This is an electronic reprint of the original article.This reprint may differ from the original in pagination and typographic detail.

Powered by TCPDF (www.tcpdf.org)

This material is protected by copyright and other intellectual property rights, and duplication or sale of all or part of any of the repository collections is not permitted, except that material may be duplicated by you for your research use or educational purposes in electronic or print form. You must obtain permission for any other use. Electronic or print copies may not be offered, whether for sale or otherwise to anyone who is not an authorised user.

Molera Calves, Guifre; Neidhardt, A.; Ploetz, C.; Khronschnabl, G.; Cimò, G.; Pogrebenko,S.V.Venus and Mars Express spacecraft observations with Wettzell radio telescopes

Published: 01/03/2016

Document VersionPublisher's PDF, also known as Version of record

Published under the following license:Unspecified

Please cite the original version:Molera Calves, G., Neidhardt, A., Ploetz, C., Khronschnabl, G., Cimò, G., & Pogrebenko, S. V. (2016). Venusand Mars Express spacecraft observations with Wettzell radio telescopes. Paper presented at IVS GeneralMeeting, Johannesburg, South Africa.

Venus and Mars Express spacecraft observations with Wettzellradio telescopes

Guifre Molera Calves 1,2, Alexander Neidhartd 3, Christian Plotz 4, Gerhard Kronschnabl 4, Giuseppe Cimo 2,5,Sergei Pogrebenko 2

Abstract The ESA Venus, Mars Express and Rosettaspacecrafts were observed at X-band with the Wettzellradio telescopes Wn and Wz in a framework of the as-sessment study of the possible contribution of the Euro-pean VLBI network to the upcoming ESA deep spacemissions and further projects. These observations wereextended to regular weekly sessions to routinely run theprocessing and analysis pipeline. Recorded data weretransferred from Wettzell to the JIVE cooperation part-ners for correlation and analysis. A high dynamic rangeof the detections allowed us to achieve a mHz level ofthe spectral resolution accuracy and extract the phaseof the spacecraft signal carrier line. Several physicalparameters can be determined from these observationalresults with more observational data collected. Apartfrom other results, the measured phase fluctuations ofthe carrier line at different time scales can assess to de-termine the influence of the Solar wind plasma densityfluctuations on the accuracy of the astrometric VLBIobservations.

Keywords VLBI, radio telescope, spacecraft signal,solar wind

1. Aalto University, School of Electrical Engineering, Depart-ment of Radio Science and Engineering, Espoo, Finland.2. Joint Institute for VLBI ERIC, Oude Hogeevensedijk 4, 7991PD Dwingeloo, The Netherlands3. Technische Universitat Munchen, Geodatisches Observato-rium Wettzell, Germany4. Bundesamt fur Kartographice und Geodasie, Germany5. ASTRON, the Netherlands Institute for Radio Astronomy,Postbus 2, 7990 AA, Dwingeloo, The Netherlands.

1 Introduction

The combination of Very Long Baseline Interferome-try (VLBI) and Doppler spacecraft tracking has beensuccessfully exploited in a number of space sciencemissions, including VLBI tracking of the descent andlanding of the Huygens Probe in the atmosphere of Ti-tan [2], VLBI observations of ESA’s Venus Expressand of the Mars Express Phobos flyby [7].

Fig. 1 Scheme of the Planetary Radio Interferometry andDoppler Experiments (PRIDE) observations.

Based on the experience acquired in this projects,the Planetary Radio Interferometry and Doppler Exper-iment (PRIDE) concept has been developed. PRIDE isan international enterprise led by the Joint Institute forVLBI in Europe (JIVE). It focuses primarily on track-ing planetary and space science missions through radiointerferometric and Doppler measurements [3]. PRIDEprovides precise estimations of the spacecraft state vec-tors based on Doppler and VLBI phase-referencingtechniques [1]. These can be applied to a wide rangeof research fields.

1

2 Molera Calves et al.

Figure 1 shows the basic configuration for thePRIDE observations, where the observations ofplanetary spacecrafts are combined with natural radiosources.

