Appendix A The Basics of Radar

Introduction Radars used by the FAA operate under some very basic principles. They transmit a quick

burst of energy into space and go into a listen mode that is long enough in duration for

the pulse of energy to travel to the range extent of the radar and back to the antenna.

When the pulse of energy bumps into an object, such as an aircraft, the energy bounces

off the object and returns to the radar as an energy echo. The range of the aircraft from

the radar is derived from the round-trip-travel time of the pulse at the speed of light from

the radar to the aircraft and back to the radar. Radar antennas focus the energy burst into

a very narrow beam 1-2.5°, so the direction of the energy burst is very specific and

determined by orientation of a rotating antenna. The azimuth of a detected aircraft

relative to the radar is known by the orientation of the antenna at the instant of

transmission. A radar transmits hundreds of pulses per second on regular intervals called

the Pulse Repetition Time period (PRT). Air traffic control radars are categorized into

two classifications: Beacon and Search.

The range of an object from the radar is determined by the

amount of time required for the energy to travel the round trip

from the antenna to the object and back to the antenna.

Range = 1mile

round trip = 12.36 micro seconds

The primary radar sends a pulse of energy into space

objects illuminated by the energy

pulse reflect a small portion

back to the antennaante

nna

ante

nna

Beacon Radar

Beacon radar is a simple communications system between a ground station interrogator at

the radar site and a transponder located in the aircraft. The interrogator sends a short

series of pulses that are coded to request information such as identity or altitude; and then

listens. If an aircraft is in the path of the interrogation pulses, the aircraft transponder

will receive and process the interrogation. Three microseconds after receiving the

interrogation, the transponder transmits the appropriate coded reply pulses. The beam

width of a beacon antenna is about 2.4°. Beacon radar systems are known as dependent

surveillance systems because they only work when aircraft are equipped with

transponders. As a throw-back to military applications, search radar is often referred to

as primary radar, and beacon as secondary radar. Regarding the mission of the FAA, this

is a misnomer because most aircraft in the US are equipped with transponders. Beacon

systems are much more versatile and robust than search systems from an air traffic

control perspective. For example, beacon systems provide unambiguous identification

and altitude of aircraft. For this reason, air traffic control relies more heavily on beacon

systems for directing aircraft.

Figure A1

A1

Search Radar Search radars are known as independent surveillance systems because they require no

cooperation from the aircraft. A search radar transmits a single high energy, high

frequency pulse of energy and listens. When the pulse collides with objects in its path,

small echoes of the pulse return to the radar antenna. A pulse of energy bounces off the

skin of an aircraft, and the position of that aircraft is painted on the controller’s display;

hence the term: skin-paint radar. A good way to understand search radar is to consider

the pulse of energy as a very bright pulse of light that our eyes ccan not detect. This

pulse of light illuminates objects which reflect a small portion of the light back to the

radar. Some materials absorb the radar energy the same way that the color black absorbs

visible light. Most materials reflect a portion of the energy, and some materials, such as

metal, are more reflective than others, such as concrete. Most objects illuminated by the

radar beam reflect echoes of the energy to the antenna. The radar detects these

reflections and converts them to voltage levels of a much lower frequency. The voltage

levels are sent to the radar signal processor. The processor determines which echoes are

from aircraft. The range of the object from the antenna is given by the round-trip travel

time of the energy at the speed of light. This is measured in microseconds where 12.36us

is equivalent to 1 nautical mile( nmi). Typical range resolution for FAA search radars is

1/16 nmi. Azimuth resolution for FAA search radars is related to the 3dB horizontal

beam width of the antenna: normally about 1.5°. Therefore, the smallest range, azimuth

increment, or bin, normally processed by a search radar is approximately 1/16 nmi X

1.5°.

The signal processor is designed to discriminate between echoes from aircraft and echoes

from other objects such as mountains and buildings. One important property of an

aircraft in flight is that it moves at a high velocity relative to mountains and buildings.

An object moving away from or toward a point has radial motion with respect to the

point. A moving object that maintains constant range from a point, such as an object in

orbit, has tangential motion. A radar echo from a stationary object or an object with

tangential motion is nearly identical to the original transmitted pulse at a fraction of the

original power. An echo from an object with radial motion is shifted slightly in

frequency according to a principal known as the Doppler Effect. The radar processor

exploits this Doppler shift to discriminate moving targets from stationary targets. A

possible search target is declared when an echo from an object with radial motion

exceeds a voltage level called the detection threshold. This phenomenon is known as a

“hit” in radar terminology. A radar processor declares hit to be a moving target (aircraft)

when successive transmit pulses result in multiple hits from the same range within a

single beam width, or beam dwell. Other sources also return Doppler shifted echoes to

the radar: swaying trees or grass, waves on water, trains and road traffic, even clouds.

