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Sapienza Space debris Observatory Network (SSON): A high coverage infrastructure for space debris monitoring
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Sapienza Space debris Observatory Network (SSON): A high coverage infrastructure for space debris monitoring Shariar Hadji Hossein a,∗, Marco Acernese b, Tommaso Cardona a, Giammarco Cialone a, Federico Curianò a, Lorenzo Mariani a, Veronica Marini a, Paolo Marzioli b, Leonardo Parisi b, Fabrizio Piergentili b, Fabio Santoni c a b c
S5LAB, Sapienza Space Systems and Space Surveillance Laboratory, Sapienza University of Rome, via Eudossiana 18, 00184 Rome, Italy DIMA – Department of Mechanical and Aerospace Engineering, Sapienza University of Rome, via Eudossiana 18, 00184 Rome, Italy DIAEE – Department of Astronautical, Electrical and Energy Engineering, Sapienza University of Rome, via Eudossiana 18, 00184 Rome, Italy
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Keywords: Space situational awareness Space surveillance Astronomy Space debris Observatory network
a b s t r a c t Optical observations represent a passive method for space debris tracking and monitoring. Although being constrained to limited time intervals, e.g. when the target is in sunlight and the observatory is in darkness, a debris observatory network distributed over multiple locations can improve the observational interval and favor the data integration for more consistent and significant results. The Sapienza Space Systems and Space Surveillance Laboratory (S5Lab) At Sapienza University of Rome has established Sapienza Space debris Observatory Network (SSON), an international network of optical observatories addressed at debris surveillance. The network is composed of six observatories owned and controlled by Sapienza, plus five observational sites controlled by collaborators (mainly University of Michigan and the University of Bern). The network operations have been carried out during several observations campaign, including the Tiangong-1 space station re-entry campaign performed in support of the Inter-Agency space Debris Committee (IADC). The heterogeneous capabilities of the different observatories represent an advantage for acquiring a wider set of debris monitoring data with different techniques. A strong synergy between our institution and the Italian specialized industry has also revealed to be very productive in the context of the network establishment. The present paper will describe the SSON infrastructure and the opportunities, improvements and future perspectives for research institutions or space industry of this wide observatories network will be discussed.
1. Introduction The Sapienza Space debris Observatory Network (SSON) , see Fig. 1, consists in eleven optical observatories distributed in four continents: Europe, Africa, North and South America, allowing to cover approximately 34 of the sky vault simultaneously, thus being capable of intercepting space debris ranging from LEO (Low Earth Orbit) to Geostationary Earth Orbit (GEO). This network is composed of two parts. The biggest one developed and controlled by the S5LAB (Sapienza Space Systems and Space Surveillance Laboratory) research team, which has several years of hands-on experience in satellite systems manufacturing and launch [1–3] and space surveillance. The second one controlled by collaborating institutions in different countries. The key feature of the network is represented by the achievable high precision and low installation cost. The applied methods of observation,
∗
collection and processing of data have been matured and optimized in a decade of research in the field of space debris. The present paper is divided into three sections. In the first one the SSON network observatories, owned by the S5LAB, will be described in detail. In the second one, the observational methodologies and technology, with focus on the achievable results in terms of data typology and accuracy will be described. In the last and final section, the reports on data analysis methodologies with regards to the extrapolation about orbital prediction data and achieved results within an example of observational campaign will be shown. 2. Network infrastructure The SSON is a network of optical astronomic observatories and one radio telescope. All the observatories are completely remotely controllable.
Corresponding author. E-mail address: [email protected] (S. Hadji Hossein).
https://doi.org/10.1016/j.jsse.2019.11.001 Received 14 June 2019; Received in revised form 9 November 2019; Accepted 27 November 2019 Available online xxx 2468-8967/© 2019 International Association for the Advancement of Space Safety. Published by Elsevier Ltd. All rights reserved.
Please cite this article as: S. Hadji Hossein, M. Acernese and T. Cardona et al., Sapienza Space debris Observatory Network (SSON): A high coverage infrastructure for space debris monitoring, Journal of Space Safety Engineering, https://doi.org/10.1016/j.jsse.2019.11.001
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Fig. 1. SSON, location of observatories.
The instrumentation selection has been carefully carried out in order to have different optical tubes, mounts and CCDs (Charge-Coupled Device) for each observatory. This feature is aimed at acquiring data with different FoVs (Field of View), thus optimizing the joint observational campaigns completed with a multiplicity of observatories. The available instrumentation mainly relies in Commercial-Off-TheShelf (COTS) components in order to reduce costs, improve the replacement and repair times, and to increasingly accurate predictive precision. This network is inherently very versatile [6], since able both to perform tracking of targets with known TLEs (Two Line Elements) considerably improving their accuracy [7] and to operate a Space Survey [8] addressed to sectors of the celestial vault that are, notoriously, of great scientific and commercial interest. The observational capability of the network core is extended by the collaborating observatories data, who provide data in the framework of observational campaigns or joint vault sectors surveys. In the next section the observatories owned by the S5LAB, divided in Equatorial and Italian Networks, will be described.
