Introduction
1. A weather radar is airborne pulse radar designed to locate turbulent clouds ahead of the aircraft so that, in the interests of
safety and comfort, they may be circumnavigated either laterally or vertically, or penetrated where the turbulence is likely to be least. The radar beam is conical, and typically scans in azimuth 75° to 90° either side of the aircraft’s heading (Fig 16-1).
Some systems can scan vertically (typically ± 25°) to give a profile display. Cloud returns are displayed as bright areas on a sector PPI display equipped with either fixed or electronically generated range and bearing markers as shown in Fig 16-2.
The scanner has limited stabilization in pitch and roll so that the scan remains horizontal and has a steady tilt angle relative to the horizon during aircraft manoeuvre.
Fig 16-1: Conical Beam Scanning in Azimuth
Fig 16-2: Cloud Formation on a PPI Display
2. Most weather radars have a secondary ground mapping application and in this mode the radar transmission is often converted to a cosecant2 beam. This type of beam is described later in the section on Ground Mapping radars of this chapter.
Principle of Operation
3. Cumuliform clouds are associated with rising and descending currents of air leading to turbulence, which can be severe in the case of cumulo-nimbus clouds. The turbulence tends to retain the water droplets within the cloud which increase in size until they fall as heavy precipitation. It is this precipitation, and in particular the large water drops, which reflect the radar energy and from which
turbulence can be inferred. A frequency of approximately 10 GHz, giving a wavelength of about 3 cm, is used in order to provide adequate reflections from these large raindrops. Hailstones are normally covered with a film of water and tend to produce the strongest echoes. Gentle rain, snow, and dry ice produce the weakest echoes. Non-turbulent, principally stratiform clouds are not usually detected by the radar as the water droplet size is too small, neither can the radar detect clear air turbulence.
Normally the radar energy will penetrate the precipitation of one cloud so as to be able to display echoes from clouds beyond. However, extremely heavy precipitation may attenuate the radar to an extent that this penetration is not achieved. Since the system is designed as a warning device, accurate range and angular discrimination is not needed. It is therefore possible to transmit long pulses, which provide high peak signal strength for low average power consumption. The antenna need not be large, so a small parabolic aerial which can fit in the nose of even a light aircraft can provide a satisfactory beam width of about 3°.
Iso-Echo Contour Display
4. Normal cathode-ray tubes provide a single colour (or 'black and white') display of the returns.
The pilot is not normally interested in weather elsewhere than along his intended flight path, so traditionally the screen covers an angle of about 75° either side of the aircraft's heading, although several extend further. White returns indicate the reflecting raindrops or hailstones. Modern black and white displays can be reversed to show the returns as black on white, which in certain lighting conditions makes the shape of the return easier to identify.
5. The strength of the returned radar signals varies according to the precipitation rate and by inference reflects the degree of turbulence. However, a normal monochrome CRT display is unable to discriminate between these different signal amplitudes and clouds with significantly different degrees of turbulence would appear the same on the display.
6. In order to overcome this shortcoming a system known as Iso-echo Contour has been developed.
In this system an amplitude threshold level is established, and all signals which exceed this level are switched to earth (Fig 16-3).
The effect is to create a ‘black hole’
on the display corresponding to those parts of the cloud return with
the greatest precipitation rate. The outer and inner edges of the surrounding return correspond to two contours of precipitation rate, and the width of the ‘painted’ return reflects the precipitation gradient in
Fig 16-3: Generation of Iso-Echo Display
the area. The narrower the paint, the steeper the gradient, and therefore the more severe the turbulence.
Multi-threshold Displays
7. An extension of the Iso-echo Contour system is to have a number of threshold levels in order to generate a series of precipitation rate bands. It is necessary to have a colour CRT to display these gradations, with a different colour used for each precipitation rate band. Conventionally, the colours range from black, indicating no or very light precipitation, through green and yellow to red, which corresponds approximately to the traditional Iso-echo Contour threshold. Increasingly, new systems add another colour, magenta, to indicate areas of most intense precipitation. One such display is shown in Fig 16-4. This sequence can be considered as being similar to that of traffic lights viz. red for 'danger', amber for 'warning', and green for 'no hazard at present but possibly changing'.
