AirborneWeather Radar 13

Weather Operation

AvoidingThunderstorms

Select maximum range to detect weather formations in good time and adjust the TILT to remove ground returns. If the storm system is extensive make an early track adjustment, in consultation with ATC, to avoid it. If this is not possible, as the clouds get nearer select the lower ranges and CONT and determine the best track to avoid potential turbulence. Ensure that short term alterations of heading steer the aircraft away from the worst areas and not deeper into them. To achieve this, constant switching between short, medium and longer ranges is necessary in order to maintain a complete picture of the storm system.

Shadow Area

There is also the danger of not being able to map the area behind heavy rain where no radar waves will penetrate; this will leave a shadow area which may contain severe weather.

Height of Storm Cloud

The height of a storm cloud can be ascertained by adjusting the tilt until the radar returns from it just disappear. The height of the top of the cloud can be calculated by using the 1 in 60 rule.

It is also worth noting that a thunderstorm may not be detected if the tilt setting is set at too high an angle.

Height Ring

With the older AWR systems where the conical beam is produced by a dish antenna there is always some vertical overspill of energy which is reflected back to the aircraft and appears as a “height ring”, which roughly indicates the aircraft’s height. It also indicates that the equipment is serviceable when there is no weather ahead.

Colour AWR Controls

The controls for a colour AWR would be similar to that for the monochrome unit in terms of range, tilt and gain but may have the following additional features such as Wx, Wx + T, Wx(var). These are for Weather, Weather plus Turbulence and Weather with variable gain control. Other sets may have WxA control which stands for Weather Alert and would give a flashing display of the areas associated with turbulence. There may also be available a Contour Intensity control to enable adjustment of the display for optimum presentation. The latest AWR controls are activated by push ON/OFF switches located around the colour screen and include:

Test

This displays the colour pattern for pre-flight serviceability check.

Airborne Weather Radar 13

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13 AirborneWeather Radar

Radar Weather Airborne 13

Figure 13.17 Typical Radar System Test Pattern for PPI displays

Hold

This allows the display to be frozen so that storm movements can be assessed. When a storm is located, at say 100 NM, HOLD is selected and a constant heading maintained. HOLD and WX then appear alternately on the screen. After two or three minutes deselect the HOLD facility; this brings back the current display and the storm position is seen to move from its held position to its actual position, thereby indicating its movement relative to the aircraft.

TGT Alert

This operates in conjunction with the WEA facility and alerts a pilot of a storm return of contour strength. When TGT ALERT is selected and no contouring clouds are present the screen shows a yellow T in a red square, (screen top right). If a contouring cloud is detected within 60 to 160 NM and +/- 15° of heading, the yellow symbol TGT, in a red square, flashes on and off once a second instead of the T.

Fault

This is controlled by a fault monitoring circuit and FAULT flashes on the screen if there is a power or transmitter failure.

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AirborneWeather Radar

AWR Summary

Components	Tx / Rx, antenna, indicator, control unit.
Functions	Turbulence, navigation.
Principle of Operation	Echo for range, sweep for relative bearing.
	Pencil beam for weather and long range (> 60 NM) mapping.
	Cosecant² beam for short range. Antenna attitude stabilized.
	Beam width dependent on antenna size.
	Effect of beamwidth on resolution.
	Frequency of 9375 MHz best for large water droplets/hail.
Weather	Turbulence where rainfall gradient is steepest.
	Few returns from wavelength of >10 cm.
	Colours in order: black, green, yellow, red, magenta.
	Beware U’s, fingers, scallops and hooks.
Mono Control Unit	Power/Stab On - antenna attitude stabilised in pitch and roll.
	Stab Off	- antenna locked to aircraft axes.
	Range	- Standby, selections up to about 150 NM.
	Tilt	- ± 15°. Tilt up for increased range or lower altitude.
	MAP	- fan-shaped beam. Use up to 60 NM.
	MAN	- Manual gain with pencil beam to map > 60 NM.
	WEA	- Pencil beam with AGC.
	CONT	- Black holes indicate turbulence.
Mapping	Tilt down for best target presentation. Beware hill shadows.
Weather Operation	Adjust tilt for best weather picture.
	Too high tilt will miss TS.
	Beware shadow area.
Colour AWR	Use of controls - Wx, Wx+ T, Wx(Var), WxA, Hold, Tgt Alert.

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Airborne Weather Radar 13

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13 Questions

Questions

	1.	A frequency used by airborne weather radar is:
		a.	8800 MHz
		b.	9.375 GHz
		c.	93.75 GHz
		d.	1213 MHz
	2.	An airborne weather radar is required to detect targets up to a maximum range of
		200 NM. Ignoring pulse length and flyback in the CRT calculate the maximum PRR.
		a.	405 pps
		b.	810 pps
		c.	1500 pps
		d.	750 pps
	3.	Using airborne weather radar the weather beam should be used in preference to the
		fan-shaped beam for mapping in excess of ................ NM:
		a.	20 to 25
		b.	60 to 70
13		c.	100 to 150
		d.	150 to 200
Questions	4.	Airborne Weather Radar is an example ................ of radar operating on a frequency
		of .............. in the ............... band.
		a.	primary	8800 MHz	SHF
		b.	secondary	9.375 MHz	UHF
		c.	secondary	9375 MHz	SHF
		d.	primary	9375 MHz	SHF

5.The correct sequence of colours of a colour Airborne Weather Radar as returns get stronger is:

a.	red	yellow	green
b.	yellow	green	red
c.	green	yellow	red
d.	red	green	yellow

6.A false indication of water may be given by the AWR display when:

a.flying over land with the Land/Sea switch in the Sea position

b.flying over mountainous terrain

c.there is cloud and precipitation between the aircraft and a cloud target

d.attempting to use the mapping beam for mapping in excess of 50 NM

7.Airborne weather radar operates on a frequency of:

a.8 800 MHz because gives the best returns from all types of precipitation

b.13 300 MHz

c.9 375 MHz because it gives the best returns from rainfall associated with Cb

d.9.375 GHz because this frequency is best for detecting aircraft in flight

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					Questions		13
	8.	The mapping mode of Airborne Weather Radar utilizes:
		a.	a pencil/weather beam from 70 NM to 150 NM
		b.	a cosecant²/fan-shaped beam which is effective to 150 NM
		c.	a pencil/weather beam with a maximum range of 70 NM
		d.	a cosecant²/fan-shaped beam effective 50 NM to 70 NM
	9.	An Airborne Weather Radar system uses a frequency of 9 GHz because:
		a.	it has a short wavelength so producing higher frequency returns
		b.	the short wavelength allows signals to be reflected from cloud water droplets
			of all sizes
		c.	the wavelength is such that reflections are obtained only from the larger water
			droplets
		d.	the frequency penetrates clouds quite easily enabling good mapping of ground
			features in the mapping mode
	10.	The antenna of an Airborne Weather Radar is stabilized:
		a.	in pitch, roll and yaw
		b.	in pitch and roll
		c.	in pitch and roll whether the stabilization is on or off
		d.	in pitch and roll but only when 0° tilt has been selected
	11.	The colours used to denote variations in rainfall rate on an Airborne Weather Radar				13
							Questions
		screen are ........... for very light or no returns, ............... for light returns, ........... for
		medium returns and ............ for strong returns.
		a.	black	yellow green magenta
		b.	black	green	yellow magenta
		c.	grey	green	yellow red
		d.	black	green	yellow red

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