Acronyms/Abbreviation ADR ALT–AZ ASI BSC CCD CMOS CNC DEC DIMA EduScope EQUO-OG EQUO-OS FoV GPS IADC INAF LEO MITO NORAD RA RESDOS S5LAB SCUDO SSON TLE
Active Debris Removal Altitude–Azimuth Italian Space Agency Broglio Space Center Charge-Coupled Device Complementary Metal-Oxide Semiconductor Computer Numerical Control Declination Department of Mechanical and Aerospace Engineering Educational Telescope Equatorial Observatory on Ground Equatorial Observatory Off-Shore Field of View Global Positioning System Inter-Agency space Debris Committee National Institute of Astro-Physics Low Earth Orbit Mid latitude Italian Observatory North American Aerospace Defense Command Right Ascension Remote Space Debris Observation System Sapienza Space Systems and Space Surveillance Laboratory Sapienza Coupled University Debris Observatory Sapienza Space debris Observatory Network Two Line Elements
2.1. Equatorial network The creation of a network of high coverage observatories has, as basic requirement, the installation of observatories at equatorial latitudes. Indeed, in addition to the coverage of a large amount of satellites in polar orbit, equatorial latitudes allow to observe geostationary satellites with a high elevation angle. The Equatorial Network consists of two observatories placed in two observation sites both in Kenya, at ASI’s (Agenzia Spaziale Italiana) Broglio Space Center: EQUO-OG (Equatorial Observatory – On Ground) located at the BSC (Broglio Space Center) base camp in Malindi coast, and EQUO-OS (Equatorial Observatory Off-shore) , located on the Santa Rita off-shore platform in Ngomeni Bay. Every observatory is provided with an autonomously professional weather station and an all-sky camera with the aim of monitoring the meteorological evolution of the observation site in real time as protection of the observatory in case of adverse weather.
The network core is composed of six observatories owned by S5LAB, four located in Italy, from now on referred as Italian Network [4], and two in Kenya, from now on referred as Equatorial Network [5]. The observatories can operate in a totally automated way by using both third party software and bespoke software. 2
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Fig. 3. EQUO-OS, located in Malindi Kenya, at the ASI Broglio Space Center.
Fig. 2. EQUO-OG, located in Malindi Kenya, at the ASI Broglio Space Center.
These observatories, in terms of precipitations quantity, frequency and for the sudden weather changes that influence the area climate, observe in difficult environmental conditions, and for this reason, are controlled by two military-grade computers, with the electrical connections coated with polymeric materials with high resistance to atmospheric agents. 2.1.1. EQUO-OG EQUO-OG (see Fig. 2), is the main observatory of equatorial scopes with a 200 mm mirror optical tube, is installed on an ALT–AZ Mount. This observatory allows to perform stereometric data acquisition campaigns with the Italian network [9]. Indeed, when commanding simultaneous observation of at least on Italian observatory and EQUO-OG, it is possible to derive the target orbital parameters and altitude with higher precision, the higher is the ground distance between the observatories (commonly referred as “baseline”) [10,11]. It is totally remotely controlled. 2.1.2. EQUO-OS EQUO-OS (see Fig. 3) is the twin observatory of EQUO-OG, with a 152 mm mirror optical tube. It is installed on an ALT–AZ mount and operate with a CMOS (Complementary Metal-Oxide Semiconductor sensor) camera in order to obtain video observation. Due to its position, on the Off-shore launch platform “Santa Rita”, about six kilometers from the Malindi coast, one of the challenges during installation was the connection to the Internet. The observatory was connected to the internet by installing a pair of high gain dish antennas with a highly polarized beacon, allowing a radio internet connection between the base and the platform.
Fig. 4. RESDOS, located in Urbe Airport of Rome.
2.2.1. RESDOS RESDOS is the main telescope of the Laboratory observation network (depicted in Fig. 4). With a carbon fiber optical tube in Ritchey-Chrétien configuration and a diameter of the primary ventilated mirror of 40 cm, it is an essential instrument for tracking satellites and space debris. It can ensure high performance and reliability by presenting a complete remote controllability. Thanks to its high-performance German equatorial mount with a go-to speed of over 6°/s, it allows to track even the smallest and fastest debris analyzing it with a CCD with sensor 1024 × 1024 pixels (~1,0 Megapixel) 13.3 × 13.3 mm (177 mm2 ). RESDOS is provided with a motorized roto-focuser which allows the instrument focus to automatically compensate the atmospheric temperature changes.
2.2. Italian network The Italian network consists in four observatories displaced in central Italy, it is constantly and continuously updated. Three of the four Italian observatories are in Rome: EduScope (Educational Telescope) is located on the rooftop of the DIMA (Department of Mechanical and Aerospace Engineering) building at Sapienza University of Rome, while RESDOS (Remote Space Debris Observation System) and MITO are placed in Urbe airport of Rome, in two separate and autonomous domes. SCUDO is located in the mountain town of Collepardo, in the region of Lazio.
2.2.2. MITO MITO (see Fig. 5) is totally remotely controlled, it works together with the RESDOS to refine acquired measurements. Equipped with a 20 cm mirror tube in Schmidt-Cassegrain configuration, mounted on German-style mount motorized with encoder for "fine" aiming. 3
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Fig. 7. EduScope located in DIMA Building, at “La Sapienza” University of Rome.
Fig. 5. MITO located in Urbe Airport of Rome.
mentioned networks, in case of particularly important observational campaigns, there is the possibility to resort to the help of Telescopes from other collaborating institutions. Four optical observatories located in Loiano (Italy) owned by INAF (National Institute of Astro-Physics), Ann Arbor (Michigan, USA) and Cerro Tololo (Chile) owned by University of Michigan, Bern (Switzerland) owned by Astronomy Institute of Bern University and a radio telescope, located in Bonn (Germany) owned by Fraunhofer-FHR institute. These institutions are collaborating with the S5LAB research team as part of the SSON network.