Echoes from such sources that display on ATC consoles are collectively called primary

moving clutter. Moving clutter as well as noise and interference emitted from other

sources can all be sent to the processor masked as possible moving targets. The

processor must determine a detection threshold above the level of the noise and clutter,

but below the level of real aircraft echoes. If the detection threshold is too high, small

aircraft might not be detected. If the threshold is too low, false detections of noise (false

A2

alarms) might overwhelm the processor and the ATC displays. The detection threshold is

set dynamically according to the level of ambient noise and clutter through a process

known as Constant False Alarm Rate (CFAR) . When false detections occur, they

display on the ATC consoles as clutter. Clutter can distract controllers and obscure

aircraft detections. Beyond radial motion detection and CFAR thresholding, older analog

radars have only limited means to edit the content of their output. As a result, they are

often susceptible to excessive clutter. Newer digital radars, such as the ASR-9, employ

numerous techniques in post processing to remove clutter.

Detection Threshold

System Noise

Re

ce

ive

r O

ff

aircraft

System Noise

Re

ce

ive

r O

ff

threshold

false alarmfalse alarm

This image is a representation of what analog Moving Target Video might look

like on an oscilloscope. In this representation, only the largest maximum

represents a return from an aircraft. The detection threshold is arbitrarily

placed. In this representation, the 2 maximums labeled false alarms would

display on an ATC console as clutter.

Analog Vs Digital Radar Processor

The ASR-8 is considered an analog radar because its output is analog video pulses. The

signal processor of the ASR-8 compares each echo with a reference frequency to

determine phase or Doppler shift. The output of this stage of the processor is echoes with

radial velocity. These echoes are digitally sampled at 1/16th

nmi increments. Each 1/16th

nmi is called a range cell and the sampled voltage level from an echo is called residue.

The processor uses a digital Moving Target Integrator (MTI) filter to combine residue

over multiple PRTs (2-4 listening periods). True targets with sufficient phase shift that

occur at the same range add over multiple PRTs while random, spurious echoes do not.

The processor of the ASR-8 also implements a digital mean filter CFAR to calculate a

detection threshold for each range, azimuth bin. The amplitude of the integrated moving

target residue in each range, azimuth bin is compared with the detection threshold

calculated for the same bin. At the conclusion of MTI and CFAR, the target data is

converted to quantized video pulses and output to the ARTS automation system. Beyond

MTI and CFAR, the ASR-8 has no further means to differentiate between moving targets

and moving clutter. The automation system doesn’t filter or track analog search video.

Any video pulses output from the ASR-8 are displayed on the ATC console.

Figure A2

A3

Range

Azim

uth

1/16 nm range cells

BEFORE INTEGRATION

Range1/16 nm range cells

AFTER INTEGRATION

+

+

+

Current PRT-2

Current PRT-1

Current PRT

Azim

uth

ThresholdThreshold Current PRT

Digital Radar

The ASR-9 is considered a digital radar because the final output from the ASR-9 is a

digital target message: a series of 8 bit words to describe a target’s positions. The ASR-9

implements Moving Target Detection (MTD) to cancel stationary targets and classify

moving targets by velocity. This allows MTD radars to identify aircraft in weather and

over other sources of slow moving clutter. MTD radars have much faster processors and

much greater memory capacity than older analog systems. This allows much more

information to be stored and accessed during processing. The echoes are compared to a

reference and sampled for amplitude and phase. This information is stored in digital

memory addressed by range, azimuth. From that point on, all the processing is

accomplished with numerical analysis that includes frequency content (velocity),

amplitude, target detection and detection thresholds. The detection threshold is

dynamically calculated for each range, azimuth bin in a process that includes ambient

noise and clutter as well as clutter or noise that is known to occur historically in a

particular bin. The on-board CPUs store and process 1 beam dwell of information at a

time (1 beam dwell or beam width ≈ 1.4° in azimuth). For each target, this could be 10 –

15 sequential hits. The MTD processor integrates residue with like velocity at the same

range across a beam dwell. The processor then determines if the integrated residue in a

particular range, azimuth bin meets the detection threshold for that bin. All the complete

targets from a single scan (1 antenna rotation) are then stored in what could be called

scan memory. This scan memory is addressed by range, azimuth and scan number and

contains amplitude and approximate velocity among other attributes. Multiple scans are

stored at a time. This scan memory is accessed by the post processor.

Post Processor Traditionally, the post processor is a group of micro-processors that are incorporated into

the digital radar system. The post processor works with detected targets as opposed to

hits. In essence, targets with radial motion that integrated across the beam dwell to

exceed amplitude thresholds. In modern systems, the post processor interfaces with the

signal processor to the point that it is difficult to distinguish between the two. Three

features of post processing will be mentioned here.

