Radio science investigations by VeRa onboard the Venus Express spacecraft

doi:10.1016/j.pss.2006.04.032

Planetary and Space Science

Volume 54, Issues 13–14, November 2006, Pages 1315-1335

https://doi.org/10.1016/j.pss.2006.04.032 Get rights and content

Abstract

The Venus Express Radio Science Experiment (VeRa) uses radio signals at wavelengths of 3.6 and 13 cm (“X”- and “S”-band, respectively) to investigate the Venus surface, neutral atmosphere, ionosphere, and gravity field, as well as the interplanetary medium. An ultrastable oscillator (USO) provides a high quality onboard reference frequency source; instrumentation on Earth is used to record amplitude, phase, propagation time, and polarization of the received signals. Simultaneous, coherent measurements at the two wavelengths allow separation of dispersive media effects from classical Doppler shift.

VeRa science objectives include the following:

(1)
Determination of neutral atmospheric structure from the cloud deck (approximately 40 km altitude) to 100 km altitude from vertical profiles of neutral mass density, temperature, and pressure as a function of local time and season. Within the atmospheric structure, search for, and if detected, study of the vertical structure of localized buoyancy waves, and the presence and properties of planetary waves.
(2)
Study of the H₂SO₄ vapor absorbing layer in the atmosphere by variations in signal intensity and application of this information to tracing atmospheric motions. Scintillation effects caused by radio wave diffraction within the atmosphere can also provide information on small-scale atmospheric turbulence.
(3)
Investigation of ionospheric structure from approximately 80 km to the ionopause (<600 km), allowing study of the interaction between solar wind plasma and the Venus atmosphere.
(4)
Observation of forward-scattered surface echoes obliquely reflected from selected high-elevation targets with anomalous radar properties (such as Maxwell Montes). More generally, such bistatic radar measurements provide information on the roughness and density of the surface material on scales of centimeters to meters.
(5)
Detection of gravity anomalies, thereby providing insight into the properties of the Venus crust and lithosphere.
(6)
Measurement of the Doppler shift, propagation time, and frequency fluctuations along the interplanetary ray path, especially during periods of superior conjunction, thus enabling investigation of dynamical processes in the solar corona.

Introduction

The Venus Express (VEX) Radio Science Experiment (VeRa) utilizes radio science (RS) techniques for studies of the Venus atmosphere, ionosphere, gravity field, and surface. Additional science objectives include radio sounding investigations of the solar corona and the near-Sun interplanetary medium. The VeRa instrument and its capabilities have been described in detail by Häusler et al. (2006). A suite of companion investigations on Mars Express (MEX) has been described by Pätzold et al. (2004).

VeRa investigations will be carried out by making use of the VEX spacecraft transponder, which is otherwise used for telemetry and tracking. This transponder has been augmented by the addition of an ultrastable reference frequency oscillator (USO) specifically to support the VeRa experiments.

RS investigations fall into three broad categories of experimentation and observation. First, for the study of planetary atmospheres and ionospheres, the spacecraft must be ‘occulted’ so that the gas or plasma lies between the radio source and receiver. In a typical occultation experiment conducted with an orbiter, the spacecraft sequentially passes behind the ionosphere, the neutral atmosphere, and finally the planetary disk as seen from the tracking station on Earth; it then re-emerges in the reverse sequence. During an occultation event one ‘senses’ the media of interest—atmosphere and ionosphere—by their effects on the radio signal (Fjeldbo and Eshleman, 1969; Fjeldbo et al., 1971; Eshleman, 1973). For the case of coherent, dual-frequency transmission, this experiment allows the separation of dispersive and non-dispersive media effects from classical Doppler effects.

Second, oblique-incidence scattering investigations using propagation paths from a spacecraft via a planetary surface to an Earth-based station can be used to explore the surface properties including the microwave scattering function. Such investigations are commonly referred to as “bistatic radar”, because the transmitter (the spacecraft) and receiver (the ground system on Earth) are separated by significant angular distances and/or ranges. Magellan observations in the early 1990s greatly improved our understanding of the microwave emissivity, reflectivity, and topography of the Venus surface (Pettengill et al., 1992; Ford and Pettengill, 1992; Tyler et al., 1992), but some of the detailed relationships among these properties remain unresolved (Carter et al., 2001).

Third, “gravity” experiments use precision measurements of the distance and velocity along the line of sight between the spacecraft and Earth to detect perturbations in the gravity field in order to determine the distribution of mass within Venus. The spherical harmonic gravity field of Venus was determined to degree and order 180 by Barriot et al. (1998) and Konopliv et al. (1999), but questions concerning the properties of the Venusian crust and lithosphere below special target areas still remain open.

Radio occultation measurements strongly complement and extend other spacecraft and Earth-based remote-sensing observations, such as infrared spectroscopy, which provide detailed information on atmospheric constituents and vertical structure at low resolution over wide regions by instrument scanning. Greatly improved results are obtained by combining radio-occultation measurements with such supplementary observations.

In all cases, a RS experiment relies on the extreme frequency stability of the signals employed and measured. VeRa will be the first RS experiment at Venus to employ an ultra-stable reference frequency source (USO) on board the spacecraft.