3. Observational methodologies Fig. 6. SCUDO, located in Collepardo, Italy.
The observational campaigns are usually aimed both at improving the orbit determination and at reconstructing the attitude of the targets [12]. Indeed, while the optical observations are constrained to peculiar lighting conditions (the observable shall be in sunlight while the observatory is in darkness) and weather, the acquirable data can improve the orbit and attitude determination of satellites and space debris. While the orbit determination mainly relies in the target identification, in the angular celestial coordinates monitoring and in the integration of multiple observations to derive an improved TLE [7], the attitude determination aims at identifying the variation of the light flux associated to the target in order to predict its attitude with respect to the velocity vector direction [13]. When determining the satellite attitude, an estimation of the object surface exposed to the aerodynamic drag (which greatly affects the orbital parameters of LEO objects) can improve the trajectory (and orbital decay) predictions of all the in-orbit objects [14]. Moreover, the optical attitude determination can be significant in the framework of future ADR (Active Debris Removal) missions, which will need a precise estimation of non-cooperative objects rotational state for assuring the removal completion. The space survey and the tracking of known objects have as their main aim to be able to determine both the attitude of the debris or of the satellite, and to operate a correct orbit determination. The SSON network is able to acquire data both as images and videos of the observed targets. The orbital determination is usually completed only by utilizing images taken to the target. Attitude reconstruction of space debris can rely both on images and videos. In fact, although it is possible to obtain a good approximation of the attitude of the target by using only images of several consecutive orbital passages from several observatories, the best approximation is achieved by merging the aforementioned data with what obtainable from a video analysis. The two observational methods are reported below.
It mounts a CCD with a FoV of almost 2° allowing a "wide angle" vision of the observables. 2.2.3. SCUDO SCUDO (see Fig. 6) is the ultimately completed observatory of the S5LAB network. It was designed with the aim of simultaneously acquiring images and videos during the observations; it is provided with an ALT–AZ mount, which has the capability to install two optical tubes, both are equipped with 200 mm mirror. The observatory is completely remotely controlled and connected to the Internet, and it can operate autonomously. 2.2.4. EduScope EduScope (see Fig. 7) is the observatory dedicated to students. The S5LAB research team periodically organizes sessions dedicated to students to make them familiar with the tools used for research in the field of space debris. EduScope taking on an important role in hand-over between under graduate and new students willing to join the laboratory research team. It is provided by a Newtonian 20 cm optical tube installed on an ALT–AZ mount. Images are taken by a professional CCD Camera. EduScope is the only observatory of the network without Remote controllability. The telescope is not usually applied to observational campaigns, but it may be occasionally utilized as support telescope. 2.3. Collaborating observatories Due to the need of improving the coverage of the celestial vault and to the related necessary network extension to locations outside the afore4
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Fig. 9. Processed image (noise removal filter).
Fig. 8. Example tracklet, with the background stellar field, obtained by the sidereal tracking technique applied during a survey.
3.1. Photographic observations Photographic observation is the main activity carried out by SSON, representing about 80% of the data collected from the network. The photographic data derived from two consequential steps that the S5LAB has refined to chase space debris: • The Space Survey, to try to detect unknown debris. • The Tracking of known objects. Both tasks provide same type of photography [15]. As reported before, every space debris, when exposed to the Sun, reflects a part of the received light radiation; If part of the radiation is reflected towards the Earth and, at the same time, the on-ground zone is in a darkness, then the object will be visible from ground. Its apparent magnitude will decrease (i.e. with increased luminosity) with the increase of the reflected light portion. Space survey is the first step for the research of debris: S5LAB has developed an algorithm which allows an automatic sky survey [16] (Fig. 8) coordinated with other network observatories [17]. Each telescope, according to its geographical position take a certain number of images, pointing to a sector of the celestial vault in which a potential debris are illuminated by sun while the observation site is in darkness. Depending on FoV it is covered a sky sector of probable interest in the time of one night. The telescope starts to maintain the “sidereal tracking”, in order to make the stellar field appear as fixed in the taken images. The images are acquired with exposures times between 1 and 3 s. Then, the mount points the telescope towards the following portion of sky sector of interest. This way observatory can collects hundreds images every night. The prolonged exposure associated with sidereal tracking, allows to filter all objects in visibility obtaining a streak resulting like the one in Fig. 9, called tracklet. The algorithm processes each image separately, transforming it in bitmap and applying a noise removal filter (Fig. 10). To separate targets and stars in the obtained, image is applied an edges detector process, based on a Canny algorithm [18] and each pixel is analyzed to identify the pixels belonging to a star or a tracklet. The A pixel by pixel analysis is performed by using a standard Connectedcomponent labeling algorithm [19]. The result is a detection and discrimination of targets (Fig. 11) from stars (Fig. 12) in FoV.
Fig. 10. Sky survey flowchart.
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Fig. 13. Example of tracking algorithm.
Fig. 11. Detected targets.
entirely by the telescope FoV or only in part. If the strip of light appears to come out of the FoV boundary, the exposure time is reduced. Obtained photos, are related on more orbital passages of the same object and are used to better refine the orbital prediction through analysis of light curves [24]. 3.2. Video observations As explained in the previous paragraph, the orbital parameters of a known target are easily obtained from the TLEs. These allow to estimate the orbit, the RA and the DEC and therefore the velocity of the target. The software developed by S5LAB receive in input the TLEs (released by NORAD or obtained from the space survey) of the target and thanks to an algorithm which, as shown in Fig. 13, receive in input target’s orbit, derived from TLEs, calculates its real position at a certain time and using a for cycle provide a continuous GO-TO commands to the telescope mount to move the alt–altazimuth axes to follow and keep the target cantered for the entire passage. From the video produced during the tracking procedure it is possible to derive the error in RA and DEC intrinsic to the TLE and based on the relative magnitude trend as a function of time, we obtain a light curve, whose analysis allows to calculate attitude of the target knowing some constructive characteristics [24]. Having a congruous number of light curves analysis related to several passages taken at the same time from more observatories, it is possible to determine, with an acceptable approximation, the structure and dynamics of the target. [25]
Fig. 12. Detected stars.