Moving Target Integration

Figure A3

A4

Tracker The tracker retains a history of each target. At a basic level, each target is classified

according to 2 categories: primitive, and track (in some tracker literature, primitives are

called plots). When a target is initially detected by the radar, the tracker makes an entry

on the primitive list for the position of that target and predicts a region in space where

that primitive might appear on the next scan. That region excludes the same range and

azimuth as the previous or current scan since trackers generally employ minimum

movement criteria for aircraft. When a target occurs within the predicted zone on the

next scan, the tracker updates the primitive’s position on the primitive list. If a primitive

is updated m out of n scans (for example, a new target appears in its predicted zone 3 out

of 4 scans), the tracker moves that target from the primitive list to the track list. If a

target from the track list is doesn’t appear in its predicted zone on a particular scan, the

tracker places that track into the coast list and predicts a new position for the next scan. If

a track remains on the coast list for a predetermined number of scans (3, for example),

that track will be dropped from the trackers inventory. Ideally, this happens when an

aircraft lands or flies out of range. For the most part, controllers only need to look at

tracks. Ideally, the processor will not upgrade random occurrences, such as clutter, noise

and interference to track status. In ASR-9 terminology, mature tracks are assigned the

title “correlated targets” implying that these targets correlated to a track in the track list.

Controllers select, by push button, to view either Correlated or Uncorrelated Search

targets. In Uncorrelated mode, all search targets are displayed; this includes primitives

and mature tracks. In Correlated mode, only mature tracks are displayed. In both cases,

this doesn’t include search targets that correspond to beacon replies. Beacon / search

merging occurs prior to the tracker in the processor path(Merging of Search and Beacon

is covered below).

Geocensor Maps Static geocensoring is a means of dealing with frequently occurring clutter that is

problematic. A geocensor map is a means of manually lowering the radar’s sensitivity in

the area where excessive clutter is generated. As an example, a roadway 10 nmi North of

a radar might generate excessive targets during commuting hours. A geocensor map is

partitioned into Range, azimuth bins – for example 1/16 nmi X 1.5°. During an

evaluation by an expert, a detection threshold override value is entered for each bin

where excessive clutter is observed. The override value is chosen according to the

magnitude of the clutter. During operation, the processor selects the larger of the

calculated detection threshold and the override value. A sufficiently high override can

ameliorate most clutter, however, at a cost. Artificially raising the threshold in over a

roadway subsequently lowers the sensitivity of the radar over the roadway, which in-turn

lowers probability of detection of aircraft over the roadway. In order for the radar to

detect an aircraft over the roadway, the echo of the aircraft must exceed the raised

detection threshold. This is to say that the echo from the aircraft must have greater

amplitude than the echoes from the rush-hour traffic. In such an instance, the expert must

decide which is more important for air traffic control in a particular area. Another

drawback to this method is that static geocensor maps are permanent and applied 100%

of the time even though the source of clutter may be transient – such as when there is not

a car on the road.

A5

While a static geocensor map remains constant, and the environment can change from

minute to minute. A roadway might have episodes of high traffic interspersed with long

periods when there is no traffic. There are also seasonal occurrences, such as bird

migration, high winds, rain and snow. The most recent upgrade to clutter processing in

the ASR-9 was introduced with the ASR-9 Processor Augmentation Card Phase II

(9PAC-II ). Included, as part of the 9PAC-II, is an adaptive geocensor map. The

adaptive geocensor map reacts dynamically to changes in the environment and adjusts

detection thresholds accordingly. The adaptive geocensor function generates a range &

azimuth map of accumulated clutter power over time to identify stationary or slow

moving emitters of clutter. The resolution of the geocensor map is 1/16 nmi X 0.7°, ½

the width of the antenna beam. This fine resolution requires hours of accumulation to

achieve. In order to improve reaction time to changes in the environment, the geocensor

map has four layers: one very-fine resolution layer, and three layers of sequentially

decreasing resolution. The coarsest map matures in about one minute. The geocencor

function incorporates all four layers into the final map with weighted averaging. In the

low resolution maps, the range azimuth bins can be one to three miles wide. Very strong

emitters of clutter, such as a freeway bridge or overpass, can raise the average clutter

power for a fairly large area. When the four map layers are merged, the received average

clutter powers blend in to areas surrounding the main source. The end result is that the

sensitivity of the radar is heavily reduced above the roadway, and gradually reduced

around the roadway with a gradient from greatly reduced sensitivity over the clutter

source to normal sensitivity some distance away from the source. One of the best affects

of the dynamic geocensor map is that sensitivity is normalized when clutter is absent.

