Extreme weather conditions are a major threat to the safe operation of an aircraft; If an aircraft passes through regions of severe turbulence it is obviously subject to mechanical stress, which may cause damage, possibly leading to a crash. Therefore flight crews need to be aware of these conditions and understand the consequences.
Weather radar is an airborne system capable of detecting the weather conditions leading to the hazards of turbulence, hail, lightning and turbulence, which help pilots to identify weather conditions and subsequently reroute for the safety and comfort of passengers.
On commercial flights, passenger comfort is also important since the number of customers would soon decline if the discomfort and sickness which turbulence may bring became commonplace.
There are three main technologies typically used in aircraft to detect weather conditions:
The word radar is derived from radio detection and ranging. Weather radar works on primary radar principle in which, energy is directed via an antenna to a target which could be an aircraft, the ground or specific weather conditions.
Weather radar systems have been developed which detect either water droplets or electrical activity, both of which are associated with convective turbulence in cumulonimbus clouds. Clear air turbulence has no detectable associated phenomena which can give a clue to its presence.
Precipitation can occur in many different forms including:
TYPES OF PRECIPITATION & ITS REFLECTIVITY
There are various factors which are to be considered for an weather radar system.
The higher the frequency (smaller the wavelength) the larger is the backscatter cross-section per unit volume of the target hence the greater the echo power. However, high frequencies suffer more atmospheric absorption than lower frequencies, and further cannot penetrate clouds to the same extent.
Thus the choice of frequency is a limited to two types
X-band (8–12.5 GHz)
X-band microwave energy pulses can provide good resolution of images; however, this means that they can only be used for weather avoidance. Higher frequencies require a smaller antenna; for this reason, larger passenger aircraft use X-band radar.
An additional consideration is the beamwidth; for a given scanner diameter a narrower beam is produced with a higher frequency
The volume of the target giving rise to an echo is directly related to the pulse width, thus use of long pulses will give improved range.
However longer pulse width cannot be used since the system uses a single antenna for both as transmitter and receiver, so the antenna must be switched to the transmitter for the duration of the pulse; thus the pulse width determines minimum range.
For example if the pulse width is 2 μs, no return can appear for the first 2 μs of the time-base, giving a minimum range of one-sixth of a nautical mile.
Range resolution also deteriorates with increasing pulse width.
For example if a pulse of 2 μs duration occupies about 2000 ft in space. When two targets are on the same bearing but within 1000 ft of one another the echo from the nearest target is still being received when the leading edge of the echo from the furthest target is received.
The result is that both targets merge on the p.p.i. display. The range of the targets does not affect the resolution
Since range resolution and minimum range are not critical in a weather radar, pulses tend to be longer than in other radars, say 2-5 μs. A shorter pulse width, say 1 μs, may be switched in when a short displayed range is selected.
The range of a weather radar system is typically 320 miles.
Microwave energy pulses are reflected from the moisture droplets and returned to the radar antenna. The system calculates the time taken for the energy pulses to be returned; this is displayed as an image on a dedicated weather radar screen, or the image can be integrated with the electronic flight display system. The strength of the returned energy is measured and used to determine the size of the target. Higher moisture content in a cloud provides higher returned energy. The antenna is scanned in the lateral plane to provide directional information about the target.
The weather radar system of an aircraft consist of the following components
Antenna (Nose Cone)
Transceivers (Nose Cone)
Control Panels (Cockpit)
The transceivers are located in the nose cone to minimising the length of the waveguide
Microwave signals are transmitted and received via the antenna.
There are two types of antenna that are used
Of the two, for a given diameter and wavelength, the flat plate has the higher gain/narrower beam/least side lobe power, but is most expensive. The flat-plate antenna projects a more focused beam than the parabolic type; this is due to the reduction in side-lobes.
Since the flat plate is almost twice as efficient as the parabolic reflector it is invariably used with a modern system except where cost is an overriding factor or the space available in the nose of the aircraft allows a large parabolic reflector to be used.
PARABOLIC ANTENNA VS FLAT PLATE ANTENNA RADIATION(LOBE) PATTERN
The parabolic-reflector works on a similar principle to a car head lamp reflector. Energy striking the reflector from a point source situated at the focus will produce a plane wave of uniform phase travelling in a direction parallel to the axis of the parabola. The feed in a weather radar parabolic antenna is usually a dipole with a parasitic element which, of course, is not a point source. The consequence of a dipole feed is that the beam departs, from the ideal and there is considerable spill-over, giving rise to ground target returns from virtually below the aircraft, the so-called height ring.
