The ESA Venus Express spacecraft was observed at X-band with the Wettzell radio telescope in October-December 2009 in the framework of an assessment study of the possible contribution of the European VLBI Network to the upcoming ESA deep space missions. A major goal of these observations was to develop and test the scheduling, data capture, transfer, processing, and analysis pipeline. Recorded data were transferred from Wettzell to Metsahovi for processing, and the processed data were sent from Mets ahovi to JIVE for analysis. A turnover time of 24 hours from observations to analysis results was achieved. The high dynamic range of the detections allowed us to achieve a milliHz level of spectral resolution accuracy and to extract the phase of the spacecraft signal carrier line. Several physical parameters can be determined from these observational results with more observational data collected. Among other important results, the measured phase fluctuations of the carrier line at different time scales can be used to determine the influence of the solar wind plasma density fluctuations on the accuracy of the astrometric VLBI observations

Ayucar, Miguel Perez

Calves, Guifre Molera

Cimo, Giuseppe

Kronschnabl, Gerhard

Neidhardt, Alexander

Pogrebenko, Sergei

Wagner, Jan

NASA Technical Reports Server

Guifre´ Molera Calve´s et al.: First Results of Venus Express Spacecraft Observations with Wettzell, IVS 2010 General
Meeting Proceedings, p.171–175
http://ivscc.gsfc.nasa.gov/publications/gm2010/molera.pdf
First Results of Venus Express Spacecraft Observations with
Wettzell
Guifre´ Molera Calve´s 1, Jan Wagner 1, Alexander Neidhardt 2, Gerhard Kronschnabl 3,
Miguel Pere´z Ayu´car 4, Giuseppe Cimo´ 5, Sergei Pogrebenko 5
1) Aalto University Metsa¨hovi Radio Observatory
2) FESG/TUM, Wettzell
3) BKG, Wettzell
4) ESA-ESAC, Madrid
5) Joint Institute for VLBI Europe, Dwingeloo
Contact author: Guifre´ Molera Calve´s, e-mail: gofrito@kurp.hut.fi
Abstract
The ESA Venus Express spacecraft was observed at X-band with the Wettzell radio telescope
in October-December 2009 in the framework of an assessment study of the possible contribution of
the European VLBI Network to the upcoming ESA deep space missions. A major goal of these
observations was to develop and test the scheduling, data capture, transfer, processing, and analysis
pipeline. Recorded data were transferred from Wettzell to Metsa¨hovi for processing, and the processed
data were sent from Metsa¨hovi to JIVE for analysis. A turnover time of 24 hours from observations
to analysis results was achieved. The high dynamic range of the detections allowed us to achieve a
milliHz level of spectral resolution accuracy and to extract the phase of the spacecraft signal carrier line.
Several physical parameters can be determined from these observational results with more observational
data collected. Among other important results, the measured phase fluctuations of the carrier line at
different time scales can be used to determine the influence of the solar wind plasma density fluctuations
on the accuracy of the astrometric VLBI observations.
1. Introduction
The ESA Venus Express spacecraft (VEX S/C) radio transmission signals are an interesting
target for exercising new science support methods usable in prospective ESA planetary probe
and deep space missions. Spacecraft observations, detections, processing and analysis technology
are based on the heritage of our team involved in the VLBI support of the VEGA and Huygens
missions [1, 2, 3]. During a campaign in 2008-09, a number of European VLBI Network telescopes
joined trial observations of VEX in the framework of the PRIDE (Planetary Radio Interferometry
and Doppler Experiments). This paper describes the analysis methods, introduces the software
developed for handling the analysis flow, and presents the most recent detections we achieved
during a single-dish VEX observation performed at Wettzell. The high dynamic range of our
phase detections—with a typical SNR of several thousands in 1 Hz or several millions in 1 mHz
as achieved in observations with Metsa¨hovi, Wettzell, Yebes, Medicina, Matera, and Noto stations
in coordination with ESA Space Astronomy Centre (ESAC), and ESTRACK station Cebreros,
Spain—allows to extract and analyze the phase of the S/C carrier line from many different physical
perspectives, among which the S/C orbitography and propagation effects are the first in line. In
particular we show how the phase-time of the S/C signals was applied to detect interplanetary
IVS 2010 General Meeting Proceedings 171
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Guifre´ Molera Calve´s et al.: First Results of VEX S/C Observations
plasma scintillation on a novel frequency band (X-band) and present the results of the 2009.11.6
Wettzell observation.
2. Theory
2.1. Spacecraft Detection
The technique employed in our observations enables us to obtain the spectra of the S/C radio
