The main purpose of the “Pi of the Sky” system is to investigate short timescale astrophysical phenomena (particularly gamma-ray bursts, optical transients and variable stars). Wide field, short exposures and full automation of the system, together with effective algorithms, give good prospects for effective identification of space debris elements. These objects can be a great danger for current and future space missions, and should be continuously monitored and cataloged. Algorithms for identifying optical transients (OT), designed for the “Pi of the Sky” experiment enable moving objects like planes, satellites and space debris elements to be identified. The algorithm verifies each OT candidate against a database of known satellites and is also able to automatically self-identify moving objects not present in this database. The data collected by the prototype in the Las Campanas Observatory enabled us to obtain a large sample of observations of moving objects. Some of these objects were identified as high-orbit geostationary (GEO) satellites, which shows that it is possible to observe even distant satellites with small aperture photo lenses. The analysis of the sample is still going on. The preliminary results and algorithms for automatic identification of moving objects will be described here

Majczyna, A.

Należyty, M.

Sokołowski, M.

Wajer, P.

Wawrzaszek, R.

Acta Polytechnica

English

CTU Open Journal Systems (Czech Technical University, Prague / České vysoké učení technické v Praze)

Acta Polytechnica Vol. 51 No. 2/2011Searching for Space Debris Elements with the “Pi of the Sky” SystemM. Sokolowski, M. Nalez˙yty, A. Majczyna, R. Wawrzaszek, P. WajerAbstractThe main purpose of the “Pi of the Sky” system is to investigate short timescale astrophysical phenomena (particularlygamma-ray bursts, optical transients and variable stars). Wide ﬁeld, short exposures and full automation of the system,together with eﬀective algorithms, give good prospects for eﬀective identiﬁcation of space debris elements. These objectscan be a great danger for current and future space missions, and should be continuously monitored and cataloged.Algorithms for identifying optical transients (OT), designed for the “Pi of the Sky” experiment enable moving objectslike planes, satellites and space debris elements to be identiﬁed. The algorithm veriﬁes each OT candidate against adatabase of known satellites and is also able to automatically self-identify moving objects not present in this database.The data collected by the prototype in the Las Campanas Observatory enabled us to obtain a large sample of observationsof moving objects. Some of these objects were identiﬁed as high-orbit geostationary (GEO) satellites, which shows thatit is possible to observe even distant satellites with small aperture photo lenses. The analysis of the sample is still goingon. The preliminary results and algorithms for automatic identiﬁcation of moving objects will be described here.Keywords: space debris, robotic telescopes, satellite observations, satellite tracking, optical transients.1 IntroductionSpace debris consists of objects originally launchedby humans, which now orbit the Earth but are nolonger in use. They can be upper stages of rock-ets, dead satellites, engine modules of geostationarysatellites, remnants from satellite collisions, etc. Col-lisions of these elements moving at a speed of sev-eral km/s can destroy active satellites and can in theworst case be dangerous for astronauts on missions(spacecraft or space stations). This problem wasspectacularly demonstrated in reality on 12 Febru-ary 2009, when the Irridium 33 and Cosmos 2 251satellites collided over northern Siberia at relativespeeds of about 11 km/s. The crash produced thou-sands of new pieces of space junk, which can bedangerous for other satellites. Such elements shouldbe cataloged and continuously monitored to enablesatellites and spacecraft to avoid them.Most space debris elements populate Low EarthOrbits (LEO), but the number of objects near to geo-stationary orbit (GEO) is constantly growing. Thereare about 13 000 artiﬁcial objects in the Earth’s or-bit, fewer than 800 of which are active satellites,while more than 12,000 are space debris elementslarger than 10 cm. In principle, the only way toavoid collisions with potentially dengerous space de-bris elements (larger than 1 cm in size) is by ma-noeuvering spacecraft. However, in order to do this,space junk elements must be continuously monitoredand cataloged. Eﬀorts are being undertaken by na-tional and international space agencies to discoverand monitor space debris elements. The largest ex-isting catalog of objects in the Earth orbit is theSpace Object Catalog, provided by NORAD (NorthAmerican Aerospace Defense