[American Institute of Aeronautics and Astronautics SpaceOps 2010 Conference Delivering on the Dream Hosted by NASA Marshall Space Flight Center and Organized by AIAA - Huntsville,

Operational Results of Automated Conjunction Analysis System using In-house Generated KOMPSAT-2 Ephemeris

Su-Jin Choi1, Ok-Chul Jung2 and Dae-won Chung3, Yong-Sik Chun4, Hak-Jung Kim5 Korea Aerospace Research Institute, Daejeon, 305-333, Korea

Because of ever-increasing number of space debris, the possibility of a satellite collision with the space debris or other objects, such as satellites and rocket bodies which are operational or non-operational, is becoming more likely. It is necessary for satellite operation team to analyze the collision risk completely and take some appropriate actions in advance to avoid catastrophic consequences. Many national space agencies, such as JAXA, CNES and ESA, have their own conjunction analysis system to prevent their satellite from collision. Since FY-1C was destroyed by anti-satellite missile test in January 2007, MOT has analyzed collision risk between FY-1C debris and KOMPSAT-2 because these debris were scattered around the KOMPSAT-2 (KOrea Multi-Purpose SATellite-2). Since the collision of Iridium 33 and Cosmos 2251 in February 2009, the collision risk about the debris generated by Iridium and Cosmos also has analyzed. The key element to predict collision probability is orbital information of debris and collision risk assessment algorithm. This paper presents the automated conjunction analysis system, which is able to automatically compute the conjunction analysis between debris and KOMPSAT-2 on a daily basis for 7-day predictions. It has been developed in a client-server environment that connects with Conjunction Analysis Tool in STK using connect module. This paper also represents the operational results of conjunction analysis using in-house generated KOMPSAT-2 ephemeris. When we carry out conjunction analysis, two types of orbital elements are available. One is catalog of NORAD as the main source for the orbital information of space debris because of its public availability. The other is in-house generated KOMPSAT-2 ephemeris based on the operational orbit determination due to its accuracy.

I. Introduction INCE plenty of spacecrafts were launched into the space in 1957, space debris such as mission-related debris, rocket bodies, spacecraft and fragmentation debris has been increased. One of the numerous threats during the

satellite operation is the risk of collision with other objects, which should be taken into account via routine conjunction analysis for the safe satellite operations.

STherefore, many national space agencies such as CNES, NASA, and ESA have their own conjunction analysis

and alert system for the safe satellite operations. For example, CNES monitors the collision risk for its 14 spacecrafts in LEO[1], NASA currently covers 24 unmanned assets with a three step conjunction assessment process[2], ESA process for the assessment of high-risk conjunction events about ERS-2 and ENVISAT[3].

After Chinese anti-satellite event, conjunction analysis between KOMPSAT-2 and FY-1C debris was required to MOT (Mission Operations Team) because the fragment of FY-1C were scattered near the KOMPSAT-2 orbit. In addition, space debris environment near KOMPSAT-2 orbit was severe according to the Fig. 1[4]. This figure shows the low earth spatial density of catalog objects. After February 12, 2009, due to catastrophic collision between Iridium-33 and Cosmos-2251 satellite, the space debris environment in LEO became worse than before.

American Institute of Aeronautics and Astronautics

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1 Flight Dynamics Engineer, Mission Operations Department, Satellite Information Research Institute 2 Flight Dynamics Engineer, Mission Operations Department, Satellite Information Research Institute 3 Head of Mission Operations Department, Satellite Information Research Institute 4 Director of Satellite Operations and Application Division, Satellite Information Research Institute 5 Principal Research Engineer, Satellite Information Research Institute

SpaceOps 2010 Conference Delivering on the Dream Hosted by NASA Mars25 - 30 April 2010, Huntsville, Alabama

AIAA 2010-2297

Copyright © 2010 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.


