NASA’s Planetary Data System (PDS) is an active archive of science data from NASA planetary missions. PDS archives and distributes peer-reviewed planetary science data to the world-wide planetary science community. The Radio Science Sub-Node (RSSN) in collaboration with the Ring Moon Systems (RMS) Discipline Node aims to maintain a long-term radio science capability within PDS. Members of the RSSN team have strong connections to the NASA Deep Space Network (DSN), a long history of participation in radio science observations, and close relationships with the radio science community. RSSN plans to develop software tools and templates to assist data providers in the creation and ingestion of consistent PDS4-compliant data products as well as preparation and maintenance of mission-independent ancillary products from the DSN. RSSN will also develop software tools to assist data users in visualization and quick-look analysis of radio science data, enabling users to assess quality and usability early in their projects. With the constantly growing volume of data and the large variety of observation types, observing conditions, and object types, the development of metadata and quick-look tools will be critical in connecting scientists and other users to data of interest before committing to detailed analyses.In this presentation, we will describe the RSSN objectives, overview of recent developments, near-term plans, and engagement with the radio science community.

Illustration of a two-member constellation of small spacecraft at Venus with crosslinks for radio occultations that can lead to global coverage with high spatial and temporal resolutions. SummaryFrom Mercury to the outer reaches of the solar system, the past six decades have witnessed a vast set of discoveries utilizing radio science (RS) methods.For example, based on key gravitational evidence, sub-surface oceans have been inferred at Titan, Enceladus, and Europa, where potential future missions may search for life.

The Mars Express spacecraft is operating in Mars orbit since early 2004. The Mars Express Radio Science Experiment (MaRS) employs the spacecraft and ground station radio systems (i) to conduct radio occultations of the atmosphere and ionosphere to obtain vertical profiles of temperature, pressure, neutral number densities and electron density, (ii) to conduct bistatic radar experiments to obtain information on the dielectric and scattering properties of the surface, (iii) to investigate the structure and variation of the crust and lithosphere in selected target areas, (iv) to determine the mass, bulk and internal structure of the moon Phobos, and (v) to track the MEX radio signals during superior solar conjunction to study the morphology of coronal mass ejections (CMEs). Here we report observations, results and discoveries made in the Mars environment between 2004 and 2014 over almost an entire solar cycle.

Spacecraft-to-ground bistatic radar provides a straightforward method for surveying planetary surfaces on scales of importance to landers and rovers. Centimeter wavelengths, currently in use for deep-space telecommunications, interact with surface structure of similar to somewhat larger scales. For the quasi-specular component of scattering and for surfaces uniformly illuminated by monochromatic signals from an orbiting or flyby vehicle, the echo Doppler dispersion is proportional to the root mean square (rms) surface slope. When the specular condition occurs within 10°-20° of the Brewster angle, the surface dielectric constant can be derived from relative echo power measured simultaneously in orthogonal polarizations and the Fresnel reflection laws. Cross spectra, computed from outputs of the orthogonally polarized receivers, may be used to calculate a complete description of the polarization properties of the scattered fields. Application to planetary studies requires accurate amplitude and phase calibration of the polarization channels, including correction for any leakage between the two receiving paths, such as from imperfectly isolated antenna feeds. We illustrate these techniques using Mars Express results from “Stealth” (Medusae Fossae), a region on Mars that has not previously been detected by Earth-based radar, and from a long profile including Acidalia Planitia. Single-location Stealth observations support previous conclusions that the surface is rough and porous (dielectric constant ≈ 1.4). But the longest experiment (in which the specular point was followed for an hour) yields relatively high dielectric constants (2.8), suggesting that the model is incomplete. The surface of Acidalia Planitia has low dielectric constants ( ≈2.6) over 60-90 W at 50 N and higher values (≈3.6) as the specular point moves south and crosses the equator.

Bistatic radar provides a simple, cost-effective way to obtain survey information about planetary surfaces on scales important to landers and rovers. The centimeter-scale waves interact most strongly with surface structure on similar and slightly larger scales yielding estimates of rms surface slopes zeta and material dielectric constant epsiv (which can be related to density). Recent experiments in the Mars north polar region show an unusually heterogeneous surface with some segments having zeta less than 0.2deg. The dielectric constants appear to vary only between 1.8 within the polar cap (snow) and 3-4 outside (sand). Uplink experiments (transmissions from ground to spacecraft) have been successfully conducted using Mars Odyssey; future possibilities include spacecraft-to-spacecraft experiments.

