Comparison of EISCAT Radar Data on Space Debris with Model Prediction by the MASTER Model of ESA

6 Pages·0.07 MB·English

by  M. Landgraf

PEDAS1-B1.4-0003-02
COMPARISON OF EISCAT RADAR DATA ON SPACE DEBRIS
WITH MODEL PREDICTIONS BY THE MASTER MODEL OF
ESA
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M.Landgraf1,R.Jehn1,andW.Flury1
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2 1ESA/ESOC,Robert-Bosch-Str. 5,64293Darmstadt,Germany

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3 ABSTRACT
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Intheefforttoobtain lowcost routine spacedebris observations inlowEarthorbit, ESAplanstoutilise theradar fa-
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v cilitiesoftheEuropeanIncoherent ScatterScientificAssociation. Firstdemonstration measurements wereperformed
0 from 11 to 23 February 2001. In total 16hours of radar signals were collected. Here we compare these initial mea-
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surementswiththepredictionsoftheESAMASTER/PROOF’99modelinordertoassessthesensitivityaswellasthe
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1 reliability ofthe data. Wefindthat while thedetermination ofobject size needs tobe reviewed, the altitude distribu-
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tion provides agood fittothemodel prediction. Theabsolute number ofobjects detected inthevarious altitude bins
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0 indicatesthatthecoherentintegrationmethodindeedincreasesthedetectionsensitivitywhencomparedtoincoherent
/ integration. In the data presented here integration times from 0.1 to 0.3s were used. As expected, orbit information
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p cannot be obtained from the measurements if they are linked to ionospheric measurements as planned. In addition
- routinespacedebrisobservations providealsousefulinformation forthevalidation oflarge-object catalogues.
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v INTRODUCTION
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X CurrentlyESAmonitorsthespacedebrispopulationinlowEarthorbit(LEO)inirregularintervals(Mehrholz etal.,
r 2002). This is done in the frame of 24 hour radar beam-park experiments. The data from these measurements are
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taken in order to constrain models of the population evolution of space debris in orbit (Klinkradetal., 2001). The
most prominent of the European space debris models is MASTER/PROOF’99,which allows spacecraft operators to
assess the risk of adebris impact for their mission. Asthe 24 hour experiments only provide a snapshot of the envi-
ronment, thetemporalbehaviour ofthespace debrispopulation isdifficulttodetermine. Routinemonitoring ofLEO
is required in order to better constrain the debris models. The most suitable technique for monitoring LEO is radar
detection byhigh-power, largeaperturefacilities.
In order to obtain inexpensive access to such observations, ESA started in 2000 a study with the Sodankyla¨
Geophysical Observatory (SGO) to use the radar facilities of the European Incoherent SCATter (EISCAT)scientific
association. These facilities operate up to 2000 hours per year for ionospheric research purposes. In the study, the
SGO team was able to demonstrate the exploitation of ionospheric measurements for space debris observations. A
comparison oftheretrieved, stillpreliminary, datawiththeESAMASTER/PROOF’99modelispresented here.
THEEISCATFACILITYATTROMSØ
Thespacedebrismeasurements wereperformedintheframeofademonstration campaigninFebruary2001. A
◦ ′ ◦ ′
radar facility located at 69 35 N and 19 14 E near Tromsø, Norway, was used. The UHF radar at Tromsø operates
at a frequency of 931MHz with a pulse repetition frequency of 200kHz. The transmitter can transmit peak powers
upto1.5MWthrough themainantenna, butitwasrestricted tojustbelow1MWduring themeasurement campaign.
◦ ′
The35mantennawaspointedparalleltothefieldlinesofthelocalgeo-magneticfield,azimuth183 18 andelevation
◦ ′
77 6. During the test campaign two different phase codes called TAU2and CP1LTwere transmitted. Of the TAU2
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data1.2hourshavebeenanalysedaswellas2.8hoursoftheCP1LTdata. Forbothexperiments thepulselengthwas
about0.5ms. TherangewindowoftheCP1LTexperimentcoveredrangesfrom400to1,400kmwithagapbetween
700and800km,andTAUcoveredrangesfrom500to1,800km withagapbetween800and1,100km.
