- Email: [email protected]
Status of CNES optical observations of space debris in geostationary orbit
Advances in Space Research 34 (2004) 1143–1149 www.elsevier.com/locate/asr
Status of CNES optical observations of space debris in geostationary orbit F. Alby
a,*
, M. Boer b, B. Deguine a, I. Escane a, F. Newland a, C. Portmann
c
a
b
CNES, 18 Avenue E. Belin, 31401 Toulouse, Cedex 4, France CESR-CNRS, 9 Avenue du Colonel-Roche, BP 4346, 31028 Toulouse, Cedex 4, France c LOGUS, 43 bd de Tournon, 83440 Montauroux, France
Received 19 October 2002; received in revised form 15 January 2003; accepted 21 January 2003
Abstract On-ground optical systems using a telescope and a CCD camera offer an effective solution to the problem of observing objects in geostationary orbit. CNES has been studying and developing such systems for several years with two main objectives: firstly to develop systems able to detect debris in the vicinity of the geostationary orbit for statistical evaluation of the population and secondly to develop a tool to determine the orbits accurately: these objectives are currently met using two different systems called TAROT and ROSACE. This paper presents the main characteristics of both systems, the principle of their image processing software, their development status and the main results obtained. Finally, perspectives for further developments and coupling of the two systems are presented. 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Space debris; Geostationary orbit; CNES optical observations
1. Introduction The geostationary orbit is becoming more and more cluttered as it offers numerous advantages for many applications. Currently, 900 objects of a size greater than 1 m have been counted close to this orbit. Precise knowledge of the satellites and debris in or near the geostationary arc is necessary to make informed decisions on the necessary steps to be taken to avoid debris impacts, and to verify the application of such decisions. Ground based telescopes offer a satisfactory solution to this knowledge acquisition problem due to the particularities of this orbit, namely the negligible movement of objects in relation to the Earth’s surface. The CNES has carried out a number of different observation experiments of satellites or debris in geostationary orbit over the last few years using ground based telescopes. These experiments have proven the feasibility of the concept and shown the advantages of *
Corresponding author. Tel.: +33-5-61-28-14-64; fax: +33-5-61-2831-82. E-mail address: [email protected] (F. Alby).
such systems. Currently, two complementary projects with different goals are being developed: • The objective of the TAROT project is to detect objects, satellites or debris of a certain minimum size located around the geostationary orbit. • The ROSACE project is concerned with the development of a precise means of determining the orbit of objects in the geostationary orbit. The prime objective of this article is to present the common principles of these two projects with regard to the data acquisition and calculation of the object localization. Then, the technical characteristics of the two systems will be described (hardware and software) as well as their performance and the main results obtained.
2. Common principles The two systems use small size telescopes fitted with a CCD camera located in the focal plane. Observations take place when the station is in the dark as the observed object is lit by the sun. In a geostationary orbit, the objects are practically immobile in relation to the Earth
0273-1177/$30 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2003.01.016
1144
F. Alby et al. / Advances in Space Research 34 (2004) 1143–1149
and the telescope remains pointed in a set direction for the exposure time. This makes it possible to detect small size objects as photons from the observed object are cumulated on the same pixel of the CCD array during the exposure time. These objects will appear in the form of dots or stains. Due to the rotation of the Earth, stars will, in contrast, appear in the form of tracks of a length dependent on the exposure time. The use of a star catalogue makes it possible to determine the telescope pointing to a high level of precision, and by calculating the direction of the observed object(s) in relation to the stars, it is possible to obtain elevation and azimuth position data for the observed object. With ROSACE, the need of greater accuracy has led to the use of a CCD camera shutter which closes at regular intervals during the exposure, giving dash-like star tracks. This process improves the determination of the position of stars and therefore measurement data accuracy (see Fig. 2).
3. Tarot system 3.1. General information The ‘‘Telescope a Action Rapide pour les Objets Transitoires’’ (TAROT-Bo€er et al., 1999) is installed on the Calern plateau above Grasse in the South of France and belongs to the CNRS (National Scientific Research Centre). The main goal of this telescope is to observe the optical counterparts of gamma ray bursts as part of a worldwide surveillance network. As such events are rare, TAROT is available for other missions that can be interrupted following an alert. 3.2. Description of the hardware TAROT is a small size telescope (25 cm aperture) with a certain number of specific features for debris tracking (see Fig. 3): • Wide field of view (2 2) that is useful for detection of objects making systematic observation campaigns easier. • Automated telescope, allowing for remote control and preparation of a work plan to be run later in differed time.
