民航气象雷达 344058_4V792_CMM_029

TASK34-40-58-870-808-A01
9.WRT-2100Simplified Functional Theory
A.General
(1)Simplified functional theory is broken down into discussions of the transmitter function,
receiver function,DSP,turbulence detection,windshear detection,windshear states,
and MultiScan operation.The transmitter,receiver,and DSP functions each have
integrated self-monitoring functions.The self-monitoring functions check signal validity,
circuit integrity,and performance parameters.
B.Transmitter Function
(1)The transmitter function produces a multiple pulse,RF signal at X-band frequencies
9327.06to9338.88MHz for WRT-2100units,CPN822-1710-001,-201,-202(or
9327.42-to9348.57-MHz for WRT-2100unit,CPN822-1710-002).The transmitter
function produces RF signals at six frequencies over this rangefor WRT-2100units,
CPN822-1710-001,-201,-202.Refer to Table10/Table34-40-58-99A-117-A01.For
WRT-2100,CPN822-1710-002,the transmitter function produces RF signals at nine
frequencies over the frequency range9327.42-to9348.57-MHz.Table11/Table
34-40-58-99A-118-A01shows the transmit frequencies used by the WRT-2100,CPN
822-1710-002.Multiple frequencies are used in detecting weather to provide improved
correlation of return data,resolve range,velocity,and amplitude ambiguities of the
returned signal,and improve ground clutter rejection.
TRANSMIT CHANNEL FREQUENCY
Weather19.3295GHz
Weather29.3301GHz
Weather39.3306GHz
Weather49.3312GHz
Windshear19.3319GHz
Test9.3389GHz
Transmit Frequencies(For WRT-2100Units,CPN822-1710-001,-201,-202)
Table10/Table34-40-58-99A-117-A01
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TRANSMIT CHANNEL FREQUENCY
Weather19.3282GHz
Weather29.3287GHz
Weather39.3293GHz
Weather49.3299GHz
Weather59.3304GHz
Weather69.3310GHz
Weather79.3315GHz
Windshear19.3323GHz
Test19.3486GHz
Transmit Frequencies(For WRT-2100,CPN822-1710-002)
Table11/Table34-40-58-99A-118-A01
(2)For WRT-2100units,CPN822-1710-001,-201,and-202,the pattern of transmitted
pulses(for non-windshear detection modes)is shown in Figure15/GRAPHIC
34-40-58-99B-436-A01.For any selected range one20microsecond pulse(25
microseconds for SB2or later)and four6microsecond pulses will be transmitted during each epoch.After transmitting the20microsecond pulse there is a4.9millisecond (leading edge of transmit pulses)space.This allows returns from targets as far as331 NM.An epoch is the time internalinterval in which a radial of radar data is processed.
The time is equal to the size of the radar processing element in degrees divided by the antenna scan rate.This pulse pattern would be transmitted at each radial of the antenna scan.The transmitted frequency would alternate between the four weather transmit channel frequencies.For example:the weather radar system is operating in a weather detection mode and the antenna is scanning clockwise.At the0°radial,the transmitter will output the pulse pattern of Figure15/GRAPHIC34-40-58-99B-436-A01 at the weather(WX)1transmit channel frequency of9.3295GHz.At the completion of that epoch the antenna moves a quarter degree clockwise and the pulse pattern transmission is repeated at one of the other three weather channel frequencies.The selection of the frequency is random.At the next quarter degree radial of the antenna, the transmit pulse pattern is repeated at one of the remaining two transmit channel frequencies.The pulse pattern and four frequencies are used for all non-windshear modes of operation such as Weather(WX),Turbulence(TURB),Weather Plus
Turbulence(WX+T),and ground Mapping(MAP)detection.
