Overview of Differential VLBI Observations of Lunar Orbiters in SELENE (Kaguya) for Precise Orbit Determination and Lunar Gravity Field Study

The Japanese lunar explorer SELENE (Kaguya), which was launched on September 14th, 2007, was the target of VLBI observations over the period November 2007 to June 2009. These observations were made in order to improve the lunar gravity field model, in particular the lower degree coefficients and the model near the limb. Differential VLBI Radio sources, called VRAD instruments, were on-board the subsatellites, Rstar (Okina) and Vstar (Ouna), and the radio signals were observed by the Japanese VERA (VLBI Exploration of Radio Astrometry) network, and an international VLBI network. Multi-frequency and same-beam VLBI techniques were utilized and were essential aspects of the successful observing program. Multi-frequency VLBI was employed in order to improve the accuracy of the orbit determination obtained from the phase delay from the narrow-band satellite signals, while the same-beam VLBI method was used to resolve the cycle ambiguity which is inherent in the multi-frequency VLBI method. The observations were made at three S-band frequencies (2212, 2218 and 2287 MHz), and one X-band frequency (8456 MHz). We have succeeded in correlating the recorded signals from Okina/Ouna, and we obtained phase delays with an accuracy of several pico-seconds at S-band.     

Japanese satellite program on the solar research for the coming solar maximum

The Institute of Space and Astronautical Science (ISAS) is developing a satellite dedicated to high-energy observations of solar flares. The Solar-A will be launched in August–September, 1991, from the Kagoshima Space Center on board a M3S-II vehicle. The instrument complement emphasizes hard X-ray and soft X-ray imaging, with both high resolution (2.4 arc sec pixel size) and full-Sun field of view. Solar-A contains instruments supplied in part by U.S. and U.K. experimenters. This paper describes the instrumentation and the tentative observing program.     

Prospects for space stellar astrometry

Astrometry is the major astronomical technique to measure distances, masses and motions of stars. Dividing astrometric techniques into five types according to the size of the field in which a single instrument can produce measurements, the present achievements of the Earth-based astrometry are described. The astrometric activities such as measurements of star diameters, double star relative positioning or stellar parallaxes, search for invisible companions, photographic plate reduction, visual and photoelectric meridian and astrolable astrometry are reviewed. Then, the methods used to construct a quasi-inertial celestial reference frame and to materialize it by a fundamental catalogue are presented and discussed. A much better definition of an absolute reference frame is made possible by VLBI, but the problem of extending it to stellar positions is not yet satisfactorily resolved.The limitations of the ground based astrometry are: the atmospheric turbulence and refraction, Earth's motions and the impossibility to view the entire sky with a single instrument. These limitations are discussed and it is shown how astrometry from space can overcome them. A priori, a gain of two orders of magnitudes in accuracy for all types of astrometry is expected, but at this new level of precision, new effects and limitations will appear, as already shown in the studies of the approved programs.Then, the ESA astrometric satellite HIPPARCOS presently under development is presented. The satellite and the payload are described as well as the observing procedures. Several limitations, specific to space borne instrumentation and to the milliarc second accuracy expected have been identified. However the main limitation in precision remains the photon noise. The data reduction methods are sketched. The data downlinked at a rate of 20 kilobits per second have to be used with an equal weight all over the 21/2 years of observation. They are expected to yield a mean accuracy of 2 milliarc seconds in position and parallax and 2 m.a.s. per year in proper motion for most of the 100000 stars of the program (M _b < 9). Stars to be observed by HIPPARCOS have to be carefully selected. The main fields in which the results of HIPPARCOS will be used are listed from the proposals made by the scientific community. The task of constructing the HIPPARCOS input catalogue from these proposals is presented.Another feature of the ESA astrometric satellite is the use of the HIPPARCOS star-mapper as a photometric and position survey of the sky. This experiment, called TYCHO, should give at least 400000 star positions with accuracies of the order of 0.03 to 0.15 depending upon the magnitudes. Two colour instantaneous magnitudes should also be obtained to 0.1–0.4 mag. precision.Several Space-Telescope on-board instruments are also capable to make small field astrometric observations. Accurate imaging is possible with the Wide Field and the Faint Object cameras. Lunar occultations will be performed with the High Speed photometer. But the main astrometric mode of the Space Telescope will be the use of the Fine Guidance Sensors to measure the relative positions of stars to ±0.002. It is described together with its main scientific applications.The establishment of an absolute reference frame is subsequently discussed. Plans using simultaneously VLBI, HIPPARCOS, and Space Telescope observations are described. They consist in linking the HIPPARCOS stellar system to quasars via radio-stars or stars in the vicinity of optical quasars.Finally, several space astrometry proposals are described: long focus space astrometry and two versions of space interferometry.     

