THE PULKOVO COOPERATION FOR RADAR AND OPTICAL OBSERVATIONS OF SPACE OBJECTS

LMolotov^1,2,3,4, A.Konovalenko⁵, G.Tuccari⁶, I.Falkovich⁵, M.Nechaeva⁷, A.Dementiev⁷, R.Kiladze⁸, V.Titenko^1,3, A.Agapov³, V.Stepanyants³, Z.Khutorovsky², S.Sukhanov², Yu.Burtsev⁹, O.Fedorov¹⁰, V.Abrosimov¹¹, A.Volvach¹², A.Deviatkin¹, A.Sochilina¹, I.Guseva¹, V.Abalakin¹, V.Vlasjuk¹³, Liu Xiang¹⁴, Yu.Gorshenkov¹⁵, G.Kornienko¹⁶, R.Zalles¹⁷, M.Ibrahimov¹⁸, P.Sukhov¹⁹, I.Shmeld²⁰, V.Samodourov²¹, S.Buttaccio⁶, C.Nicotra⁶, A.Pushkarev^1,12, A.Tsyukh¹¹, V.Nesteruk¹¹, A.Erofeeva¹⁶, N.Marshalkina¹⁸

¹ Central (Pulkovo) Astronomical Observatory, Russian Academy of Sciences, St.-Petersburg, Russia
² JSC “Vimpel” International Corporation, Moscow, Russia
³ Keldysh Institute of Applied Mathematics, Russian Academy of Sciences, Moscow, Russia
⁴ Central Research Institute for Machine Building, Korolev, Russia
⁵ Institute of Radio Astronomy, National Academy of Sciences of Ukraine, Kharkiv, Ukraine
⁶ Istituto di Radioastronomia, Noto, Italy
⁷ Radiophysical Research Institute, N. Novgorod, Russia
⁸ Abastumani Astrophysical Observatory, Georgian Academy of Sciences, Tbilisi, Georgia
⁹ Space Force of Russian Ministry of Defence, Moscow, Russia
¹⁰ National Space Agency of Ukraine, Kyiv, Ukraine
¹¹ National Control and Space Facilities Test Center, Vitino Village, Sacskiy Region, Ukraine
¹² Scientific-research institute “Crimean Astrophysical Observatory”, Simeiz, Ukraine
¹³ Special Astrophysical Observatory, Russian Academy of Sciences, N. Arkhyz, Karachaevo-Сherkessia, Russia
¹⁴ Urumqi Astronomical Observatory, National Astronomical Observatories, CAS, Urumqi, China
¹⁵ Special Research Bureau of Moscow Power Engineering Institute, Moscow, Russia
¹⁶ Ussuriysk Astrophysical Observatory, Far-Eastern Branch of RAS, Gornotaeznoe, Primorsky Kray, Russia
¹⁷ National Astronomical Observatory of Bolivia, Tarija, Bolivia
¹⁸ Ulugh Beg Astronomical Institute, Uzbekistan Academy of Sciences, Tashkent, Uzbekistan
¹⁹ Astronomical Observatory, Shevchenko University, Odessa, Ukraine
²⁰ Institute of Astronomy, University of Latvia, Riga, Latvia
²¹ Puschino Radioastronomy Observatory, P.N. Lebedev Physical Institute, RAS, Puschino, Moscow region, Russia

There is a long history of satellite astrometry observations at the Pulkovo Observatory since October 10, 1957, when the first photographic frame of the “Sputnik” rocket body was taken (see Figure 1) and its position was determined. The list of activities included optical observations, the treatment of measurements, the orbit determination, the development of the motion theory, and the compilation of a catalogue of geostationary objects.

Fig. 1. The photo frame of the “Sputnik” rocket body taken on October 10, 1957 by Dr. T.P. Kiseleva using the 10-cm telescope AKD. The exposure time is 21 s.

In last years, Pulkovo Observatory has tried to realize the multiform concept of study of space objects in close collaboration with the Ballistic Center of the Keldysh Institute of Applied Mathematics and the JSC “Vympel” International Corporation.

The Central Astronomical Observatory at Pulkovo (CAO) is investigating the space objects on the basis of optical and radar observations. Radar observations are carried out in the framework of the LFVN that includes the 70-m antenna in Evpatoria with the 6-cm transmitter facility and the international array of receiving radio telescopes (RT-64 at the Bear Lakes, RT-32 at Noto, RT-25 at Urumqi, RT-22 at Simeiz, RT-22 at Puschino (Russia), RT- 32 at Ventspils (Latvia)). More detailed information may be found in [1,2]. As a part of this work, CAO supports the radio astronomical station disposing of the 64-m antenna at the Bear Lakes (Special Research Bureau of Moscow Power Engineering Institute) near Moscow (see Figure 2).