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Figure 1. Low Earth Spatial density of the catalog objects. The KOMPSAT-2 was launched into space July 28, 2006, by a Rocket launch vehicle at the Plesetsk

Cosmodrome in northern Russin. It weights 800 kg and has 1,000 watts of power and is now successfully operating without any anomaly. The resolution of the satellite image is 1m for panchromatic and 4m for multi-spectral scene[5]. Mission orbit of KOMPSAT-2 is a sun-synchronous circular orbit with the altitude of 685.13 ± 1 km. Orbit inclination is 98.13° ± 0.05 degree, its eccentricity is 0 to 0.001 and its ascending orbit crossing is at 10:50 a.m.

In order to analysis the collision risk between KOMPSAT-2 and space debris, we has developed ACAS (Automated Conjunction Analysis System). This system is able to achieve the conjunction analysis automatically using catalog of NORAD (North American Aerospace Defense Command). Due to lack of the accuracy of NORAD TLE, results of conjunction analysis faced with somewhat uncertainty. MOT, therefore, applies in-house generated KOMPSAT-2 Ephemeris (KARI TLE) to conjunction analysis because the accuracy of KARI TLE is more precise than NORAD TLE.

In this paper, the level of accuracy both NORAD TLE and in-house generated ephemeris of KOMPSAT-2 is described. Moreover, the operational results of conjunction analysis using KARI TLE are to be shown with the results using NORAD TLE.

II. Space Debris to be Analyzed MOT specially has monitored two types of space debris because there space debris are scattered near KOMPSAT-

2 altitude. One is FY-1C debris which was generated January 2007 and the other is from Cosmos 2251 and Iridium 33 space debris, which were collided over northern Siberia February 2009.

FY-1C was Chinese weather satellite and was polar orbit satellites of the Fengyun series at an altitude 865 km with a mass of 750 kg. But it was destroyed by anti-satellite missile test on January 11, 2007. As a result, it was generated more than 2500 space debris as of February 2010 in space and it was scattered between 200 km and 4, 000 km altitude.

On February 10, 2009, the Iridium 33 and Cosmos 2251 communications satellites were collided each other. This was the first accidental hypervelocity collision of two intact spacecraft occurred, leaving two distinct debris clouds extending through much of low Earth orbit. As a result, space debris was generated more than 1500 space debris as of February 2010 and were scattered between 200 km and 1600 km altitude.

Figure 2. The number of space debris to be monitored.

Fig. 2 shows the number of space debris to be monitored containing FY-1C, Cosmos 2251 and Iridium 33 from Oct, 2009 to Feb, 2010. The reason why the numbers of space debris were increased that non-observed space debris were continuously observed as time goes by.

III. Type of Orbital Information This section describes the types of orbital information and its accuracy. One is NORAD TLE provided by the

official website and the other is KARI TLE based on the orbit determination result using GPS on-board navigation solution of KOMPSAT-2. Firstly, operational orbit determination of KOMPSAT-2 is to be illustrated in detail. And orbital accuracy of NORAD TLE and KARI TLE are subsequently explained as follows.

This section also shows the error covariance by comparing not only between orbital trajectory of NORAD TLE and reference which means orbital trajectory of OOD prediction, but between orbital trajectory of KARI TLE and reference. When it propagates orbital elements, TLE is propagated by SGP4 model and OOD is propagated by STK High-Precision Orbit Propagator (HPOP) model with perturbations such as JGM-3(36ⅹ36), Jacchia 1971, Sun/Moon attraction and solar radiation pressure.

A. Operational Orbit Determination The operational orbit determination (OOD) provides orbits of KOMPSAT-2 using GPS based navigation

solutions from dumped telemetry. The OOD uses a batch type minimum variance estimator for deriving definitive orbit of the satellite. The OOD includes editing and smoothing the measurement data followed by a batch least square curve fit using iterative differential correction to optimize the estimated trajectory. As part of the optimization process, the corrected orbital elements are integrated in order to re-compute the residual data. GPS navigation solutions in ECEF coordinate system are transformed to True of Date (ECI) coordinate system.