Fig. 1. Example of the TAROT image (ASTRA satellites).
Fig. 2. Example of a ROSACE image (ASTRA satellites).
Fig. 3. The TAROT telescope.
F. Alby et al. / Advances in Space Research 34 (2004) 1143–1149
• Agility: this feature is not useful for geostationary orbit observations but could be used, for example, for tracking objects located on lower orbits during an image collection cycle. Both a and d axis speed can be adjusted, to up to 80 per second. TAROT is fitted with a Thomson THX 7899 CCD camera with the following features: Size: 2048 2048 pixels, Pixel size: 15 lm (3.6 arcsec), Readout time: 2 s (slow mode), Readout noise: 13e , Cooling with three stage Peltier device down to )45 C. 3.3. Description of software The overall organization of the TAROT telescope control and image analysis software is represented in Fig. 4. The ‘‘debris’’ application software interfaces with the basic application (‘‘system control’’) as follows: • observation requests prepared in the ‘‘debris’’ application program are sent to the ‘‘system control’’ application monitor and inserted in the telescope schedule, • raw images obtained are sent back to the ‘‘debris’’ application for processing. Only the ‘‘viewer’’ function of the ‘‘system control’’ application is used. This function enables quick viewing of small images available in jpeg format via the web, then displaying or downloading of the actual image data. 3.3.1. General architecture The general architecture of the ‘‘debris’’ application software is illustrated in Fig. 5. Four main functions are provided: • request preparation, • detection/identification, • processing of catalogued objects, • processing of non-catalogued objects.
1145
3.3.2. Request preparation function Requests may be from two different sources, the user or the system: First of all, the user indicates his/her requirements such as the date of observation, the pointing direction, the number of exposures, the interval between each exposure and the choice of the filters for the telescope. The user is assisted by the ‘‘pointing’’ module in preparing the request. This module calculates the date, right ascension and declination for the observation using the information provided by the user, namely the satellite name and the last two-line bulletin in the database for the given satellite, a user-provided bulletin or the longitude of the specified satellite in geostationary orbit. This module then performs the extrapolation, geometric visibility calculation and pointing direction calculation functions. Requests can also be generated automatically by the system. This corresponds to the case of the detection of a new object, and necessitates that the object be observed several times during the same night to determine its orbit (see Section 3.3.5). The system can also create automatic requests to update the orbit bulletins of the objects in the database. The requests are inserted in the TAROT timeline. Results from the observations are in form of FITS format files (Flexible Image Transport System), since this standard is the normalized format for telescope observations. This file contains the image itself as well as information such as the telescope pointing direction and date. The user also has access to the list of requests already deposited and can make a link with observations carried out. A database contains all observations already made, which offers a user interface allowing the user to choose the name of the object and to list the corresponding image. This interface equally allows the user to run the processing operations described below on any selected image.
Fig. 4. The TAROT software.
1146
F. Alby et al. / Advances in Space Research 34 (2004) 1143–1149
consecutive images, with those in the database. Filters on longitude, for example, may be used to reduce the computing time. 3.3.4. Processing of catalogued objects If an object is already catalogued, an initial orbit bulletin will be available in the database. It is then possible to launch a restitution of the orbit to update the former bulletin taking the new measurements into account. This is a standard process based on the least square method.
Fig. 5. General architecture of TAROT software.
3.3.3. Detection of objects The ‘‘satdetect’’ module processes the image in order to detect objects and to produce an angular measurement (date, elevation and azimuth). The main processing steps from the raw image are as follows (Bijaoui et al., 2000): • calculation of the theoretical track of stars according to the exposure time and the pointing direction (star catalogue), • appropriate filtering with the theoretical track profile to reset the image and determine the position of stars, • calculation of the average track then the real track of each star, taking its magnitude into account, • subtraction of real track, • determination of the average background of the image and related dispersions, • threshold setting taking dispersion into account: detection of objects, • making several consecutive exposures (typically three exposures of 10 s each, spaced 10 s apart) to confirm the detection (elimination of CCD faults or cosmic radiation). Detected objects must then be identified. Two sources of information are available: the two-line database and the database built using TAROT observations. The principle is to extrapolate the orbit bulletins available at the date of measurement and to compare the elevation and azimuth values of detected objects,over several
3.3.5. Processing of non-catalogued objects For the moment, the processing of non-catalogued objects has still to be developed. Its principle, however, is as follows: When a non-catalogued object (new object) is detected, a number is given to it and the measurement made is stored in a file. As it is not possible to determine the orbit of the object with a single measurement, it is necessary to make other observations during the same night before launching the orbit restitution. A possible approach is as follows: • With three consecutive exposures already available the theoretical elevation and azimuth position of the object is calculated using the assumption of a linear drift of these angles over a period of a few minutes (see Fig. 6). A new request is made and sent to TAROT. • Since the process may be repeated several times, then coarse assumptions on the orbit shape are made to find the object about one hour later. • The ‘‘glosat’’ module calculates the initial orbit bulletin using the few measurements initiate orbit restitution. • Several measurements are thus made during-the night. • The orbit restitution software is subsequently run to verify that the various observations do indeed concern the same object. • The orbit bulletin of the new object is stored in the TAROT database and observations are programmed for the following night.