CATALOGUEDOBJECT
The strongest of the high-altitude TAU2 events during the test campaign began at 22:19:06 UTC, 20 Febru-
ary 2001. We identified the target as a large catalogued object, a Tsyklon upper stage with the COSPARdesignator
1994-11G. According to the catalogue the object has a total mass of 1,390kg and is cylindrical in shape, with a di-
ameterof2.7m,aheight of2.2m,andaradarcrosssection of8.3m2. Theorbitalinformation intheTLEcatalogue,
withTLEepochonedaybefore theexperimentepoch, letsuspredict thatitshould havepassedtheradarbeamatthe
◦ ′
off-axis angular distance of 1 16. Figure 1 shows the analysis summary plot of the event. The analysis was done
using0.27scoherentintegration.
Tau2 20-Feb-2001 22:19:06
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42.7
2
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ati
R
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g
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1
10
0 2 4 6 8 10 12 14 16 18 20
Time (s)
1535
1530 R = 0.010 t2 - 1.42 t + 1530
1525
m)1520 EISCAT
k
(
R 1515
1510
Catalogue
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1500
0 2 4 6 8 10 12 14 16 18 20
Time (s)
-800
-1000 v = 27.1 t - 1493
r
)
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-s
m -1200
(
v r
-1400
-1600
0 2 4 6 8 10 12 14 16 18 20
Time (s)
Fig. 1. Event at 22:19:06, 20 February 2001, compared against the catalogue. The top panel shows a quantity
proportional to the signal to noise power ratio. The middle panel shows the measured slant range (small circles), a
parabolicfit,andthecatalogueprediction(largecircles). ThebottompanelshowsthemeasuredDopplervelocity(small
circles),alinearfit,andthecatalogue predictionfortherangerate(largecircles).
ThetoppanelofFigure1showsaquantityproportionaltothesignaltonoisepowerratioduringthebeamtransit.
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The antenna side-lobe structure is clearly visible. Markers indicate those scans where the ratio was larger than the
detectionthresholdof4.5. Theoff-centreangulardistanceofthepasswasfittothedata(Markkanen etal.,2002). The
′
best fit was achieved by assuming that the transit occurred 31 off-axis. It seems difficult to reconcile the predicted
◦ ′ ′
offset1 16 withtheinferred valueof31.
The middle and bottom panels of Figure 1 show the measured slant range and Doppler velocity. The solid
dark curves represent quadratic and linear fits to the data points. The large dots represent the predicted values. The
measured range is about 7km larger than predicted. The slope of the velocity curve is as predicted, but there is
a discrepancy of about 0.1kms−1 in the actual velocity values. The circular-orbit, vertical-beam acceleration is
27.5ms−2,whichisconsistentwiththevalue27.1ms−2 fromthevelocityfit. Thetimingaccuracyforthecatalogued
objects is of the order of 10s, while we believe the measured timing to be accurate to within about 0.1s. However,
therangeandvelocity discrepancies cannotberemovedsimplybyadjusting therelativetiming,because therequired
correction wouldbeabout sixseconds fortherangedata, butonly about threeseconds fortheDoppler data. Asmall
misalignmentoftheantennacan,however,explaintheobserveddeviation.
Thisexample of the detection ofalarge catalogued object bythe EISCATradar demonstrates the usefulness of
routinespacedebrisobservations forthevalidation oflarge-object catalogues.
SMALL-SIZEDEBRIS
Whilecollisionsofoperationalspacecraftwithlargecataloguedobjectsliketheupperstagediscussedabovecan
be avoided by manoeuvring the spacecraft, objects smaller than 10cm, that normally are not found in the catalogue,
pose a serious threat to manned and unmanned spacecraft. Complex spacecraft like the International Space Station
Alphaareshielded fromimpactsofobjectssmallerthanabout1cm. Thus,thepopulation inthesizerangebetween1
and10cmisparticularly important toobserve.