For example, when there is no traffic on a road, or there is no wind over a wind farm, the

sensitivity of the radar will gradually increase to normal levels over those areas.

Merging Beacon and Search For ACK, both the ASR-9 and the Mode S are capable of merging the search targets to

the beacon replies. When the Mode S is in Interim Beacon Interrogator (IBI), the ASR-9

digitizes and decodes the reply video pulses from the Mode S interrogator. Using a best

curve fit function; the ASR-9 examines each beacon target to determine if there is a

corresponding search target at the same range and azimuth. Merging takes place before

the tracker and before the geo-map. There is no further processing for merged search

targets. In normal operation or Mode S mode, the Mode S receives the ASR-9 search

messages, and determines for each beacon target if there is a corresponding search target.

The expression used to describe a merged target is to say that the beacon was reinforced

by the search. For a merged target, the search message is dropped, and a beacon target

message with the reinforced bit set is sent to the automation system.

In the case of the ASR-8 and the BI-5 at FMH, there is no merging. Any search video

output from the ASR-8 is sent to the automation system and displayed on the ATC

console

A6

Common Automated Radar Terminal System-2E (ARTS-IIE) The ARTS-IIE performs some processing functions and distributes radar and beacon

video to the air traffic control consoles. The ARTS-IIE can be configured to accept raw

search and beacon video or digital target messages. There are two ARTS-IIEs at the

FMH TRACON: One for the FMH ASR-8, B-5 video and a second for the ACK ASR-9,

Mode S data. The ARTS that receives FMH Radar video feeds 3 air traffic control

consoles that are dedicated to approach control into Hyannis and Otis AFB. The other

ARTS feeds 2 displays that are dedicated to approach control into Nantucket. All the

displays are Cathode Ray Tube (CRT) analog video display systems. The two ARTS

systems at the FMH TRACON function independently and have different capabilities

related to their different input configurations.

ARTS with Analog Video Input This system receives beacon video reply pulses and ASR target video. The ARTS

processor contains a beacon video DEFRUITER and decoder. The output from the

decoder is digital reply messages. This system has a beacon tracker that correlates the

beacon identification codes to flight plans and flight numbers. This information is can be

displayed along with ID codes, altitude, ground speed and raw reply pulses. ASR-8

target video is displayed at the appropriate range and azimuth. There is no processing of

ASR-8 target video. If an aircraft is seen by both the beacon and the ASR, a search paint

will appear adjacent to the beacon symbol. The ARTS will display aircraft without

transponders at their appropriate range and azimuth as search paints. Any false alarms

will also display as clutter.

ARTS with Digital Input

Currently, this system is capable of receiving inputs from up to 2 different beacon

systems and 1 search radar, giving the controller the ability to switch between two

different beacon radar feeds almost instantaneously. This system has a search tracker and

will display a data block for search only targets when a controller selects that search track

with a track-ball curser on the display. For aircraft that are picked up by both the beacon

and the search radars, only the beacon symbol is displayed. Merging is accomplished at

the radar site by the Mode S or the ASR processor.

A7

WinPlot

WinPlot is a software application created by Steve Smith, FAA Central Region Radar

Engineer, for the FAA for the purpose analyzing radar coverage. WinPlot reads the radar

data and simultaneously displays the targets on a range azimuth plot (X,Y plot) to give

the effect of watching targets on an ATC console. The user can select to view,

independently or all at once, X,Y plot, Elevation plot, a Message viewer or Statistics.

WinPlot has many tools to help examine radar coverage.

Figure A4 shows the Winplot Graphical User Interface.

Table A1

Format of WinPlot message viewer

Scan Message Range Degrees Code Altitude RL Port Time Delta

scan number

message type (beacon,search,equipment status)

range (nmi)

Azimuth (deg)

Mode 3 ID

pressure alditude

number of replies received (X2 due to an error with the BEXR)

NA

Greenwich Mean (the GPS clock was 15 sec faster than the BEXR clock)

Time (sec) since previous reply from this aircraft (1 rotation of the antenna = 4.7 sec)

Figure A4

A8

Appendix B

Wind Turbine Impacts to Radar

obstruct a radar’s view of the coverage volume. This is known as shadowing. If erected

close enough to a radar, tall metal towers, such as wind turbines, can distort or even

reflect the beam pattern of the radar antenna to other directions. These phenomena can

result in false targets and speed jumps. A speed jump occurs when an aircraft appears to

travel much farther during one particular scan compared to all the other scans. One

possible cause for speed jumps is a slight bending or deforming of the beam pattern by a

metal tower placed too close to the radar antenna.

Photo of single 2-section wind turbine blade on a flat-bed trailer [3]

Figure B2

An aspect

angle from the

side of the

wind turbine

provides the

greatest

component of

radial motion

of the blades

Wind Turbines Wind Turbines present a special set of problems for search radars.