Flap Plate Type
The antenna comprises a flat steerable plate with a large number of radiating slots, each equivalent to a half-wave dipole fed in phase. The antenna is mounted on the forward pressure bulkhead behind the radome; this is a streamlined piece of structure constructed of materials that have low attenuation of the radar signals. The mechanical condition of the radome is very important to the effectiveness of the weather radar system, e.g. de-lamination will affect signal attenuation.
In both types of antenna, scanning is achieved ,by rotating the complete antenna assembly. The antenna automatically traverses from left to right on a repetitive basis to be able to scan the weather patterns ahead of the aircraft.
A weather radar may scan up to 300 nautical miles ahead of the aircraft .within azimuth scan angles of typically± 90°. Unless the beam is controlled to move only in or above the horizontal plane part or all of the weather picture may be masked by ground returns.
The reference position is to scan the antenna so as to provide images across the horizon; inputs from the aircraft’s attitude reference system are used to provide the stabilisation. In fact stabilization holds the beam not in the horizontal plane but at a constant elevation with respect to the horizontal. This constant elevation is determined by the tilt control as set by the pilot.
Motors are used as part of a drive mechanism to traverse the antenna in azimuth and to tilt the antenna in pitch. Synchro transmitters are used to relay the various positions of the antenna back to the transceiver.
WEATHER RADAR ANTENNA
Energy pulses are carried between the antenna and transceiver via a waveguide. This is because losses in a coaxial cable would be high at frequencies above 3 GHz, and prohibitive at frequencies above 10 GHz. Coaxial cables are also limited in terms of the peak power handling capability.
Waveguides have the following disadvantages
they are bulky
require more maintenance
The transceiver is a combined transmitter and receiver. Modern transceivers are solid-state devices, incorporating video processing for the display and stabilisation signals for the antenna. Since the energy received from a given size of water droplet varies with range, the energy returns from closer ranges will be higher than those received from droplets further away.
The transceiver will automatically compensate for returns from targets that are near or far from the aircraft. This is achieved by altering the gain as a function of time from when the energy pulse is transmitted. Pulses of radar energy are transmitted on a repetitive basis; the interval between pulses depends on the range selected by the crew. Time has to be allowed for the energy pulse to be reflected from water droplets at the limit of the selected range before the next pulse is transmitted.
WEATHER RADAR TRANSCEIVER
The control panel can different in different types of aircraft however the basic functions remain same. A typical weather radar control panel is shown.
WEATHER RADAR CONTROL PANEL
Used to select displayed range. Will also change the range mark spacing. Selection may be by pushbutton or rotary switch, the latter possibly incorporating 'OFF', 'STANDBY' and 'TEST' positions.
Pushbutton or incorporated in the range switch. With standby selected there will be no transmissiOn while indicator extra high tension (e.h.t.) may or may not be on.
There are different types of modes of operation of weather radar
Used to set gain of receiver manually. A continuously rotatable or click-stop control is normal. The control may incorporate contour on-off; by rotating the knob past the maximum gain position contour plus preset gain will be selected. In this latter case a separate spring return push button may be used to turn contour off momentarily. In other systems the gain control may simply incorporate a preset gain on-off switch at its maximum position.
A special pattern specified by the manufactures replace weather (or mapping) picture when test is selected.
Scanner Stab Switch
Adjustment of scanner elevation angle typical + 15 deg
Freeze or Hold Switch
Data update of display stopped, last updated picture displayed. Transmission and scanner rotation continues. Warning lamp may be provided.
The basic display used for primary radar systems is the plan-position-indicator (PPI). As the beam sweeps from side to side, a radial image on the display (synchronised with each sweep) moves across the display. The image on the display depends on the amount of energy returned from the target. Original weather radar systems had dedicated monochrome displays based on a cathode ray tube (CRT); these have evolved over the years into full colour displays, often integrated with other electronic flight instruments. The full benefits of a weather radar system can be appreciated when the system is used on an aircraft with an electronic flight instrument system (EFIS) display, Figure 20.7. A symbol generator is used to provide specific weather radar images as determined by the transceiver. An electronic display control panel allows each pilot to select the range of weather radar in increments of 10, 20, 40, 80, 160 and 320 miles.
The electronic display is overlaid onto the map mode allowing the pilot to relate the aircraft’s heading with the weather images. These images are colour coded to allow the pilot to assess the severity of weather conditions. Colours (ranging from black, green, yellow, red and magenta) are used to indicate rainfall rates that can be interpreted as a level of turbulence.