Fig. 6. Short term pointing.
F. Alby et al. / Advances in Space Research 34 (2004) 1143–1149
3.4. Results obtained Initially, observations of ASTRA satellites were made to validate the reference frame changes between the various modules and to highlight any possible bias. Thanks to the special shape of the ASTRA constellation (see Fig. 1) it is possible to verify that the telescope was effectively pointed in the direction requested by the user. Observations were subsequently made of the TC2 and INMARSAT satellites for which a precise reference orbit is available. The purpose of these ongoing tests is to highlight any possible datation problems and to determine the accuracy of measurements made. The minimum size of objects detected by TAROT is estimated at 50 cm. The limiting magnitude is 17 for 10 s exposure and can reach 19 for 1 min exposures. 3.5. Current plans A certain number of technical problems with the telescope and its control system have slowed the system development. The short-term goal is to carry out a systematic exploration campaign of the geostationary arc visible from the observation site in order to count the objects and estimate the proportion of non-catalogued objects. Participation in the next observation campaigns organized by IADC is also scheduled. Further into the future, we plan to extend the use of TAROT for the observation of objects located on lower orbits.
4. ROSACE 4.1. General information ROSACE is an optical ground station concept that is able to provide precise traditional azimuth and elevation measurements for satellites and other objects in or near the geostationary arc at low cost relative to other systems of equivalent precision (e.g., turnaround systems), both in terms of construction and installation, and operation and maintenance. The station is controlled by a station management computer and controlling electronics, allowing the station to operate entirely autonomously during campaign operation. The ROSACE ground station concept was developed as part of a CNES Research and Development initiative for the STENTOR research satellite (experimental telecommunication satellite) due to be launched at the end of 2002. At this time ROSACE will be ready for routine orbit determination operations to support the STENTOR mission and to provide redundancy capability for other geostationary satellite ground stations. Once validated, it is also intended to use the station for calibrating other stations, for monitoring collocated
1147
satellites and, due to its passive nature, for producing precise measurements for debris in or near the geostationary arc. 4.2. Description of the hardware The ROSACE station consists of a Newton telescope and CCD camera and the necessary surrounding hardware and controlling electronics and software to allow the telescope to be pointed precisely, the camera to produce images and the images to be analysed to produce satellite measurements; all without operator intervention. For the prototype station, a Newton telescope of 500 mm aperture and a focal length of 1900 mm has been used (Fig. 7). The detector at the centre of the telescope is a CCD camera of 1024 1536 pixels. The field of view provided by the telescope and covered by the CCD array is 0.3 0.4, each pixel thus covering 1 s, or 170 m in the geostationary arc (Escane et al., 2001). The station’s GPS receiver integrated into the control box provides the time reference, and allows the image capture to be time stamped precisely. The first operational prototype was installed at the Observatory of Haute–Provence early in 2001. 4.3. Description of software The planning software is run prior to starting a campaign at the remote control centre. This uses approximate satellite ephemerides to produce a list of all possible images that could be collected over the specified campaign period for the satellites selected. The image list contains the ideal pointing vector for the telescope and the duration of image collection and shutter cycles, optimised to avoid bright stars or planets and using a sunrise/sunset mask and lunar mask to avoid collecting images under conditions potentially dangerous for the CCD array.
Fig. 7. The ROSACE telescope.