The blades of wind turbines can be 140’ long by 9’wide. This

presents a large radar cross-section that is larger than many aircraft.

The tips of the blades can travel at speeds in excess of 150 knots.

The worst case aspect angle from the radar to the wind turbine is in

profile, such that the motion of the blade tips with respect to the

radar is purely radial. The Doppler shifted echoes from rotating

wind turbine blades share many characteristics with those of

aircraft. As a result, a radar processor can not discern between a

wind turbine blade and an aircraft. This produces false alarms at

the range and azimuth of the wind turbine. Echoes from the blades

combine with echoes from aircraft flying over the same range and

azimuth. The echoes from the aircraft are masked by the wind

turbine blade echoes. Wind turbines are tall structures that can

Figure B1

B1

Clutter, Obscuration, and Misses Though the blades are generally made of fiber glass or carbon fiber composite, which is

less reflective than metal, it has been observed that the echoes from wind turbine blades

are sufficiently large to meet moving target detection criterion of air search radars at

ranges greater than 70 nmi. Moving targets in the radar processor generated by wind

turbine blades pose two problems: 1. Clutter: wind turbine blades generate clutter on the

display that is a distraction for air traffic controllers because it is difficult for a controller

to distinguish between a dot of clutter and a dot that represents an aircraft without a

transponder. 2. Obscuration: A search radar processor also ccan not discriminate

between an echo from a wind turbine blade and one from an aircraft. Terminal radars

used by the FAA such as the ASR-8/9 are 2-dimensional radars that classify targets by

range, azimuth (bearing). Such radars do not discriminate by altitude. The processor of

an ASR-8/9 would classify two aircraft at the same range, azimuth, but different altitudes,

as a single target. The same is true of an aircraft flying over a rotating wind turbine

blade. Since the echo from a rotating wind turbine blade matches many characteristics of

an echo from an aircraft, a radar processor would classify an aircraft and a rotating wind

turbine blade at the same range & azimuth as a single target. In this way, wind turbine

blades can obscure aircraft flying over them.

Track Seduction The problem of having to discriminate between an aircraft and a wind turbine blade near

the same range & azimuth is repeated for the tracker in post processing or the automation

system. The tracker records the current position of a tracked target, and predicts where

that target will appear during the next antenna scan. If multiple targets appear in an area

where the tracker is expecting a tracked target, the tracker must select one target to

update the track. It is possible for the tracker to select clutter in place of an aircraft.

Selecting the wrong target can shift the prediction box away from the aircraft’s trajectory

during the next scan. If the new prediction box includes the position of another wind

turbine, the tracker can update the track with wind turbine clutter for a second time. This

scenario is most likely to occur when an aircraft is obscured by wind turbine clutter or

missed due to raised detection thresholds in the vicinity of a wind farm. If there are many

wind turbines close together, it is possible for a tracker to track wind turbine clutter all

the way across the wind farm while losing track of a real aircraft in the process. This

phenomenon is called track seduction.

Raised Detection Thresholds Excessive false alarms from wind turbine blades can be reduced by raising the detection

threshold in the range, azimuth bin of a wind turbine. The trade-off is that decreasing the

sensitivity near a wind turbine reduces the probability of detecting an aircraft at the same

range & azimuth. Both static and dynamic geocensor maps can also reduce processor

sensitivity in the proximity of and between wind turbines. Static geocensor maps

generally have low resolution. Therefore, the cost of ameliorating one clutter source is to

desensitize the radar over a region that is wider than the space occupied by the wind

turbine. Because the ASR-9 dynamic geocensor map includes low resolution layers, the

same principal serves to lower the sensitivity of the radar, to a lesser degree, over a wind

B2

farm. A wind turbine is a very strong point emitter of clutter. In the low resolution

layers, a wind turbine may substantially raise the average clutter power over time for a

large area. In a wind farm, there may be many point emitters spread across one or more

low-resolution cells. This would result in the low resolution maps showing high levels of

clutter throughout the farm rather than at the position of each wind turbine. When the

four layers are merged by weighted averaging into a single map, the low resolution maps

contribute high clutter counts between the wind turbines and in the vicinity of the wind

farm. Areas with high clutter counts are assigned higher detection thresholds. The result

is a maximum sensitivity reduction occurring over each wind turbine with sensitivity

blending of reduction between the wind turbines.