USB与VLBI联合确定“探测一号”卫星轨道

我国绕月探测工程“嫦娥一号”卫星将以统一S波段(USB)为主,辅以甚长基线干涉仪(VLB I)测轨分系统来完成测控任务。由于“探测一号”卫星轨道与“嫦娥一号”调相轨道段相似,有关单位于2005年3月17日—20日进行了USB和VLB I联合跟踪“探测一号”试验。通过对联合测轨数据的处理,研究了USB—VLB I联合定轨方法,分析了联合定轨和预报精度,得出了一些结论。     

Hubble Space Telescope at twelve years of age

The Hubble Space Telescope was deployed from the Space Shuttle Discovery into a 380-mile high Earth orbit on April 25, 1990. It subsequently made outstanding astronomical discoveries with its 8-foot (2.4-meter) telescope and other scientific instruments. Critical to the successful observations was continuous availability of power from its solar arrays during sunlit periods, and from nickel-hydrogen batteries when the satellite was in the Earth's shadow. The adopted nickel-hydrogen batteries were carefully selected and tested to confirm their depth-of-discharge and operating temperature that delivered the longest life in charge/discharge cycling service. These batteries had a design life of 7 years. At 12 years after launch the Hubble batteries have delivered more charge/discharge cycles than any other batteries in low-Earth orbit. However, the Hubble batteries have been subjected to many unexpected stresses, and peculiar reductions in battery capacity have been observed. Battery replacement requires a costly trip to the Hubble Space Telescope by astronauts, so the remaining useful life of the batteries must be predicted. Already in four servicing missions, astronauts have replaced or modified optics, solar arrays, a power control unit, and various science packages. A fifth servicing mission is scheduled in 2004. This paper discusses battery charging hardware and software controls, history of battery events in Hubble, cell performance model and spare battery tests, and capacity walkdown.     

The Radio Frequency Subsystem and Radio Science on the MESSENGER Mission

The MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) Radio Frequency (RF) Telecommunications Subsystem is used to send commands to the spacecraft, transmit information on the state of the spacecraft and science-related observations, and assist in navigating the spacecraft to and in orbit about Mercury by providing precise observations of the spacecraft’s Doppler velocity and range in the line of sight to Earth. The RF signal is transmitted and received at X-band frequencies (7.2 GHz uplink, 8.4 GHz downlink) by the NASA Deep Space Network. The tracking data from MESSENGER will contribute significantly to achieving the mission’s geophysics objectives. The RF subsystem, as the radio science instrument, will help determine Mercury’s gravitational field and, in conjunction with the Mercury Laser Altimeter instrument, help determine the topography of the planet. Further analysis of the data will improve the knowledge of the planet’s orbital ephemeris and rotation state. The rotational state determination includes refined measurements of the obliquity and forced physical libration, which are necessary to characterize Mercury’s core state.     

OPPORTUNITIES FOR MAGNETOSPHERIC RESEARCH WITH COORDINATED CLUSTER AND GROUND-BASED OBSERVATIONS

ESA's first multi-satellite mission Cluster is unique in its concept of 4 satellites orbiting in controlled formations. This will give an unprecedented opportunity to study structure and dynamics of the magnetosphere. In this paper we discuss ways in which ground-based remote-sensing observations of the ionosphere can be used to support the multipoint in-situ satellite measurements. There are a very large number of potentially useful configurations between the satellites and any one ground-based observatory; however, the number of ideal occurrences for any one configuration is low. Many of the ground-based instruments cannot operate continuously and Cluster will take data only for a part of each orbit, depending on how much high-resolution (burst-mode') data are acquired. In addition, there are a great many instrument modes and the formation, size and shape of the cluster of the four satellites to consider. These circumstances create a clear and pressing need for careful planning to ensure that the scientific return from Cluster is maximised by additional coordinated ground-based observations. For this reason, the European Space Agency (ESA) established a working group to coordinate the observations on the ground with Cluster. We will give a number of examples how the combined spacecraft and ground-based observations can address outstanding questions in magnetospheric physics. An online computer tool has been prepared to allow for the planning of conjunctions and advantageous constellations between the Cluster spacecraft and individual or combined ground-based systems. During the mission a ground-based database containing index and summary data will help to identify interesting datasets and allow to select intervals for coordinated studies. We illustrate the philosophy of our approach, using a few important examples of the many possible configurations between the satellite and the ground-based instruments.     