1148
F. Alby et al. / Advances in Space Research 34 (2004) 1143–1149
The principal software component of the station is the supervisor, whose role is to manage the station during a campaign. This is started remotely from the CNES control centre upon generation and transfer of the necessary campaign planning files. It is then responsible for commanding the station’s electronics box to drive the station hardware as necessary, and it monitors the returned data from the electronics box to ensure a continued satisfactory state for the system. The image selection agent is responsible for selecting the most appropriate next image from the list of possible images generated during campaign planning, based on the current time of the request, the number of times each satellite direction has been viewed and the delay required between subsequent images to allow the collection and analysis cycle to finish. The image processing agent is the second major software element, responsible for analysing each image and generating satellite measurements. The analysis consists of filtering the raw photometric data to improve the signal-to-noise ratio of the star and satellite content in the image, determination of the correspondence between the star catalogue and the star tracks in the image, identification of the satellites in the image and subsequent generation of traditional angular measurements of satellite position. 4.4. Validation campaigns The performance of the station has been determined by reconstituting the satellite orbits from the measurements obtained from the ROSACE station and also by passing the ROSACE measurements through the orbits
reconstituted with traditional ground station measurement sources (Newland et al., 2002). To reconstitute satellite orbits reliably from purely angular measurements requires a relatively long period of observation (at least three nights of observation) to provide good coverage of the satellite through a sufficiently large portion of the geostationary arc. The first validation campaign was performed over 14 days from April 16th to April 30th for the French Telecom 2D satellite located at 8W, with support from its traditional ranging and turnaround measurement stations. During this period, two station-keeping manoeuvres were performed for Telecom 2D. The satellite orbit was reconstituted both from ROSACE-only measurements, traditional station only and a combination of the two. In all cases and for all lengths of data analysed, the different measurements were found to be coherent and the resulting orbits consistent with each other. The estimated parameters were observable in each case, as indicated by the eigenvalues of the least squares calculation. The residuals resulting from the orbit restitution with the combination of ROSACE and traditional measurements do not display any remaining signal or slope, demonstrating coherence over the 11 days illustrated that cover both manoeuvres. The residuals for ROSACE measurements are 1 arcsec at 1r in elevation and 2 arcsec at 1r in azimuth. The second validation campaign was performed for the INMARSAT III F2 satellite located at 15.5W. This offers the benefit over the Telecom satellites of constantly broadcast satellite ephemerides precise to better than 10 m. Fig. 8 shows the residuals resulting from
Fig. 8. Residuals of ROSACE measurements.
F. Alby et al. / Advances in Space Research 34 (2004) 1143–1149
passing the ROSACE measurements through the INMARSAT broadcast ephemerides, again showing no signal or slope apart from a possible slight bias in elevation which is currently under further investigation. The order of standard deviation of the measurements can be seen to be similar to that seen for the Telecom campaign. The station validation campaigns are ongoing, however, and a more reliable statement on station precision and performance will be possible on completion of the validation exercise. Preliminary results from the measurement validation campaigns have demonstrated the coherence of the ROSACE measurements with existing ground station measurements, and a measurement precision of the order of 1 arcsec at 1r in elevation and 2 arcsec at 1 r in azimuth.
5. Conclusions The CNES has developed two optical observation systems for geostationary orbit objects. The TAROT system has a wide field of view, fast and easy pointing features and operates automatically. Thanks to these characteristics, the TAROT system is well adapted to the detection of objects. The initial results show that the minimum size of observable objects is around 50 cm
1149
close to the geostationary orbit. The ROSACE system makes it possible to carry out precise measurements of the location of passive objects in a geostationary orbit. For this operation the system needs approximate sighting directions. This information can be provided by TAROT. The two systems can therefore be considered as complementary: TAROT detecting objects and ROSACE determining their orbits. These two projects thus make it possible to develop the basic components of a future system for the observation and cataloguing of the population of objects in geostationary orbits.
References Bijaoui, A., Alby, F., Vandame, B., et al. Optical observations in geostationary orbit, B0. 1-0004, in: 33rd COSPAR Scientific Assembly, Warsaw, 2000. Bo€er, M., Bringer, M., Klotz, A., et al. Tarot: observing gamma-ray bursts in progress. Astron. Astrophys. Suppl. Ser. 138, 579, 1999. Newland, F., Escane, I., Burnel, Y., et al. The ROSACE Optical Ground Station, in: SpaceOps 2002, Houston, Texas, 9–12 October, 2002. Escane, I., Delong, N., Newland, F. First results for the ROSACE autonomous orbit determination system using deviation on CCD, in: 16th International Symposium on Space Flight Dynamics, NASA JPL, Pasadena, California, 3–7 December, 2001.