Once the star pattern has been solved [20,21], by associating images to the GPS (Global Positioning System) UTC timestamp of the acquisition time, it is possible retrieve the celestial coordinates of the detected object [22] with an error standard deviation of approximately 5 arcseconds. The accuracy of the optical setup has been characterized by comparing the retrieved optical measurements with publicly available precision orbit ephemerides. Once, for a give object, both the precise ephemerides, assumed to represent the true state, and a sufficiently large number of optical measurements have been retrieved, it is possible to estimate the standard deviation of the measurement error. On the base of the analyzed data, it is possible to derive an approximate TLE of the target [23]. After a comparison with the NORAD (North American Aerospace Défense Command) database, either the identity of the observed object is determined, or the target is recognized as unknown. If unknown objects are detected, a new observation is scheduled to track such objects the following night. The tracking command is based on the TLE estimations that can be achieved on the base of the optically determined celestial coordinates stand-alone. By reconstructing the TLE of an unknown object it is possible to propagate its orbit through an SGP4 (Simplified General Perturbations 4) [23] routine and to know the date, the time and the exact altazimuth coordinates of its successive passages over the observation site. In this way, through a GO-TO command (i.e. a command that determines the azimuth and elevation of a telescope), it is possible to point the telescope to the precise area in which the target will pass. In this case, images are acquired with exposures times that, depending on the orbit, can vary from 1 to 20 s, depends on case if the tracklet is framed
4. Methods of data analysis and extrapolation of results Once images are obtained, they can be processed to retrieve the celestial coordinates of the captured objects [22]. These are computed by providing the raw images as input to an astrometry software developed by the S5LAB [16] and thus solving the star field [20,21]. Since times of start and stop of exposure are known with errors in the order of 1 millisecond, it is possible to associate the obtained coordinates to precise times. The measurements, which are constituted by the object angular coordinates in terms of either topocentric right ascension–declination or azimuth–elevation, can be used to improve the dynamic state estimate through an orbit determination process [23]. Different programs have been developed by the S5LAB to perform orbit determination [16,17]. It is possible to improve the accuracy of the state estimate [12] also in the space of TLE parameters. As a result, it is possible to use TLE improvement for further space surveillance analyses. Moreover, in order to understand the observed target attitude, the actual acquired light curve is compared to a synthesized light-curve that is generated by simulation [24]. The synthesized attitude considers the geometric properties of the observable. The analysis is aimed at minimizing, in several iterations, the differences between the actual and the synthesized light curves, in 6
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Fig. 14. Observing days from Italian network.
7. Conclusions
order to gather more information on the observed spacecraft in-orbit rotation [13]. The obtainable results can be of key importance during re-entry campaigns, in order to improve the re-entry time prediction precision [26].
The SSON is composed of optical telescopes and radio telescopes totally remotely controlled. It is consisting of two parts, one owned by the S5LAB research group and the other owned by independent universities and institutions, which work in close collaboration and are able to trace most of the existing space debris in conditions of solar visibility. The network allows observation of known targets, such as satellites and rocket bodies, and unknown objects through a series of long exposure photographs that can capture the light streaks due to the solar reflection on the target. Stereo-metric observation from multiple observatories allows to obtain more precise orbital parameters of known targets. The analysis of the light curves, obtained from the video observations, allows to know the attitude, permitting to improve the orbital prediction of the target. When observing unknown targets, from a single observatory, it is possible to determine a first approximation TLE and then track it for the following passes, as already mentioned for the usual known targets schedule. The whole network well-functioning has been verified with several observation campaigns, including the Tiangong-1 re-entry campaign performed in support to the IADC (Inter-Agency space Debris Committee) in 2018. The network, with a low installation and operational cost, manages to obtain good results in the field of space debris surveillance and to improve the orbital parameters of most of the in-orbit objects.
5. Past participations in observational campaigns Over the years, the search for new methodologies and the refinement of the techniques in use have allowed us to participate to a great number of observational campaigns aimed at determining the dynamics and kinematics of known satellites, among which we remember the missions: Thor Agena, Skymed1, Envisat, Idefix Ariane 42P, Cosmos 482, of which in Fig. 14 the number of observing days from the Italian network is given as an example. Between the end of 2017 and the beginning of 2018 when, by providing observational data, SSON actively participated in one of the most important observational campaigns ever carried out in Italy, related to the uncontrolled re-entry of the Chinese station Tiangong 1 [26], where due to a loss of the attitude maintenance systems, the Chinese space station quickly began to degrade its orbit with the concrete danger that some parts of it, surviving the re-entry, could affect the Italian territory.
6. Ongoing and future operational task The deployment of a network of telescopes like SSON has an intrinsic potential for expansion and growth. The S5LAB has the operational task for the next few years, that of being able to obtain total coverage of the earth’s orbital space. In future, a network with this peculiarity, could increase the accuracy of orbital prediction of a target through a continuous monitoring of its orbit of a target, carried out in relay by several telescopes [27] The main goals of S5LAB project are essentially two:
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The activity related to the projects EQUO is supported by the Italian Space Agency, in the framework of the ASI-Sapienza Agreement for the Broglio Space Center in Malindi (Accordo Attuativo della Convenzione Quadro n. 2013-079-C.0). RESDOS observatory was acquired thanks to the funding support of Fondazione Roma in the framework of the “RemoteLab” project.