Command), which isbased on a network of optical and radio telescopesof various apertures. Systems like NORAD typicallywork in two modes. In wide-ﬁeld mode they can dis-cover new space debris elements, which are later pre-cisely tracked by narrow ﬁeld instruments. Long termnarrow mode observations enables precise determi-nation of orbital parameters. The orbital elementsare stored in the Two Line Elements (TLE) formatwhich, besides identiﬁers, consists of ten parametersfully describing the object’s orbit [1]. There are alsoinitiatives in Europe to create a system similar toNORAD. One of the ideas is the Space SituationalAwareness program (SSA). The ﬁrst stage of such asystem is the European Space Surveillance System(ESSS) [2], which was proposed for automatic detec-tion and identiﬁcation of space debris pieces, and fordetermining orbital elements. It will track objectsfrom LEO orbits and predict their movements. Nat-ural candidates to join such a system are robotic tele-scopes. These instruments work automatically withalmost no human attention. They also perform auto-matic data analysis, which is expected in the ESSS.In many cases they run algorithms very similar tothose required for discovering space debris. Examplesof European telescopes already tracking elements inthe Earth orbit are TAROT (FOV of 2◦ × 2◦) andStarbrook (FOV of 10◦ × 6◦) [2].Another component of this system will be wide-ﬁeld optical systems like “Pi of the Sky”. The pur-pose of this study is to verify the ability of the “Pi103Acta Polytechnica Vol. 51 No. 2/2011Parameter Prototype Finalfocal length 85 mm 85 mmfocal ratio 1.2 1.2CCD Sensor brand Fairchild STACCD size 2 048× 2 048 px 2 048× 2 048 pxFOV 20◦ × 20◦ 1.5–2 steradiansCCD pixel size 15× 15 μm 15× 15 μmtime of exposure 10 s 10 srelative accuracy of 20 ms 20 msshutter synchronizationlimiting magnitude 12m 12mfor stationary objectsaverage accuracy of astrometry 10arcsec 10arcsec# of CCD cameras 2 2× 12 (goal 2× 16)# of mounts 1 2× 3 (goal 2× 4)Fig. 1: Basic parameters of “Pi of the Sky” telescopes. The image on the left shows the prototype system installed inthe Las Campanas Observatory in Chile between 2004 and 2010of the Sky” system to observe and automatically dis-cover objects in the Earth’s orbit (satellites or spacedebris).2 “Pi of the Sky” systemThe “Pi of the Sky” system [3, 4] was origi-nally designed for observing short timescale opticalevents, particularly optical counterparts of gamma-ray bursts (GRBs), optical transients, and to monitorvariable objects (stars, blazars etc.). The system isremotely controlled, fully automatic, to a large degreeautonomous, and performs automatic data analysis.Special algorithms were designed and developed inorder to automatically discover optical transients anddiscover new objects in the sky [5, 6, 4]. The ﬁnalversion of the system is currently under construction.The prototype system was installed in the Las Cam-panas Observatory in Chile (Figure 1) between 2004and 2010. The prototype consists of two cameras,which observe the same ﬁeld in the sky and collectimages in synchronized mode. Besides its primaryscientiﬁc goals, the data from the prototype has al-lowed to study the potential of the system for discov-ering and identifying space debris. The ﬁnal systemwill have a substantially larger FOV (the ﬁnal goalis 2 steradians), which will allow the system to lookfor new pieces of space debris more eﬀectively (moreinformation can be found in [3]). The data from theprototype veriﬁed the system capabilities, and fur-ther developed the algorithms and software tools forspace debris oriented analysis. Figure 1 gives a sum-mary of the parameters of the prototype and the fullversion of the system.3 Satellites in “Pi of the Sky”dataThe algorithms designed for identifying the short op-tical transients in the “Pi of the Sky” data are de-scribed in detail in [6]. The on-line algorithm analy-ses images while they are being collected by the cam-era, and looks for new objects appearing in the sky.In the ﬁrst step, the algorithm ﬁnds objects which ap-pear in the new image, but were not present in previ-ous images. After this step, a list of ﬂash candidatesis created, then further cuts are applied in order toreject the background (mostly cosmic rays hits, hotpixels, sky background, star ﬂuctuations etc.). Oneof the most important cuts at this stage is the coinci-dence of two cameras, requiring the optical transientto be visible in two cameras. This cut eliminatescosmic rays striking one of the CCD chips. Afterthe coincidence cut, a list of real ﬂashes from thesky is obtained. Most of them are due to movingobjects, e.g. planes, satellites, and perhaps space de-bris elements. The primary goal of the algorithm isto identify OTs from natural astrophysical