spacescience-report2007-08-lig.doc
Space Science at JIVE
1. Reconstruction of the descent trajectory of the Huygens Probe in the atmosphere of Titan
The ESA Huygens Probe, a part of the NASA/ESA/ASI Cassini-Huygens mission was launched toward the Saturnian System in 1997. The European part of the mission, the Huygens Probe has arrived to its final destination, Titan, on 14 January 2005. VLBI observation of its descent and landing were performed with a global array of radio telescopes in Australia, China, Japan and the USA. The Huygens VLBI tracking experiment was led by JIVE. The work was supported by the ESA contract 18386/NL/NR.
The final analysis of the Huygens VLBI tracking data was completed during the reporting period. The results were delivered to ESA and presented at a number of international scientific conferences. The final report on this project is published as the JIVE Research Note 0011. Fig. *.1 represents the 3D reconstructed descent trajectory of the Probe.
Fig. *.1. An artist impression of the Huygens Probe descent (left) and the VLBI-based reconstructed 3D trajectory of the descent (right).
The 1 σ stochastic error scatter ellipse of position determination in Titanographic coordinates in the reconstructed trajectory is 0.5 × 2 km, with the minor semi-axes oriented along the longest baseline of the VLBI array - Mopra (Australia) to Green Bank (USA). This translates into the angular accuracy ellipse of 100 by 400 microarcseconds. Such the accuracy of measurements provides a valuable input into the studiers of the air mass dynamics of Titan’s atmosphere. The Huygens VLBI tracking experiment paved the way for future multi-task applications of VLBI techniques for planetary and space science missions
2. Analysis of the Huygens VLBI tracking data with high velocity resolution
High spectral resolution Doppler data from the Huygens VLBI tracking experiment, obtained by JIVE scientists, made possible investigation of the probe’s velocity variation pattern with a sub-cm-per-second accuracy. The velocity data provide valuable input into the study of the flight dynamics effects of parachute and balloon systems in planetary atmospheres. The results of the study have direct applications to the design of prospective planetary probes on Mars, Venus and Titan.
The results of high velicity resolution study were presented at 4th International Workshop on Tracking, Telemetry and Command Systems Applications (ESOC, Darmstadt, Germany, 2007). Fig. *.2 illustrates the detection of a 1.2 rpm probe’s spinning with the rotational velocity of 2 cm/s.
Fig. *.2. Upper left panel: Auto-correlation functions (ACF) for the post-landing (dash line) and parachuting (solid line), Parkes tracking data. Middle left panel: the velocity deviation spectra – for post-landing (boxes) and parachuting (circles) phases, showing the 0.02 Hz and 2 cm/s probe’s spin spectral feature. Lower left panel: an overlay of the estimated spin-induced radial velocity variation component (blue line) over the parachuting velocity deviation curve (black line). Right panel: an artist’s impression of the Huygens Probe on Titan
3. Spacecraft radio occultations
The ESA Smart-1 Lunar orbiter was observed with the EVN telescopes (Medicina, Metsähovi, Westerbork) in 2006. At several occasions, an occultation of the spacecraft by the Moon was observed. High spectral resolutions processing was conducted with the Huygens VLBI software correlator. It was fascinating to see the diffraction patterns clearly appearing in the data. A proper description of the VLBI-detected diffraction patterns requires a 3-Dimentional wave-optics relativistic model of VLBI delay. Jian Nianchuan, a summer student from the Shanghai Astronomical Observatory (China) visited JIVE in the summer of 2007 and worked under supervision by Sergei Pogrebenko on the first order approach to the problem He developed a simulation model of a spacecraft signal diffraction by lunar surface, using a high resolution 3D map of the Moon created by a laser altimeter onboard the USAF Clementine Moon orbiter. The results of the simulation work were in agreement with the experimental data. They were presented at the 9th ILEWG International Conference on Exploration and Utilisation of the Moon (Sorrento, Italy, 22-26 October, 2007),
Fig. *.3. A simulated diffraction pattern of a S/C radio signal on different diffraction screens.
4. Demonstration radio observations of spacecraft
An observing campaign on spacecraft tracking with the Metsähovi (TKK-MRO, Finland) and Medicina (INAF-IRA, Italy) radio telescopes, coordinated by JIVE, has started in 2008 as a part of development activities in preparation for future involvement of EVN and JIVE in ESA/NASA/JAXA deep space missions.
ESA Venus Express (VEX) spacecraft was observed with the Metsähovi radio telescope at X-band on 11 June 2008 using multi-bit data sampling, a purpose-built data capture instrumentation and high performance processing software, developed at the Metsahovi Radio Observatory in collaboration with JIVE. The data analysis tools were developed at JIVE. Fig. *.4 shows a summary plot of results of this test run.
Fig. *.4. Top left corner inset: a preview spectrum of 50 Hz resolution over a 8 MHz video band, with more than 40 dB dynamic range. Top right inserts: the detected phases of the carrier line and sub-carrier harmonic at the -35 dBc level. Central part of the plot: the final spectra of the carrier line and one of sub-carriers with 0.6 mHz resolution and SNR of 5×106.
