A METHOD
FOR AUTOMATICALLY
BIRDS
ON RADAR
BY RONALD
P. LARKIN
AND LAWRENCE
DETECTING
EISENBERG
INTRODUCTION
Radar,sinceit canhavea greatrangeand doesnot depend• special
conditions
tobeuseful,
isthemost
versatile
means
wehave
or'studying
migratorybehavior(Alerstam,1972).Asidefrom trackingradar, which
is presentlyusedto followindividualbirdsor sometimes
compactflocks,
other radar types are used to estimatethe densityof bird migration,
provideinformationon the altitudesof flight, and estimatethe direction
of flight of bird targets.
In radar studies,birdsappearassmalldotsor brightareason a display
screen.The imageson the screenare photographedor visuallydescribedby techniquesreviewedbelow, and estimatesof the number of
dotspresentare recordedby hand.The writtenrecordis necessarily
of
a statisticalnature, being almostalwaysa descriptionof the migration
of many speciesand subjectto many uncertainties.Often the investigatorturnsto the useof digitalcomputersto analyzethe writtenrecord,
transferringit onto punchedcardsor other computermedia.Thus most
contemporarytechniquesstart with an electronicsignalin the radar
representinga bird target or targetsand end with an electronicsignal
in a digitalcomputer,but interposeseveraltediousand often subjective
stepsbetween.
In the presentreport we describea way of improving this clearly
unsatisfactory
stateof affairs,a techniquethat allowsechoesfrom bird
targetsto be recordeddirectlyby a computer or microprocessor,
permitting the biologistto studythe spatialand temporalpatternsof bird
movementswith increasedaccuracyand objectivity.We review present
methodsof collectingornithologicaldata from radar displaysand discusssomereasonsfor the unsophisticated
natureof the techniques
used.
Then we list somemethodsdevelopedby othersto processradar signals
automaticallyand describea devicewe have built that allowsgreat flexibility of operationat a reasonablecost.
METHODS
OF
ESTIMATING
OR
BIRD
DENSITY
VISUALLY
PHOTOGRAPHICALLY
Immense numbers of birds in the sky and our almost universalinability to distinguishwith certaintybetweentaxa of birds from their
radar echoesmakedescriptivestatistics
an importantpart of radar studiesof bird migration.A basicdatumin manysuchstudiesis the number
of birds in a certain altitude stratum,at a certain place,or at a certain
time of day or night. Givenan accuraterepresentationof the amplitude
of the signalpresentat the receiverof a radar and a knowledgeof the
calibratedcharacteristics
of the radar, in many casesone shouldbe able
to make an accurate estimation
of the numbers
and size distribution
of
targetswithin the spacesurveyedby the radar. Yet, unlesscomplicated
172
Vol.49,No.2
Detecting
BirdEchoes
[ 173
proceduresare followedto compareradar and visualmethods(Nisbet,
1963; Gauthreaux, 1970), reliable absolute estimates of bird densities
are not often obtainedusingradar alone. As a result, many studiesof
bird migrationreport onlyrelativenumbersof birds,oftenon an ordinal
scale,or give estimatesthat are basedon unjustifiedassumptions.
Somecausesfor thissituationare inherent in propertiesof sometypes
of radar: anomalouspropagation(bendingof the radar beam), insufficientrangeresolution,insufficientangularresolutiondue to the shape
of the radar beam, and insufficientsensitivityat the rangesof interest.
Other causes,however,are avoidableif quantitativeinformation isavailable on the intensities
of bird echoes and the characteristics
of the radar.
These include: lack of absolutecalibrationof the radar on known spher-
ical reflectors,lack of accurateknowledgeof the shapeof the radar
beamor the settingsand electroniccharacteristics
of the radar, and lack
of sufficientdynamicrange in the radar display.We expect that the
latter classof avoidabledifficultiescan be minimized by taking advantage of the numericaltechniquesafforded by direct computerinput of
the radar video signal.The devicedescribedhere can be programmed
to aid in calibratingand measuringthe performanceof a radar aswell
as in collectingdata.