sources,so such objects must be identiﬁed, ﬂagged, excludedfrom the sample of OT candidates, and saved to logﬁles. Then another algorithm can simply be used toanalyze the objects in detail and look for space debriselements.In order to reject most of these events, databasesof orbital elements in Two Lines Element (TLE) for-mat are retrieved from the Internet (mainly from theNORAD website) every evening. They are combinedinto a single large database containing ≈ 13 000 or-104Acta Polytechnica Vol. 51 No. 2/2011Distance to closest satelliteEntries  9660Mean     8346RMS      4750Distance from event to closest satellite [arcsec]0 2000 4000 6000 8000 10000 12000 14000 16000 18000Number of events       210 = 1800’’satRNight 20070526MCDistance to closest satellite : night 20070526 and MCFig. 2: The image on the left shows the distribution of the distance from a ﬂash event candidate to the closest satellitefrom the catalog, for events found by the coincidence algorithm during night of 2007.05.26/27. For comparison, distancesfrom randomly generated ﬂashes to the nearest satellite are shown as red dots — the combinatorial background is nicelyreproduced. For the purposes of this analysis, the signal threshold was decreased to have more accidental coincidences.The image on the right shows the distribution of the distances from real data zoomed in the range [0, 3000] arc sec onlyFig. 3: Five subsequent 10s exposures showing fast, low orbit satellite SL-12 observed by the “Pi of the Sky” prototypeFig. 4: Five successive 10s exposures showing geostationary satellite XM-4 BLUES observed by the “Pi of the Sky”prototype. The satellite is a large object, with solar panels exceeding 10 mbital elements. For each image, the positions of allsatellites in the database are calculated (using thepredict package [7]), each ﬂash candidate is veriﬁedand, if it is closer than Rsat = 1 000 arc sec fromany of the satellites, it is ﬂagged and excluded fromfurther OT analysis. This rejection radius was de-termined according to the distribution of angulardistances from ﬂashes to the closest satellite fromthe database, which is clearly peaked around zero(Figure 2).The red dots on the plot represent the distance distri-bution for randomly generated ﬂashes to the closestsatellite from the catalog, and illustrate the size ofthe combinatorial background.The satellites identiﬁed according to the TLEdatabase are mostly LEO and GEO satellites. Im-ages of low orbit satellite SL-12 are shown in Fi-gure 3. There are also many geostationary satellitesin the sample (mostly large, with solar panels ex-ceeding 10 m). However, these observations showthat it is possible to observe even geostationary ob-jects with the small aperture (70 mm) instrument.Images of geostationary satellite XM-4 BLUES areshown in Figure 4.The orbital element databases are not complete,and many satellites are not included in them. In or-der to identify them, event candidates are examinedagainst track criteria. There are currently two pro-cedures for track identiﬁcation:• Normal track — tries to form a track out ofevents from diﬀerent images, it ﬁnds slowly mov-ing satellites (like the geostationary satellite in105Acta Polytechnica Vol. 51 No. 2/2011CCDX0 200 400 600 800 100012001400160018002000CCDY     0200400600800100012001400160018002000normal_tracks.lstCCDX0 200 400 600 800 100012001400160018002000CCDY     0200400600800100012001400160018002000single_camera_tracks.lstFig. 5: Tracks identiﬁed by the two algorithms described in the text during the same night. The image on the left shows160 normal tracks and the image on the right presents 40 single camera tracksCCDX0 200 400 600 800 100012001400160018002000CCDY     0200400600800100012001400160018002000track00010.lst     Track Info    −−−−−−−−−−−−−−−−−−−Frame = 847Point# = 435Line type = PParams : a=−0.00003399   b=−0.00004576   c=701.24591028Chi2 = 0.15Oper = UCCDX0 200 400 600 800 100012001400160018002000CCDY     0200400600800100012001400160018002000track00455.lst     Track Info    −−−−−−−−−−−−−−−−−−−Frame = 309Point# = 88Line type = LParams : a=49.10953497   b=−1.00000000   c=−67695.77024120Chi2 = 8.18Oper = UFig. 6: An example of a normal track with a ﬁtted parabola (left image) and a single camera track with a ﬁttedstraight line (right image)Figure 4). If it is possible to ﬁt a track to aset of events from diﬀerent or same images andthe velocity of the object is approximately con-stant, all events matching the track are ﬂaggedas moving objects. An example of 160 normaltracks ﬁtted during a single night is shown inFigure 5.