Fig. 2. The 64-m dish, receiver cabin, and apparatus room at the Bear Lakes.
Fig. 3. The specialized satellite telescope SR-220 at Pulkovo Observatory.

The optical observations are made in the framework of the PULCOO [2] including the optical observation stations of many observatories of the former Soviet Union and are intended to control the most part of geostationary orbit, the telescopes used are as follows: Zeiss-400 astrograph at Ussuriysk, UAFO; Zeiss-600 at Maidanak, UBAI; Maksutov-700 at Abastumani; AAO, Zeiss-1000 at Zelenchuk, SAO; AT-64 and 2.6-m ZTSh at Nauchnyi, CrAO; SR-220 at Pulkovo, CAO; RK-300 at Mayaki, AOSU; Zeiss-600 at Tarija, NAOB and others). As part of this work, CAO maintains the 22-cm optical telescope SR-220 with the wide field of view at Pulkovo (see figure 3, it is installed in the top of the AKD telescope, with which the first satellite photograph frame in 1957 was made ) and prepares the 50-cm MTM-500 telescope for observations in Kislovodsk, Northern Caucasus.

All necessary software for the CCD-frame processing, ephemeris support, and orbit determination are developed in CAO, together with the “LAPLACE” analytical theory of motion of uncontrolled GEO-objects, and with a model of explosions and the space debris database.

The main directions of the work include the development of the VLBI (very long baseline interferometry) method for the coordinated and non-coordinated measurements of catalogued high-orbit space objects, and the adjustment of the beam-park and beam-track techniques for the search for non-catalogued objects. The three main VLBI principles consist in the following:

• simultaneous observations of a space object by use of an array of spaced dish antennas,
• the radio signals are received and recorded on the tapes/disks together with the precise clock indications, all frequencies being connected to an H-maser,
• the tapes/disks from all antennas are cross-correlated to measure the time delays between the times of arrival of wavefronts to antennas, and the frequencies of interference (the fringe rates).

The VLBI- radar combines the “classic” radar and VLBI techniques to provide the instrument for 3-D measurements: radar has the resolution for range and radial velocity; VLBI provides the angle and angular rate. The Evpatoria RT-70 included in the LFVN radar system can measure the Doppler shifts at five receiving points (with a precision of 0.003 Hz), the angular positions of objects (with a precision up to 0.01”); the ranges (up to 50 m), the rotation periods, the orientation of the rotation axes, the information on sizes and the structure of surfaces. This research started in 1999, the first VLBI-radar experiment for space debris was carried out in May, 2001. Stages of the correlation processing procedure are described in [4], where the precision of the Doppler shift measurements has been evaluated. These data were obtained by the cross-correlations of the transmitted sounding signal recordings and the echo-signals received at radio telescopes. The VLBI-radar experiments on space debris objects moving in various orbits were carried out in 2004-2005, and the final stage of the correlation processing of radar echoes was adjusted. The fringe rate measurements were obtained by the cross- correlations of echo-signals received by pairs of radio telescopes (see Figures 4, 5) on the baselines Bear Lakes-Noto-Urumqi and Bear Lakes-Noto-Simeiz.

Fig. 4. The cross-spectrum of echo from Cosmos-1366 on the baselines Bear Lakes- Noto, Urumqi-Noto and Bear Lakes-Urumqi. The fringe rates are measured as frequencies of spectral maximums: -373:703 Hz, -176:524 Hz and -195:890 Hz, respectively. VLBR03.1.

Fig. 5. Time dependence of the cross-spectrum maxima of Cosmos-1366 on the baselines Bear Lakes-Noto (mono), Bear Lakes-Urumqi (mour) and Noto-Urumqi (nour). VLBR03.1.

The shifts of cross-spectrum maxima, obtained on the baselines between the receiving antennas, (Fig. 5) with respect to the initial count point 22:23:11 are –3.35 s for Bear Lakes- Noto, +1.65 s for Bear Lakes-Urumqi and –5.5 s for Noto-Urumqi baselines. Accordingly, with respect to the Bear Lakes point, the echo signals are ahead in Noto by 2.15 s and delayed in Urumqi by 5 s. It may be explained by the fact that the reflecting area of the Raduga 9 facility has a narrow beam directivity (about a few degrees wide) and successively passed the receiving points during the rotation. This fact is demonstrated with respect to the initial orbit of an object reconstructed from optical data to evaluate the direction of the object rotation axis. In Table 1 the results of comparisons of precisions of the Doppler shift and fringe rate measurements are given. The Doppler shift measurements were transformed into the half- sums of the object radial velocities with respect to the sounding and receiving antennas. The fringe rate measurements were transformed into the radial velocity differences of the object with respect to two receiving antennas.