Techniquesused to estimatebird densitiesfrom radar displayshave
variedin precisionand tediousness
of applicationbut are basicallyvisual
estimations(Eastwood,1967). Severalstudieshave useda pencil beam
in combinationwith an A-scope(a radar displayof echo intensityas a
function of slant range) to count individual echoes(reviewedin Blok-
poel, 1969;Brudererand Steidinger,1972;Blokpoeland Burton, 1975;
Blokpoel,1976). Other investigators
havecountedthe numbersof individualdotsappearingon the PPI display(Plan-PositionIndicator, the
conventionalmap-like radar display; Nisbet, 1963) or measuredthe
rangeat whichsaturationof the PPI displayoccurs(Gauthreaux,1974).
Gauthreaux(1970) describes
a methodof adjustingthe gain of a search
radar sothat the PPI matchesa predeterminedstandarddensityof bird
echoesat a givenrange.The attenuationnecessary
to producea density
match is recorded and converted into an estimate of bird density. A
similar method is describedby Hunt (1973). Lesstedious,but alsoless
accurateand lesseasilycalibrated,is the methodof establishing
a graded
ordinal seriesof dot-densitiesand matchingphotographsof the PPI
displaywith one of the membersof the series.The resultingdot-density
estimateis related to, but not necessarilya simple function of, the bird
migrationdensity.The scalesusedhave employedthree (Williamset al.,
1974),five (Gauthreaux,1970),six (Alerstam,1972), or nine (Blokpoel,
1969;Richardson,1972)categories.
Severalreasonsare apparent for the presentlack of sophistication
in
radar ornithology:(1) Like weather radar (Smith et al., 1974) radar
ornithologyhas developedas a "spinoff"from military, space,and airtraffic-controlapplications.Consequently,
techniquesthat are now routine in other radar applicationsare untried in the studyof birds. (2)
174]
R. P. LarkinandL. Eisenberg
Bird-Banding
Spring 1978
Except for a few installations,the radars used by ornithologistsare
meant to monitor weather systemsor aircraft, and are used for ornithologyin slackperiodsor asan ancillaryproject.In fact, improvements
in the designof radars for aircraft detectionhave often been suchas to
reduceclutteron the screenssuppressing
birds alongwith ground targetsand water droplets(Miller, 1975). Somedayit may be difficult to
observebird movementson many radarsunlessthe specialcircuitsdesigned to suppressbird echoescan be bypassedand the "raw radar"
(amplified video) signal displayedfor the ornithologist(Gauthreaux,
1977).(3) Studentsof birdsare primarilyinterestedin biologicalquestions and have neither the backgroundnor the funds to apply highly
sophisticatedtechniques.(4) Even the conventionaltechniquesallowthe
investigatorto collectan enormousamount of data in a shorttime. Then
the sheerquantityof informationfilling one'sfile cabinetsdiscourages
one from gatheringmore detaileddata by more sophisticated
means.
(5) Most importantly, the currently used techniquesare useful in that
they continueto provide new insightsinto migration and other highaltitude flights of birds.
METHODS
OF
DETECTING
TARGETS
ON
RADAR
AUTOMATICALLY
Visual displayssuchasthe PPI scopediscardmuch of the information
that is availablefrom the radar signal;for example,the amplituderange
of a PPI dot is only about 6 dB. Since echoeson radar are electrical
signals,it is desirableto substitutea deviceto processthesesignalselectrically, insteadof displayingthem visuallyand recording this visual
representationon photographicfilm.
To our knowledge,other techniquesfor dealingwith the radar video
signalhave suffered from low resolution,high cost,or both. Many devicesgenerateonly target/notarget information (discussed
in Hansen,
1974). Boardman (1970) and Smith et al. (1974) describea cloud "profiler" that recordsecho strengthson computer disk from range bins
which are 1,280 m in length. The system,designedto map cloudsout
to 111 km, usesa fixed bin length and cannotbe adapted for high
resolutionwithout prohibitiveexpense.Glover (1972) describesa speciallybuilt devicethat achievesa 75 m rangebin length. Hunt (1975,
1977) deviseda bird-countingsystemfor aid in predictionof bird-aircraft collisions.It is hardwired and is designedto count numbersof
birds in 305-m bins along the radar beam. A new "Moving Target Detector" (ARTS-3) designedfor air traffic control expresslysuppresses
slow-movingtargetssuchasbirds,while recordingradar echoesto 100
m resolution(Anon., 1975). The Moving Target Detectormaintainsa
"clutter map" similar to the idea describedbelow. Its productioncostis
about$70,000.Similarly,a numberof computer-based
targetdetectors
on military equipmentare designedto detectlarge, fast-movingtargets
at great range, and to reject birds as "clutter."
