Technical Paper J. Astron. Space Sci. 28(4), 333-344 (2011) http://dx.doi.org/10.5140/JASS.2011.28.4.333 A Study on the Strategies of the Positioning of a Satellite on Observed Images by the Astronomical Telescope and the Observation and Initial Orbit Determination of Unidentified Space Objects Jin Choi1†, Jung Hyun Jo1, Young-Jun Choi1, Gi-In Cho1, Jae-Hyuk Kim1,2, Young-Ho Bae1, Hong-Suh Yim1, Hong-Kyu Moon1, and Jang-Hyun Park1 1Space Situational Awareness Center, Korea Astronomy and Space Science Institute, Daejeon 305-348, Korea 2Astrodynamics and Control Laboratory, Department of Astronomy, Yonsei University, Seoul 120-749, Korea An optical tracking system has advantages for observing geostationary earth orbit (GEO) satellites relatively over other types of observation system. Regular surveying for unidentified space objects with the optical tracking system can be an early warning tool for the safety of five Korean active GEO satellites. Two strategies of positioning on the observed image of Communication, Ocean and Meteorological Satellite 1 are tested and compared. Photometric method has a half root mean square error against streak method. Also null eccentricity method for initial orbit determination (IOD) is tested with simulation data and real observation data. Under 10 minutes observation time interval, null eccentricity method shows relatively better IOD results than the other time interval. For follow-up observation of unidentified space objects, at least two consecutive observations are needed in 5 minutes to determine orbit for geosynchronous orbit space objects. Keywords: optical satellite observation, photometric method, initial orbit determination, null eccentricity method 1. INTRODUCTION over a certain altitude. The optical technology enables to observe space objects regardless of their orbits, but only The methods for the indirect observation of space the relative angular distance information can be obtained objects from ground station without communication and the objects that do not reflect the sunlight are hardly include radar, laser and optical observation. Space ob- observed (Lee et al. 2004). jects refer to not only satellite under operation but also The optical observation is often used for the observa- all kinds of artificial objects in the space such as decom- tion of high altitude space objects. The optical observa- missioned satellites, rocket bodies and space debris. To tion is not limited by the range to the observation target. observe such space objects, suitable observation tech- The optical observation is appropriate for the observa- nology should be used according to the orbital charac- tion of high altitude space objects since they stay in the teristics of the target space object. The radar provides the Earth’s shadow for a relatively short time. Most space advantage to find out the range to an object immediately, surveillance facilities including the Ground-Based Elec- but the observation of space objects beyond a certain low tro-Optical Deep Space Surveillance (GEODSS) perform earth orbit is difficult by this method. The laser facility surveillance of high altitude spaces by means of optical like satellite laser ranging allows precise measurement of observation (Schildknecht 2007). the range to an object, but it is difficult to survey a wide A geostationary orbit, which is useful in various areas, area at one time and to surveil high altitude space objects needs continued surveillance for the efficient utilization. This is an Open Access article distributed under the terms of the Received Nov 07, 2011 Revised Nov 22, 2011 Accepted Nov 24, 2011 Creative Commons Attribution Non-Commercial License (http://cre- †Corresponding Author ativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, E-mail: [email protected] provided the original work is properly cited. Tel: +82-42-865-2080 Fax: +82-42-861-5610 Copyright © The Korean Space Science Society 333 http://janss.kr pISSN: 2093-5587 eISSN: 2093-1409 J. Astron. Space Sci. 28(4), 333-344 (2011) A geostationary orbit is about 42,164 km far from the cen- the tracked space objects are active satellites (Celestrak ter of the Earth and has the ring shape extended from the 2011). The size of space objects that can be tracked is Earth’s equator. The space objects on the geostationary 10~20 cm on a low earth orbit and 1 m or greater on a high orbit always maintain fixed elevation and azimuth when altitude orbit. The space objects that are smaller than 1 viewed from a ground station. Hence, they can be used for m may be detected but they are not tracked continuously various purposes including communication and weather (Schildknecht 2007). observation, and production of geostationary satellites Follow-up observation is required to decide the risk of cost a considerable amount of resources. Thus, the coun- the unidentified space objects that have not be listed on tries that operate geostationary satellites are taking many the United States Strategic Command (USSTRATCOM) efforts