(3)For WRT-2100,CPN822-1710-002,the pattern of transmitted pulses(for non-windshear
detection modes)is shown in Figure16/GRAPHIC34-40-58-99B-437-A01.A fixed pulse pattern is used for all weather,turbulence,and ground map modes of operation.The pulse pattern is used in manual operation or automatic MultiScan operation.The pulse pattern consists of a single25microsecond pulse(for long-range weather detection)and
a series of four six-microsecond pulses.The transmitted frequency is selected randomly
from the seven weather frequencies listed in Table11/Table34-40-58-99A-118-A01.A
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small variable delay occurs between transmit pulse patterns.This variable(and random)
delay varies(or dithers)the pulse timing.Dithering of the pulse timing reduces the
interference from other radar systems operating within the region.
(4)For FLW detection the transmit pulse pattern will consist of2microsecond wide pulses
transmitted at a PRF of3000Hz.A total of64pulses will be transmitted per each
windshear epoch.Table12/Table34-40-58-99A-119-A01provides a summary of epoch
characteristics.Figure17/GRAPHIC34-40-58-99B-438-A01shows a windshear pulse
pattern.When windshear mode is activated,windshear pulse patterns are transmitted
during the counter-clockwise scan of the antenna.
(5)The transmitter function of the WRT-2100must be able to produce multiple pulse
widths,at variable PRF rates,and at several different frequencies.To accomplish this,
DDS of RF signals is used.
CHARACTERISTIC NON-WINDSHEAR WINDSHEAR
RANGE ALL RANGES ALL RANGES
ELEMENT SIZE0.375degree 1.0degree
SCAN RATE45degree/second45degree/second
EPOCH PERIOD8.333ms±5μsec22.975ms±5μsec
Epoch Timing
Table12/Table34-40-58-99A-119-A01
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Non-Windshear Transmit Pulse Pattern(For WRT-2100,CPN822-1710-001,-201,and-202)
Figure15/GRAPHIC34-40-58-99B-436-A01
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Non-Windshear Transmit Pulse Pattern(For WRT-2100,CPN822-1710-002)
Figure16/GRAPHIC34-40-58-99B-437-A01
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Windshear Transmit Pulse Pattern
Figure17/GRAPHIC34-40-58-99B-438-A01
C.Receiver Function
(1)After the transmission of a set of RF pulses,the RT is switched to the receive function.
The receiver function operates between transmit pulse patterns.The receiver"listens"
during these periods for reflected pulses to return after hitting a precipitation(or ground) target.The strength of the return signal is related to the reflective properties of the
target and the distance the pulse has to travel.Radar systems compensate for the
attenuation of the signal due to the distance traveled,by a circuit called Sensitivity
Time Control(STC).The STC circuit controls the receiver sensitivity with respect
to time and thus range.
(2)The receiver function produces digital I and Q return signals that will be used by the
processing circuits.The receiver function amplifies,filters,and down converts the
received signal to an IF signal.The second IF signal is mixed with in-phase signal and
a quadrature-phase(90degrees out-of-phase)signal to produce the I return and Q
return signals.
D.DSP Function
(1)The I and Q return signals are then applied to the DSP function.The received signal
provides strength of signal information and the range to target.The range of the target
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is determined by the amount of elapsed time that occurs between the transmission of
a pulse and the reception of the reflected or return signal.The direction or azimuth
bearing of the target is determined by noting the azimuth pointing position of the
antenna.Bearing and range information is then coupled with the reflectivity information.
Refer to Figure18/GRAPHIC34-40-58-99B-439-A01for a representation of the return
signal from one radial(or azimuth position).
(2)The reflectivity information is placed into memory and the reflectivity data is correlated
with data from adjacent azimuth positions to produce display data that will be sent to an indicator.The DSP function also processes other reflectivity data information besides
strength of signal and range to target.The following paragraphs discuss the processing of reflectivity data to detect turbulence,windshear,and provide MultiScan operation.