Fig 16-4: Weather Radar Colour Display
Sensitivity Time Control
8. As well as the nature of the cloud target, the strength of the returning radar signal is dependent on the range of the cloud. In order to eliminate this variable, sensitivity time control (STC), or swept gain techniques are used in which the receiver gain is lowered at the instant each pulse is fired, and then progressively increased according to a predetermined law. This ensures that echoes from distant ranges are amplified more than those from close range.
9. At long ranges a cloud is likely to fill the radar beamwidth only partially and the echo signal will vary inversely as the fourth power of range whereas at lesser ranges, where the beamwidth is completely filled, the reflected signal varies inversely as the square of the range. Therefore, there is no universal law for all ranges to which STC can be made to conform and any installation will have a display that is compensated only over a limited, fairly short, range (e.g. 25 nm).
Display Interpretation
10. Radio waves are only reflected by cloud if there are water droplets above a certain size, or hail, but rapidly building storms will typically contain ice in their upper levels which reflects very little radar energy. When cruising at high altitude it is therefore important to use the tilt control to scan downwards or to use the ‘profile’ capability of some radars to intercept the lower portion of the storm containing the water droplets.
11. Having detected a cloud which is likely to be turbulent a course of action must be determined. The best option would be to avoid the cloud altogether, however this may not be possible in practice and consideration must be given as to the best part of the cloud to penetrate. Fig 16-5 shows a typical Iso-echo display cloud return diagrammatically. There are two areas, marked W, where the amplitude of the received signal has exceeded the threshold level, and these therefore,
show as ‘black holes'. Although these areas can be assumed to be areas of high precipitation and therefore of turbulence, greater consideration should probably be given to areas where the precipitation gradient is highest. This is indicated by the width of the ‘paint’. The upper part of Fig 16-5 illustrates the returning signal strength and it will be seen that the gradient is higher to the left than to the right. On the display this variation is shown by the narrower band at A than at B. By implication, the particularly narrow band at Y can be considered to be the area of greatest turbulence.
Fig 16-5: Diagram of Typical Cloud Return Indicating Zones of Differing Turbulence
12. Area Y should, therefore, be the first priority for avoidance. The two areas labelled W represent returns above the Iso-echo threshold level and are therefore areas of high precipitation and turbulence, and although area X between these ‘holes’ is of a lower level, the degree to which the amplitude has dipped below that of W is not apparent from the display; it may easily be very nearly as turbulent. The best area for penetration is likely to be B where the paint is wide (wider than A), and the amplitude continues to fall to below the video threshold level on the right.
Determining Cloud Vertical Extent
13. The vertical extent of cloud is most simply determined on screen if the equipment has a profile scanning capability. On azimuth only systems it is possible to make an approximate estimation of the vertical extent of a cloud by tilting the aerial both above and below the horizontal until the echo
just disappears as shown in Fig 16-6. The solution to the trigonometrical equation involving the tilt angles and range is normally solved using a table or a graph, such as that shown in Fig 16-7. One in sixty rule may also be used for an approximate result. For e.g. If the beam just misses the precipitation when it is angled upwards at 1°
at a range of 60 nm, one could calculate that the top of the precipitation is 1 mile (6080 ft) above the aircraft. However, if the beam has a vertical width of 3°, the bottom of the beam is 1.5° below the centre. In this example, the top of the precipitation is actually 0.5°, and therefore 0.5 miles or 3000 ft approx below the aircraft.
Fig 16-6: Cloud Height Measurement
Cockpit Controls
14. A control panel of a COO is depicted in Fig 16-8. The brightness (BRT) knob may be replaced by a 'colour intensity' control. There is a rotary control to select the maximum range of the display, and the range rings will appear on the display at proportions of that maximum range.