Status of CNES optical observations of space debris in geostationary orbit F. Alby
a,*
, M. Boer b, B. Deguine a, I. Escane a, F. Newland a, C. Portmann
c
a
b
CNES, 18 Avenue E. Belin, 31401 Toulouse, Cedex 4, France CESR-CNRS, 9 Avenue du Colonel-Roche, BP 4346, 31028 Toulouse, Cedex 4, France c LOGUS, 43 bd de Tournon, 83440 Montauroux, France
Received 19 October 2002; received in revised form 15 January 2003; accepted 21 January 2003
Abstract On-ground optical systems using a telescope and a CCD camera offer an effective solution to the problem of observing objects in geostationary orbit. CNES has been studying and developing such systems for several years with two main objectives: firstly to develop systems able to detect debris in the vicinity of the geostationary orbit for statistical evaluation of the population and secondly to develop a tool to determine the orbits accurately: these objectives are currently met using two different systems called TAROT and ROSACE. This paper presents the main characteristics of both systems, the principle of their image processing software, their development status and the main results obtained. Finally, perspectives for further developments and coupling of the two systems are presented. 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Space debris; Geostationary orbit; CNES optical observations
1. Introduction The geostationary orbit is becoming more and more cluttered as it offers numerous advantages for many applications. Currently, 900 objects of a size greater than 1 m have been counted close to this orbit. Precise knowledge of the satellites and debris in or near the geostationary arc is necessary to make informed decisions on the necessary steps to be taken to avoid debris impacts, and to verify the application of such decisions. Ground based telescopes offer a satisfactory solution to this knowledge acquisition problem due to the particularities of this orbit, namely the negligible movement of objects in relation to the Earth’s surface. The CNES has carried out a number of different observation experiments of satellites or debris in geostationary orbit over the last few years using ground based telescopes. These experiments have proven the feasibility of the concept and shown the advantages of *
Corresponding author. Tel.: +33-5-61-28-14-64; fax: +33-5-61-2831-82. E-mail address: [email protected] (F. Alby).
such systems. Currently, two complementary projects with different goals are being developed: • The objective of the TAROT project is to detect objects, satellites or debris of a certain minimum size located around the geostationary orbit. • The ROSACE project is concerned with the development of a precise means of determining the orbit of objects in the geostationary orbit. The prime objective of this article is to present the common principles of these two projects with regard to the data acquisition and calculation of the object localization. Then, the technical characteristics of the two systems will be described (hardware and software) as well as their performance and the main results obtained.
2. Common principles The two systems use small size telescopes fitted with a CCD camera located in the focal plane. Observations take place when the station is in the dark as the observed object is lit by the sun. In a geostationary orbit, the objects are practically immobile in relation to the Earth
0273-1177/$30 2004 COSPAR. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.asr.2003.01.016
1144
F. Alby et al. / Advances in Space Research 34 (2004) 1143–1149
and the telescope remains pointed in a set direction for the exposure time. This makes it possible to detect small size objects as photons from the observed object are cumulated on the same pixel of the CCD array during the exposure time. These objects will appear in the form of dots or stains. Due to the rotation of the Earth, stars will, in contrast, appear in the form of tracks of a length dependent on the exposure time. The use of a star catalogue makes it possible to determine the telescope pointing to a high level of precision, and by calculating the direction of the observed object(s) in relation to the stars, it is possible to obtain elevation and azimuth position data for the observed object. With ROSACE, the need of greater accuracy has led to the use of a CCD camera shutter which closes at regular intervals during the exposure, giving dash-like star tracks. This process improves the determination of the position of stars and therefore measurement data accuracy (see Fig. 2).
3. Tarot system 3.1. General information The ‘‘Telescope a Action Rapide pour les Objets Transitoires’’ (TAROT-Bo€er et al., 1999) is installed on the Calern plateau above Grasse in the South of France and belongs to the CNRS (National Scientific Research Centre). The main goal of this telescope is to observe the optical counterparts of gamma ray bursts as part of a worldwide surveillance network. As such events are rare, TAROT is available for other missions that can be interrupted following an alert. 3.2. Description of the hardware TAROT is a small size telescope (25 cm aperture) with a certain number of specific features for debris tracking (see Fig. 3): • Wide field of view (2 2) that is useful for detection of objects making systematic observation campaigns easier. • Automated telescope, allowing for remote control and preparation of a work plan to be run later in differed time.