Return Signal For One Azimuth Position
Figure18/GRAPHIC34-40-58-99B-439-A01
E.Turbulence Detection
(1)The WRT-2100uses a greater number of pulses(increased PRF)to produce
information about target turbulence and windshear events.Refer to Figure19/GRAPHIC 34-40-58-99B-014-A01.The frequency of the return signal will be offset from the
transmitted frequency because of the Doppler shift caused by the velocity of the aircraft with respect to the target.In addition to the frequency shift caused by the aircraft velocity,
a frequency shift caused by the movement of the precipitation.To measure the spectrum
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width of the frequency shifts caused by precipitation movement,the Doppler shift due to the aircraft must be filtered out.Refer to Figure20/GRAPHIC34-40-58-99B-015-A01.
(2)To provide an accurate spectrum of return signal frequencies,a large number of
samples(returns)must be used to produce accurate and reliable results.The threshold of turbulent targets is precipitation velocities of5meters/second.This threshold
translates into a Doppler frequency shift of312.5Hz.For this reason,the PRF for the four pulse set of pulses,is increased to1838pulses/second.With this large number of transmit pulses,the WRT-2100will process every pulse for precipitation information and provide turbulence data from the spectrum of Doppler shifts caused by precipitation movement.Because of the high PRF rate,the maximum range for turbulence detection is approximately44miles(16.09kilometers).This range limit is established to prevent range ambiguity and so subsequent pulsed transmissions do not saturate the receiver.
(3)Once an accurate spectrum of return signals is obtained,the turbulence processing
circuits must determine if the spectrum represents the spectrum of a turbulent target.A turbulent target is one that exhibits a wide variance in particle(raindrop)velocity.The velocity variance can be thought of as the range or spectrum of velocities.The broader the spectrum,the greater the turbulence.The alert threshold for passenger carrying air transport aircraft is approximately five to six meters per second(11.2to13.4mph).The five meters/second threshold corresponds to the threshold between light and moderate turbulence that could cause food and beverage spillage or possibly minor injury.
(4)Refer to Figure21/GRAPHIC34-40-58-99B-016-A01for a diagram showing the
spectrums of various targets.Target number1shows the spectrum of a typical
non-turbulent target.That is,the frequency shift due to precipitation movement is
minimal and the majority of returns are at the mean frequency.If the width of the
spectrum is much narrower than target number1then the target shows so little variance in velocities that it may be assumed to be a stationary target or ground target return.
Target number2shows the spectrum of the threshold turbulence target.This target shows68%of the return signal frequencies are present within the spectrum width.
Target number3shows a turbulent target with the majority of returns occurring beyond the spectrum width.Target number3would be identified as being turbulent and the TURB ALERT signal would identify the return as a turbulence target.
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Frequency Spectrum Shift
Figure19/GRAPHIC34-40-58-99B-014-A01
Return Signal with Target Spectrum for One Azimuth Position
Figure20/GRAPHIC34-40-58-99B-015-A01
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Target Spectrums
Figure21/GRAPHIC34-40-58-99B-016-A01
F.Windshear Detection
(1)Another hazardous weather-related target is a windshear event,such as a microburst.
Windshear is a condition in which the wind abruptly changes its speed or direction(or
both)over a small distance.It can be associated with frontal systems,occurring over a
large area,or with thunderstorms,occurring over a small area.Windshear can occur at an altitude without the presence of clouds,where it is referred to as clear air turbulence, or near ground level where it has an impact on the takeoffs and landings of aircraft.It is the windshear occurring near the ground level,referred to as a downburst,microburst,
or macroburst,that presents the greatest danger to aircraft.
(2)A downburst refers to one of two types of windshear:macroburst and microburst.A
macroburst is a large downburst with the horizontal component of the outburst winds
extending in excess of2.5miles(4km).Damaging winds,lasting5to30minutes,could have velocities up to134mph.A microburst is a small downburst with the horizontal
component of the outburst winds extending only2.5miles or less.An intense microburst could induce winds as high as168mph,normally lasting less then ten minutes.