The result of OOD is the estimation of spacecraft state for a specific epoch based on observational data obtained by GPS receiver. Gravitational, solar flux and drag error can be estimated when needed.

The OOD uses Cowell’s method, which is direct numerical integration of the satellite equations of motion in rectangular coordinates. The force models include the geopotential up to 70 X 70, the lunar solar potentials, solar radiation pressure, drag using Jacchia 71 model, and general accelerations. The numerical integration procedure is a predictor-corrector type.

Since the degree of accuracy of OOD is 2~3m on the average during low solar activity, it is reasonable to adopt as the reference trajectory[6].

B. NORAD TLE United States Space Surveillance Network (U.S. SSN) involves detecting, tracking, cataloging and identifying

man-made objects orbiting the Earth[7]. Anyone is able to access to NORAD website in order to get the orbital elements of space debris larger than 10 cm. Due to the limitations in the sensitivities of the Space Surveillance


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Network radar and telescopes, the lower size threshold of objects which can be observed and correlated with the catalog is on the order of 10 cm in the LEO, and 1 m in the GEO[8]. Most of all, as NORAD is the only public source to get orbital information. Therefore, most of the space agency uses NORAD TLE.

The advantage of NORAD TLE is that updating cycle of LEO satellite is frequent. For example, KOMPSAT-2 is updated 4 times per day on the average. Other advantage of NORAD TLE is its consistency, which means that all space objects in LEO tend to have similar uncertainty.

Fig. 3 shows the difference of orbital trajectory of KOMPSAT-2 between NORAD TLE and OOD with 7-day prediction on 11-Feb, 2010. In order to explain the difference of orbital trajectory, Radial, In-Track Cross-Track (RIC) coordinate is to be introduced. According to Fig. 3, the differences in radial and cross-track direction are somewhat within the boundary, on the other hand, in-track is gradually divergent.

Figure 3. The difference of orbital trajectory between OOD and NORAD TLE.

C. KARI TLE Most space agency use their own tracking system when space debris approaches to their satellite in order to

determine the orbital elements of space debris accurately. But those agencies do not need to get additional tracking data of their satellite if they already know their satellite’s orbital information processed by observations from GPS receiver or antenna tracking measurement. MOT can get OOD of KOMPSAT-2 by processing navigation solution.

In order to obtain KARI TLE, orbit prediction for a week using STK HPOP model is needed in advance. After then, based on the predicted trajectory, we can obtain KARI TLE from ‘Generate TLE’ function of STK.

Fig. 4 shows the difference of orbital trajectory between KARI TLE and OOD with 7-day prediction on 11-Feb, 2010. According to Fig. 4, the difference of radial, cross-track and in-track is somewhat changed. Therefore, applying to KARI TLE must give us more accurate results.


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Figure 4. The difference of orbital trajectory between OOD and KARI TLE.

D. Error Covariance Table 1 shows the results of error covariance for 1 day and 7 days respectively. This result obtained by each 5

other days such as 2-Jan, 12-Jan, 22-Jan, 01-Feb and 11-Feb, 2010 so as to investigate the tendency of the difference and then it obtained by mean value of five data. As shown up in Table 1, error covariance for 1 day has similar level of accuracy but error covariance for 7 days has large-scale difference at in-track, which means that carrying out conjunction analysis using KARI TLE give us more accurate results.

Table 1 Error covariance 1 day

Contents Radial (km2) In-Track (km2) Cross-Track (km2) NORAD TLE 0.018035 0.368286 0.064650

KARI TLE 0.021248 0.389270 0.026899 7 days

Contents Radial (km2) In-Track (km2) Cross-Track (km2) NORAD TLE 0.032867 2.332930 0.040673

KARI TLE 0.015241 0.351914 0.022292

IV. Automated Conjunction Analysis System Overview MOT has been analyzed the KOMPSAT-2 conjunction analysis using catalog of NORAD and STK/CAT module

in manual for 7 days into the future since Chinese ASAT event was occurred. The development of ACAS is required in order to reduce the task and attention of flight dynamics. Fig. 5 shows the architecture of ACAS.

