LFVN ::: Astrometry of the Solar System Bodies with VLBI Radar

Igor Molotov⁽¹⁾, Maria Nechaeva⁽²⁾, Igor Falkovich⁽³⁾, Alexander Konovalenko⁽³⁾,
Vladimir Agapov⁽⁴⁾, Gino Tuccari⁽⁵⁾, Giuseppe Pupillo⁽⁶⁾, Stelio Montebugnoli⁽⁵⁾,
Gennadiy Kharlamov⁽⁷⁾, Lance Benner⁽⁸⁾, Viacheslav Fateev⁽⁹⁾, Yuriy Burtsev⁽⁹⁾,
Alexander Volvach⁽¹⁰⁾, Xiang Liu⁽¹¹⁾, Vasiliy Oreshko⁽¹²⁾, Ivars Shmelds⁽¹³⁾,
Pietro Bolli⁽⁵⁾, Alexander Dementiev⁽²⁾, Alexander Antipenko⁽²⁾, Nikolay Dugin⁽²⁾,
Vladimir Jazykov⁽¹⁾, Dmitriy Bezrukov⁽¹³⁾

⁽¹⁾ Central Astronomical Observatory at Pulkovo
⁽²⁾ Radiophysical Research Institute, Russia
⁽³⁾ Institute of Radio Astronomy, Ukraine
⁽⁴⁾ Keldysh Institute of Applied Mathematics, Russia
⁽⁵⁾ Istituto di Radioastronomia, Italy
⁽⁶⁾ Osservatorio Astronomico di Torino, INAF, Italy
⁽⁷⁾ Special Research Bureau, Russia
⁽⁸⁾ Jet Propulsion Laboratory, USA
⁽⁹⁾ International Vimpel Corporation, Russia
⁽¹⁰⁾ Crimean Astrophysical Observatory
⁽¹¹⁾ Urumqi Astronomical Observatory
⁽¹²⁾ Pushchino Radio Astronomy Observatory, Russia
⁽¹³⁾ Institute of Astronomy, Latvia

Abstract

Three VLBI sessions of Low Frequency VLBI Network (LFVN) were carried out in 2006-2007 (VLBR06.1, VLBR07.1, VLBR07.2) having the main goal to adjust the application of the VLBI radar method for astrometry of the solar system bodies. The transmitter of Evpatoria RT-70 provided the radio sounding of the Venus, 2004 XP14 and 2007 DT103 asteroids, and space debris objects at 6-cm wavelengths. Also LFVN joined to the radar experiments of Goldstone RT-70 at 3.6 wavelengths for NEA during VLBR06.1 and VLBR07.1 sessions. The echo-signals were recorded by array of the radio telescopes of in Russia, Ukraine, Italy, China and Latvia using NRTV, Mk-5 and Mk-2 terminals. The number of experimental results is presented in the paper.

1. Introduction

The project of the Low Frequency VLBI Network (LFVN) was started in 1996 in order to arrange the international VLBI cooperation with participation of former USSR radio telescopes [1]. One from the LFVN goals is developments of the VLBI radar (VLBR) for the investigations of the Solar system bodies [2]. The VLBR combines the radar sounding of space objects with powerful transmitter and the receiving of radar echoes by array of radio telescopes in VLBI mode. VLBI radar is a scientific instrument for the 3-D measurements: the radar has the resolution for range and radial velocity, and the VLBI provides the angle and angular rate and can measure the variations of proper rotation of the Earth group planets; determine the trajectories of planets and asteroids at Radio Reference Frame, obtain the data on the space debris object sizes and structure of surface [3]. LFVN arranges VLBR experiments since 1999 with help of the C-band transmitter of Evpatoria RT-70. VLBR program of research of new promising space debris problem was started in 2001 [4]. The main tasks were the VLBI measurements of the Doppler shifts and fringe rates of echoes from the space debris objects and determination of the object rotation period and orientation of rotation axis, as well as the size of objects and its basic constructions [5].