A summary of the observation possibilities and scientific objectives, including RF-transmission modes of the VEX spacecraft has been presented in Häusler et al. (2006).

Section snippets

Background

The Venusian atmosphere consists mainly of CO₂ (∼96.5%) and N₂ (∼3.5%) with small amounts of other gases (see Fig. 1). The lower and middle atmosphere display a strong zonal wind structure with a period of 4–5 days (“super-rotation”, in the sense of planetary rotation). Venus is shrouded by a 20 km thick cloud layer; the surface temperature is ∼735 K (Seiff et al., 1985; Moroz and Zasova, 1997, Moroz and Zasova, 1997). There is no sublimation/condensation of the atmosphere as is the case for

Background

The absence of a planetary magnetic field leads to important differences between the ionospheres of Earth and Venus. The upper atmosphere of Venus, unprotected by a magnetic field from direct interaction with the solar wind, is subject to strong atmospheric escape processes.

Solar radiation creates a hot neutral atmosphere which extends into the solar wind. The ionospheric pressure, consisting of both thermal and magnetic components, balances the dynamic pressure of the solar wind (Fig. 8). The

Background

Magellan addressed and answered fundamental questions about the geophysics of Venus and its geological history but presented us with new problems. We know that Venus is a “one-plate” planet and does not have plate tectonics as on Earth. The tectonic style of this planet, however, is not clearly understood. A strong link with the underlying mantle dynamics is suspected. In such a context the structure of the lithosphere is a key parameter because it plays a major role in the mantle dynamics and

Background

The first radar scattering cross-section measurements of the unresolved Venus disk took place in 1961 from Earth (Victor and Stevens, 1961; Pettengill et al., 1962, Pettengill et al., 1997).

Due to limitations imposed by Earth-based observing geometry, echoes at normal incidence could be obtained only near the equator. At radar wavelengths longer than 15 cm (f<2 GHz), a disk-averaged reflectivity of approximately 0.15 was obtained. Beginning in 1978, the PVO carried out vertical radar altimeter

Solar corona

Venus passes through superior solar conjunction once during the nominal mission and once during the extended mission. The conjunction geometries, showing the position of Venus in the plane of the sky relative to the Sun, are shown in Fig. 12. Mostly northern ecliptic latitudes are sounded during the nominal mission solar conjunction in October 2006 when the minimum solar offset is 3.6 solar radii. In contrast, the Venus-to-Earth ray path moves nearly diametrically along a radial during the

The VEX radio communication system

At the NNO ground station data are received with a 35 m dish providing a gain of 55.8 dBi in S-band and 68.2 dBi in X-band at a system noise temperature of 47.9 and 74 K, respectively. Carrier to noise analyses have been presented in Häusler et al. (2006).

A block diagram of the VEX onboard radio subsystem is shown in Fig. 14. The 1.30 m diameter main high gain antenna HGA1 with two feeds (X- and S-band) will be used for the VeRa operations. The transponder X-band signals are being amplified by two

VeRa experiment planning within the VEX-mission

The VEX spacecraft was launched from Baikonur, Kazakhstan, by a Soyuz-Fregat vehicle on 9 November 2005, 04:33 UTC. Following a 5-month interplanetary cruise, VEX will be injected into a 24-h polar orbit with semi-major axis a=39,494 km (6.5 Venus radii) and eccentricity e=0.84 on 11 April, 2006. The height of pericenter will be adjusted with thruster firings such that it stays in the range 250–400 km.

In contrast to the mission baseline which calls for the Cebreros ground station for command and

Conclusions

The VEX mission provides an opportunity to extend our knowledge of the atmophere, gravity, and surface of Venus—a decade after radar mission Magellan and several decades after missions such as Veneras 4-16, Mariner 5, 10, and Pioneer Venus.

The Venus Express Radio Science Experiment (VeRa) will use radio signals to sound the atmosphere and ionosphere of Venus during Earth occultations, to probe the surface with bistatic radar, to measure gravity anomalies near pericenter, and to study the

Acknowledgements

The authors are thankful to ESA and EADS-Astrium for supporting the development of the instrument.

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Radio science investigations by VeRa onboard the Venus Express spacecraft

Abstract

Introduction

Section snippets

Background

Background

Background

Background

Solar corona

The VEX radio communication system

VeRa experiment planning within the VEX-mission

Conclusions

Acknowledgements

Icarus

Icarus

Planet. Space Sci.

Icarus

Icarus

Icarus

Icarus

Icarus

Icarus

Icarus

Adv. Space Res.

Adv. Space Res.

Adv. Space Res.

Adv. Space Res.

Icarus

Adv. Space Res.

Icarus

Icarus

Adv. Space Res.

Some results of dual-frequency radio occultation exploration of the nighttime ionosphere of Venus with satellites Venera-9 and 10

Space Res.

Middle Atmosphere Dynamics

The geology and geomorphology of the Venus surface as revealed by the radar images obtained by Veneras 15 and 16

J. Geophys. Res.

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Astron. J.

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