ACAS programmed by Visual Studio C++ and consists of TLE Gathering Module, ACAS Graphical User Interface, Connect Module, STK/CAT and Report Management Module. The characteristics and the function of each module will be described in more detail.

TLE Gathering Module collects the space debris to be monitored from NORAD website. TLE is a set of orbital elements that describes the orbit of an earth satellite. The format of TLE is specified by NORAD. Orbital elements of thousands of space debris are determined by NORAD. Whenever it evaluates the conjunction analysis, orbital elements of the space debris collected by TLE Gathering Module are sent to Connect Module.

ACAS Graphical User Interface is able to set relevant parameters such as monitoring and warning threshold, conjunction analysis execution time, start time and stop time. User can choose not only automated mode or manual


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mode, but also data source such as NORAD TLE or KARI TLE. Default mode is automated and NORAD TLE mode.

Connect Module is capable of driving the STK/CAT using information of ACAS GUI and relevant TLE obtained by TLE Gathering Module.

STK/CAT is Commercial-Off-The-Shelf software made by Analytical Graphics, Inc. This software provides conjunction analysis systems which can be performed conjunction analysis against the entire NORAD TLE database. Because of pre-filtering of orbit elements, it is possible to quickly exclude those space objects that can’t possibly have a miss due to their orbit shape, distance between orbits, and the relative passage times when each object is close to the orbit of the other. Using pre-filter provides unprecedented computation speed without reducing fidelity by decreasing the number of objects for which STK generates ephemeris data for the final stage of proximity calculation. STK/CAT can report on any potential conjunction from the satellite database. Based on threshold inputs, the results will be included the time, nearest approach, and number of the spacecrafts. Detailed information can be provided from custom report styles or the related spacecraft, which has the closest approach, can be directly imported into STK for future analysis and visualization.

Report Management Module is capable of extracting important information the contents of the file generated from STK/CAT and recording the information on stack file. Information of stack file consists of date of conjunction analysis, NORAD ID of space debris, minimum range that is the minimum distance between satellite and space debris, epoch of satellite, epoch of space debris, closest time, name of space debris and the total number of space debris to be considered in the analysis.

Figure 5. Architecture of automated conjunction analysis system.

V. Operational Results of Conjunction Analysis This section represents the operational results of conjunction analysis using KARI TLE compared with result of

NORAD TLE. In order to operate KOMPSAT-2 safely from space debris, MOT classified two kinds of threshold as follows. One is monitoring threshold which means that space debris approaches to satellite within 5km and the other is warning threshold which means that space debris approaches to satellite within 1 km in RIC coordinate. In case space debris is violated monitoring threshold, MOT will perform the trend analysis roughly. Subsequently in case space debris is violated warning threshold, MOT will also perform the trend analysis more frequent and take some appropriate actions in advance to avoid catastrophic consequences if it is needed.

This section, first of all, shows the case study of warning threshold. And then the number of violations and the percent of the kinds of space debris in which it approached to KOMPSAT-2 will be shown.


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A. Case Study of Violated Warning Threshold This is a case study of space debris violated warning threshold for a few days. The second object was FY-1C

debris, which satellite number is 32371. Space debris violated warning threshold was observed on 26 Dec, 2009 and its closest time was 01 Jan 2010 16:34:09 according to operational results of conjunction analysis.

Table 2 shows the detail results of conjunction analysis using NORAD and KARI TLE from 26 Dec, 2009 00:00:00 to 1 Jan, 2010 00:00:00. In Table 2, the satellite epoch of NORAD TLE changed each day but the debris epoch was not updated regularly. When it comes to updating frequency of KOMPSAT-2 from catalog of NORAD, orbital information of KOMPSAT-2 has updated 4 times for a day on the average while orbital information of debris has updated from 1 times for every other day to even 1 times for two weeks. In case of satellite epoch of KARI TLE, its epoch is exactly 1 day because it was made from in-house generated ephemeris.