Table 1 shows the main characteristics of the radiotelescopes at the Wettzell observatory used for space-craft observations. The 20 m Radio Telescope Wettzell(RTW) with station code Wz was finished in the year1983 and is one of the main systems of the Inter-national VLBI Service for Geodesy and Astrometry(IVS). It is equipped with an S/X-receiving system.The antenna Wn with a 13.2 m dish is the northerntelescope of the newly build TWIN Radio TelescopeWettzell (TTW), which was finished inaugurated in theyear 2013. TTW is a fast-slewing broadband telescopefollowing the specifications of the VLBI Global Ob-serving System (VGOS). The antenna Wn is currentlyequipped with a standard S/X-receiving system, whichis extended with a Ka-band receiver.

Table 1 The radio telescopes at Wettzell used for the spacecraftobservations.

Station code latitude longitude altitude dish SEFD-X(m) (Jy)

Wettzell Wz 49◦08’42.0” 12◦52’37.9” 661.20 20.0 700Wettzell Wn 49◦08’38.1” 12◦52’39.7” 672.57 13.2 840

2 Observations

The PRIDE team initiated systematic observations ofplanetary spacecraft in 2009. ESAs VEX spacecraftwas selected due to its high-quality signal, suitabletransmission frequency, and possibility to observe withEuropean VLBI radio telescopes. The 20 m RTW (Wz)has participated since 2010 in 57 sessions on VEX untilthe mission ended in 2014, and in 23 sessions on MEXbetween 2014 and 2016. In 2015, the Wn was addition-ally included into the network of planetary spacecraftobservations. A total number of 17 sessions have beenarranged since then with Wn. In our observations, thespacecraft signals are observed in X-band (λ = 3.6cm,fo = 8.4GHz).

On 20.05.2015, we operated the first simultaneousobservation of spacecraft by using both Wn and Wz.After that, eight similar experiments were arranged.

Fig. 2 Detection of the spacecraft signal transmitted by Rosettaby the Wettzell radio telescopes on 24.07.2015.

The figure below shows the relative power to noise ra-tio, the Doppler frequency residuals, and their differ-ence detected on 26.10.2015.

Fig. 3 Upper panel: relative power Wz (blue) and Wn (red). Mid-dle panel: residual frequencies for same data. Lower panel: dif-ference between both telescopes residuals. MEX data observedat Wz and Wn on 26.10.2015. Standard deviation of Frequencydetections are 1.78 mHz for Wn, 1.76 for Wz and 0.5 mHz forthe difference.

Spacecraft observations are research and develop-ment approaches to open new domains for geodeticVLBI. The geodetic benefit is the interrelation to othertechniques to tie the different reference frames. VGOSsupports spacecraft tracking, while mostly Earth orbit-ing satellites in the near field are in the focus [6]. Butthe IVS encourages developments which use VLBI onsources beside quasars (see different sessions of thepast IVS General Meetings). It opens doors to space-

IVS 2016 General Meeting Proceedings

VEX/MEX spacecraft observations 3

craft navigation which is strongly related to the appli-cation of reference frames. Besides this, the observa-tions of spacecrafts are checks of the accuracy of thetechniques at the location of the antennas and through-out the whole processing chain. Additionally, they aretechnical tests for possible future domains of VLBI,which are currently already possible. Another aspectis that one of the pillars of geodesy is the gravity andthe gravitational field [10]. In a wider sense, spacecraftobservations with VLBI enable the determination ofgravitational fields of planetary objects. From the pointof view of a geodetic observatory the observations arequite valuable. Using two telescopes provides the pos-sibility of more observations, because one telescopemight be able to observe while the other is occupiedby another session. It would also offer possibilities ofnew observation modes for differential VLBI, so thatone telescope observes quasars close to the spacecraftwhile the other tracks the spacecraft itself. Current im-plementations do not take too much care on these pos-sibilities.

3 Analysis

The data processing is conducted using the on-purposesoftware developed for multi-tone tracking of plane-tary spacecraft signals. The software is divided intothree parts: the SWspec, the SCtracker and the digitalPhase-Locked-Loop. All three software packages weredescribed in [7]. Figure 3 shows a block diagram of thespacecraft tracking software.

Fig. 4 Illustration of the data analysis steps and software mod-ules used for spacecraft tracking purposes: SWspec, SCtrackerand digital PLL. The complete software is developed and main-tained jointly by JIVE and Aalto University.

The three most important parameters out of our de-tections are the Doppler noise, the Signal-to-Noise Ra-tio (SNR) of the carrier and the phase scintillation in-dices. The two first output results of a regular observa-tion are shown in Figure 2. The results are utilized fora wide field of research:

• studies of the interplanetary scintillation caused bythe solar wind at different solar elongations and po-sitions with respect to the Sun [7].