to maintain and protect geostationary orbit. catalogue (Musci et al. 2004, Space-Track 2011). Initial Each geostationary satellite is to be operated within orbit determination (IOD) is necessary to perform follow- its own positional area, which is to prevent collision be- up observation. However, IOD is hardly performed at the tween satellites and communication or broadcasting ra- precision level of general orbit determination because the dio frequency interference. Therefore, the number of geo- observation data for unidentified space objects are not stationary satellites that can be launched is limited and sufficiently many. Moreover, a small number of observa- thus each country is taking various efforts to occupy the tion data are used for IOD, the observation error affect the positions favorable to their purposes. For example, the orbit determination significantly. Hence, the positioning KOREASAT 1 satellite, after the mission was completed, precision should be improved through the satellite posi- has been rented to France for the purpose of occupying tioning method and the appropriate observation strategy the country’s own orbit. Some countries have illegally for it. placed their own satellites on the unpermitted longitude The research on the space object IOD in Korea began (Gunter’s Space Page, Lee et al. 2011a,c). with ‘The Study on Preliminary Orbit Determination’ by Station keeping is performed periodically to place geo- Kim et al. (1988). Thereafter, Lee et al. (2004) published stationary satellites within the designated zones. For in- the research results on IOD and precise orbit determina- stance, Korea performs station keeping for the Commu- tion using actual optical observation results. Choi et al. nication, Ocean and Meteorological Satellite (COMS) in (2009) presented the method for the satellite position the north-south direction and in the east-west direction determination on the observed image, IOD and the dif- one time per week and for the KOREASAT satellite one ferential correction results by means of actual optical ob- time per week with the ±0.05 degree zone (Lee et al. 2003, servation. Hwang & Jo (2009) compared the precision of 2011b). Thus, the geostationary satellites currently in op- IOD methods depending on the time spans using three eration are always within the designated zones. Surveil- different observation data and reported the differences. lance of the designate zones is the first step to surveil the Space surveillance is generally based on the indirect objects that may collide with the satellite for the safety’s observation of the target space objects which is greatly sake. different from actual operation of satellites. In particular, The actual risk of collision between geostationary satel- space objects surveillance using an optical telescope is lites in operation with other space objects has been iden- the method that has been continuously used since hu- tified in various cases. The other space objects include mans launched artificial space objects. The method is ap- space debris and decommissioned satellites. In January plied in various research areas including the calculation and February in 2011, an inclined geosynchronous satel- of the orbit information with a precision level at which lite of a foreign country came into the orbit of the COMS space objects can be tracked for a certain period of time (Lee et al. 2011a). The risk of collision between the two and the surveillance of the distribution of space debris of satellites was very high. The size of such an unidentified small sizes. space object is various ranging from rocket bodies to In this study, we observed space objects with an opti- space debris that is too small to be detected. Some are on cal telescope that is used for celestial body observation the list of the Space Surveillance Network, but other are and performed IOD in order to carry out the follow-up hardly tracked. observation. For this, we firstly compared the positioning Currently, the number of space objects that are un- methods that are used to determine the position of satel- der tracking is about 15,000 according to the U.S. Stra- lites from the observed images by the optical telescope tegic Command including active satellites, decommis- images. The observation target was the COMS, and the sioned satellites and other space debris. About 1,100 of objective of orbit determination was to acquire the ini- http://dx.doi.org/10.5140/JASS.2011.28.4.333 334 Jin Choi et al. Unidentified space objects optical observation and IOD tial orbital elements that can be used in the follow-up was performed with an appropriate level of exposure so observation. Hence, the orbit determination precision that the satellite could be shown as a streak with a suf- was evaluated with reference to the possibility of the ficient number of fixed stars. The second observation follow-up observation. The null-eccentricity method was strategy was a ‘photometric method’ in which the obser- employed in the IOD.