• Single camera tracks — identify tracks us-ing events identiﬁed by a single camera. Thetracking starts with events from a single im-age, and requires at least 5 events in a track.If a new single camera track is identiﬁed, itis matched to earlier tracks of the same type.If they match each other, the earlier track is ex-tended by the new one and a single larger track isformed. This procedure rejects fast satellites (orplanes) which produce “long line” signatures ina single image (Figure 3). An example of 40 nor-mal tracks ﬁtted during a single night is shownin Figure 5.In the initial version only straight lines were ﬁt-ted. For many moving objects, however, FOV of20◦ × 20◦ is large enough to observe signiﬁcant cur-vature of the track. Thus, in order to make the ﬁtprocedure more eﬃcient, a parabolic curve is nowalso ﬁtted. If the track consists of at least 20 events106Acta Polytechnica Vol. 51 No. 2/2011Fig. 7: Example of a text ﬁle containing satellite observational dataand χ2par/χ2line < 2, parabolic ﬁt parameters are cho-sen. Examples of a straight line and parabolic tracksare shown in Figure 6. The eﬃciency of identifyingmoving objects by the normal track procedure wasstudied by checking how many of the satellites iden-tiﬁed in the TLE database were later identiﬁed inthe track procedure. The procedure is already veryeﬃcient and 99 % of objects that are moving objectsaccording to the TLE database were also identiﬁedby the normal track procedure.Software tools were created in order to obtain thepositions and times of all observations of the givenTLE-satellite. The observational data (right ascen-sion, declination, time and some additional informa-tion) is saved to text ﬁles (Figure 7). Optionally,the data may be saved to the database in order toallow for fast searches of multi-night observations ofsatellites. Php scripts were developed to access thedata via the web browser. For example, it extractsall observations of the given satellite (from multiplenights).The data was later used to ﬁt the orbital parame-ters. Two programs were tested: FindOrb [8] and thecustom developed orbfinder software. Preliminarytests show that it is possible to ﬁt orbital parametersusing “Pi of the Sky” data if at least 0.7–1.0 % of thewhole orbit is observed.4 ConclusionsThe data from the prototype “Pi of the Sky” systemwas used to study the capability of the system to dis-cover space debris elements. Preliminary estimatesshow that the system is able to observe at least halfof the objects from the NORAD database. It is evenpossible to observe large and bright satellites on thegeostationary orbit. Algorithms for automatic iden-tiﬁcation of moving objects were developed, testedand seem to be eﬃcient. The analysis is still goingon, but it seems that a wide ﬁeld system like “Pi ofthe Sky” could be used successfully for survey tasksfor discovering and calculating the orbit of large andeasy targets.AcknowledgementWe are very grateful to G. Pojmanski for giving ac-cess to the ASAS dome and for sharing his experiencewith us. We would like to thank the staﬀ of the LasCampanas Observatory for their help during instal-lation of the apparatus. This work was ﬁnanced bythe Polish Ministry of Science from 2005 until 2010as a research project.References[1] Website The Source for Space Surveillance Data,http://www.spacetrack.org/tle format.html[2] Klinkrad, H., et al.: Europe’s Eyes on the Skies —The Proposal for a European Space SurveillanceSystem, ESA Bulletin 133 (2008).[3] Z˙arnecki, A. F., et al.: Improving photometry ofthe Pi of the Sky, this journal.[4] Malek, K., et al.: Pi of the Sky Detector, Ad-vances in Astronomy, Volume 2010 (2010), Arti-cle ID 194946, 9 pages,http://www.hindawi.com/journals/aa/2010/194946.html[5] Sokolowski, M.: Investigation of astrophysicalphenomena in short time scales with Pi of theSky apparatus, PHD thesis, Institute for NuclearStudies, Jan 2008, http://arxiv.org/abs/0810.1179[6] Sokolowski, M., et al.: Automated Detectionof Short Optical Transients of AstrophysicalOrigin in Real Time, Advances in Astronomy,Vol. 2010 (2010), Article ID 463496, 11 pages,http://www.hindawi.com/journals/aa/2010/463496.html[7] Web page of Predict package,http://www.qsl.net/kd2bd/predict.html[8] Web page of FindOrb package,http://www.projectpluto.com/ﬁnd orb.htm107Acta Polytechnica Vol. 51 No. 2/2011M. SokolowskiE-mail: msok@fuw.edu.plThe Andrzej Soltan Institutefor Nuclear StudiesHoz˙a 69, 00-681 Warsaw, PolandM. Nalez˙ytyUniversity of Warsaw Astronomical ObservatoryAl. Ujazdowskie 4, 00-478 Warsaw, PolandA. MajczynaThe Andrzej Soltan Institutefor Nuclear StudiesHoz˙a 69, 00-681 Warsaw, PolandR. WawrzaszekSpace Research Center of thePolish Academy of SciencesBartycka 18A, 00-716 Warsaw, PolandP. WajerSpace Research Center of thePolish Academy of SciencesBartycka 18A, 00-716 Warsaw, Poland108

Searching for Space Debris Elements with the “Pi of the Sky” System

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