(3)Both macrobursts and microbursts can cause extensive damage at ground level.
The macroburst is characterized by a succession of downdrafts beneath the parent
raincloud.Since the cold air dome is heavier than the warm air surrounding it,the
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atmospheric pressure inside the dome is higher than its environment.This causes the cold air to push outward,inducing gusty winds behind the leading edge of the cold air outflow.A downburst can be detected on a Doppler weather radar because of its characteristic velocity signature where areas of positive and negative velocity over a small distance would indicate the impact signature of the downburst.Refer to Figure 22/GRAPHIC34-40-58-99B-017-A01.This figure shows what happens to an aircraft landing with a microburst present.
(4)As the aircraft approaches the microburst it encounters the strong headwinds from the
outflow.The headwind increases the aircraft's lift and airspeed.The increased airspeed from the headwinds,would normally cause the pilot to decrease throttle in order to maintain the descent.After the strong headwinds the aircraft enters,momentarily,
the center of the microburst with a strong downflow.The aircraft then encounters the outflow again,but this time a strong tailwind decreases the airspeed and the lift of the aircraft.The aircraft has already reduced speed to maintain its flight along the glidepath against the headwind.Now with the tailwind,the aircraft loses much of its remaining lift.The pilot must respond with full throttle and increase the angle of attack to recover the lost airspeed.The danger of the microbursts is the quick transition of airspeed.If the outflow of the microburst induces winds that are40mph,the aircraft must handle a sudden80mph change in airspeed(from a40mph headwind to a40mph tailwind).
For this reason,the pilot needs as much warning as possible that a windshear event exists in the flight path.
(5)Refer to Figure23/GRAPHIC34-40-58-99B-018-A01.In previous paragraphs it was
shown how Doppler processing of returns would indicate the distribution of velocities within a target.This same technique is used now to detect the windshear event by using the characteristic velocity signature where areas of positive and negative velocity over
a small distance would indicate the impact signature of the downburst.The strong
headwinds of the target will produce a positive Doppler frequency shift,while the strong tailwinds will produce a negative Doppler frequency shift.The distance between these two events is determined by the amount of elapsed time between the return pulses. (6)For windshear detection,a higher number of transmit pulses are required to gather the
windshear data.In the windshear mode of operation,the pulse pattern of the RT is a pulse set of64pulses transmitted once per epoch.The PRF rate for the64pulses is3000Hz.The PRF rate is dithered between pulse sets to decorrelate alien radar returns.The PRF dither will vary the dither period between pulse sets from0-to-384 microseconds,in10-microsecond increments.
(7)For windshear operation the epoch is changed.The epoch is based upon the scan rate
and the element size.The element size is the area of the radar scan being processed.
For windshear operation the element size is one-degree.The scan rate is also changed because during windshear operation the antenna scans only a120-degree area(±60°) instead of the normal weather mode area of180-degrees(±90°).During windshear operation,the weather processing is operating during left-to-right scans and windshear processing is operating during right-to-left scans.Additionally,the targets detected as
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being windshear targets are displayed only in the area±30°from the aircraft center line.
The result of these changes is the scan,produce an epoch period of approximately23 milliseconds.
(8)The return pulse information is then used to produce a distribution of the positive and
negative velocity targets that are detected.The spread of target velocities will indicate if
a downburst is present.
(9)Figure24/GRAPHIC34-40-58-99B-019-A01shows a graph of the headwinds and
tailwinds encountered from a windshear event.The weather radar looks for a
specific signature of outflow velocities.It looks for outflow nearest the ground,where outflow velocities would be highest.The grade of the slope of Figure22/GRAPHIC 34-40-58-99B-017-A01represents the hazard to the aircraft.A steep slope(fast
transition)indicates more hazard.The hazard may also be expressed as the hazard factor,F.A hazard factor to describe the severity of a windshear event was developed by NASA.The horizontal component of windshear and the vertical component of windshear are placed in a ratio to produce a hazard factor that would be analogous to a negative climb ratio.This hazard Factor(F)is used to define a threshold for windshear events that would be hazardous to an aircraft.This threshold is defined as F=0.13.