Fig. 1. Example of the TAROT image (ASTRA satellites).
Fig. 2. Example of a ROSACE image (ASTRA satellites).
Fig. 3. The TAROT telescope.
F. Alby et al. / Advances in Space Research 34 (2004) 1143–1149
• Agility: this feature is not useful for geostationary orbit observations but could be used, for example, for tracking objects located on lower orbits during an image collection cycle. Both a and d axis speed can be adjusted, to up to 80 per second. TAROT is fitted with a Thomson THX 7899 CCD camera with the following features: Size: 2048 2048 pixels, Pixel size: 15 lm (3.6 arcsec), Readout time: 2 s (slow mode), Readout noise: 13e , Cooling with three stage Peltier device down to )45 C. 3.3. Description of software The overall organization of the TAROT telescope control and image analysis software is represented in Fig. 4. The ‘‘debris’’ application software interfaces with the basic application (‘‘system control’’) as follows: • observation requests prepared in the ‘‘debris’’ application program are sent to the ‘‘system control’’ application monitor and inserted in the telescope schedule, • raw images obtained are sent back to the ‘‘debris’’ application for processing. Only the ‘‘viewer’’ function of the ‘‘system control’’ application is used. This function enables quick viewing of small images available in jpeg format via the web, then displaying or downloading of the actual image data. 3.3.1. General architecture The general architecture of the ‘‘debris’’ application software is illustrated in Fig. 5. Four main functions are provided: • request preparation, • detection/identification, • processing of catalogued objects, • processing of non-catalogued objects.
1145
3.3.2. Request preparation function Requests may be from two different sources, the user or the system: First of all, the user indicates his/her requirements such as the date of observation, the pointing direction, the number of exposures, the interval between each exposure and the choice of the filters for the telescope. The user is assisted by the ‘‘pointing’’ module in preparing the request. This module calculates the date, right ascension and declination for the observation using the information provided by the user, namely the satellite name and the last two-line bulletin in the database for the given satellite, a user-provided bulletin or the longitude of the specified satellite in geostationary orbit. This module then performs the extrapolation, geometric visibility calculation and pointing direction calculation functions. Requests can also be generated automatically by the system. This corresponds to the case of the detection of a new object, and necessitates that the object be observed several times during the same night to determine its orbit (see Section 3.3.5). The system can also create automatic requests to update the orbit bulletins of the objects in the database. The requests are inserted in the TAROT timeline. Results from the observations are in form of FITS format files (Flexible Image Transport System), since this standard is the normalized format for telescope observations. This file contains the image itself as well as information such as the telescope pointing direction and date. The user also has access to the list of requests already deposited and can make a link with observations carried out. A database contains all observations already made, which offers a user interface allowing the user to choose the name of the object and to list the corresponding image. This interface equally allows the user to run the processing operations described below on any selected image.
Fig. 4. The TAROT software.
1146
F. Alby et al. / Advances in Space Research 34 (2004) 1143–1149
consecutive images, with those in the database. Filters on longitude, for example, may be used to reduce the computing time. 3.3.4. Processing of catalogued objects If an object is already catalogued, an initial orbit bulletin will be available in the database. It is then possible to launch a restitution of the orbit to update the former bulletin taking the new measurements into account. This is a standard process based on the least square method.
Fig. 5. General architecture of TAROT software.
3.3.3. Detection of objects The ‘‘satdetect’’ module processes the image in order to detect objects and to produce an angular measurement (date, elevation and azimuth). The main processing steps from the raw image are as follows (Bijaoui et al., 2000): • calculation of the theoretical track of stars according to the exposure time and the pointing direction (star catalogue), • appropriate filtering with the theoretical track profile to reset the image and determine the position of stars, • calculation of the average track then the real track of each star, taking its magnitude into account, • subtraction of real track, • determination of the average background of the image and related dispersions, • threshold setting taking dispersion into account: detection of objects, • making several consecutive exposures (typically three exposures of 10 s each, spaced 10 s apart) to confirm the detection (elimination of CCD faults or cosmic radiation). Detected objects must then be identified. Two sources of information are available: the two-line database and the database built using TAROT observations. The principle is to extrapolate the orbit bulletins available at the date of measurement and to compare the elevation and azimuth values of detected objects,over several
3.3.5. Processing of non-catalogued objects For the moment, the processing of non-catalogued objects has still to be developed. Its principle, however, is as follows: When a non-catalogued object (new object) is detected, a number is given to it and the measurement made is stored in a file. As it is not possible to determine the orbit of the object with a single measurement, it is necessary to make other observations during the same night before launching the orbit restitution. A possible approach is as follows: • With three consecutive exposures already available the theoretical elevation and azimuth position of the object is calculated using the assumption of a linear drift of these angles over a period of a few minutes (see Fig. 6). A new request is made and sent to TAROT. • Since the process may be repeated several times, then coarse assumptions on the orbit shape are made to find the object about one hour later. • The ‘‘glosat’’ module calculates the initial orbit bulletin using the few measurements initiate orbit restitution. • Several measurements are thus made during-the night. • The orbit restitution software is subsequently run to verify that the various observations do indeed concern the same object. • The orbit bulletin of the new object is stored in the TAROT database and observations are programmed for the following night.