1) Accurately determine the geographical area of re-entry for satellites or large space debris. 2) Increase more the resolution on debris trajectory so that is possible to provide, to satellites operator, alert notifications in the event of probable collision with space debris or other uncontrolled satellites. 7
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Contents lists available at ScienceDirect
Journal of Space Safety Engineering journal homepage: www.elsevier.com/locate/jsse
Sapienza Space debris Observatory Network (SSON): A high coverage infrastructure for space debris monitoring Shariar Hadji Hossein a,∗, Marco Acernese b, Tommaso Cardona a, Giammarco Cialone a, Federico Curianò a, Lorenzo Mariani a, Veronica Marini a, Paolo Marzioli b, Leonardo Parisi b, Fabrizio Piergentili b, Fabio Santoni c a b c
S5LAB, Sapienza Space Systems and Space Surveillance Laboratory, Sapienza University of Rome, via Eudossiana 18, 00184 Rome, Italy DIMA – Department of Mechanical and Aerospace Engineering, Sapienza University of Rome, via Eudossiana 18, 00184 Rome, Italy DIAEE – Department of Astronautical, Electrical and Energy Engineering, Sapienza University of Rome, via Eudossiana 18, 00184 Rome, Italy
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Keywords: Space situational awareness Space surveillance Astronomy Space debris Observatory network
a b s t r a c t Optical observations represent a passive method for space debris tracking and monitoring. Although being constrained to limited time intervals, e.g. when the target is in sunlight and the observatory is in darkness, a debris observatory network distributed over multiple locations can improve the observational interval and favor the data integration for more consistent and significant results. The Sapienza Space Systems and Space Surveillance Laboratory (S5Lab) At Sapienza University of Rome has established Sapienza Space debris Observatory Network (SSON), an international network of optical observatories addressed at debris surveillance. The network is composed of six observatories owned and controlled by Sapienza, plus five observational sites controlled by collaborators (mainly University of Michigan and the University of Bern). The network operations have been carried out during several observations campaign, including the Tiangong-1 space station re-entry campaign performed in support of the Inter-Agency space Debris Committee (IADC). The heterogeneous capabilities of the different observatories represent an advantage for acquiring a wider set of debris monitoring data with different techniques. A strong synergy between our institution and the Italian specialized industry has also revealed to be very productive in the context of the network establishment. The present paper will describe the SSON infrastructure and the opportunities, improvements and future perspectives for research institutions or space industry of this wide observatories network will be discussed.
1. Introduction The Sapienza Space debris Observatory Network (SSON) , see Fig. 1, consists in eleven optical observatories distributed in four continents: Europe, Africa, North and South America, allowing to cover approximately 34 of the sky vault simultaneously, thus being capable of intercepting space debris ranging from LEO (Low Earth Orbit) to Geostationary Earth Orbit (GEO). This network is composed of two parts. The biggest one developed and controlled by the S5LAB (Sapienza Space Systems and Space Surveillance Laboratory) research team, which has several years of hands-on experience in satellite systems manufacturing and launch [1–3] and space surveillance. The second one controlled by collaborating institutions in different countries. The key feature of the network is represented by the achievable high precision and low installation cost. The applied methods of observation,
∗
collection and processing of data have been matured and optimized in a decade of research in the field of space debris. The present paper is divided into three sections. In the first one the SSON network observatories, owned by the S5LAB, will be described in detail. In the second one, the observational methodologies and technology, with focus on the achievable results in terms of data typology and accuracy will be described. In the last and final section, the reports on data analysis methodologies with regards to the extrapolation about orbital prediction data and achieved results within an example of observational campaign will be shown. 2. Network infrastructure The SSON is a network of optical astronomic observatories and one radio telescope. All the observatories are completely remotely controllable.
Corresponding author. E-mail address: [email protected] (S. Hadji Hossein).
https://doi.org/10.1016/j.jsse.2019.11.001 Received 14 June 2019; Received in revised form 9 November 2019; Accepted 27 November 2019 Available online xxx 2468-8967/© 2019 International Association for the Advancement of Space Safety. Published by Elsevier Ltd. All rights reserved.
Please cite this article as: S. Hadji Hossein, M. Acernese and T. Cardona et al., Sapienza Space debris Observatory Network (SSON): A high coverage infrastructure for space debris monitoring, Journal of Space Safety Engineering, https://doi.org/10.1016/j.jsse.2019.11.001
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Fig. 1. SSON, location of observatories.
The instrumentation selection has been carefully carried out in order to have different optical tubes, mounts and CCDs (Charge-Coupled Device) for each observatory. This feature is aimed at acquiring data with different FoVs (Field of View), thus optimizing the joint observational campaigns completed with a multiplicity of observatories. The available instrumentation mainly relies in Commercial-Off-TheShelf (COTS) components in order to reduce costs, improve the replacement and repair times, and to increasingly accurate predictive precision. This network is inherently very versatile [6], since able both to perform tracking of targets with known TLEs (Two Line Elements) considerably improving their accuracy [7] and to operate a Space Survey [8] addressed to sectors of the celestial vault that are, notoriously, of great scientific and commercial interest. The observational capability of the network core is extended by the collaborating observatories data, who provide data in the framework of observational campaigns or joint vault sectors surveys. In the next section the observatories owned by the S5LAB, divided in Equatorial and Italian Networks, will be described.