Table 1. The precision of two kinds of VLBI-radar measurements – the Doppler shift and fringe rate for Cosmos-1366.

Date	UTC	O-C mm/s
Doppler shift		Evpatoria-Bear Lakes	Evpatoria-Noto	Evpatoria-Urumqi
2002/07/25	12:44:00	-0.41	2.21	-2.98
2002/07/25	12:45:00	-3.29	1.66	-4.70
2002/07/25	12:46:00	4.37	-3.34	6.78
Fringe rate		Bear Lakes-Noto	Urumqi-Noto	Urumqi-Bear Lakes
2003/07/25	22:23:14	20.2	24.2	-94.0
2003/07/25	22:23:15	20.1	24.9	-94.6
2003/07/25	22:23:16	13.2	78.0	-96.2

One can see that the precision of fringe rate measurements is dozens of times worse than the precision of Doppler shift data while it should be quite the reverse. It may be explained by insufficient current frequency resolution of the correlator. Nevertheless both kinds of VLBI-radar measurements may be used for the improvement of the initial orbit of an object.

The procedures of the beam-park and beam-track searching are adjusted using the newly designed recording terminals for the e-VLBI named NRTV (Near Real Time VLBI). It can register the echo-signals on the PC-disks and then transfer them into Internet for further analysis at the VLBI data processing center. The recorded signals are auto-correlated to the high frequency and time resolutions, and the obtained data are presented in the form of the “frequency vs.time” diagram. It is supposed that possible space objects will leave the tracks in the form of lines on this diagram, and the slope of the line will reflect the value of the Doppler shift of echo-signals. The beam-park mode (i.e. the fixed beam direction with respect to the rotating Earth) was used in an attempt to find the LEO objects. The beam track mode (i.e. the fixed beam direction with respect to the inertial frame) was used in an attempt to detect the GEO objects. In the beam track mode the antenna beams are slowly moving along the GEO. During VLBR04.2 in July 2004 the GEO region around the point with coordinates RA. 12^h 08^m 43^s.0, Dec. +00° 50' 45" has been observed. Processing the measurements in this experiment allowed to clearly detect the echoes from 6 catalogued GEO objects and to determine the time-moment of the signal maximum, the duration of the beam crossing and the Doppler shift. A sample of the “frequency vs. time” diagram is shown in figure 6.

Fig. 6. The sample of the “frequency vs. time” diagram for analyzing the beam-track experiments. On the vertical axis is time from 22:12:31 to 22:16:41 of day 206, 2004. On the horizontal axis are frequencies 247802.734 – 262451.172. The two points are identified with COSMOS 1961 and TELESAT-5.

The geographic position and aperture of the telescopes working with PULCOO are presented in figure 7.

Fig. 7. The geographic position and diameter of telescopes working with PULCOO.

In the first place, the PULCOO was organized to improve the ephemerides of the objects selected as targets for the VLBI-radar experiments. Two other major research directions are the precise tracking of the GEO-objects in order to develop the dynamical control method and to search for the small fragments produced by GEO-object explosions on the basis of the barrier method [5]. There were obtained about 20’000 measurements for the GEO and objects in highly-elliptical orbits in the past year. Also regular observations of the GEO fragments were carried out. The 64-cm AT-64 and 2.6-m ZTSH telescopes at Nauchnyi (CrAO) are used to search for the fragments in the barriers calculated by use of the “LAPLACE” theory. Zeiss-600 in Nauchnyi (SAI MSU), Zeiss-1000 in Zelenchuk (SAO RAS) and Zeiss-600 in Maidanak (UBAI) are used for follow-up observations of the fragments. In common 22 fragments of 16^m -18.5^m were discovered and the orbital parameters were determined for 25 of them. Some fragments are tracked already on time intervals of several months. This work is carried out in collaboration with the ESOC (6, 7). The program of the PULCOO modernization is now in progress. The first stage of the program foresees the purchase of 10 CCD-matrixes (six matrixes have already been purchased for Nauchnyi, Pulkovo, Maidanak, Mayaki, Ussuriysk and Abastumani), and the upgrading of three telescopes (the 50-cm and 1-m telescopes in Pulkovo and the 60-cm telescope in Mayaki). The measurements are accumulated in the “LAPLACE” space debris database in Pulkovo and transferred to the “Vympel” International Corporation and Ballistic Center of the Keldysh Institute of Applied Mathematics, where the Center on collection, processing, and analysis of information on space debris of the Russian Academy of Sciences was arranged this year.