signal, to determine the apparent topocentric frequency of the S/C signal carrier line and accom-
panying “ranging” tones with sub-mHz accuracy, and to determine the phases of these tones with
respect to the phase of the Earth-based station’s H-maser clock. Further on, the phases can be
calibrated on a baseline basis using a VLBI multi-station phase referencing technique.
2.2. Interplanetary Plasma Effects
Interplanetary scintillation (IPS) originates from the diffraction of radio waves by fluctuations
within the solar wind. Measurements of IPS have been conducted during many years to probe
the solar wind throughout the inner heliosphere. The phase variations by the density fluctuations
scatter the signal and cause the fluctuation in received power and phase. [Density variations and
turbulence in the plasma scatter the S/C signal and cause fluctuations in received power and
phase.] Large variations (>> 1 radian) introduced across the wave front are known as “strong”
scattering. When the variations are small (<< 1 radian) the scattered waves add constructively to
generate much larger fluctuations in the signal received, known as “weak” scattering. The standard
measurement of the level of IPS is the scintillation index m. This is the ratio of the rms variation
in the strength of the source signal due to IPS and the average strength of the signal [4, 5]. It
should be noted that, especially at higher frequencies, the scintillation index depends on the source
angular size and structure. In this respect, the spacecraft, an almost ideal point source, provides
a good opportunity. Although its emitting power is subject to internal power variations which
depend on the transmitting data content, the phase of its carrier line is a stable quantity because
it is locked to a high precision atomic clock at the communication station. We note that the phase
variations in this case are the results of a two-way propagation: from the communication station
to the spacecraft and back to the observing station.
3. Methods: Data Processing and Analysis
The VEX observations at Wettzell started on 2009.11.16 using the standard VLBI Mark IV
data acquisition rack with four 8 or 16 MHz channels and 2-bit Nyquist sampling for aggregate
data rates of 128 or 256 Mbps. The Mark-IV-formatted data were recorded with Mark 5A and
PCEVN disk systems. Immediately at the end of the observational run, files were electronically
transferred through the Internet with Tsunami UDP to the Metsa¨hovi Radio Observatory (MRO)
for processing. Data were processed with the high-performance, ultra-high spectral resolution
spectrometer-correlator software under development at MRO. Processing tasks ran in two iterations
between MRO and JIVE, with the JIVE analysis software involved in the iterative loop.
With our current facilities the full processing cycle takes 5-10 times longer than the observa-
tional time, but we are constantly improving it.
172 IVS 2010 General Meeting Proceedings
Guifre´ Molera Calve´s et al.: First Results of VEX S/C Observations
3.1. Spacecraft Detection
Initial detection of tones was performed using the high-resolution spectrometer software de-
veloped at MRO (SWSpec). A Python PyQt4 GUI allows fast reconfiguration for new processing
jobs. SWSpec supports several input file formats such as the (still common) VLBA and Mark
IV magnetic tape formats on Mark 5A and PCEVN and the more modern Mark 5B and VDIF
formats. In addition, it can handle various “raw” multi-channel, multi-bit formats. A SWSpec
pass extracts one selected raw data channel and performs accurate windowed-overlapped discrete
Fourier transforms and spectrum time integration. All parameters are fully adjustable. The S/C
processing of 8 or 16 MHz baseband used 3.2M DFT points and cosine-squared windowing. Fig-
ure 1 shows the S/C Doppler detection with the Wettzell antenna, and Figure 2 the resulting
spectrum (after Doppler model removed) with a spectral resolution of 0.9 mHz.
Figure 1. Top graph: Frequency detections of the
VEX carrier signal at 8.412 GHz on 2009.11.16.
Peak frequency spread is 8.7 kHz over 1 hour.
Bottom graph: Post-fit stochastic residual ex-
hibiting an RMS of only 9 mHz at 5s sampling.
Figure 2. The resulting spectrum of the S/C car-
rier line in a zoom band after the Doppler correc-
tion. Spectral resolution at 0.9 mHz.
3.2. Tone Tracking, Filtering and Phase Detection
The output spectra of SWSpec are processed using MathCAD and Matlab scripts developed
at JIVE. They extract S/C tone frequencies (detected by visual inspection) from the series of
integrated spectra. A 4 to 7 order phase stopping polynomial is fitted to the S/C carrier line
frequency detections using a weighted Least Mean Squares (LMS) method with weight coefficients
depending on SNR and RFI considerations. The resulting polynomial coefficients are used as input