Acronyms/Abbreviation ADR ALT–AZ ASI BSC CCD CMOS CNC DEC DIMA EduScope EQUO-OG EQUO-OS FoV GPS IADC INAF LEO MITO NORAD RA RESDOS S5LAB SCUDO SSON TLE
Active Debris Removal Altitude–Azimuth Italian Space Agency Broglio Space Center Charge-Coupled Device Complementary Metal-Oxide Semiconductor Computer Numerical Control Declination Department of Mechanical and Aerospace Engineering Educational Telescope Equatorial Observatory on Ground Equatorial Observatory Off-Shore Field of View Global Positioning System Inter-Agency space Debris Committee National Institute of Astro-Physics Low Earth Orbit Mid latitude Italian Observatory North American Aerospace Defense Command Right Ascension Remote Space Debris Observation System Sapienza Space Systems and Space Surveillance Laboratory Sapienza Coupled University Debris Observatory Sapienza Space debris Observatory Network Two Line Elements
2.1. Equatorial network The creation of a network of high coverage observatories has, as basic requirement, the installation of observatories at equatorial latitudes. Indeed, in addition to the coverage of a large amount of satellites in polar orbit, equatorial latitudes allow to observe geostationary satellites with a high elevation angle. The Equatorial Network consists of two observatories placed in two observation sites both in Kenya, at ASI’s (Agenzia Spaziale Italiana) Broglio Space Center: EQUO-OG (Equatorial Observatory – On Ground) located at the BSC (Broglio Space Center) base camp in Malindi coast, and EQUO-OS (Equatorial Observatory Off-shore) , located on the Santa Rita off-shore platform in Ngomeni Bay. Every observatory is provided with an autonomously professional weather station and an all-sky camera with the aim of monitoring the meteorological evolution of the observation site in real time as protection of the observatory in case of adverse weather.
The network core is composed of six observatories owned by S5LAB, four located in Italy, from now on referred as Italian Network [4], and two in Kenya, from now on referred as Equatorial Network [5]. The observatories can operate in a totally automated way by using both third party software and bespoke software. 2
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Fig. 3. EQUO-OS, located in Malindi Kenya, at the ASI Broglio Space Center.
Fig. 2. EQUO-OG, located in Malindi Kenya, at the ASI Broglio Space Center.
These observatories, in terms of precipitations quantity, frequency and for the sudden weather changes that influence the area climate, observe in difficult environmental conditions, and for this reason, are controlled by two military-grade computers, with the electrical connections coated with polymeric materials with high resistance to atmospheric agents. 2.1.1. EQUO-OG EQUO-OG (see Fig. 2), is the main observatory of equatorial scopes with a 200 mm mirror optical tube, is installed on an ALT–AZ Mount. This observatory allows to perform stereometric data acquisition campaigns with the Italian network [9]. Indeed, when commanding simultaneous observation of at least on Italian observatory and EQUO-OG, it is possible to derive the target orbital parameters and altitude with higher precision, the higher is the ground distance between the observatories (commonly referred as “baseline”) [10,11]. It is totally remotely controlled. 2.1.2. EQUO-OS EQUO-OS (see Fig. 3) is the twin observatory of EQUO-OG, with a 152 mm mirror optical tube. It is installed on an ALT–AZ mount and operate with a CMOS (Complementary Metal-Oxide Semiconductor sensor) camera in order to obtain video observation. Due to its position, on the Off-shore launch platform “Santa Rita”, about six kilometers from the Malindi coast, one of the challenges during installation was the connection to the Internet. The observatory was connected to the internet by installing a pair of high gain dish antennas with a highly polarized beacon, allowing a radio internet connection between the base and the platform.
Fig. 4. RESDOS, located in Urbe Airport of Rome.
2.2.1. RESDOS RESDOS is the main telescope of the Laboratory observation network (depicted in Fig. 4). With a carbon fiber optical tube in Ritchey-Chrétien configuration and a diameter of the primary ventilated mirror of 40 cm, it is an essential instrument for tracking satellites and space debris. It can ensure high performance and reliability by presenting a complete remote controllability. Thanks to its high-performance German equatorial mount with a go-to speed of over 6°/s, it allows to track even the smallest and fastest debris analyzing it with a CCD with sensor 1024 × 1024 pixels (~1,0 Megapixel) 13.3 × 13.3 mm (177 mm2 ). RESDOS is provided with a motorized roto-focuser which allows the instrument focus to automatically compensate the atmospheric temperature changes.
2.2. Italian network The Italian network consists in four observatories displaced in central Italy, it is constantly and continuously updated. Three of the four Italian observatories are in Rome: EduScope (Educational Telescope) is located on the rooftop of the DIMA (Department of Mechanical and Aerospace Engineering) building at Sapienza University of Rome, while RESDOS (Remote Space Debris Observation System) and MITO are placed in Urbe airport of Rome, in two separate and autonomous domes. SCUDO is located in the mountain town of Collepardo, in the region of Lazio.
2.2.2. MITO MITO (see Fig. 5) is totally remotely controlled, it works together with the RESDOS to refine acquired measurements. Equipped with a 20 cm mirror tube in Schmidt-Cassegrain configuration, mounted on German-style mount motorized with encoder for "fine" aiming. 3
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Fig. 7. EduScope located in DIMA Building, at “La Sapienza” University of Rome.
Fig. 5. MITO located in Urbe Airport of Rome.
mentioned networks, in case of particularly important observational campaigns, there is the possibility to resort to the help of Telescopes from other collaborating institutions. Four optical observatories located in Loiano (Italy) owned by INAF (National Institute of Astro-Physics), Ann Arbor (Michigan, USA) and Cerro Tololo (Chile) owned by University of Michigan, Bern (Switzerland) owned by Astronomy Institute of Bern University and a radio telescope, located in Bonn (Germany) owned by Fraunhofer-FHR institute. These institutions are collaborating with the S5LAB research team as part of the SSON network.