InthedemonstrationcampaignperformedbySGO,56objectsinthesizerangeabove1cmweredetectedwithin
the3hoursofanalyseddata. Themeasureddistributioninsizeandaltitudedistributionsdiscussedbelowarecompared
tothepredictionbytheESAMASTER/PROOF’99model. Duetothepointingoftheradarbeamalongthelocalgeo-
magneticfieldline,orbitalparameterscouldnotbedetermined fromthemeasurements. Thepredicted distribution of
orbitalelementsisdiscussed belowanyhow.
SizeDistribution
Figure2showsthedistributionofobjectdiameter,asdeterminedbytheanalysisofthereflectedsignalamplitude,
aswellastheprediction bytheMASTER/PROOF’99model. Onlyobjectsdetectedintherangewindowfrom800to
1,400kmhavebeenconsidered.
Fig. 2. Distribution of object sizes. The prediction of the absolute number of beam-crossing objects as a function of
objectsizebytheMASTER/PROOF’99modelisshownasthedarkgrayhistogram,thelightgrayhistogramshowsthe
detectable objects, according to the model, and the distribution of sizes of actually detected objects is shown as the
diamondsymbols.
Under the assumption of incoherent integration, objects larger than about 7cm are detectable reliably in the
range window between 800 and 1,400km. Whilethe majority ofthe objects predicted tocross the beam aresmaller
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than 2cm, sizes of 2 to 3cm have been derived for the objects actually detected in the experiment. From the figure
it is evident that while no object with a derived size larger than 6cm has been detected, more than 20 of those large
objectsshouldhavecrossedthebeamaccordingtothepredictionbyMASTER/PROOF’99. Also,thenumberof3cm
objects is three times higher than predicted, while the total number of detected objects is in agreement. Apparently,
thesizesderivedfortheobjectslargerthan10cmhavebeenunderestimated. Becauseitisunknownwheretheobjects
crossthebeampattern, theEISCATradarscanonlygivealowerboundfortheradarcrosssection. Consequently, the
translation ofthemeasuredradarcrosssectiontoobjectdiametersystematically underestimates theobjectsize.
AltitudeDistribution
The distribution of orbital altitudes of detected objects is shown in Figure 3. At the altitudes of maximum
debris density between 850 and 950km, 12 objects were detected. According to the MASTER/PROOF’99 model,
the predicted number of objects detectable in this altitude bin by incoherent integration is below 8, significantly
less than actually detected. This is also true for all but the altitude bin around 1,200km. The sensitivity of the
radar appears to be higher than for the standard incoherent detection method. This can be explained by the fact
that the detection algorithm exploits the phase information by coherently integrating the detected signal over 0.1s.
MASTER/PROOF’99 can only simulate radar detection with incoherent integration. Since the detection algorithm
usedbytheEISCATteamisbasedonacoherent integration technique, thepredicted numberofobjectsislowerthan
theEISCATdata.
Fig. 3. Distribution of orbital altitudes. The prediction of the absolute object number as a function of orbital altitude
bytheMASTER/PROOF’99modelisshownasthegrayhistogram. Thelightgraybarsgivethenumberofobjectsthat
wouldbedetectable by theTromsø radar ifanincoherent detection method wasused. Thedark gray part ofeach bar
indicates thepredicted number ofobjects undetectable bythismethod. Thedistribution oforbital altitudes ofdetected
spacedebris objectisshownbythediamond symbols.
InclinationDistribution
Assuming circular orbits, the inclination ofanobject’s orbit canintheory bederived from theDoppler velocity
measured by the radar. However, the functional dependence of the Doppler velocity on the orbit inclination varies
withtheorientation oftheradarbeam. AscanbeseeninFigures4and5themappingfromDopplervelocitytoorbit
◦ ◦
inclination is ambiguous for an antenna azimuth around 180 (South). An easterly pointing (antenna azimuth 90 )
willprovideanunambiguous relationship betweenthemeasuredDopplervelocity andorbitinclination.
As the EISCAT transmitter at Tromsø was pointed parallel to the geo-magnetic field lines, which follow the
north-south direction, orbitinclinations cannotbederivedfromthedataobtainedduringthemeasurementcampaign.