The European Columbus Programme

This paper describes the current status of the COLUMBUS Programme, Europe's contribution to the U.S. Space Station, which is being studied under contract to the European Space Agency. Twelve European nations are involved in and are contributing to this new space undertaking. The elements of the COLUMBUS Space Segment presently being considered by ESA are a Pressurized Laboratory Module (4 segment), permanently attached to the U.S. Space Station, dedicated to materials science, fluid physics and compatible life sciences, and a Polar Platform, configured to accommodate Earth observation, meteorology, communications and space science payloads. The reference launch vehicles are the Space Shuttle for the attached Module and Ariane 5 for the Polar Platform. The more recently added COLUMBUS flight configuration, the Man-Tended Free Flyer (MTFF), consists of a combination of two programme elements, the Resource Module and a 2-segment Pressurized Module. It is designed to provide all required resources and services to the various payloads in a continuous microgravity environment to perform material science, fluid physics and compatible life sciences experiments. The MTFF is carried into orbit by the European launcher Ariane 5. As an option, studies of an enhanced ground based EURECA carrier as a small co-orbiting platform, launched by the Space Shuttle, will be initiated. The primary function would be to accommodate space science and/or micro-g payloads. These EURECA studies are performed currently outside the COLUMBUS programme scope, and this option is therefore not addressed in detail in this paper.     

科学实验卫星高速上行链路设计

为保障在卫星与地面之间开展空间量子科学实验的条件,必须为科学实验载荷建立一条星—地高速数据双向传输链路.通过跟踪CCSDS-SLS-NGU（空间数据系统咨询委员会-空间链路业务-下一代上行链路）工作组对NGU的研究进展,结合科学实验卫星有效载荷建立高速上行链路的需求,采用高带宽利用率调制技术、高效信道编码方式及适用的链路数据传输协议,设计了一种传输速率为1Mbit/s、误码率优于1×10-9的高速上行链路方案,并给出星载设备实现方案和地面初步测试结果.该方案在技术体制上兼容已有CCSDS规范,便于地面站及星载接收机实现,完全满足开展空间量子科学实验的需要.     

GPS Program Status

The Navstar Global Positioning System (GPS) Program is composed of three segments ? Space, Control, and User Equipment. The Space segment is responsible for the development and launch of the GPS satellite constellation. The Control segment is responsible for monitoring the satellite telemetry and providing updated navigation information to the satellites. The User Equipment (UE) segment is responsible for the development and procurement of the GPS receivers for a variety of host vehicle platforms. Recently, approval was given to the User segment to enter Low Rate Initial Production (LRIP). This approval marks the beginning of Phase III (production and deployment) of the GPS program. This paper will discuss the overall status of all three segments with an emphasis on the User Equipment segment as it enters the production phase of the program.     

Water in Space: The Water World of ISO

In this review we present the main results obtained by the ISO satellite on the abundance and spatial distribution of water vapor in the direction of molecular clouds, evolved stars, galaxies, and in the bodies of our Solar System. We also discuss the modeling of H₂O and the difficulties found in the interpretation of the data, the need of collisional rates and the perspectives that future high angular and high spectral resolution observations of H₂O with the Herschel Space Observatory will open.     

Very Long Baseline Interferometry: Dependencies on Frequency Stability

Very Long Baseline Interferometry (VLBI) is a differential technique observing radiation of compact extra-galactic radio sources with pairs of radio telescopes. For these observations, the frequency standards at the telescopes need to have very high stability. In this article we discuss why this is, and we investigate exactly how precise the frequency standards need to be. Four areas where good clock performance is needed are considered: coherence, geodetic parameter estimation, correlator synchronization, and UT1 determination. We show that in order to ensure the highest accuracy of VLBI, stability similar to that of a hydrogen maser is needed for time-scales up to a few hours. In the article, we are considering both traditional VLBI where extra-galactic radio sources are observed, as well as observation of man-made artificial radio sources emitted by satellites or spacecrafts.     