The efforts of three LFVN sessions during 2006-2007 were concentrated on adjustment of the VLBR method for the planets, asteroids, and small fragments of space debris at Geostationary orbit. Also the complimentary works were carried out, which included the development of an e-VLBI technique with dedicated Near-Real-Time-VLBI (NRTV) terminal, improvement of the correlation center of the Radiophysical Research Institute (RRI) in Nizhnij Novgorod, elaboration of the new data processing center of the International Vimpel Corporation in Moscow (for Mk-5 format VLBI data) and testing of the Ventspils RT-32 in Latvia with new equipment (new the C-band receiver, feed-horn and digital BBC).

2. Observations and results

VLBR06.1 (July 3-9, 2006), VLBR07.1 (July 28 - August 3, 2007) and VLBR07.2 (November 10-14, 2007) experiments were carried out with participations of Evpatoria RT-70 and Simeiz RT-22 in Ukraine, Kalyazin RT-64 and Zelenchuckskaya RT-32 (06.1) in Russia, Noto RT-32 and Medicina RT-32 (07.1 & 07.2) in Italy, Urumqi RT-25 (06.1 & 07.2) in China, and Ventspils RT-32 (07.2) in Latvia. Twice (06.1&07.1) for asteroid radar experiments LFVN was joined with JPL arranged with Goldstone RT-70 in USA at X-band. The recorded VLBI data was partially received on videocassettes and partially downloaded through Internet, converted into uniform format and processed in RRI correlation center. Vimpel processing center was used for the primary autocorrelation of VLBR data and for Mk-5 data extraction.

In the course of data processing the correlation of the transmitter signal model and the echo signal, received at each receiving point, is fulfilled. Then the spectral analysis of correlation signal is performed for further calculation of frequencies. Measurement of the frequency of the maximum spectral response allows to evaluate the Doppler frequency shift, conditioned by the object radial velocity.

One of the tasks of the VLBR sessions was research of orbital parameters of Earth's group planets. During July of 2007 Venus was on comparatively small distance from the Earth (about 0.3 AU), that allowed to successfully implement the cross-correlation of transmitted signal of Evpatoria radar and received signal at station Evpatoria (the radio telescope in Evpatoria was switched on receive mode on period ~5 minute in the end of each scan) and Kalyazin. The cross-spectra are presented on Fig.1-a. The estimation of Doppler shift frequencies was made in result of spectral analyses. Fig. 1-b shows the Doppler shift frequencies in dependence on time.

Fig. 1. The spectrum of signal, obtained in result of correlation of Evpatoria transmitter signal and echo from Venus, received in Kalyazin RT-64 in 08:11:25 UT (Fig 1-a) and the time dependence of Doppler shift frequency of this echo (Fig. 1-b). The Doppler shift frequency F_doppl is measured according the frequency of spectral main maximum. (Experiment VLBR 07.1, 31 of July, 2007, 08:10:00 UT).

The successful radar detection of asteroid 2004 XP14 was carried out in July 3 of 2006, when asteroid approached on minimal distance from Earth (near 400000 km). Assumed diameter of asteroid is 430 m. The radar of such large asteroid on small distance did not carried out before. In session VLBR06.1 the radar sounding was provided in two frequency range: at 5010.024 MHz by Evpatoria RT 70 transmitter and at 8560.0 MHz by Goldstone RT-70 transmitter. The correlation and spectral analysis of data allowed to obtain the response from asteroids in both frequency ranges. The cross-correlation of model of transmitter signal and signal, reflected from asteroid, has been performed. The spectral response of Evpatoria on echo from asteroid at radar of Goldstone emission is on Fig. 2-a. Evaluations of Doppler shift frequencies are carried out according main spectral maximum (see Fig. 2 b).

Fig.2. The spectrum of cross-correlated signal of transmitter signal model and echo from asteroid 2004XP14 received in Evpatoria RT-70 in 08:39:00 UT (Fig 2-a) and the time dependence of Doppler shift frequency of this echo (Fig 2-b). The radar sounding provided with Goldstone RT-70 at X-band. (Experiment VLBR 06.1, 3 of July, 2007, 08:37 UT).

