======Space Science======1. SPACE SCIENCE 1.1. VLBI AND SPACE SCIENCE The Space Science and Innovative Applications (SpaSIA) group continued developments of two space-oriented applications, the near-field VLBI for multi-disciplinary scientific applications as well as support to the operational (RadioAstron) and design studies of prospective Space VLBI missions.
The near-field VLBI technique allows researchers to determine state-vectors of target sources (spacecraft) with high accuracy. The interest of the planetary and space science communities to this technique translates into an enlargement of the user base of VLBI facilities.
Over the reporting period of 2013-14, the SpaSIA group continued developing several key components of the near-field VLBI technique. These include all steps of spacecraft VLBI tracking experiments, from planning to post-processing. In particular, the team has developed and tested a set of software tools, which allow efficient scheduling of near-field VLBI tracking experiments and pipelining of single-dish data in order to estimate radial Doppler-shift of the spacecraft signal. In collaboration with the R&D group of JIVE and Department of Astrodynamics and Space Missions of the Delft University of Technology, the software correlator SFXC has been upgraded with a set of modules specific for near-field VLBI processing. These modules include the correlator delay model tested in VLBI experiments on targets at distances from several astronomical units (e.g. Mars Express) down to Earth satellites (e.g. RadioAstron). The results of these are described in the papers published by the SpaSIA group in 2013-14 (Molera Calvés et al. 2014 AA 564, A4; Duev et al. 2015, AA 573, A99).
Over the reporting period, most of the JIVE activities in the area of space science applications of VLBI were supported via EC FP7 project ESPaCE (grant agreement 263466, on-going) as well as collaborations between JIVE and Chinese radio astronomy observatories co-sponsored by the NWO and Shanghai Astronomical Observatory (ShAO).
1.2. PLANETARY RADIO INTERFEROMETRY AND DOPPLER EXPERIMENT (PRIDE)
The Planetary Radio Interferometry and Doppler Experiment is based on the near-field VLBI developments described above. In 2012 PRIDE has been selected and in 2014 adopted as a part of the science suit of the ESA’s mission JUICE (Jupiter Icy Satellites Explorer). PRIDE for the JUICE mission is an instrument with zero demand on the science payload mass, and only ad hoc demand on other S/C resources (onboard power, commands, telemetry). The experiment is designed as an enhancement of the science output of the mission by means of exploiting the available, mostly the service onboard instrumentation and the infrastructure of Earth-based radio astronomy facilities (Fig. 1).
Fig. 5.1. Generic configuration of PRIDE-JUICE.
Fig. 5.2. PRIDE-JUICE measurables
PRIDE will provide additional measurements in the areas of prime focus for other JUICE experiments. In certain applications its “deliverables” are unique. The latter is most obvious in the astrometric domain where PRIDE can provide precise estimates of the S/C celestial position directly in the ICRF frame thus enabling precise determination of the celestial mechanics parameters of the Solar System bodies, in particular, the Jovian satellites. All scientific applications of PRIDE are based on two measurables: the lateral (transverse) celestial position of S/C and its radial velocity (Doppler). The former is the main outcome of VLBI racking of S/C, while the latter is an “inevitable” ad hoc product of VLBI tracking (Fig. 2).
Together with oyher JUICE instruments, PRIDE-JUICE will address the following scientific objectives: 1). Improvement of the ephemerides of the Galilean satellites; 2). Providing accurate input into definition of the Solar System reference frame; 3). Determination of the Ganymede shape (in concurrence with GALA); 4). Measurements of surface slope, near surface dielectric constant and surface density; 5). Characterization of the gravity field parameters of Ganymede, Callisto and Europa; 6). Determination of the vertical structure of the Jovian atmosphere; vertical structure (electron density) of the Jovian ionosphere and characterization of electron density profiles in ionospheres of Ganymede, Callisto and Europa by means of radio occultation. The specifics of PRIDE measurements make it convenient to represent all tasks assigned to PRIDE-JUICE in the following four groups: • Positioning measurements o Characterisation of surface motion over Ganymede's tidal cycle. o Investigation of the core and rocky mantle. o Improvement of Jovian system ephemerides. • Range rate (Doppler) for flight dynamics measurements o Determination of the amplitude and phase of the gravitational tides. o Characterisation of surface motion over Ganymede's tidal cycle. o Investigation of the core and rocky mantle. o Investigation of the interior of Callisto, with a special emphasis on its degree of differentiation. • Radio occultation experiments o Determination of the sources and sinks of the ionosphere and exosphere. o Characterisation of surface organic and inorganic chemistry, including abundances and distributions of materials. o Determination the thermodynamics of atmospheric meteorology. o Determination of the three-dimensional temperature, cloud and aerosols structure from Jupiter’s upper troposphere to the lower thermosphere. • Ad hoc interplanetary plasma diagnostics o Study of the interactions between Jupiter's magnetosphere and Io, Europa, Ganymede, and Callisto by means of characterisation of the turbulence in the Jovian system and inter-planetary electron plasma via radio scintillation study of the JUICE transmission. The first group above requires PRIDE to provide measurements of the S/C differential lateral position relative to the ICRF2 background extragalactic radio sources with the accuracy of 100-10 μas (1 sigma RMS) over integration time 60-1000 s.