Magellan (MGN) bistatic radar observations in 1994 confirmed earlier Pioneer Venus reports of unusual Venus surface reflectivity and emissivity at elevations above 6054 km radius. They also revealed that the anomalous values of surface dielectric constant ɛ near Cleopatra Patera included a large imaginary component (ɛ ≈ −i 100) at 13 cm wavelength, consistent with a semiconducting surface material. The MGN observations were conducted using a linearly polarized wave, canted at 45° with respect to the plane of incidence and radiated by the MGN synthetic aperture radar antenna toward the specularly reflecting region of the mean planetary surface. In 2006 similar experiments were conducted using 13 cm circularly polarized transmissions from Venus Express (VEX). The VEX signal‐to‐noise ratio (SNR) was lower than that of MGN, but elevated ∣ɛ∣ has been inferred broadly over Maxwell Montes. A quasi‐specular echo was detected near Cleopatra but with insufficient SNR to address the question of conductivity. An early failure of the VEX 13 cm radio system precludes further measurements with VEX.

Bistatic radar provides a simple, cost-effective way to obtain survey information about planetary surfaces on scales important to landers and rovers. The centimeter-scale waves interact most strongly with surface structure on similar and slightly larger scales yielding estimates of rms surface slopes zeta and material dielectric constant epsilon (which can be related to density). Recent experiments in the Mars north polar region show an unusually heterogeneous surface with some segments having zeta less than 0.2 degrees. The dielectric constants appear to vary only between 1.8 within the polar cap (snow) and 3-4 outside (sand). Uplink experiments (transmissions from ground to spacecraft) have been successfully conducted using Mars Odyssey; future possibilities include spacecraft-to-spacecraft experiments.

Between August and December 2005, we conducted 76 oblique‐incidence scattering experiments using the SRI International 46‐m antenna in the Stanford foothills to illuminate Mars for 20‐min periods with an unmodulated, 75‐cm λ, circularly polarized wave. The direct signal and a Martian surface echo, separated by differential Doppler frequency shifts, were received simultaneously by the one‐bit receiver on board the Mars Odyssey spacecraft. Four of the proposed Phoenix landing regions, denoted A–D, were probed by at least one experiment. The surface echoes are characterized by both fluctuating amplitude and varying spectral width, which are responses to surface reflectivity and roughness variations. Our analysis of the echo data is based on quasi‐specular scattering theory and makes use of high‐resolution Mars Orbiter Laser Altimeter (MOLA) topographic maps to model the scattering surface in three dimensions along the specular track of the echo. We find MOLA topography sufficient to model scattering at 75‐cm λ, indicating that the effective horizontal scales being sensed by the radar are at or larger than the MOLA grid spacing of 150–500 m. We find that the RMS surface slopes for the proposed Phoenix landing regions, as determined using the Hagfors scattering law and applicable to those scales, rarely exceed 1°, except in the presence of large craters and other surface irregularities.

Radio Science is an opportunistic discipline in the sense that the communication link between a spacecraft and its supporting ground station can be used to probe the intervening media remotely. Radio science has recently expanded to greater, cooperative use of international assets. Mars Express and Venus Express are two such cooperative missions managed by the European Space Agency with broad international science participation supported by NASA's Deep Space Network (DSN) and ESA's tracking network for deep space missions (ESTRAK). This paper provides an overview of the constraints, opportunities, and lessons learned from international cross support of radio science, and it explores techniques for potentially optimizing the resultant data sets.

Still delivering ESA's Venus Express probe has been in orbit since April 2006. Eight research papers in this issue present new results from the mission, covering the atmosphere, polar features, interactions with the solar wind and the controversial matter of venusian lightning. Håkan Svedham et al . open the section with a review of the similarities and (mostly) differences between Venus and its 'twin', the Earth. Andrew Ingersoll considers the latest results, and also how the project teams plan to make the most of the probe's remaining six years of life.

EXPRESS BISTATIC RADAR. R. A. Simpson, G. L. Tyler, B. Hausler, and M. Patzold, Stanford University (Packard Bldg Room 332, MC 9515, Stanford, CA 94305; rsimpson@magellan.stanford.edu), Stanford University (Packard Bldg Room 331, MC 9515, Stanford, CA 94305; len.tyler@stanford.edu), Universitat der Bundeswehr Munchen (Institut fur Raumfahrttechnik, D-85577 Neubiberg, Germany; bernd.haeusler@unibw-muenchen.de), Universitat zu Koln (Institut fur Geophysik und Meteorologie, Albert Magnus Platz, D-50923 Koln, Germany; paetzold@geo.uni-koeln.de).

Introduction: Mars Express (MEX) bistatic radar (BSR) ground tracks will not move to far northern (e.g., Phoenix) latitudes until June 2006. But the generally uniform appearance of these plains areas suggests that certain surface characteristics may be inferred from observations already in hand. Of particular interest are two MEX experiments conducted in the vicinity of the Viking Lander 2 site (Table 1).