嫦娥二号再拓展试验测定轨精度研究

基于＂嫦娥二号＂卫星再拓展试验的设计轨道,研究各种摄动力对轨道确定精度的影响,得出的结论是：若要达到km量级的轨道确定精度,必须考虑除天王星和海王星之外所有大行星以及日月的质点引力。文章进一步利用数值分析法研究再拓展任务的轨道确定精度,分析结果表明：基于目前的测控条件,使用30 d以上的测轨弧段可以得到稳定可靠的轨道解,而短弧（小于20 d）稳定轨道的获取需要VLBI（甚长基线干涉）测轨数据支持;当＂嫦娥二号＂距离地球700万km时,测控精度可优于30 km;虽然每天测轨弧段的增加可以改善轨道精度,但是当增加到8 h以上时,定轨精度将不再有明显改善。     

Deuterium Observations in the Galaxy

An accurate measurement of the primordial value of D/H would provide one of the best tests of nucleosynthesis models for the early Universe and the baryon density. Such evaluations have been traditionally made using present estimations of the deuterium abundance in the interstellar medium, extrapolated backwards in time with the use of galactic evolution models. Direct estimations of the primordial deuterium abundance have been carried out only recently in QSOs absorbers at high redshift.We will summarize galactic observations of deuterium and suggest that, perhaps, a single D/H value for the interstellar medium is not representative. These evaluations mainly came from observations completed in the far UV with first the Copernicus satellite over the Lyman lines series followed then by H and D Lyman-alpha lines observations with both the IUE and the GHRS on the Hubble Space Telescope. We discuss different known systematics and show that the situation is not yet clear. It is not possible today to claim that we know "the" D/H value in the interstellar medium, if any.Overall and in the context of additional D observations made in the solar system, we conclude that the actual evolution of deuterium from Big-Bang nucleosynthesis to now is not yet understood. More observations, recently made with IMAPS (the Interstellar Medium Absorption Profile Spectrograph) and hopefully to be made with FUSE (the Far Ultraviolet Spectroscopic Explorer to be launched in the fall of 1998), at higher spectral resolution or in many different galactic sites are certainly needed to help us reach a better global view of the evolution of that key element, and thus better constrain any evaluation of its primordial abundance.     

The infrared space observatory

The Infrared Space Observatory (ISO), a programme of the European Space Agency, is an astronomical satellite operating at wavelength from 2.5 to 200 m. It will be launched in 1995.The ISO optical subsystem is a cryogenically cooled telescope with its baffling system (main baffle and sunshade). The telescope, a 60 cm Ritchey-Chrétien type, focuses the beam to the four scientific instruments located in its focal plane. The extremely low temperature, 1.8 K, is provided by the payload module (PLM) cryostat, filled with superfluid He.This paper presents the main choices done for the telescope design together with their rationale and the performances achieved on the flight model (FM) of the telescope. The FM telescope is presently installed inside the payload module, ready for the system final verifications.     

The W-band data collection experiment of the DAVID mission

The data collection experiment (DCE) of the scientific mission DAVID (DAta and Video Interactive Distribution) of the Italian Space Agency (ASI) will pioneer the use of the W-band for telecommunications experiments. In particular the collection of high volumes of data from remote or virtually remote sites will be achieved through the exploitation of a W-band link in a time window of a few minutes per satellite pass. The experiment will hence demonstrate the capability of the W-band channel to be used reliably for a telecommunication link. At the same time, the experiment will provide useful elements for the characterisation of the W-band channel, in order to be able to design properly future operational systems working at W-band.     

Chemical Composition of Icy Satellite Surfaces

Much of our knowledge of planetary surface composition is derived from remote sensing over the ultraviolet through infrared wavelength ranges. Telescopic observations and, in the past few decades, spacecraft mission observations have led to the discovery of many surface materials, from rock-forming minerals to water ice to exotic volatiles and organic compounds. Identifying surface materials and mapping their distributions allows us to constrain interior processes such as cryovolcanism and aqueous geochemistry. The recent progress in understanding of icy satellite surface composition has been aided by the evolving capabilities of spacecraft missions, advances in detector technology, and laboratory studies of candidate surface compounds. Pioneers 10 and 11, Voyagers I and II, Galileo, Cassini and the New Horizons mission have all made significant contributions. Dalton (Space Sci. Rev., 2010, this issue) summarizes the major constituents found or inferred to exist on the surfaces of the icy satellites (cf. Table 1 from Dalton, Space Sci. Rev., 2010, this issue), and the spectral coverage and resolution of many of the spacecraft instruments that have revolutionized our understanding (cf. Table 2 from Dalton, Space Sci. Rev., 2010, this issue). While much has been gained from these missions, telescopic observations also continue to provide important constraints on surface compositions, especially for those bodies that have not yet been visited by spacecraft, such as Kuiper Belt Objects (KBOs), trans-Neptunian Objects (TNOs), Centaurs, the classical planet Pluto and its moon, Charon. In this chapter, we will discuss the major satellites of the outer solar system, the materials believed to make up their surfaces, and the history of some of these discoveries. Formation scenarios and subsequent evolution will be described, with particular attention to the processes that drive surface chemistry and exchange with interiors. Major similarities and differences between the satellites are discussed, with an eye toward elucidating processes operating throughout the outer solar system. Finally we discuss the outermost satellites and other bodies, and summarize knowledge of their composition. Much of this review is likely to change in the near future with ongoing and planned outer planet missions, adding to the sense of excitement and discovery associated with our exploration of our planetary neighborhood.     