UsingthePROOFapplicationoftheMASTER/PROOF’99model,wecanhoweverpredicttheinclinationdistribution
◦
expectedforthemeasurementcampaign. ThepredictionisshowninFigure6. Itcanbeseenthatinclinationsfrom60
◦
to110 arecoveredbybeam-parkexperiments withthetransmitteratTromsø. Thiscoversdebriscreatedinclassical
Sun-synchronous orbitsaswellasdebriscreatedbymostlaunches fromPlesetsk.
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-0.4
FGAN (50.6 deg lattitude)
at 60 deg latitude
at 70 deg latitude
-0.6
-0.8
e (km/s) -1
at
R
ge -1.2
n
a
R
-1.4
-1.6
-1.8
55 60 65 70 75 80 85 90 95 100
Inclination (deg)
Fig. 4. Mapping oforbitalinclination toDopplervelocityforasouthernantennapointing.
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FGAN (50.6 deg lattitude)
at 60 deg latitude
at 70 deg latitude
1.5
1
e (km/s) 0.5
at
R
ge 0
n
a
R
-0.5
-1
-1.5
55 60 65 70 75 80 85 90 95 100
Inclination (deg)
Fig. 5. Mapping oforbitalinclination toDopplervelocityforaneasternantennapointing.
CONCLUSIONANDOUTLOOK
The test campaign performed by SGO resulted in raw data on one large space debris object as well as in the
size and altitude distribution of 55 small space debris objects. The comparison with the ESA MASTER/PROOF’99
modelshowsthattheconversionfromsignalamplitudestoobjectsizeshastobereviewed. Themorestraightforward
measurementoftheobjects’altitudeprovidesadistributionthatiswellinaccordwiththepredictionbythemodel. As
the model is regularly checked against results from other beam-park experiments, we can conclude that the demon-
strationmeasurementswiththeEISCATtransmitteratTromsøprovidesreliablemeasurements. Theabsolutenumber
of detections is significantly larger than the predicted number when assuming incoherent integration as a detection
technique. Thus,thecoherent integration ofthereceivedsignalindeedincreases thedetection sensitivity.
As expected, we find that, when linked to ionospheric measurements, we will not obtain orbital information
using theEISCATradars. Thisrestriction can, however, easily beremovedifdedicated observation timeisacquired.
Inthiscaseanoptimumantennapointing willallowustodetermineatleastDopplerinclinations, whichareidentical
totrue orbit inclinations for circular orbits. AsthePROOFtool ofthe newMASTER/PROOF-2001model allowsto
predict the distribution of Doppler inclinations, this would allow us to constrain the statistical orbit properties of the
small-sizeLEOspacedebrispopulation.
In summary we find that the measurements performed during the demonstration campaign in February 2001
provethevalueofspacedebrisdataobtainedbyexploitingionospheric measurements. Themereamountofdata,that
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Fig. 6. Distribution oforbitalinclinations aspredictedbytheMASTER/PROOF’99model.
willbeavailable incase simultaneous operations with ionospheric measurements intheorder of1000hours peryear
canbeperformed, provides animportant contribution totheunderstanding oftheevolution ofLEOspacedebris.
In order to realise measurements that operate simultaneously with the ionospheric investigations, a real-time
detectionmethodhastobedeveloped,becausethevolumeofdatathatwouldbeneededtorecordtherawsignalfrom
the radar for 1000hours per year is prohibitive. With real-time detection only the object parameters as well as the
signalofthepassagewillberecorded. Astudytodevelopreal-timemeasurementswillbestartedbytheendof2002.
REFERENCES
Klinkrad, H., J. Bendisch, K. D. Bunte, H. Krag, H. Sdunnus, and P. Wegener, The MASTER-99Space Debris and
MeteoroidEnvironment Model. Adv.SpaceRes.,28(9),1355–1366, 2001.
Markkanen, J.,M.Lehtinen, A.Huuskonen, andV.Va¨a¨na¨nen, Measurements ofSmall-SizeDebriswithBackscatter
ofRadioWaves. Finalreport,Sodankyla¨ Geophysical Observatory, 2002.
Mehrholz, D.,L.Leushacke, D.Banka,Beam-ParkExperimentsatFGAN,thisissue.