Table 2 Detail results of conjunction analysis Conjunction Analysis Using NORAD TLE

Date Satellite Epoch Debris Epoch Minimum Range Closest time (UTC) 20091226 09359.77749 09359.43697447 0.189226 01 Jan 2010 16:34:09.466 20091227 09360.87274 09359.43697447 0.265563 01 Jan 2010 16:34:09.474 20091228 09361.76263 09359.43697447 0.353774 01 Jan 2010 16:34:09.480 20091229 09362.65252 09359.43697447 0.584304 01 Jan 2010 16:34:09.487 20091230 09363.74777 09359.43697447 0.756167 01 Jan 2010 16:34:09.548 20091231 09364.15849 09363.99190605 0.609228 01 Jan 2010 16:34:09.832 20100101 09365.80137 09363.99190605 0.609811 01 Jan 2010 16:34:09.788

Conjunction Analysis Using KARI TLE Contents Satellite Epoch Debris Epoch Minimum Range Closest time (UTC) 20091226 09360.00000 09359.43697447 0.349965 01 Jan 2010 16:34:09.336 20091227 09361.00000 09359.43697447 0.419542 01 Jan 2010 16:34:09.402 20091228 09362.00000 09359.43697447 0.484817 01 Jan 2010 16:34:09.415 20091229 09363.00000 09359.43697447 0.594923 01 Jan 2010 16:34:09.455 20091230 09364.00000 09359.43697447 0.691689 01 Jan 2010 16:34:09.481 20091231 09365.00000 09363.99190605 0.774974 01 Jan 2010 16:34:09.798 20100101 10001.00000 09363.99190605 0.713687 01 Jan 2010 16:34:09.811

Fig. 6 shows the time histories of minimum range for 7 days. The result of NORAD TLE represented that minimum range was less than 0.2 km on 26 Dec, 2009. In order to know more accurate result, it was necessary to analyze using KARI TLE because it could be more trusted in case the time when a collision is predicted remains over 4 days according to Table 1. On the first day (26-Dec) of conjunction analysis, KARI TLE shows that minimum range is bigger than NORAD TLE and minimum range gradually increases as time goes by. Therefore, we didn’t need to prepare mitigation actions.


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Figure 6. Time histories of minimum range.

B. The Number of Violation Fig. 7 shows the violation number of the monitoring and warning threshold using KARI TLE and NORAD TLE

respectively. Red and blue graphs represent violation number of the monitoring threshold and yellow green and violet graphs represent violation number of the warning threshold. According to the conjunction analysis result, the violation number in which it is over 100 numbers was observed in the monitoring threshold every month and in which it is less 20 numbers was observed in the warning threshold every month respectively.

Figure 7. The Number of Violations.

C. The Percent of the Kinds of Space Debris In Fig. 8, it shows statistical result what kinds of space debris approaching to KOMPSAT-2. In case of monitoring,

the major kind of space debris violated monitoring threshold is FY-1C. In case of warning, the major kind of space debris violated warning threshold is also FY-1C, but the percent is more than 80%. It is, first of all, because the number of FY-1C is more than half in the space debris to be monitored and it regards that orbital elements of KOMPSAT-2 and FY-1C debris are similar. Table 3 shows the orbital elements of KOMPSAT-2 and kinds of space debris to be monitored. The orbital element of kinds of space debris was calculated using NORAD TLE roughly.


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Figure 8. The percent of the kinds of space debris.