The other three groups are based essentially on range rate (Doppler) measurements which for all applications require PRIDE to provide supplementary multi-static Doppler range-rate measurements for S/C OD with an accuracy of no worse than 0.015 mm/s (1 sigma RMS) at 60 s integration time.
Scientific applications of PRIDE measurements and some methodological topics of the experiment are described in the recent publications [Duev et al. 2012], [Molera Calvés et al. 2014].
Being large and massive bodies, the Galilean moons will strongly influence the JUICE orbit at different phase of the Jovian tour. This is especially the case during Galilean satellites’ flybys and the Ganymede's orbital phase. PRIDE will monitor the JUICE S/C will detect the gravitational perturbation by the moons, allowing in turn accurate determination of their positions. The moons’ positioning accuracy will be highly dependent on the accuracy of the S/C state vector. Flybys will provide the position of the related moon at central epoch, while orbital phase around Ganymede will provide continuous tracking of Ganymede. In that last case, Jupiter's ephemeris accuracy will affect Ganymede's observed position too and will need to be solved at the level of accuracy of the PRIDE measurements. The lateral S/C position measurements (ICRF) with the 1-sigma accuracy of about 10-100 μas translates into 30-300 meters at opposition every 60-1000 seconds. This is 103 times better than ground based observations including mutual event observations. During the Ganymede's orbital phase, this will be an improvement of a factor 10,000 compared to Earth-based optical astrometry. Moreover, direct velocity measurements of Ganymede will be available for the first time, owing to Doppler measurements of JUICE during the Ganymede's orbital phase (PRIDE range rate accuracy of ~0.015 mm/s over 60 s integration, X-band tracking in the two-way Fig. 5.3. RadioAstron mission (Spektr-R spacecraft) in orbit, artist’s impression. Courtesy Lavochkin Association.
X/X regime). It is noteworthy that the amount of data to be produced PRIDE-JUICE will enlarge significantly the overall astrometric database on the Jovian system. JIVE leads the PRIDE-JUICE development in cooperation with the Royal Observatory of Belgium, France Laboratoire d'Astrophysique de Bordeaux, Observatoire de Paris and CNES (France), DLR and TU Berlin (Germany), FÖMI Satellite Geodetic Observatory (Hungary), Delft University of Technology (The Netherlands), Institute for Space Sciences (Romania), and UC Berkeley (USA).
1.3. SPACE VLBI: RADIOASTRON
After completion of the in-orbit checkout period in the beginning of 2012, the Space VLBI mission RadioAstron began implementation of its Early Science Programme, Key Science Programmes and regular PI-led science experiments. The SpaSIA group at JIVE took part in all three stages of the science operations of the RadioAstron mission. The special task addressed by the group dealt with enhancements of the RadioAstron orbit determination by conducting PRIDE-style tracking of the spacecraft. This activity resulted in a publication by Duev et al. (2015). It has been shown that PRIDE tracking can enhance, as necessary for particularly demanding long-baseline experiments, orbit determination enabling sub-nanosecond delay model predictions (Fig. 5.4). After completion of verification tests described in the paper by Duev et al. 2015, the SFXC correlator at JIVE has been declared as available for correlating user-led RadioAstron experiments. Based on this, several RadioAstron proposals submitted in January 2015 have requested correlation at JIVE.
Fig. 5.4. Improved versus nominal orbital solution: difference in measured residual delays (blue dots) compared to the difference in modeled delays (green dots) (top), and their double difference (bottom). Baseline Effelsberg – RadioAstron. Experiment GK047A, 2013.03.09-10.
1.4. TOWARD SPACE-BORNE ULTRA-LONG-WAVELENGTH INTERFEROMETER
The Ultra-Long-Wavelength (ULW) regime (300 to 10 m in wavelength or 1 to 30 MHz in frequency) is one of the last major unexplored bands in electromagnetic spectrum of cosmic emission. Opening this unique band will enable transformational scientific results. A joint Sino-European proposal called DSL, Discovering the Sky at the Longest Wavelengths, have been worked on in 2013-14 with participation by JIVE radio astronomers. This joint proposal has been built up on the long productive cooperation between Chinese and European scientists and engineers in radio astronomy, space sciences and technology.
This proposal describes the scientific impact and rationale of a mission in the unique observing window of astronomy, cosmology, Solar- and geo-physics. The technology involved is interferometric in its fundamental nature is consistent with the expertise accumulated at JIVE. The mission concept aims at a broad variety of topics ranging from cosmology (“dark ages”) to astrophysics of ultra-steep spectrum extragalactic radio sources and neutron stars to physical processes in solar-terrestrial plasma. The concept of the DSL mission is based on the advanced technology of micro-satellites (Fig. 5.5). They form a regular constellation on the Moon orbit taking the advantage of partial shielding from terrestrial radio-frequency interferences (RFI). It is expected that in the coming decade the concept will mature and result in the creation of the new space-based radio astronomy facility.
Fig. 5.5. An artist’s impression of the DSL mission. A formation of 8 micro-satellites equipped with ULW (Ultra-Long Wavelegth) antennas and receivers creates an interferometric array. A larger spacecraft serves, in particular, as an autonomus data processing centre of the DSL mission.