The Pulkovo Observatory carries out the radar and optical research of high-orbit space objects in wide cooperation with other institutes and observatories. The Low Frequency VLBI Network applied for the VLBI-radar experiments has been equipped with the new NRTV recording terminals that allow to obtain the measurements in quasi-real time using the Internet for transfer of the VLBI signals from the receiving antennas to the correlation center. The procedure of the correlation processing was adjusted for the radar echo signals to measure the Doppler shift and fringe rate. The Pulkovo cooperation of optical observers initiated the routine observations of space objects practically along the entire geostationary ring. The “barrier” method of searching for the faint non-catalogued GEO fragments was adjusted. The potentialities of the PLCOO will increase after finishing the first modernization stage. This work was supported by the National Space Agency of Ukraine under project “Interferometer”, by the grant of the Russian Ministry of Education and Science, the INTAS-2001-0669, INTAS 03-70-567, RFBR 05-02-16832 grants.

Fig. 8. Average magnitude distribution of discovered GEO-fragment.

Fig. 9. Mass-to-area ratio value distributed for 21 fragments.

1. Molotov I.E., Konovalenko A.A., Lipatov B.N., Tuccari G., Agapov V.M., Sochilina A.S., Falkovich I.S., Gorshenkov Y.N., Molotov E.P., Stepaniants V.A., Liu X., Zhang J., Volvach A.E., Fedorov O.P., Bukreev A.N., Tsyukh A.M., Nesteruk V.N., Malevinsky S.V., Dementiev A.F., Antipenko A.A., Buttaccio S., Nicotra C., Stepka I.D., Zinoviev A.N., Saurin V.P., Pushkarev A.B., Nechaeva M.B., Sika Z.K., Shmeld I.K. First results of the space debris radar observations using Evpatoria RT-70 transmitter and Low frequency VLBI network. Proceedings of Fifth US/Russian Space Surveillance Workshop. Central Astronomical Observatory at Pulkovo. September 24-27, 2003. P. Kenneth Seidelmann (Ed.). Saint- Petersburg: VVM. co. Ltd., 2003, pp. 294-304.
2. Molotov I., Konovalenko A., Agapov V., Sochilina A., Lipatov B., Gorshenkov Yu., Molotov E., Tuccari G., Buttaccio S., Liu X., Zhang J., Hong X., Huang X., Kus A., Borkowski K., Sika Z., Abrosimov V., Tsyukh A., Samodurov V., Falkovich I., Litvinenko L., Stepaniants V., Dementiev A., Antipenko A., Snegirev S., Nechaeva M., Volvach A., Saurin V., Pushkarev A., Deviatkin A., Guseva I., Sukhov P. Radar interferometer measurements of space debris using the Evpatoria RT-70 transmitter. Advances in Space Research, Volume 34, Issue 5, 2004, p. 884-891.
3. Molotov I. Pulkovo cooperation of optical observers. Programme & Abstracts of Fourth European Conference on Space Debris, ESOC, Darmstadt, Germany, 18-20 April, 2005, ESA Publication Division, p.173.
4. Molotov I., Tuccari G., Nechaeva M., Dugin N., Konovalenko A., Falkovich I., Gorshenkov Y., Liu X., Volvach A., Agapov V., Pushkarev A., Titenko V., Buttacio S., Rumyantsev V. & Shmeld I. First results of European VLBI radar observations of space objects. Proceedings of 7th European VLBI Network Symposium on VLBI Scientific Research & Technology. Toledo, Spain, October 12-15, 2004. Rafael Bachiller, Francisco Colomer, Jean-Francois Desmurs, Pablo de Vicente (Editors), Observatorio Astronomico Nacional., p.329-330.
5. Sochilina A., Kiladze R., Grigoriev K., Molotov I., Vershkov A. On the orbital evolution of explosion fragments. Advances in Space Research, Volume 34, Issue 5, 2004, p. 1198-1202.
6. Agapov V, Dick J., Guseva I., Herridge P., Khutorovskiy Z., Molotov I., Ploner M., Rumyantsev V., Schildkneht T., Stepanyants V., Sukhov P., Titenko V. Joint RAS/PIMS/AIUB GEO survey results. Submitted to: Proceedings of Fourth European Conference on Space Debris, ESOC, Darmstadt, Germany, 18-20 April, 2005, 6 pages.
7. Agapov V., Biryukov V., Kiladze R., Molotov I., Rumyantsev V., Sochilina A., Titenko V. Faint GEO objects search and orbital analysis. Submitted to: Proceedings of Fourth European Conference on Space Debris, ESOC, Darmstadt, Germany, 18-20 April, 2005, 7 pages.

Размещен 8 декабря 2006.