The gain control is usually left at 'automatic' in the weather mode, but when using the radar for mapping the operator should adjust the gain manually by use of the knob to obtain the best possible picture. The function
control on this unit has positions to display weather (WX), ground returns (MAP) and a combination of weather and TCAS information (WX / T), as well as a test facility. Additional functions may include the display of windshear warnings, as described in paragraph 17.
Fig 16-8: AWR Control Unit
Fig 16-7: Cloud Height Measurement Graph
15. The tilt control should be used to search for the most likely and most severe weather returns.
These would usually appear in the middle of a cumulonimbus cloud, where the updraughts are strongest, so if the aircraft is at high altitude, the most intense returns would appear below the aircraft, and above it if flying at low altitude. The crew can tilt the antenna accordingly. Another reason for
adjusting the tilt would be to remove unwanted ground returns from the weather display.
Requirements
16. As per ICAO, certain aircraft must carry airborne weather radar equipment when operating at night or in instrument meteorological conditions in areas where thunderstorms or other potentially hazardous weather conditions, regarded as detectable with airborne weather radar, may be expected to exist along the route. These aircraft are:
(a) All pressurised aeroplanes.
(b) Unpressurised aeroplanes with a maximum certificated take-off mass of > 5700 kg.
(c) Unpressurised aeroplanes with a maximum approved passenger seating configuration of more than 9 seats.
Unpressurised propeller-driven aeroplanes lighter than the above and with fewer seats may be operated with other equipment capable of detecting thunderstorms, for example the lightning detector, if the equipment is approved by the relevant authority.
Windshear Detection
17. Turbulence associated with windshear is a major safety problem during the approach and departure phases of flight. Even at high altitudes, windshear in frontal zones can have a considerable affect on passenger safety. Systems have been developed which compare airspeed information from the Air Data Computer (ADC) with groundspeed information from the navigation computer. This comparison can detect changes in wind affecting the aircraft. Pressure altitude information from the ADC can provide information about rates of climb or descent, and when coupled to airspeed (and air temperature) changes can give an indication of the effect that a gust is having on the aircraft.
18. TAWS (Terrain awareness and avoidance system) computers will alert the crew if an increasing headwind (or decreasing tailwind) and / or a severe updraught exceed a defined threshold.
These are characteristics of conditions which might be expected just before an encounter with a microburst. A decreasing headwind (or increasing tailwind) and / or a severe downdraught are characteristic of conditions that might be experienced within a micro burst itself, or just afterwards, and result in a more urgent warning.
19. The alert and warning thresholds depend on available climb performance, flight path angle, airspeed changes, and fluctuations in static air temperature. A windshear alert on a weather radar display will often take the form of bands of colour across the screen in the aircraft's direction of flight.
Lightning Detectors
20. For light aeroplanes, alternative systems have been developed, some of which have been incorporated in systems for larger aircraft also. One commercially available system uses a small external aerial incorporating directional antennas (on a similar principle to ADF) to detect electrical discharges (lightning) from thunderstorms in line-of-sight range. The system computer uses the signal strength of this discharge to determine not only the direction from which it has come, but also its range. The equipment can then display each discharge on a screen.
21. By keeping each discharge in the computer's memory, a map can be built up of the discharges over a period of time. This shows the areas of most frequent discharge, which can be assumed to be the most intense thunderstorms. The thunderstorm map may be shown on an independent screen or as part of any other horizontal situation display (Fig 16-9). In some installations, the pilot may have the choice of displaying either the normal map of historical information, or a screen which shows only the discharges which have occurred from the moment
he started a 'stopwatch' on the equipment. This facility is useful if the pilot is committed to flight through a stormy air mass and wishes to know where the storm activity is reducing. An area of reducing activity is likely to be less dangerous than one where the activity is increasing. Fig 16-9 shows such a display of 'strikes' on a simple EHSI MAP display fed from a navigation computer.
Fig 16-9: Discharges on Nav Display