Two constraintsmake radar signalsof smalltargetslike birds somewhat difficult to capture by conventionalmeans in digital form at a
Vol.49,No.2
Detecting
BirdEchoes
[ 175
reasonableprice. First, the analog-to-digital(A/D) converterthat samplesthe radar signalmustbe very fast comparedto the onesnormally
availablein laboratorycomputers,and secondlythe memoryunit that
receivesthe radar data mustbe equallyfast to copewith the output of
the A/D converter. As an illustration of the cost of such devices, an A/
D converterthat can sample8 bits every 33 nseccurrentlycostsabout
$6,000 and a transientrecorderwith computer-compatible
output,operatingevery 50 nsec,costsabout$5,600.
THE
VIDEO
SAMPLING
UNIT
By usinga samplingprocedureinsteadof a fastconversion,we have
constructed
a relativelyinexpensivedevicewhichallowsthe information
containedin the radar videosignalto be transferreddirectlyto a computer or microprocessor,
permittingaccurate,detailed,and quantitative
analysisof numbers,sizedistributions,and spatialdistributionsof bird
echoes.The device,whichwe havecalleda Video SamplingUnit (VSU),
is simplein principle.It is containedon a singleprintedcircuitboard
and functionsas a peripheraldeviceto a PDP 8/E minicomputer,although it could easilybe adapted to work with many typesof digital
machinesincludingthe inexpensiveand rapidly developingmicroprocessors.
The prototypewas constructedin the RockefellerUniversity
ElectronicsLaboratoryand costlessthan $2,000 includinginstallation
and modifications,but not including the costof the minicomputer.(A
circuitdiagramand other detailsare availablefrom the authors.)
The VSU canbe adaptedto any pulsedradar. It hasbeendesigned
to be usedwith an AN/MPQ-29 pencil-beamtrackerwith a 4 kHz nominal pulserepetitionfrequency(PRF), and 0.25 /.•secpulsewidth (further described in Larkin and Sutherland, 1977).
A simplifiedblock diagramof the VSU is shownin Figure 1. Its operationis similarto sweepsamplingunits whichare soldas accessories
to somesophisticated
oscilloscope
systems
(alsocalled"serialsampling";
Findlay and Ornstein, 1975). It convertsthe radar echoamplitude into
a digitalform by collectingone sample,at a narrowand specifiedrange,
for each pulse transmittedby the radar. The VSU differs from the
conventional
sweep-sampling
devicebecausethe rangeof eachsample
is determinedby the computerprogram.It canbe programmedto sample eachrange'bin' in turn, producingthe informationcontainedin an
A-scopedisplayor, with antennapositioninformationavailableto the
computer,that in a PPI or Range-Heightdisplay.Or it can be pro-
grammedto sampleoneor a few rangebins,generating
dataidealfor
wingbeator target "signature"analysis.
The initiationof a sampleis precededby transferringan 8-bit number, correspondingto the complementof a number representinga
rangeof 0-255 units,from the computerto a fast8-bitcounterin the
VSU. When the next radar trigger occurs,an enablingflip-flopis set
that servesto gatea 20 MHz crystaloscillatorinto the counter.The
counter counts down until it reaches zero at which time three events are
176]
RADAR
R. P. LarkinandL. Eisenberg
Bird-Banding
Spring 1978
VSU
COMPUTER
A/D
Video
Converter
I
i•
'
T I]
20MH•,
Clock
Pulse
' BI!
Trigger
FIGURE1. A block diagram of the principal elementsin the VSU interface.The path
alongthe top (radar video,amplifier,S/H, A/D converter)carriesthe analogsignal,
whereasthe rest of the circuitis basicallydigital. Abbreviations:S/H = sample-andhold,A/D = analog-to-digital,
O.S. = one-shot,F.F. = flip-flop.
initiated:(1) the enablingflip-flopis resetstoppingthe clockentry into
the counter; (2) a fast sampleand hold unit is triggered and put into a
samplemodefor a periodof 50 nsec(7.5 m) by meansof a 50 nseconeshot;(:3)the A/D converterof the computeris alsotriggered,initiating
a conversionand settingthe computerinterrupt requestline when the
conversionis complete.As a result, the VSU collectsa very fast sample
of the radar video signalat a specifiedtime followingthe radar pulse
and presentsit to the computer'sA/D converter.The A/D converter,
whichis the standardmodelsoldby the computermanufacturer,takes
up to 30/zsecto perform the conversion.The computercan then transfer a new 8-bit value to the VSU, arming it for another conversionat
the next radar trigger pulse.