to the next processing steps: narrow band adaptive filtering and Phase-Lock-Loop (PLL).
The spacecraft tone tracking software (SCTracker) developed at MRO accepts the original raw
input data files, a list of S/C tone frequencies (relative to carrier), and the stopping polynomial
coefficients for the carrier tone. A double-precision polynomial evaluation is applied for stopping
tone phases. Adaptive filtering extracts and filters stopped tones down to a several kHz wide
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Guifre´ Molera Calve´s et al.: First Results of VEX S/C Observations
bandwidth using a 2nd order WOLA (Window-Overlap-Add) DFT-based algorithm of the Hilbert
transform approximation. The time-domain sample sequence of every extracted and filtered tone is
written as an output data set for further post-processing using digital PLL software at JIVE. The
residual phase in a stopped band is determined with respect to a set of subsequent frequency/phase
polynomials applied for phase stopping. Depending on the SNR of the carrier line and individual
tones, the final bandwidth of phase detections can range from several kHz to several mHz. In the
case of VEX carrier line, this bandwidth is in the range of 10-100 Hz. Figure 3 illustrates the
carrier line residual (after Doppler correction) phase variation.
Figure 3. Narrow band phase detections after PLL tone tracking and band filtering of the S/C carrier line.
3.3. Scintillation Analysis
The post-processing analysis of the spacecraft tracking data enables us to study several pa-
rameters of the S/C signal with milliHz accuracy, among which the phase fluctuations of the S/C
carrier line can be used for characterization of the Interplanetary Plasma (IP) density fluctuations
along the signal propagation line at different spatial and temporal scales and at different solar
elongations. These fluctuations are well represented by a near-Kolmogorov [6] spectrum. From
our VEX measurements we retrieved such essential parameters as the phase scintillation index and
bandwidth of scintillations and their dependence on the solar elongation, distance to the target,
position of the source in the solar system and the solar activity index. Multi-station observations
can distinguish the up- and down-link plasma contributions, because they will observe the S/C
through different Fresnel channels. Figure 4 shows the spectral power density of the slow fluctu-
ation phase turbulence below 10 Hz. The S/C carrier line phase scintillations measurements are
complementary to the classical power scintillation measurements of signals from the natural radio
sources and are crucial for the optimization of the design characteristics of PRIDE.
4. Conclusion
The first results are encouraging, although more test and development observations are needed
before obtaining physically significant results. The initial study focused more on the measurement
techniques and data analysis. But the physical interpretation of the results is a task that we wish
to continue in upcoming studies when more observational data is collected.
174 IVS 2010 General Meeting Proceedings
Guifre´ Molera Calve´s et al.: First Results of VEX S/C Observations
Figure 4. Spectral power density of slow fluctuation phase turbulence below 10 Hz. Note the high dynamic
range of 10e06.
5. Acknowledgements
We are thankful to several members of the VLBI radio astronomy, space science, and technology
communities. In particular we thank Ed Himwich, Ari Mujunen, Jouko Ritakari, Giuseppe Mac-
caferri, Stelio Montebugnoli, Salvatore Pluchino, Francesco Schilliro, Giuseppe Colucci, Giuseppe
Bianco, Cristiano Cosmovici, Pablo de Vicente, and Leonid Petrov for their contributions to the
ongoing work.
References
[1] Sagdeev, R.Z., et al. (1992), “Differential VLBI Measurements of the Venus Atmosphere Dynamics by
Balloons: VEGA Project”, Astron. Astrophys. 254, 387
[2] Pogrebenko, S.V., et al. (2004), “VLBI tracking of the Huygens probe in the atmosphere of Titan,
in Planetary Probe Atmospheric Entry and Descent Trajectory Analysis and Science”, ed. A. Wilson,
ESA SP-544, 197–204
[3] Lebreton, J.P., et al. (2005), “An overview of the descent and landing of the Huygens probe on Titan”.
Nature 438, 758-764.
[4] Hewish, A., Scott, P.F., and Wills, D. (1964), “Interplanetary Scintillation of Small Diameter Radio
Sources”. Nature 203: 1214-1217.
[5] Fallows, R.A., Williams, P.J. and Breen, A.R. (2002), “EISCAT measurements of solar wind velocity
and the associated level of interplanetary scintillation”, Ann. Geophys. 20: 1279-1289.
[6] Kolmogorov, A.N.(1941) “The local structure of turbulence in incompressible viscous fluid for very large
Reynolds numbers”. Proceedings of the USSR Academy of Sciences 30: 299-303 (Russian), translated
into English in Proceedings of the Royal Society of London, Series A: Mathematical and Physical
Sciences 434, 9-13 (1991).
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