Fig. 6. Short term pointing.
F. Alby et al. / Advances in Space Research 34 (2004) 1143–1149
3.4. Results obtained Initially, observations of ASTRA satellites were made to validate the reference frame changes between the various modules and to highlight any possible bias. Thanks to the special shape of the ASTRA constellation (see Fig. 1) it is possible to verify that the telescope was effectively pointed in the direction requested by the user. Observations were subsequently made of the TC2 and INMARSAT satellites for which a precise reference orbit is available. The purpose of these ongoing tests is to highlight any possible datation problems and to determine the accuracy of measurements made. The minimum size of objects detected by TAROT is estimated at 50 cm. The limiting magnitude is 17 for 10 s exposure and can reach 19 for 1 min exposures. 3.5. Current plans A certain number of technical problems with the telescope and its control system have slowed the system development. The short-term goal is to carry out a systematic exploration campaign of the geostationary arc visible from the observation site in order to count the objects and estimate the proportion of non-catalogued objects. Participation in the next observation campaigns organized by IADC is also scheduled. Further into the future, we plan to extend the use of TAROT for the observation of objects located on lower orbits.
4. ROSACE 4.1. General information ROSACE is an optical ground station concept that is able to provide precise traditional azimuth and elevation measurements for satellites and other objects in or near the geostationary arc at low cost relative to other systems of equivalent precision (e.g., turnaround systems), both in terms of construction and installation, and operation and maintenance. The station is controlled by a station management computer and controlling electronics, allowing the station to operate entirely autonomously during campaign operation. The ROSACE ground station concept was developed as part of a CNES Research and Development initiative for the STENTOR research satellite (experimental telecommunication satellite) due to be launched at the end of 2002. At this time ROSACE will be ready for routine orbit determination operations to support the STENTOR mission and to provide redundancy capability for other geostationary satellite ground stations. Once validated, it is also intended to use the station for calibrating other stations, for monitoring collocated
1147
satellites and, due to its passive nature, for producing precise measurements for debris in or near the geostationary arc. 4.2. Description of the hardware The ROSACE station consists of a Newton telescope and CCD camera and the necessary surrounding hardware and controlling electronics and software to allow the telescope to be pointed precisely, the camera to produce images and the images to be analysed to produce satellite measurements; all without operator intervention. For the prototype station, a Newton telescope of 500 mm aperture and a focal length of 1900 mm has been used (Fig. 7). The detector at the centre of the telescope is a CCD camera of 1024 1536 pixels. The field of view provided by the telescope and covered by the CCD array is 0.3 0.4, each pixel thus covering 1 s, or 170 m in the geostationary arc (Escane et al., 2001). The station’s GPS receiver integrated into the control box provides the time reference, and allows the image capture to be time stamped precisely. The first operational prototype was installed at the Observatory of Haute–Provence early in 2001. 4.3. Description of software The planning software is run prior to starting a campaign at the remote control centre. This uses approximate satellite ephemerides to produce a list of all possible images that could be collected over the specified campaign period for the satellites selected. The image list contains the ideal pointing vector for the telescope and the duration of image collection and shutter cycles, optimised to avoid bright stars or planets and using a sunrise/sunset mask and lunar mask to avoid collecting images under conditions potentially dangerous for the CCD array.
Fig. 7. The ROSACE telescope.