5

3

5

2

2

2

1

1

Merged mapHigh resolution bin

over low resolution bin

a detection threshold value is assigned to each

range, azimuth bin according to the average

amount of clutter received from that bin over time

Figure B4 (next page) shows playback of a 1-hour data recording from the ASR-9 at

Palm Springs, CA. Figure B4 shows aircraft flying over a wind farm on approach to

Palm Springs Intl. The red dots indicate ASR-9 search misses. The light blue dots

represent uncorrelated primitives which are not displayed on the air traffic control

console unless the controller selects uncorrelated search at the control consol. A close

examination of the data reveals that many of the search misses do not occur directly over

a wind turbine. This implies that most of the search misses in this recording are the result

of raised thresholds across the wind farm as well as directly over each wind turbine. It is

interesting to note that there is very little wind turbine clutter displayed on the

controllers’ screens, and still there are many search misses over the wind farm.

Figure B3

B3

Fig

ure B

4

Fig

ure B

4 sh

ow

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f aircraft flyin

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ver w

ind tu

rbin

es on ap

pro

ach to

Palm

Sprin

gs In

ternatio

nal A

irport. A

red d

ot

indicates a search

miss. T

he lig

ht b

lue d

ots rep

resent u

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rrelated p

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at do n

ot sh

ow

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the fin

al disp

lay. M

ost

unco

rrelated p

lots w

ithin

the w

ind farm

are due to

rotatin

g w

ind tu

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e blad

es. A clo

se look at th

e data rev

eals that m

ost

of th

e search m

isses are due to

raised th

reshold

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in th

e win

d farm

.

B4

Tra

ffic Over P

alm

Sp

rings W

ind

Farm

Shadowing

Since wind turbine towers are tall and up to 16’ wide, they can obstruct a radars view of

the coverage volume. This loss of coverage occurs behind the wind turbine in the

shadow of the illuminating radar beam. In theory, wind turbines do not completely

screen a radar’s view because the energy from the radar is diffracted, or bends, around

the wind turbine tower. Due to diffraction, the shadow behind the wind turbine is

partially illuminated. This partial illumination behind the tower results in a region of

signal reduction, also called attenuation, due to fading Signal reduction in the shadow

varies inversely with the squares of the distances from the radar to the wind turbine and

the wind turbine to the aircraft. Signal reduction varies directly with the diameter of the

tower. The wind turbine creates a shadow at both the search and beacon frequencies;

however, the search signal is attenuated much more than the beacon. The beacon

interrogations and replies are separate transmissions and each makes one pass through the

shadow. The energy from a single search transmission makes a full round trip between

the radar and the aircraft; thus passing through the shadow twice (Figure B5).

Mathematically, the signal attenuation at any point in the shadow of the wind turbine is

squared for the round-trip case.

Interrogate

Reply

Beacon Search

Transmit Pulse

Echo

shadow shadow

shadow

shadow

Figure B5

The beacon interrogations and replies each

have a direct path from a transmitter to

receiver. Therefore, each transmission

passes through the shadow once during

each interrogate reply sequence.

The energy from the search transmit pulse

must make a full round-trip from the antenna

to the target and back to the antenna. For an

aircraft in the shadow of a wind turbine, that

energy must pass through the shadow twice.

One-way Travel of Beacon Round-Trip Travel of Search

B5

The formula for calculating the relative illumination in the shadow of

the wind turbine is based on the ratio of the scattered electric field

that versus the direct electric field. The direct electric field

represents total amount of energy that would arrive at a particular

point if there were no obstruction. The scattered energy represents

the portion of the total energy that is scattered by the tower.

Where

Relative Illumination in the shadow is the ratio of Escattered:Edirect=

dir

scat

E

E =

the strength of the electric field behind the wind turbine relative to the unobstructed field at the same range.

Rr2p = range from radar to plane

Rr2w = range from radar to wind turbine

Rw2p = range from wind turbine to plane

D= diameter of wind turbine tower

λpwwr RR

Dshadowtheinationillu

22

2

r2pR1___min −=

B6

Attenuation Behind a Wind Turbine

0 2 4 6 8 10-70

-60

-50

-40

-30

-20

-10

0Signal Loss with Wind Turbine 1nm From Radar

Distance from Wind Turbine to Aircraft(nm)

Att

enuation (

dB

)

0 2 4 6 8 10-70

-60

-50

-40

-30

-20

-10

0Signal Loss with Aircraft 1nm From WT

Distance from Radar to Wind Turbine (nm)

Att

enuation (

dB

)