3. Observational methodologies Fig. 6. SCUDO, located in Collepardo, Italy.
The observational campaigns are usually aimed both at improving the orbit determination and at reconstructing the attitude of the targets [12]. Indeed, while the optical observations are constrained to peculiar lighting conditions (the observable shall be in sunlight while the observatory is in darkness) and weather, the acquirable data can improve the orbit and attitude determination of satellites and space debris. While the orbit determination mainly relies in the target identification, in the angular celestial coordinates monitoring and in the integration of multiple observations to derive an improved TLE [7], the attitude determination aims at identifying the variation of the light flux associated to the target in order to predict its attitude with respect to the velocity vector direction [13]. When determining the satellite attitude, an estimation of the object surface exposed to the aerodynamic drag (which greatly affects the orbital parameters of LEO objects) can improve the trajectory (and orbital decay) predictions of all the in-orbit objects [14]. Moreover, the optical attitude determination can be significant in the framework of future ADR (Active Debris Removal) missions, which will need a precise estimation of non-cooperative objects rotational state for assuring the removal completion. The space survey and the tracking of known objects have as their main aim to be able to determine both the attitude of the debris or of the satellite, and to operate a correct orbit determination. The SSON network is able to acquire data both as images and videos of the observed targets. The orbital determination is usually completed only by utilizing images taken to the target. Attitude reconstruction of space debris can rely both on images and videos. In fact, although it is possible to obtain a good approximation of the attitude of the target by using only images of several consecutive orbital passages from several observatories, the best approximation is achieved by merging the aforementioned data with what obtainable from a video analysis. The two observational methods are reported below.
It mounts a CCD with a FoV of almost 2° allowing a "wide angle" vision of the observables. 2.2.3. SCUDO SCUDO (see Fig. 6) is the ultimately completed observatory of the S5LAB network. It was designed with the aim of simultaneously acquiring images and videos during the observations; it is provided with an ALT–AZ mount, which has the capability to install two optical tubes, both are equipped with 200 mm mirror. The observatory is completely remotely controlled and connected to the Internet, and it can operate autonomously. 2.2.4. EduScope EduScope (see Fig. 7) is the observatory dedicated to students. The S5LAB research team periodically organizes sessions dedicated to students to make them familiar with the tools used for research in the field of space debris. EduScope taking on an important role in hand-over between under graduate and new students willing to join the laboratory research team. It is provided by a Newtonian 20 cm optical tube installed on an ALT–AZ mount. Images are taken by a professional CCD Camera. EduScope is the only observatory of the network without Remote controllability. The telescope is not usually applied to observational campaigns, but it may be occasionally utilized as support telescope. 2.3. Collaborating observatories Due to the need of improving the coverage of the celestial vault and to the related necessary network extension to locations outside the afore4
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Fig. 9. Processed image (noise removal filter).
Fig. 8. Example tracklet, with the background stellar field, obtained by the sidereal tracking technique applied during a survey.
3.1. Photographic observations Photographic observation is the main activity carried out by SSON, representing about 80% of the data collected from the network. The photographic data derived from two consequential steps that the S5LAB has refined to chase space debris: • The Space Survey, to try to detect unknown debris. • The Tracking of known objects. Both tasks provide same type of photography [15]. As reported before, every space debris, when exposed to the Sun, reflects a part of the received light radiation; If part of the radiation is reflected towards the Earth and, at the same time, the on-ground zone is in a darkness, then the object will be visible from ground. Its apparent magnitude will decrease (i.e. with increased luminosity) with the increase of the reflected light portion. Space survey is the first step for the research of debris: S5LAB has developed an algorithm which allows an automatic sky survey [16] (Fig. 8) coordinated with other network observatories [17]. Each telescope, according to its geographical position take a certain number of images, pointing to a sector of the celestial vault in which a potential debris are illuminated by sun while the observation site is in darkness. Depending on FoV it is covered a sky sector of probable interest in the time of one night. The telescope starts to maintain the “sidereal tracking”, in order to make the stellar field appear as fixed in the taken images. The images are acquired with exposures times between 1 and 3 s. Then, the mount points the telescope towards the following portion of sky sector of interest. This way observatory can collects hundreds images every night. The prolonged exposure associated with sidereal tracking, allows to filter all objects in visibility obtaining a streak resulting like the one in Fig. 9, called tracklet. The algorithm processes each image separately, transforming it in bitmap and applying a noise removal filter (Fig. 10). To separate targets and stars in the obtained, image is applied an edges detector process, based on a Canny algorithm [18] and each pixel is analyzed to identify the pixels belonging to a star or a tracklet. The A pixel by pixel analysis is performed by using a standard Connectedcomponent labeling algorithm [19]. The result is a detection and discrimination of targets (Fig. 11) from stars (Fig. 12) in FoV.
Fig. 10. Sky survey flowchart.
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Fig. 13. Example of tracking algorithm.
Fig. 11. Detected targets.
entirely by the telescope FoV or only in part. If the strip of light appears to come out of the FoV boundary, the exposure time is reduced. Obtained photos, are related on more orbital passages of the same object and are used to better refine the orbital prediction through analysis of light curves [24]. 3.2. Video observations As explained in the previous paragraph, the orbital parameters of a known target are easily obtained from the TLEs. These allow to estimate the orbit, the RA and the DEC and therefore the velocity of the target. The software developed by S5LAB receive in input the TLEs (released by NORAD or obtained from the space survey) of the target and thanks to an algorithm which, as shown in Fig. 13, receive in input target’s orbit, derived from TLEs, calculates its real position at a certain time and using a for cycle provide a continuous GO-TO commands to the telescope mount to move the alt–altazimuth axes to follow and keep the target cantered for the entire passage. From the video produced during the tracking procedure it is possible to derive the error in RA and DEC intrinsic to the TLE and based on the relative magnitude trend as a function of time, we obtain a light curve, whose analysis allows to calculate attitude of the target knowing some constructive characteristics [24]. Having a congruous number of light curves analysis related to several passages taken at the same time from more observatories, it is possible to determine, with an acceptable approximation, the structure and dynamics of the target. [25]
Fig. 12. Detected stars.
