Ground weather radars are surveillance sensors that are used to discover, assess and track hazardous weather (mostly CB clouds and associated phenomena such as thunderstorms and hail). They use the same operating principle as the Primary Surveillance Radar (PSR)), i.e. derive information from echoes received after transmitted electromagnetic pulses are reflected by the hydrometeors forming the clouds and the precipitations. However, instead of detecting aircraft and filtering out clutter (e.g. terrain and clouds) they focus on the airborne clutter. Since the size of the water particles in the clouds is much smaller than that of aircraft, weather radars operate much higher frequencies (3-10 GHz) and therefore, shorter wavelengths (10-3 cm) than PSR. Shorter wavelength radars can detect smaller particles at the expence of reduced range.
While the basic operational principle of the ground weather radar is similar to that of the PSR, it can provide a much wider variety of information about the meteorological phenomena. This includes:
- Location (bearing and distance from the radar). These are determined by a calculation based on the echo delay (distance) and the azimuth of the antenna (bearing). As with the primary radar, the slant range is measured. Horizontal distance is determined by knowing the elevation angle.
- Level (or level band). Unlike the PSR, the emitted signal is narrrow in the vertical plane. Therefore, by knowing the distance of the phenomenon and the antenna attidude, it is possibe to calculate the level. Note that this value is geometric, unlike the information derived from an aircraft's transponder (which is pressure-based). The vertical development of the phenomenon is determined by performing scans at different angles. This can be done by either moving the antenna vertically or by using a phased array to electronically steer the emitted signal.
- Intensity/type. This is determined by processing the returns. The results need to be adjusted to the range, since a closer situated phenomenon would return the same signal as one that is of greater intensity but is farther. The intensity provides an idea about the phenomenon type. It should be noted that there is no exact match between these though.
- Direction and speed of movement. This is determined by the doppler shift of the received signal.
ATC weather radar principle of operation
Example of weather data on the situational display. Note that dull colours are used so that distraction is minimized. Different intensities are shown using a variety of fill patterns.
Example of weather data on a separate display. Note the use of different colours as well as the vertical view (below and left of the planar view).
- Making consecutive scans at different antenna attitudes takes time. The more the angles, the more precise the data but also the longer the period of information gathering. This can be somewhat mitigated by rotating the antenna slower and using a phased array for the vertical scan (which is faster compared to the mechanical adjustment of the antenna)
- Processing the weather data for a 360 degree coverage at multiple angles requires a lot of computing power. While modern computers can perform this task quickly, older equipment often needs several (e.g. 5-10) minutes.
- Non-weather objects (e.g. flocks of birds, large groups of insects, wind turbines, etc.) produce returns that are similar to weather phenomena.
- Only the radial component (i.e. the closing speed) can be determined. This can be mitigated if multiple radars are used. Another option is calculate the speed by comparing the consecutive positions of a particular phenomenon but this takes much longer.
- Accuracy decreeses with distance. This happens because the beam becomes wider at longer distances. Therefore, accuracy at the end of the radar range is several times smaller than at close range, i.e. the position of distant phenomena is determined with greater error.
Cumulunimbus clouds usually develop very fast, often within 10-20 minutes (from a clear sky). They may also move quickly with the associated weather front (at speeds of around 30 knots).
Depending on the equipment, collecting and processing of weather information can be time consuming to the extent that when the output data is available, the phenomenon:
- has travelled a considerable distance;
- has developed but there is no current data to inform the controller about that;
- has ceased to exist.
Use of Ground Weather Radars by Controllers
Air traffic controllers may be presented with information from the weather radars integrated in their situational display or on a separate monitor. The advantage of the former is that the weather data and traffic situation are integrated. The drawback is that the display becomes more cluttered so it is more likely to cause a distraction or to have some important information obscured. To mitigate this, the weather data is normally presented in a more basic way compared to the view on a dedicated display. A specialized monitor usually uses a colour palette to present different intensities and may also provide 3-D information about the phenomenon while the integrated display is sometimes limited to just one colour with different patterns showing the different intensities.
The use of this information is described in the local instructions (e.g. manual of operations). Depending on the reliability of the information, its use may vary from enhancing the controllers' Situational Awareness (e.g. what traffic begaviour to expect) to providing information to pilots or guidance on weather avoiding. For example, in the US controllers provide information about the location and intensity (light/moderate/heavy/extreme) of the precipitation. Radar navigational guidance is provided on request by pilots.
Because of limitations described in the section above, weather information on the radar screen may be obsolete by the time it is available. Depending ot the situation, it may be useless, or, in some cases, even dangerous to rely on. For example, if a controller is trying to guide an aircraft to a gap between two CBs, it is possible that due to their development and/or movement (and the delayed information), the safe path has also moved or does not even exist anymore. As a result, controllers may only be allowed to use this information for enhancing their situational awareness.