(10)The hazard factor,F,is expressed in the following equation:
F=(Wx/g)-(Wh/v)
(11)Where:
Wx=horizontal component relative to the aircraft horizontal flight path.
Wh=the vertical component of the wind velocity
g=gravity
v=aircraft true airspeed
(12)Because of the hazard a windshear event presents to an aircraft during takeoff and
landings,the detection of windshear events is automatically enabled when the aircraft is taking off or landing.The radar is automatically turned on when the radio altimeter reports an altitude is less than2300feet(701.04meter)and the aircraft is on the runway or in the air(and the aircraft is not in the maintenance hanger).Windshear operation will be active anytime the weather radar system is powered on,the system is operating in a weather detection mode,qualifier logic is valid,and WINDSHEAR FUNCTION ENABLE (P1B-1G)is grounded and AUTO TURN-ON INHIBIT(P1B-3C)is open.
(13)The windshear detection mode is automatically activated below an altitude of2300feet
(701.04meter)Above Ground Level(AGL),anytime the weather radar is"ON".The windshear detection mode is activated,if the radio altimeter reports an altitude less than 2300feet(701.04meter),and the aircraft is not in the maintenance hangar and is on the runway or in the air.It should be noted if the radar is"ON"and in the maintenance hangar,the RT will still transmit.
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(14)If the radar is already operating in a weather detection mode when a windshear
hazard event is detected,no pilot intervention will be required.If the radar is on,but in Standby(STBY),MAP,or TEST mode,when a windshear hazard event is detected, the radar operation will automatically change to the WX+T mode.The selected range does not change.
(15)The detected windshear events are categorized by the location of the event relative to
the longitudinal axis of the aircraft and distance from the aircraft.There are three levels of alerts for windshear conditions.Figure25/GRAPHIC34-40-58-99B-020-A01indicates the level of the issued alert should a windshear hazard be detected within an area5 miles(8.04kilometers)ahead of the aircraft but within±30°of the aircraft heading. (16)The least severe level is the windshear advisory alert(level1).The windshear advisory
alert is generated whenever a detected windshear event occurs within the scan area but outside the regions where a windshear hazard event would generate a windshear warning or windshear caution alert.The output generated for a windshear advisory alert is the display of the windshear icon on the display.
NOTE:For Boeing-certified systems,the advisory alert level is deactivated.
(17)The second level of alert is the windshear caution alert(level2).The windshear caution
alert is generated whenever a detected windshear event occurs outside the windshear warning alert region but within±30°of the aircraft heading and3NM from the aircraft.
During takeoff for Boeing-certified aircraft,the windshear caution alert is inhibited from the time the aircraft reaches80knots airspeed until the aircraft reaches400feet(121.92 meter)AGL.For Boeing-certified aircraft only,during landing the windshear caution alert is inhibited below400feet(121.92meter)AGL.For Airbus-certified aircraft,during
takeoff the windshear caution alert is inhibited from the time the aircraft reaches100 knots airspeed until the aircraft reaches50feet(15.24meter)AGL.For Airbus-certified aircraft during landing,the windshear caution alert is inhibited below50feet(15.24 meter)AGL.There are no windshear caution alerts generated above1200feet(365.76 meter)AGL.The output for a windshear caution alert consists of the windshear icon displayed and an aural alert output of a chime or the synthesized phrase"MONITOR RADAR DISPLAY".