Successful detection and phase extraction of the -35 dB subcarrier the VEX S/C from the distance of 1.5 AU demonstrates a possibility of scientific experiments with Metsähovi-class telescopes and deep space missions at the distances of up to 50 AU. This is a very encouraging conclusion in preparation for the EVN participation in deep space gravity theory test missions (Pioneer-anomaly), like the Odyssey mission, proposed for the ESA Cosmic Vision programme (B.Cristophe, Experimental Astronomy, in press). JIVE scientists, involved in this study, showed that high-precision VLBI measurements could vectorise the mysterious anomalous accelerations of spacecraft at high velocities and large distances from Sun.
The NASA/ESA Ulysses spacecraft was observed with the Medicina telescope at S-band on 28 November 2008. For these observations, 8-bit data capture employed a device based on iBob hardware and FPGA firmware developed at Metsähovi. Two hours of the captured data were electronically transferred from Medicina to Metsähovi for high performance processing; while intermediate data were send from MRO to JIVE for post-processing and analysis. At the time of the observations Ulysses was 4.5 AU away from Earth, and its signal power level was more than 1000 times less than that from Venus Express. Preliminary results of this test are shown in Fig. *.5.
Fig. *.5. A dynamic spectrum of the Ulysses carrier line observed with the Medicina telescope, processed at Metsähovi and analysed at JIVE. The spectral resolution is 10 Hz over the 8 MHz video band. The received signal power was 3 dBTsys in 1 Hz.
5. Simulation of the tropospheric limitations on the accuracy of differential VLBI on multiple spacecraft
Several planned ESA/NASA/JAXA/RSA missions will involve multiple spacecraft and planetary probes. VLBI on the multiple transmitters on and around the same planet can provide ultra-precise differential measurements of the state vectors of the S/C and probes. One of the limiting factors in such measurements is the propagation effects in the Earth atmosphere. To characterise these propagation effects one can use simulations of electromagnetic wave propagation through the turbulent media with the Kolmogorov spectrum of density fluctuations. The JIVE summer student Dmitry Duev (Astronomical Department of the Moscow State University), supervised by Sergei Pogrebenko performed these simulations at JIVE in July-August 2008. Fig. *.6 shows the typical case of differential fluctuations of electrical length of the troposphere for 1 arcminute angular separation between two point-like radio sources. A separation of 1 arcminute (or less) is typical for multiple S/C in Mars, Mercury or Venus environments. It was shown that RMS of differential fluctuations in such cases is at a level of 10 micron, thus adding a positional noise for differential VLBI measurements at the level of sub-microarcseconds, or less than a meter at a distance of 2-3 AU.
Fig. *.6. Simulated tropospheric differential electrical lengths for 1 arcmin separation near local zenith.
6. Water masers in the Saturnian system
A search for 22 GHz water maser emission from the Saturnian system triggered by the recent discovery of a water plum from Enceladus was conducted by a JIVE-led international group of researchers with the Medicina (INAF-IRA, Italy) and Metsähovi (TKK-MRO, Finland) radio telescopes. The experiment employed data handling algorithms and procedures originally developed for the Huygens VLBI tracking experiment. M ore that 300 hours of “on-Saturn” data were collected during the 2006-2008 observing campaigns. A direct-FFT on-line hardware spectrometer was used at the Medicina Observatory. At Metsähovi, spectral analysis was done off-line: the data were first recorded on disks using different data capture units and then processed with the high performance software spectrometer, developed at the Metsähovi Observatory. The resulting spectra were analysed at JIVE. The water maser emission was detected in the areas associated with different bodies of the Saturnian system: Titan, Hyperion, Enceladus and Atlas (Fig. *.7). The results were published in the beginning of 2009 (Pogrebenko, Gurvits, Elitzur et al, 2009, Astronomy & Astrophysics 294, L1).
Fig. *.7. The 22 GHz spectra reduced for the orbital motion of Atlas, the most secure detection of the observing campaign. Orbital phase 5, which has the highest SNR detection (6.5σ), corresponds to the orbital segment shown in red in the upper left panel. The high SNR of this detection and its persistence over one year of observation (as illustrated on the right panel) is indicative on a possible association of the maser emission with a spot lagging the position of Atlas by several thousand km along its orbit rather than Atlas itself. The emission might originate in the edge regions of rings A and F, disturbed by the Atlas's motion.
7. Planetary Radio Interferometry and Doppler Experiment (PRIDE)
The Earth-based global network of radio telescopes and processing facilities will conduct Planetary Radio Interferometry and Doppler Experiment (PRIDE) aimed at providing ultra-precise estimate of the state-vectors of the in situ elements in the framework of planetary missions proposed for the ESA Cosmic Vision 2015-2025 programme. JIVE-led PRIDE is a component of the following proposals: Kronos (in situ study of Saturn), TandEM (Titan and Enceladus Mission), Laplace (the mission to study Europa and the Jovian system) and EVE (European Venus Explorer). Fig. *.8 presents a generic configuration of PRIDE. PRIDE is based on the heritage of the Huygens Doppler Wind (DWE) and VLBI tracking experiments. Today’s technology and very conservative projection of capabilities of VLBI radio telescopes for the next two decades lead to the following guaranteed 1σ accuracy of positional measurements: 500 m based on S-band (2 GHz signal), 100 m at X-band (8 GHz) and 30 m at Ka-band (32 GHz) at the distance of up to 8 AU. The advantage of PRIDE is that it poses minimal requirements for the on-board instrumentation.