1148
F. Alby et al. / Advances in Space Research 34 (2004) 1143–1149
The principal software component of the station is the supervisor, whose role is to manage the station during a campaign. This is started remotely from the CNES control centre upon generation and transfer of the necessary campaign planning files. It is then responsible for commanding the station’s electronics box to drive the station hardware as necessary, and it monitors the returned data from the electronics box to ensure a continued satisfactory state for the system. The image selection agent is responsible for selecting the most appropriate next image from the list of possible images generated during campaign planning, based on the current time of the request, the number of times each satellite direction has been viewed and the delay required between subsequent images to allow the collection and analysis cycle to finish. The image processing agent is the second major software element, responsible for analysing each image and generating satellite measurements. The analysis consists of filtering the raw photometric data to improve the signal-to-noise ratio of the star and satellite content in the image, determination of the correspondence between the star catalogue and the star tracks in the image, identification of the satellites in the image and subsequent generation of traditional angular measurements of satellite position. 4.4. Validation campaigns The performance of the station has been determined by reconstituting the satellite orbits from the measurements obtained from the ROSACE station and also by passing the ROSACE measurements through the orbits
reconstituted with traditional ground station measurement sources (Newland et al., 2002). To reconstitute satellite orbits reliably from purely angular measurements requires a relatively long period of observation (at least three nights of observation) to provide good coverage of the satellite through a sufficiently large portion of the geostationary arc. The first validation campaign was performed over 14 days from April 16th to April 30th for the French Telecom 2D satellite located at 8W, with support from its traditional ranging and turnaround measurement stations. During this period, two station-keeping manoeuvres were performed for Telecom 2D. The satellite orbit was reconstituted both from ROSACE-only measurements, traditional station only and a combination of the two. In all cases and for all lengths of data analysed, the different measurements were found to be coherent and the resulting orbits consistent with each other. The estimated parameters were observable in each case, as indicated by the eigenvalues of the least squares calculation. The residuals resulting from the orbit restitution with the combination of ROSACE and traditional measurements do not display any remaining signal or slope, demonstrating coherence over the 11 days illustrated that cover both manoeuvres. The residuals for ROSACE measurements are 1 arcsec at 1r in elevation and 2 arcsec at 1r in azimuth. The second validation campaign was performed for the INMARSAT III F2 satellite located at 15.5W. This offers the benefit over the Telecom satellites of constantly broadcast satellite ephemerides precise to better than 10 m. Fig. 8 shows the residuals resulting from
Fig. 8. Residuals of ROSACE measurements.
F. Alby et al. / Advances in Space Research 34 (2004) 1143–1149
passing the ROSACE measurements through the INMARSAT broadcast ephemerides, again showing no signal or slope apart from a possible slight bias in elevation which is currently under further investigation. The order of standard deviation of the measurements can be seen to be similar to that seen for the Telecom campaign. The station validation campaigns are ongoing, however, and a more reliable statement on station precision and performance will be possible on completion of the validation exercise. Preliminary results from the measurement validation campaigns have demonstrated the coherence of the ROSACE measurements with existing ground station measurements, and a measurement precision of the order of 1 arcsec at 1r in elevation and 2 arcsec at 1 r in azimuth.
5. Conclusions The CNES has developed two optical observation systems for geostationary orbit objects. The TAROT system has a wide field of view, fast and easy pointing features and operates automatically. Thanks to these characteristics, the TAROT system is well adapted to the detection of objects. The initial results show that the minimum size of observable objects is around 50 cm
1149
close to the geostationary orbit. The ROSACE system makes it possible to carry out precise measurements of the location of passive objects in a geostationary orbit. For this operation the system needs approximate sighting directions. This information can be provided by TAROT. The two systems can therefore be considered as complementary: TAROT detecting objects and ROSACE determining their orbits. These two projects thus make it possible to develop the basic components of a future system for the observation and cataloguing of the population of objects in geostationary orbits.
References Bijaoui, A., Alby, F., Vandame, B., et al. Optical observations in geostationary orbit, B0. 1-0004, in: 33rd COSPAR Scientific Assembly, Warsaw, 2000. Bo€er, M., Bringer, M., Klotz, A., et al. Tarot: observing gamma-ray bursts in progress. Astron. Astrophys. Suppl. Ser. 138, 579, 1999. Newland, F., Escane, I., Burnel, Y., et al. The ROSACE Optical Ground Station, in: SpaceOps 2002, Houston, Texas, 9–12 October, 2002. Escane, I., Delong, N., Newland, F. First results for the ROSACE autonomous orbit determination system using deviation on CCD, in: 16th International Symposium on Space Flight Dynamics, NASA JPL, Pasadena, California, 3–7 December, 2001.