• characterization of the properties of Coronal MassEjections (CME) and monitoring of similar eventsin almost real-time [8].

• determination of the speed of the solar wind usingVLBI techniques.

• studies of the planetary atmosphere on Venus, usingradio signal and aerobreaking techniques [9].

• determination of gravitational fields of plane-tary objects (moons, comets) by using fly-bytechniques [5].

• radio occultation experiments to evaluate ingressand egress phenomena.

• orbit determination of the space missions, such asRadio Astron [3]

• enhancement and support of future planetary spacemissions: JUICE, ExoMars, Bepi-Colombo, etc.

4 Conclusions

Planetary spacecrafts as targets for radio observationsof ground-based radio telescopes provide an unique op-portunity to investigate a wide range of scientific fields.

Tracking observations of spacecraft, ESA’s MEXand Rosetta, will continue in order to improve detec-tion measurements and the processing pipeline. Futurespace mission, like ESAs Jupiter Icy Moon Explorer(JUICE), will benefit of such precise knowledge.

Radio science studies like this one allows us todisentangle the contributions from the interplanetaryplasma and the Earth’s ionosphere. The research willcontinue with the goals of characterizing the atmo-spheric and ionospheric structure of planets and media,small bodies fly-byes, atmospheric drag campaigns, ra-dio occultations, detection of coronal mass ejectionsor precise orbit determination of satellites. Besides thescientific output, the tests are technical feasibility studyof the technical workflow from scheduling to analysis.

IVS 2016 General Meeting Proceedings

4 Molera Calves et al.

Acknowledgements

We would like to thank the operators at each of the sup-porting stations that they made the observations possi-ble. We are also thankful to the project team of ESAVEX and MEX missions for their support and assis-tance of our research.

References

1. BEASLEY, A.J., & CONWAY, J.E. (1995). VLBI Phase-Referencing, eds. ASP, 82, 327.

2. BIRD, M. K., ALLISON, M., ASMAR, S. W. ET AL.(2005). Nature, 438, 8

3. DUEV, D.A., MOLERA CALVES, G., POGREBENKO, S.V.ET AL. (2012). Spacecraft VLBI and Doppler tracking: al-gorithms and implementation, A&A, 541, A43

4. DUEV, D.A., ZAKHVATKIN, M.V., STEPANYANTS, S.V.ET AL. (2015). Radio Astron as a target and as an instru-ment: Enhancing the Space VLBI missions scientific output,A&A 573, A99

5. DUEV, D.A., S.V. POGREBENKO, G. CIM, ET AL. (2016).Planetary Radio Interferometry and Doppler Experiment(PRIDE), submitted to A&A. technique: a test case of theMars Express Phobos fly-by.

6. PLANCK, L. (2013). VLBI satellite track-ing for the realization of frame ties, Dis-sertation, Technische Universitat Wien,http://ww2.erdrotation.de/SharedDocs/Downloads/EN/Publications/Dissertations/Plank Dissertation.pdf? blob=publicationFile, Download 2016-05-15.

7. MOLERA CALVES, G., CIMO, G., POGREBENKO, S.V. ET

AL. (2014). Observations and analysis of phase scintillationof spacecraft signal on the interplanetary plasma, A&A, Vol.564, id.A4, 7.

8. MOLERA CALVES, G., KALLIO, E., CIMO, G. ET AL.(2016). Exploring Interplanetary Coronal Mass Ejectionswith spacecraft radio signals, submitted to A&A.]

9. ROSENBLATT, P., BRUINSMA, S.L., MULLER-WODARG,I.C.F. ET AL. (2012). First ever in situ observations ofVenus’ polar upper atmosphere density using the trackingdata of the Venus Express Atmospheric Drag Experiment(VExADE), vol. 217, 831-838.

10. ROTHACHER, MARKUS, PLAG, HANS-PETER,NEILAN, RUTH (2015). The Global Geodetic Ob-serving System. Geodesys contribution to Earth Ob-servation. GGOS. International Association of Geodesy.http://www.ggosportal.org/lang en/nn 261454/SharedDocs/Publikationen/EN/GGOS/Introduction to GGOS,templateId=raw,property=publicationFile.pdf/Introduction to GGOS.pdf, Download 2015-09-27.

IVS 2016 General Meeting Proceedings