(18)The third level(most severe)of alert is the windshear warning alert(level3).The
windshear warning alert is generated whenever a detected windshear event occurs within±0.25NM of the longitudinal axis of the aircraft and within±30°of the aircraft heading.When the aircraft is on the ground(takeoff roll),the windshear warning alert occurs for windshear events within3NM.When the aircraft is in the air,the windshear warning alert occurs for windshear events within1.5NM.During takeoff,windshear warning alerts are inhibited from the time the aircraft attains100knots airspeed until the aircraft reaches50feet(15.24meter)AGL.During approach,windshear warning alerts are inhibited below50feet(15.24meter)AGL.There are no windshear warning alerts above1200feet(365.76meter)AGL.
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(19)The output for a windshear warning alert consists of a windshear icon displayed on the
indicator and an aural alert output of a synthesized phrase "WINDSHEAR AHEAD,WINDSHEAR AHEAD"during takeoff,or "GO-AROUND.WINDSHEAR AHEAD"during approach.The aural warning is repeated when a different event causes a new level 3alert.TPF7759_01
WINDSHEAR ENTRY WINDSHEAR AVOIDANCE
RECOVER OR CRASH
GLIDESLOPE
Approaching a Microburst During Landing
Figure 22/GRAPHIC 34-40-58-99B-017-A01
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Detecting Microbursts with Doppler Shift
Figure23/GRAPHIC34-40-58-99B-018-A01
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Determining Windshear Hazard
Figure24/GRAPHIC34-40-58-99B-019-A01
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Windshear Alert Regions
Figure25/GRAPHIC34-40-58-99B-020-A01
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G.Windshear States
(1)Windshear operation can be divided into four states.Refer to Figure26/GRAPHIC
34-40-58-99B-021-A01.The first state(State0)is the inactive or off state.The second
state(State1)is the takeoff state.The third state(State2)is the flight state,and the
fourth state(State3)is the landing state.Windshear mode is entered automatically
when qualifiers are valid and the aircraft is in a landing or takeoff configuration.The
radar is automatically turned on when the radio altimeter reports an altitude less than2 300feet(701.04meter)and the aircraft is on the runway or in the air(with qualifiers
valid).The radar system will automatically exit windshear operation when the aircraft is at an altitude above2400feet(731.52meter)AGL.The mode transitions to or from
windshear occur after the altitude and qualifier conditions are met and at the next end of the antenna scan.The windshear mode may also be entered by a mode7command
from an ARINC429control bus.When commanded in this manner,the windshear will
be active regardless of altitude or qualifier conditions.
NOTE:The FAA requirement for automatic turn-on of windshear operation
is1200-feet(365.76meter)AGL.The2300-feet(701.04meter)
threshold used as the altitude to initiate power to the unit allows time
for system initialization and processor boot-up.
(2)State0or the inactive state is when the altitude and qualifier conditions are not valid.
The radar system may be turned on or off.If the radar is off,the remote radar-on
processor will initiate power to the unit and windshear mode selection.
(3)The remote radar-on processor is powered by the BITE KA alive power supply
separately from the other unit processors,and receivers radio altitude information from
two ARINC429low-speed digital radio altitude inputs.If the radar is on,the system
will transition from a weather or mapping mode to the windshear mode when valid
altitude and qualifier conditions are met.
(4)State1or the takeoff mode is entered when the qualifiers are valid,and the aircraft is
on the ground or reporting a radio altitude of50feet(15.24meter)or less.The takeoff
mode can also be entered if commanded by ARINC429windshear selection.During
takeoff mode,the maximum range for windshear warning alerts is3NM.The windshear warning will be inhibited from the time the aircraft attains100-knots airspeed until the
aircraft reaches50-feet(15.24meter)AGL.State2or the flight mode is entered when
the landing gear is up,the altitude and qualifier conditions are valid or if commanded by ARINC429windshear selection.During normal flight mode,the maximum range for
warning alerts is1.5NM.The windshear warning is inhibited from the time the aircraft
attains100-knots airspeed until the aircraft reaches50-feet(15.24meter)AGL.
(5)State3or the landing mode is entered when the landing gear is down,the altitude and
qualifier conditions are valid,or if commanded by ARINC429windshear selection.