The LISA Pathfinder Mission

LISA Pathfinder, formerly known as SMART-2, is the second of the European Space Agency’s Small Missions for Advance Research and Technology, and is designed to pave the way for the joint ESA/NASA Laser Interferometer Space Antenna (LISA) mission, by testing the core assumption of gravitational wave detection and general relativity: that free particles follow geodesics. The new technologies to be demonstrated in a space environment include: inertial sensors, high precision laser interferometry to free floating mirrors, and micro-Newton proportional thrusters. LISA Pathfinder will be launched on a dedicated launch vehicle in late 2011 into a low Earth orbit. By a transfer trajectory, the sciencecraft will enter its final orbit around the first Sun-Earth Lagrange point. First science results are expected approximately 3 months thereafter. Here, we give an overview of the mission including the technologies being demonstrated.     

James Webb Space Telescope: Project Overview

The James Webb Space Telescope (JWST) project at the NASA, Goddard Space Flight Center (GSFC) is responsible for the development, launch, flight, and science operations for the telescope. The project is in phase B with its launch scheduled for no earlier than June 2013. The project is a partnership among NASA, the European Space Agency (ESA), and the Canadian Space Agency (CSA). The JWST mission team is fully in place, including major ESA and CSA subcontractors. This provides an overview of the planned JWST science, current architecture focusing on the instrumentation, and mission status, including technology developments, and risks.     

2001 Mars Odyssey Mission Summary

The 2001 Mars Odyssey spacecraft, now in orbit at Mars, will observe the Martian surface at infrared and visible wavelengths to determine surface mineralogy and morphology, acquire global gamma ray and neutron observations for a full Martian year, and study the Mars radiation environment from orbit. The science objectives of this mission are to: (1) globally map the elemental composition of the surface, (2) determine the abundance of hydrogen in the shallow subsurface, (3) acquire high spatial and spectral resolution images of the surface mineralogy, (4) provide information on the morphology of the surface, and (5) characterize the Martian near-space radiation environment as related to radiation-induced risk to human explorers. To accomplish these objectives, the 2001 Mars Odyssey science payload includes a Gamma Ray Spectrometer (GRS), a multi-spectral Thermal Emission Imaging System (THEMIS), and a radiation detector, the Martian Radiation Environment Experiment (MARIE). THEMIS and MARIE are mounted on the spacecraft with THEMIS pointed at nadir. GRS is a suite of three instruments: a Gamma Subsystem (GSS), a Neutron Spectrometer (NS) and a High-Energy Neutron Detector (HEND). The HEND and NS instruments are mounted on the spacecraft body while the GSS is on a 6-m boom. Some science data were collected during the cruise and aerobraking phases of the mission before the prime mission started. THEMIS acquired infrared and visible images of the Earth-Moon system and of the southern hemisphere of Mars. MARIE monitored the radiation environment during cruise. The GRS collected calibration data during cruise and aerobraking. Early GRS observations in Mars orbit indicated a hydrogen-rich layer in the upper meter of the subsurface in the Southern Hemisphere. Also, atmospheric densities, scale heights, temperatures, and pressures were observed by spacecraft accelerometers during aerobraking as the spacecraft skimmed the upper portions of the Martian atmosphere. This provided the first in-situ evidence of winter polar warming in the Mars upper atmosphere. The prime mission for 2001 Mars Odyssey began in February 2002 and will continue until August 2004. During this prime mission, the 2001 Mars Odyssey spacecraft will also provide radio relays for the National Aeronautics and Space Administration (NASA) and European landers in early 2004. Science data from 2001 Mars Odyssey instruments will be provided to the science community via NASA’s Planetary Data System (PDS). The first PDS release of Odyssey data was in October 2002; subsequent releases occur every 3 months.