Once the star pattern has been solved [20,21], by associating images to the GPS (Global Positioning System) UTC timestamp of the acquisition time, it is possible retrieve the celestial coordinates of the detected object [22] with an error standard deviation of approximately 5 arcseconds. The accuracy of the optical setup has been characterized by comparing the retrieved optical measurements with publicly available precision orbit ephemerides. Once, for a give object, both the precise ephemerides, assumed to represent the true state, and a sufficiently large number of optical measurements have been retrieved, it is possible to estimate the standard deviation of the measurement error. On the base of the analyzed data, it is possible to derive an approximate TLE of the target [23]. After a comparison with the NORAD (North American Aerospace Défense Command) database, either the identity of the observed object is determined, or the target is recognized as unknown. If unknown objects are detected, a new observation is scheduled to track such objects the following night. The tracking command is based on the TLE estimations that can be achieved on the base of the optically determined celestial coordinates stand-alone. By reconstructing the TLE of an unknown object it is possible to propagate its orbit through an SGP4 (Simplified General Perturbations 4) [23] routine and to know the date, the time and the exact altazimuth coordinates of its successive passages over the observation site. In this way, through a GO-TO command (i.e. a command that determines the azimuth and elevation of a telescope), it is possible to point the telescope to the precise area in which the target will pass. In this case, images are acquired with exposures times that, depending on the orbit, can vary from 1 to 20 s, depends on case if the tracklet is framed
4. Methods of data analysis and extrapolation of results Once images are obtained, they can be processed to retrieve the celestial coordinates of the captured objects [22]. These are computed by providing the raw images as input to an astrometry software developed by the S5LAB [16] and thus solving the star field [20,21]. Since times of start and stop of exposure are known with errors in the order of 1 millisecond, it is possible to associate the obtained coordinates to precise times. The measurements, which are constituted by the object angular coordinates in terms of either topocentric right ascension–declination or azimuth–elevation, can be used to improve the dynamic state estimate through an orbit determination process [23]. Different programs have been developed by the S5LAB to perform orbit determination [16,17]. It is possible to improve the accuracy of the state estimate [12] also in the space of TLE parameters. As a result, it is possible to use TLE improvement for further space surveillance analyses. Moreover, in order to understand the observed target attitude, the actual acquired light curve is compared to a synthesized light-curve that is generated by simulation [24]. The synthesized attitude considers the geometric properties of the observable. The analysis is aimed at minimizing, in several iterations, the differences between the actual and the synthesized light curves, in 6
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Fig. 14. Observing days from Italian network.
7. Conclusions
order to gather more information on the observed spacecraft in-orbit rotation [13]. The obtainable results can be of key importance during re-entry campaigns, in order to improve the re-entry time prediction precision [26].
The SSON is composed of optical telescopes and radio telescopes totally remotely controlled. It is consisting of two parts, one owned by the S5LAB research group and the other owned by independent universities and institutions, which work in close collaboration and are able to trace most of the existing space debris in conditions of solar visibility. The network allows observation of known targets, such as satellites and rocket bodies, and unknown objects through a series of long exposure photographs that can capture the light streaks due to the solar reflection on the target. Stereo-metric observation from multiple observatories allows to obtain more precise orbital parameters of known targets. The analysis of the light curves, obtained from the video observations, allows to know the attitude, permitting to improve the orbital prediction of the target. When observing unknown targets, from a single observatory, it is possible to determine a first approximation TLE and then track it for the following passes, as already mentioned for the usual known targets schedule. The whole network well-functioning has been verified with several observation campaigns, including the Tiangong-1 re-entry campaign performed in support to the IADC (Inter-Agency space Debris Committee) in 2018. The network, with a low installation and operational cost, manages to obtain good results in the field of space debris surveillance and to improve the orbital parameters of most of the in-orbit objects.
5. Past participations in observational campaigns Over the years, the search for new methodologies and the refinement of the techniques in use have allowed us to participate to a great number of observational campaigns aimed at determining the dynamics and kinematics of known satellites, among which we remember the missions: Thor Agena, Skymed1, Envisat, Idefix Ariane 42P, Cosmos 482, of which in Fig. 14 the number of observing days from the Italian network is given as an example. Between the end of 2017 and the beginning of 2018 when, by providing observational data, SSON actively participated in one of the most important observational campaigns ever carried out in Italy, related to the uncontrolled re-entry of the Chinese station Tiangong 1 [26], where due to a loss of the attitude maintenance systems, the Chinese space station quickly began to degrade its orbit with the concrete danger that some parts of it, surviving the re-entry, could affect the Italian territory.
6. Ongoing and future operational task The deployment of a network of telescopes like SSON has an intrinsic potential for expansion and growth. The S5LAB has the operational task for the next few years, that of being able to obtain total coverage of the earth’s orbital space. In future, a network with this peculiarity, could increase the accuracy of orbital prediction of a target through a continuous monitoring of its orbit of a target, carried out in relay by several telescopes [27] The main goals of S5LAB project are essentially two:
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The activity related to the projects EQUO is supported by the Italian Space Agency, in the framework of the ASI-Sapienza Agreement for the Broglio Space Center in Malindi (Accordo Attuativo della Convenzione Quadro n. 2013-079-C.0). RESDOS observatory was acquired thanks to the funding support of Fondazione Roma in the framework of the “RemoteLab” project.
1) Accurately determine the geographical area of re-entry for satellites or large space debris. 2) Increase more the resolution on debris trajectory so that is possible to provide, to satellites operator, alert notifications in the event of probable collision with space debris or other uncontrolled satellites. 7
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