During landing mode,the maximum range for windshear warning alerts is1.5NM.
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Events beyond the end of the runway will not be announced.In this mode,windshear warning alerts are inhibited below aircraft altitudes of50-feet(15.24meter)AGL. (6)Activation of windshear mode is dependent on the software configuration of the RT.
Refer to Table13/Table34-40-58-99A-012-A01for a summary of windshear activation requirements.
AIRCRAFT
WEATHER
RADAR SYSTEM
POWER
SELECTED
MODE
QUALIFIER(QUAL)AIRCRAFT ALTITUDE
ALL ON WX,WX+T,
TURB,or
MAP One QUAL A,and One
QUAL B valid.
Less than2300feet
(701.04meter)AGL.
ALL OFF50Feet(15.24meter)
<ALTITUDE<2300feet
(701.04meter)AGL.
AIRBUS OFF Longitudinal
acceleration>0.07g or
Airspeed>30kts.ALT<2300feet(701.04 meter)AGL.
BOEING OFF At least one QUAL A
and QUAL B valid.Less than2300feet (701.04meter)AGL.
Windshear Operation Activation Requirements
Table13/Table34-40-58-99A-012-A01
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State Diagram for Windshear Mode
Figure26/GRAPHIC34-40-58-99B-021-A01
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H.MultiScan Operation
(1)Automatic MultiScan operation is intended to allow the flight crew to view weather
targets,from takeoff to landing,without the workload of manually adjusting tilt,gain,and mode.The WRT-2100accomplishes this by using multiple transmit frequencies,multiple pulse widths,multiple scans,and digital processing to merge this reflectivity data into a composite weather radar map that shows short,mid-,and long-range weather targets.
(2)To understand MultiScan operation,it is necessary to understand the typical anatomy of
a thunderstorm.The reflective objects within a thunderstorm are forms of precipitation
such as rain,hail,snow,and ice crystals.The amount of energy reflected depends on
the reflective quality of the target.When the weather radar transmitted pulses strike a
target,some of the energy is absorbed,some of it is refracted,and the remainder is
reflected.It has been found that heavy rainfall produces the strongest reflections for
weather radar.Light rainfall,snow,and ice crystals produce the weak returns.Figure
27/GRAPHIC34-40-58-99B-022-A01shows the anatomy of a thunderstorm.As seen in this figure,the strongest reflections occur closest to the terrain.
(3)Standard tilt operation has been to adjust the radar beam so the main lobe of the beam
skims the edge of the terrain so the flight crew can view the most reflective area of the
storm.The problem with this approach has been ground clutter affecting weather target presentation,or having the radar beam miss targets.
(4)Figure28/GRAPHIC34-40-58-99B-023-A01shows an aircraft flying at33000feet
(10058.4meter)AGL using an upper tilt and lower tilt beam to illuminate weather
(precipitation)targets.During MultiScan operation the RT transmits during the clockwise sweep,the non-windshear pulse pattern at the upper tilt angle.During the next sweep,
the RT transmits the non-windshear pulse pattern at the lower tilt angle.The tilt angles
used during MultiScan operation are dependent upon the Baro altitude and the terrain
height.Reflectivity data is gathered from both tilt scans,at four different frequencies
using2different pulse widths.The data is placed into memory planes.Each scan
generating a short pulse memory phase(0to44NM)and a long pulse memory phase(0 to320NM).So for an upper and lower tilt beam four memory planes will be generated.
(5)The digital processing function performs several algorithms that identify ground clutter
within the reflectivity data using scan-to-scan correlation,and beam-to-beam correlation.
The data stored in the memory planes is used to support all selected ranges,modes of operation,and selected display options.The transmit pulse pattern uses2pulse widths (6and20microseconds)(6to25microseconds for SB2or later)to build512sample
bins for long range data and256sample bins for short range data.
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