Fig. *.8. A generic configuration of PRIDE, involving an Earth-based VLBI network and planetary spacecraft of probes of different types (orbiter, lander, atmosphere balloon).
Elements of PRIDE will be used in the ESA-JAXA mission BepiColombo on the Japanese-led Mercury Magnetospheric Orbiter (MMO) by the JIVE team in collaboration with the Institute of Space and Astronautical Sciences (Japan) and Shanghai Astronomical Observatory (China) in 2011-2019.
8. “Direct to Earth” mode as an SKA role in planetary exploration
For many planetary missions, the nominal data broadcast scenario assumes relay of science and house-keeping data from a low-power probe transmitter to an orbiter or fly-by spacecraft with re-transmission to Earth via more powerful transmitter and high-gain antenna. Such the scheme can support delivery to Earth large volumes of data with the data rate of tens of kbps. However, as an efficient backup able to provide support to critical mission operations and experiments, a low data-rate link can be achieved with the nominal transmission from the low-power in-situ elements and received by the large Earth-based radio telescopes – the so called Direct-to-Earth (DtE) regime. The most attractive option of DtE would involve the Square Kilometre Array as the Earth-based facility able to operate at the S band (2.3 GHz). Fig. *.9 illustrates the potential of SKA as a DtE facility for a mission to an outer planet mission. As shown by preliminary assessment estimates, SKA will be able to receive data streams from such the mission at the rate of 30-100 bps (Fridman, Gurvits, Pogrebenko, 2008, SKA Memo No. 104). The DtE regime is included in the preliminary design study of the ESA Cosmic Vision proposals for the Europa Jupiter System Mission (EJSM) and Titan Saturn System Mission (TSSM). JIVE is involved in both these design studies with PRIDE and DtE components.
Fig. *.9. Bit error rate (BER) as a function of transmission bit rate (in units of bps) for three different modulation and coding schemes. The bit rate of 30-50 bps with the BER=10-4 – 10-3 can be achieved with SKA receiving a signal from an omnidirectional transmitting antenna at the distance of 5 AU (typical distance to a Jovian probe) and transmitter power 1 – 3 W.
9. Toward Moon-based Ultra-Long-Wavelength Astronomy (ULWA) interferometer
Under a joint project co-funded by the Royal Dutch Academy of Sciences (KNAW) and Chinese Academy of Sciences (CAS) PhD student Linjie Chen co-supervised by Leonid Gurvits (JIVE), Richard Strom (ASTRON and University of Amsterdam), Heino Falcke (ASTRON and Radboud University, Nijmegen), Yan Yihua and Huang Maohai (both National Astronomical Observatories of China, Beijing) works on the demonstrator for a Moon-based radio interferometer for frequencies below 10 MHz. This frequency domain is the last unexplored region of cosmic electromagnetic spectrum. The Moon is an attractive base for an Ultra-Long-Wavelength Astronomy (ULWA) facility and is being considered in preliminary studies of the next wave of lunar missions in Europe, China and other countries (Fig. *.10).
Fig. *.10. Left: a hypothetic Moon-based ULWA interferometer, resembling a LOFAR-style configuration of antenna elements. Right: a simulated 3D beam pattern of a Beverage antenna, investigated by L.Chen as a possible element of a demonstrator for a Moon-basxed ULWA system.
The work is a natural extension of JIVE and ASTRON expertise since the ULWA facility will be an interferometric system, a kind of LOFAR extension toward ultimately low frequencies unreachable for Earth-based radio telescopes. The PhD project materials provide inputs into pre-design studies conducted by the European and Chinese space agencies.
10. Space VLBI
Preparation for the next generation Space VLBI mission Astro-G/VSOP-2, led by the Japanese Aerospace Agency (JAXA) accelerated in the reporting period as the launch date, fixed for 2013 is approaching. In December 2008, the leading mission organization, JAXA’s Institute for Space and Astronautical Sciences (ISAS) convened the first meeting of a newly established VSOP-2 International Science Council (VISC-2, Fig. *.11) which involves a representative of JIVE (Leonid Gurvits). The expertise in Space VLBI available at JIVE will be focused at VSOP-2 science operations, data processing and user support.
Fig. *.11. Top: an artist’s impression of the VSOP-2 Astro-G spacecraft. Bottom: VISC-2 members at ISAS in December 2008. In the background – a mockup of the historic rocket M-V that put in orbit the first dedicated Space VLBI mission HALCA/VSOP in February 1997.
In October 2008, JIVE scientists participated in the conference “Radio Universe at Ultimate Angular Resolution” dedicated to the scientific programme of the Space VLBI mission RadioAstron (Fig. *.12), held in Moscow, Russia.
insert figure 12 
Fig. *.12. RadioAstron antenna at the Lavochkin Association assembly facility, Khimki, Moscow region, October 2008.