Figure B6a,b show how the strength of a signal making a round-trip through the shadow

of a wind turbine changes with range. Figure B6a simulates an aircraft flying a radial

behind a 5 meter diameter tower positioned 1nmi from a search radar. Figure B6b

simulates an aircraft 1nm behind a 5 meter diameter tower as the position of the tower

varies in range. Note that the plots are identical

The theory that governs illumination in the shadow of a wind turbine is based on

communications principals where the radar transmitter and the aircraft transponder form

a communications link, and the wind turbine is an obstruction between them. The

equations are derived in Reference [1a]. According to the theory, most of the in-phase

energy that reaches the aircraft from the radar travels within an ellipsoid shell known as

the first Fresnel zone. If the obstruction between them blocks an entire vertical column

of this Fresnel zone, that component of the signal is scattered. Part of the signal is

scattered away and lost, and part of the signal is scattered to the receiver, but arrives out

of phase with the direct path signal. Both features of scattering serve to weaken the

signal at the receiver. The result is a partial shadow that gets brighter with range due to

diffraction. If more than one-half of the first Fresnel zone is obstructed, then the shadow

behind the wind turbine is very dark, which constitutes screening? The positions of the

antenna and the aircraft are actually the foci of the ellipsoid.

Figure B6a

Figure B6b

B7

Most of the energy that reaches the aircraft from the antenna

travels within an ellipsoid shell. The radius of the ellipsoid center increases

with the range of the aircraft.

If an obstruciton blocks a portion of the first Fresnel zone,

there is partial shadowing behind the wind turbine. The

shadow is partially illuminated due to diffraction.

The illumination in the shadow behind the wind turbine beyond 10 miles approaches a

constant value that depends on the distance between the radar and the wind turbine. For

example, in the case of a 5 meter diameter tower at a range of 1 nmi, the signal loss for a

search radar beyond 11 nmi is about 8dB. A Lear jet seen from the nose, flying along a

radial towards the radar, has an approximate radar cross section of 1m2.

For detection of

such a target, 8dB of additional attenuation can be significant, especially near the

maximum range of the radar. If a strong STC is employed, 8dB could have an impact

throughout the coverage volume for marginal targets. STC is a systematic reduction in

sensitivity for close range targets. STC begins with some initial value of signal

attenuation. The sensitivity is then successively increased by 6 dB for every range octave

(doubling of range). For example, if 45dB of attenuation is applied at range= 1 nmi.

Between range=1 nmi and range=2 nmi, attenuation is reduced to 39dB, therefore,

sensitivity is increased by approximately 6dB. If the initial STC value is high, then it is

possible for small targets to be near the minimum discernible signal (MDS) level of the

radar receiver. In this situation, the shadow from a wind turbine could push smaller

targets below the MDS – which means they could go undetected. Also, 8dB of

attenuation from a wind turbine could theoretically reduce the maximum detection range

for marginal targets. In this example, maximum range for a 1m2 target in the nose of the

beam is reduced by 20 nmi.

Figure B7

B8

0 10 20 30 40 50 60 70-140

-130

-120

-110

-100

-90

-80

-70Power in Sarch Receiver; Frequency: 2.9GHz; STC initial value: 45dB; Range to wind turbine: 1nm; Diameter: 5 meter

Range from Radar to Target(nm)

Sig

nal Level at

Receiv

er

(dB

m)

Rx Sig STC No Shadow

Rx Sig STC In Shadow

MDS

Figure B8 shows the approximate power in the receiver after STC of an echo from a 1m2

search target as range to the target varies from 1nm to 65nm. The blue curve represents

the power from an echo with no wind turbine. The green curve represents the power in

the receiver from the same target in the shadow of a 5 meter diameter tower. The

horizontal line at -108 dBm represents the smallest echo that the radar can distinguish

from noise, called the minimum Discernible Signal or MDS. This plot doesn’t represent

an accurate model of the ASR-9 receiver. This plot was generated using the radar

equation with some easily known ASR-9 parameters. The aggressive STC begins at 1nm

and decays at 20dB/decade (approximately 6dB / octave) and doesn’t fully recover before

end of range. For Figure B8, it is assumed that the aircraft is in the nose of the vertical

beam where the antenna gain is maximal. The parameters chosen to create this plot place

the blue curve at MDS just beyond the 60nm maximum range. This implies that the radar

can see a 1m2 target just beyond 60nm with no atmospheric attenuation. The shadowing

function employed is simple, well-known and discussed in many papers on wind turbine

impacts to radar. The purpose of this plot is to demonstrate that the simple equation

generally used to model the illumination behind a wind turbine tower predicts that there

is attenuation behind the wind turbine that extends for the full range of the radar. This

could effectively reduce the maximum detection range for marginal targets, especially

during inclement weather.

