Radar Basics

Airborne Weather Radar

It is common that the majority of commercial aircraft nowadays carry an Airborne Weather Radar system that is most often built into the aircraft nose. Airborne Weather Radar provides the pilot with a local (ahead only) weather picture in the cockpit and allows him to identify and avoid specific, undesirable weather formations. A maximum range of 180 NM is common although the commonly used range (as selected by pilots) would normally be in the 30 to 80 NM range.

Antenna and Receiver

Figure 2: Nose radar parabolic dish

There are two different types of nose-mounted antenna. One is a parabolic dish and the other is a flat “phased array” (or plate) system. Both are in widespread use but the flat phased array system is more commonly found than the parabolic antennas.

It is common for the antenna inside the aircraft’s nose to rotate from side to side (perhaps covering a maximum azimuth of between 120 to 180 degrees - although this is very much dependent of the implementation).

Processing Equipment

Figure 3: 3D- Weather pattern

A processing box is required to be installed in the avionics rack to process the radar data and present the required picture to the display (and hence the pilot). This is installed beside all of the other common aircraft “boxes” such as the SSR- transponder, the FMS, etc.

For this task extensive and convenient software (e.g. for this one 3D- weather picture shown in the graphic) was already developed.

Display in the cockpit

Figure 4: Display in the cockpit

A weather radar commonly used in aircraft usually uses a sector display. It is similar to a PPI - display and shows a map-like presentation of the received weather image. However, the “radar site” is not in the middle of the screen, but at the bottom edge. The deflection beam swings back and forth synchronously to the antenna swivel. With freely selectable zoom, the pilot can choose the scale that suits him best.

In most cases, the display is a multifunctional device and also shows further data.

Stabilization (tilt and roll)

Figure 5: radar beam during tilt and roll
unstabilisated vs. stabilisated

Clearly, aircraft in manoeuvres (such as during take off) would be 'pointing' their antenna beam at an area of airspace which they were not physically going to fly through. Maintaining accuracy during manoeuvres and other flight phases is important.

There are however considerable problems with maintaining this stability whilst 'in flight'. It is necessary to have some form of motorized equipment on-board in order to support this. Coping with mild manoeuvres such as shallow banked turns is less difficult than more pronounced roll and pitch changes.

Stabilization can therefore only be expected to maintain a constant antenna sweep relative to the horizon during moderate manoeuvres. If stabilization is not switched on, the display seen will mimic the direction and attitude of the aircraft.

For the vertical plane, it is normally quite clear that a manoeuvre could often be more pronounced than the eventual direction of the antenna in terms of the region of airspace to be intended. The vertical aspect of stability can be selected by the pilot if required. Airborne Weather Radar systems have a number of known problems during the take-off and landing phases of flight. Some systems may have a link to the Autopilot (otherwise, a static table will be built in to help make automated decisions on how to operate the system).

For the horizontal plane, it is possible that complex and frequent manoeuvres might be required in certain phases of flight to meet ATC instructions (for example, in a stack). There are certainly several phases where the displayed information may not be useful to the pilot and this is well understood.