. The amount that is 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. But this depends highly on the radar frequency and power used. Water vapor, turbulence (just dry air) gives very little to no reflection at all.
The AWR transmits a pulse with a duration of approx. 5-10 microseconds in the X (3 cm) 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.
Even radar is unable to penetrate a solid rain cloud as radio signals are attenuated by the water drops. You will therefore see a dark shadow like area behind heavy precipitation which looks clear but could be filled with rain. See below.
Radar range in clear air is restricted due to the size of the antenna dish and the amount of power generated in GA aircraft, you may expect about 40 to 50 nautical miles in these installations.
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.
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 and bottom of the Cb's. This can be done by the pilot on the control panel.
As the radome is one of the most sensitive part of the radar installation, a lightning strike can destroy the antenna when it is needed the most, rendering the installation useless.
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 any electrical activity.
As the price of this device is much lower than the Airborne Weather Radar it is of particular interest for GA. It is lightweight and with low power usage its ideal for general aviation aircraft. It is also possible to display the information of a stormscope and AWR on one display. Ideally on the navigation display!
Today we see technologies where radar images and other weather related information is downloaded via satellite link to the aircraft. XM Satellite Weather is such an example in the U.S. A possible disadvantage can be the delay involved in the transmission of the information, this can be five minutes or longer.
Another factor to think about is that radar images are created from a station with the antenna on the ground which can give a different picture compared to the aircraft radar.
A number of companies are developing this technology for handheld devices like the Garmin 496, bringing actual weather to the GA and experimental pilot.