Fig. 3. The spectrum of mutual-correlation of Evpatoria transmitter signal and signal reflected from GEO object 95120 and received in Noto (thin curve). The main maximum is shifted from center of spectrum on an interval near 15 Hz. The solid line on Fig. 3 presents the Gaussian, approximating spectrum. The Doppler shift frequency F_doppl_g determined by maximum of Gauss function (Experiment VLBR 06, 6 of July, 2006, 01:20:05.3 UT).

Other goal of the sessions was improving of the measurement precision for the cases, when the spectra of space debris objects are widening up to hundreds of kilohertz. This effect is become especially apparent at large sizes and irregular shape of investigated objects and at radar of revolving objects. Besides, the main maximum may be essentially shifted from central frequency of spectrum (see Fig. 3). To exclude the error in frequency Doppler shift determination, the procedure of spectrum approximation by Gauss function is applied. The solid line on Fig. 3 presents the Gaussian, which makes best fit the wide spectrum. The Doppler shift, measured by frequency of Gaussian maximum, carries information about average velocity of space debris object.

Figures 4 presents the time dependence of spectral maximum amplitude (Fig. 4-a) and Doppler shift frequency (Fig. 4-b) at radar of GEO debris fragment 95034 with size about 1 m. Analysis of temporal dynamics (see Fig. 4-a) of the frequency spectrum gives an additional possibility to obtain information on the general state of a spacecraft, including its rotation velocity, the presence of individual reflecting fragments on the object body, etc. Rotation period for fragment 95034 is equal to 59.5±6.7 seconds. The Doppler shift frequency is linear dependence on Fig. 4-b. The time intervals, where scattered signal is missed, may be excluded or changed by interpolated values. The accuracy of measurements of Doppler shift frequencies is sufficiently high in this example.

Fig.4. The time dependence of spectral maximum amplitude (Fig. 4-a) and Doppler shift frequency (Fig 4-b), calculated from cross-correlated signal of Evpatoria transmitter signal and echo-signal from fragment 95034, received in Urumqy. Period of object rotation P is near 60 s. (Experiment VLBR 07.1, 4 of August, 2007, 06:33 UT.)

Fig. 5. The time dependence of spectral maximum amplitude (Fig. 5-a) and Doppler shift frequency (Fig 5-b), calculated from cross-correlated signal of Evpatoria transmitter signal and echo-signal from object 90032, received in Kalyazin. (Experiment VLBR 07.1, 4 of August, 2007, 02:46 UT).

The time dependence of spectral maximum amplitude and Doppler shift frequency for object 43125 is displayed on Fig.6-a and Fig. 6-b. The Doppler shift frequency is changed irregularly by leaps. It may be conditioned by chaotic pattern of movement, as well as by complicated form of investigated object. Behavior of graph in Fig. 6-a confirms this suggestion. Application of mentioned procedure of spectra approximation does not lead to improvement of results and the accuracy of measurements is not high in such cases.

Fig.6. The time dependence of spectral maximum amplitude (Fig. 6-a) and time dependence of Doppler shift frequency (dotted line on Fig. 6-b), calculated from cross-correlated signal of Evpatoria transmitter signal and echo-signal from object 43125, received in Kalyazin. The boxes on Fig. 6-b indicate the Doppler shift frequencies predicted from optical observations. (Experiment VLBR07.2, 13 of November, 2007, 09:31 UT).

3. Conclusions

The LFVN experiments of 2006-2007 provided a next step in the VLBR method development. The first VLBR results were obtained for the asteroid, Earth group planet and the small space debris fragments. Also the procedure of spectrum approximation by Gauss function was proposed that allowed to improve the precision of Doppler shift and fringe rate frequency measurements.

The precise measurements for space debris fragments are important for the prognosis of the dangerous approaches with operational satellites. During VLBR08.1 (September 3-12, 2008) it is planned to arrange the VLBR observations of the objects with high AMR recently discovered into international optical observing campaigns [5].

References

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Molotov I. E., Volvach A. E., Konovalenko A. A. et al. International experiments on development of VLBI radar method for research of near-Earth bodies. Kosmichna Nauka i Tekhnologiya, Vol. 10, No. 2/3, p. 87 - 92 (2004) (In Russian).
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