The accuracyand resolution of the VSU were determined more by
convenience
and economics
rather than by whatis possiblewith modern
electronics.The 20-MHz clockhasa stabilityof 5 x 10-5 over the range
of 0 to 50øC. The 50-nsecinterval between counts results in a range
uncertaintyof ___7.5
m, plusa negligibleuncertaintydue to jitter in the
radar trigger pulse.The 8-bit counterprovidesa total range intervalof
7.5 x 256 or 1,920 m, of which the first 180 m is currently uselesssince
it precedesthe outgoingradar pulsein time. A completesequentialscan
of the 1,920-m range takes 64 msec (256 samples/4,000PRF). If
usedwith a radar with longer range and longer pulselength, different
trade-offsof theseparametersmight be chosenwithout an increasein
cost.Ten-bit amplitude resolution,determined by the A/D converter,
Vol.49,No.2
Detecting
BirdEchoes
[ 177
provides a measure of echo strength that is far more accurate than
needed.
The temporal resolution of the VSU is also sufficient for studying
birds in the radar beam. As mentioned above,the VSU can generatea
completesequentialscaneach64 msec.Sincethe maximumspeedof a
bird might be about 40 m/sec,the bird would move a maximum of only
about 2.6 m or V3of a range bin width, between one sweepand the
next. It is only when movingthe radar beam at high angular velocities
that a completesequentialscanmight provideinsufficientdata ratesfor
detectingand locatingbird targets.In this case,programmingsolutions
to concentrateon certain areasof the beam would be easyto implement.
Trials using a rotating antenna show that 2-8 recognizableblips, or
"hits"are attainedon singledistantbird targets,evenwhen conducting
completesequentialscanswhile rotating the antennaat 10-20 degrees/
sec.
In practice,we have found that the digitized radar signalis nearly as
useful as the actual radar video signal even for purposesof visually
observingmigratingbirds on radar. A singlecompletesequentialscan
during a night of fall migration is shownin Figure 2. The displayis a
FIGURE2. A photographof a digital reconstruction
of the A-scopedisplay,storedby a
singleVSU scan.Eachdot represents
the signalstrengthat one 50 nsecintervalfrom
the radar pulsetrigger correspondingto approximately7.5 m range.The displayis
1,920m in total range.(a), The outgoingradar pulse,or "mainbang,"definingrange
zero. (b), A peakrepresenting
groundclutterat shortrange.(c) and (d), Two bird
targetsat more distant ranges.
178]
R. P. LarkinandL. Eisenberg
Bird-Banding
Spring 1978
digital versionof the conventionalA-scopedisplayof echostrengthas
a functionof slantrange. Sincethe trigger precedesthe outgoingradar
pulse ("main bang") in time, the values at the extreme left contain no
echoes.Becauseadjacentsamplesare taken only 250 txsecapart, the
shapeof the bird echoesis not distortedby the samplingtechnique.The
displayshownin Figure2 is generatedin real time and the operatorcan
compareit to the A-scopedisplayon the radar to verify that the VSU
is operating correctly.
QUANTITATIVE
RECORDS OF BIRD MIGRATION
Potentially,all the information availablefrom the conventionalradar
displays,locationof targetsin range,target echostrength,rangevelocity, and echo signature,is availablefrom the video samplingunit as
accuratenumerical quantities.These data can then be processedimmediatelyby a computer or microprocessorto provide information
which is more meaningfulthan is availablefrom a PPI or A-scopedisplay analyzedby hand.
As an exampleof what is possibleafter a modestamountof processing
is performed on the raw VSU data, Figure 3 illustratesthe passageof
two birds through a stationaryconicalradar beam on a night of heavy
fall migration, 11 November 1976. Severalsimpleoperationswere performed to produceFigure 3. On the day previous,a recordof several
VSU scanshad been taken, averaged,and stored for later use. This
stored record provided a sampleof the radar noiseand clutter at the
radar site.Then similarrecordswere taken during the period of nocturnal passefinemigration and the noise record was subtractedfrom
each,providinga distribution-free(Vogel et al., 1975) meansof identifying the bird targets.The data were then scaledto arbitrary values
0-9 in order to constructthe printed imageof target height and range
as a function of time in Figure 3. (Figure 2 is a record of one scan
occurringabout second6 in Figure 3.)