Table 3 Orbital element of KOMPSAT-2 and space debris to be monitored Orbital Element KOMPSAT-2 FY-1C DEB Cosmos 2251 DEB Iridium 33 DEB

Mean Motion (Rev/Day) 14.6174 14 ± 1 14 ± 1 14 ± 1

Inclination(Deg) 98.0134 98 ± 1 74 ± 0.5 86 ± 0.5 Eccentricity 0. 0019024 < 0.1 < 0.05 < 0.03

VI. Conclusion and Future Plans As plenty of space debris became severe threat to operational satellite including KOMPSAT-2, most satellite

operation team developed conjunction analysis system to analyze the collision risk. KARI mission operation team also developed ACAS so as to monitor the approaching space debris to KOMPSAT-2 and perform appropriate action in advance. When it comes to space debris to be monitored, space debris mainly induced from FY-1C, Iridium 33 and Cosmos 2251 were chosen because these space debris were scattered near KOMPSAT-2 altitude. In order to perform conjunction analysis, two types of TLE were used. One is NORAD TLE publicly released website and the other is in-house generated TLE. It is necessary for MOT to use KARI TLE because it was more accurate than NORAD TLE in the long term. If, therefore, we use KARI TLE, we can obtain more reliable results. In this paper, we would like to show the results compared with the results of NORAD TLE.

MOT has been analyzed collision risk from October 2009 using ACAS which is able to perform conjunction analysis automatically at every noon with 7 days into the future. According to operational results of conjunction analysis, there was a case in which space debris violated warning threshold was observed on 26 Dec, 2009 and its closest time was 01 Jan, 2010. Because its minimum range was less than 0.2 km using NORAD TLE on the first day of conjunction analysis, we used KARI TLE so as to obtain more accurate results. As a result, we found that the minimum range on 26 Dec, 2009 was more than 0.3 km and the tendency of minimum range gradually increased as time went by.

In case of the violation number, more than 100 cases are observed, which violate the monitoring threshold and less than 20 cases are observed, which violate the warning threshold every month respectively. Most space debris approaching to KOMPSAT-2 is FY-1C debris. The reason why most of space debris violated in monitoring and warning threshold is that the majority of space debris analyzed in ACAS is FY-1C and the orbital elements of FY-1C is similar to orbital elements of KOMPSAT-2.

In the future, ACAS will be extended its ability not only by using space debris to be monitored but also by using all catalogs of NORAD to monitor and prevent collision with other spacecraft and debris.


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References 1T. Laporte, E. Sasot, “Operational management of collision risks for LEO satellites at CNES”, SpaceOps 2008 Conference,

Heidelberg, Germany, 2008. 2L.K Newman, “The NASA robotic conjunction assessment process: overview and operational experiences”, 59th

International Astronautical Congress, Scotland, United Kingdom, 2008.. 3T. Flohrer, H. Krag, H. Klinkrad, “ESA’s process for the identification and assessment of high-risk conjunction events”,

Advances In Space Research, 2009. 4 “Current Debris Environment in Low Earth Orbit,” Orbital Debris Quarterly News, Vol. 13, Issue 3, Jul. 2009, pp. 7. 5Byoung-Sun Lee, Jae-cheol Yoon, Yoola Hwang, Jaehoon Kim, “Orbit determination system for the KOMPSAT-2 using

GPS measurement data,” Acta Astronautica, 2005. 6 Hae-Dong Kim, Ok-Chul Jung, Eun-Kyou Kim, Hyochoong Bang., “Orbit Determination and Orbit Maneuver Performance

for the KOMPSAT-2” KSAS(The Korean Society for Aeronautical and Space Sciences fall Conference), KSAS, Busan, 2006, pp. 579-583

7“HANDBOOK FOR LIMITING ORBITAL DEBRIS”, NASA-HANDBOOK 8719.14, 2008. 8H, Klinkrad, Space Debris Models and Risk Analysis, 1st ed., Springer-Verlag, New York, 1997, Chaps. 2.

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[American Institute of Aeronautics and Astronautics SpaceOps 2010 Conference Delivering on the Dream Hosted by NASA Marshall Space Flight Center and Organized by AIAA - Huntsville,