Figure B8

Power in Search Receiver

B9

Shadow Width

For determination of shadow width, this report relies on the method recommended for

Eurocontrol [2a]

. This method assumes that the greatest contribution to the shadow effect

comes from the bistatic forward scatter of the wind turbine tower. The bistatic radar

cross section of an object is the measured or calculated amount of energy scattered in

other directions when illuminated from a particular direction. The bistatic radar cross

section of a wind turbine with the illuminator directly in front of the wind turbine, shown

in Figure 10, predicts that the greatest amount of reflected energy is a highly focused

lobe directly behind the tower. This energy, called forward scatter, is scattered in the

direction of incident propagation. The path of the forward scatter energy around the wind

turbine tower is slightly farther than direct incident path. The worst case assumption for

shadowing is that the peak of the forward scatter lobe has a path-length difference of λ/2

with respect to the direct path signal. This means that there is nearly complete

cancellation of the two signals when they combine on the back side of the wind turbine.

This cancelation produces the effect of fading (signal reduction, attenuation). Another

worst case assumption is that the effective width of the shadow ends where path-length

difference is at the mid-point between destructive interference, λ/2, and constructive

interference, λ. This value occurs at +-λ/4.

Bistatic RCS dBsm windturbine from the front

illuminated: 0 az, 0 el

observed: 0-359 azimuth, 0 elevation

illuminator

windturbine

Figure B9

Figure B9 shows the computed bistatic radar cross section of a wind turbine illuminated from the front

using computational electromagnetism (Comp EM) software. Note that the largest lobe or magnitude of

scattered energy due to the tower is directly behind the wind turbine relative to the illuminator. This

lobe is the forward scatter region.

B10

Therefore, the suggested effective width of the shadow behind a wind turbine lies

between the lines where the path-length difference between the direct signal and the

scattered signal is 3λ/4. This effective width, which varies with range and frequency,

relies on an assumption that the darkest part of the shadow is directly behind the wind

turbine. According to this model, the darkness of the shadow across the effective width

varies as (sin(x))/(x) with the darkest point in the center. For simplicity and

consideration of worst case, no effort has been made to quantify the variance of

illumination across the effective width of the shadow for this report. Instead, the

assumption has been made that the shadow is uniform across the effective width.

Where two shadows overlap, their combined destructive interference will be the vector

sum of the phases and amplitude of the two interfering signals with the direct signal.

Since the shadow is assumed uniform across the width of the effective angle, it is implied

that the resultant attenuation from overlapping shadows in decibels (dB) will add directly.

According to the model used for this report, a wind turbine shadow would be darkest only

at center. From the center toward the edges, attenuation would decrease rapidly

according to a sync function. This means that the shadow gets brighter toward the edges.

Sinc Function

-4 -3 -2 -1 0 1 2 3 40

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1The peek of a sinc function from -pi to pi

Figure B10a is a sinc function. Figure B10b is an example of a gradient from light to dark to

light at the rate of a sinc function from –π to π. In this example, very light blue would represent

no shadow

Figure B10

B11

Computational EM models predict that the darkest portions of the shadow behind the

wind turbine are two narrow lobes along the angular extremities of the effective shadow.

The analysis for this report will make use of the simpler approach that relies on the ratio

of the direct field and the scattered field and the path-lengths of their respective wave-

fronts.

Tower Illumination BeamShadow

Figure B11 shows the approximate shape that

Computational Electromagnetism models give for the

shadow behind a cylinder

Figure B11

B12

0 5 10 15 20 250

0.5

1

1.5

2Effective Angle of Shadow Width with respect to range behind a wind turbine at 2.9 GHz

distance from wind turbine (nm)

Eff

ective A

ngle

of

Shadow

Wid

th (

deg)

The equations used to model shadowing for this report do not incorporate

electromagnetic anomalies associated with Radio Frequency (RF) propagation. The

model relies on many worst case predictions. Empirical evidence of shadowing has been

difficult to observe. There are not many instances in the US where wind turbines directly

obstruct the coverage volume of a radar. Reduction in maximum range of a radar due to

wind turbines has not been observed. When drawing conclusions regarding the impacts

to radar coverage due to wind turbines, mathematical modeling can not replace empirical

evidence. Currently, there is very little empirical evidence regarding the propagation of

RF through a wind farm.

Figure B12

Figure B12 shows the width in degrees of the effective angle of the shadow as a

function of distance behind the wind turbine where effective shadow width is

defined by the path-length difference of the incident and scattered wave fronts.

B13

References

1. NTIA Technical Report TR-08-XXX, “Assessment of the Effects of Wind Turbines on

Air Traffic Control Radars”; John J. Lemmon, John E. Carroll, Frank H. Sanders.

a. Section 3.4 p11

2. Eurocontrol Document, “Assessment Methodology to Determine the Impact of Wind

Turbines on ATC Surveillance Systems”, 5/18/2007

a. Section C.5 p73

1. Photo of wind turbine on truck, taken by Brad Moon and posted August 25, 2008

“Windmills: Coming to a Shoreline Near You” on Wired Blog Network.

4. Report for NPR, “Nantucket Air Traffic”, by Kate Splaine, December, 2008

xx