Ground clutter is presentat extremelycloserangeseven after the
noisesubtractionoperation;nearby vegetationprobablybecamemore
reflectiveat night becauseof dew formation. Two birds appear and
disappeargraduallyasthey fly through the verticalbeam,not changing
in range sincethey are presumablyflying nearly level. The closerbird
is either a much smaller target than the farther bird or is passing
through the edge of the beam rather than its center.
By taking advantageof the flexibilityafforded by the computercontrol overthe VSU, it will be possibleto gathercompletelydifferent kinds
of data than thoseillustratedin Figures2 and 3. For example,the VSU
canbe usedwith a trackingradar in a signatureanalysismodeto gather
dataon the wingbeatsof bird targetsand analyzethem usingfrequencydomaintechniques,withoutthe phaselags,filtering,and nonlinearities
usuallyinherent in data taken from automaticgain control circuits.In
this mode, the computercan conducta sequentialscanof all or part of
the beam,locatingone or severaltargetsof interest.It can then sample
Vol.40,No.2
Detecting
BirdEchoes
_
[ 179
1221
111
111
Ground clutter
11
111 /
11
Birds
1111
_
111
•
121
121
221
2321
1432
13431
234421
23432
23321
11121
11121
4-
I0-
11111
11111
1111
1212
11221
•
I00
121
1121
13441
12421
13552
13542
145521
245531
124&521
245521
1231
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2442
2321
12431
13442
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122
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11
11
11111
11121
11111
1211
1111
11221
11111
21111
8-
•
11
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21
I 131
12232
11232
111132
6-
•
1
357741
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1357741
134542
24642
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11
1
•
200
i
300
i
400
i
500
Range (rn)
i
600
1.
700
I
800
•
F•OURE
3. A printed recordof two birds passingthrough a stationaryradar beam.The
VSU, running continuously,obtaineda scanapproximatelyevery qt6sec.Each tbur
consecutive
scanswere averagedtogetherto generatea line approximatelyevery ¬
sec,starting at the top of the figure. On the horizontal axis, each two adjacentrange
bins were averagedtogetherso that a characterrepresents15 m range. Where the
radar video signalwasbelow the averagenoise(measuredpreviouslyat each range
intervalby the VSU), the record is blank. Signalsabovethis thresholdare indicated
by a number correspondingto the echo height in arbitrary linear units 1-9.
at high rates (up to the PRF) near each target, recording amplitude
fluctuationsas a function of time. Since the data stored represent absoluteecho strengths,the task of correlatingtarget sizewith wingbeat
characteristics
would becomemanageableinsteadof extremelylaborious.
The use of devicessuch as the VSU will allow significantimprovementsin radar ornithology.Large amountsof precisedata can be gathered and analyzedquickly, insteadof requiring monthsof tediouscatalogingand scoringof photographs.In order to gainsuchimprovements,
180]
R. P. LarkinandL. Eisenberg
Bird-Banding
Spring 1978
the investigatormustinvestin someelectronichardware(minicomputer
or microcomputerplus VSU) and must further investtime in program-
mingthe computerto controlthe VSU in conjunctionwith a givenradar
unit. But the continuingincreasein popularityand decreasein priceof
small computerswill make the initial investmenteasier in the future.
SUMMARY
Mostof the presentlyavailablemethodsfor takingornithologicaldata
from radar displaysare time-consuming
and only partly quantitative.A
deviceis describedthat costsunder $2,000 and allowsthe videosignal
from any pulsedradar to be transferreddirectly to a computeror microprocessor.All the information available from conventional radar
A-scopedisplaysmay be derivedfrom the device,termed a Video Sampling Unit, as accurate numerical quantities. With appropriate
programming,distributions
of bird altitudesand sizes,wingbeatanalysis
and other informationrelevantto bird migrationmay be obtained.
ACKNOWLEDGMENTS
The constructionof the VSU wassupportedby a grant from the Mary
Flagler Cary Trust of Millbrook, NY. We thank William Kocsisfor technical assistance
and Donald R. Griffin for commentson the manuscript.
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CITED
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TheRockefeller
University,
NewYork,NY 10021. Received4 August 1977,
accepted24 January 1978.