Distributed Space Segment for High Energy Astrophysics: Similarities And specificites

This paper analyses two height energy astrophysics missions, MAX and SIMBOL-X, which have been studied in CNES in the frame of a large formation flying study program. It is particularly interesting to notice that the scientific specifications of two different missions lead to the same engineering solutions for the whole mission aspects and then advocate for a similar space segment architecture and re-use of common elements, thus allowing potential cost reductions for a second mission.In deed, the same level of data to download and a similar signal-to-noise ratio requirements leads to the same orbit and communications system, the same level of pointing precision and distance inter satellites lead to the same formation flying Guidance Navigation and Command (GNC) architecture. At the end, the same level of mass and thermal constraints leads to the same range of platform and the same propulsion systems and finally to the same launcher.     

PROBA2: Mission and Spacecraft Overview

Within the European Space Agency’s (ESA) General Support and Technology Programme (GSTP), the Project for On-Board Autonomy (PROBA) missions provide a platform for in-orbit technology demonstration. Besides the technology demonstration goal, the satellites allow to provide services to, e.g., scientific communities. PROBA1 has been providing multi-spectral imaging data to the Earth observation community for a decade, and PROBA2 provides imaging and irradiance data from our Sun to the solar community. This article gives an overview of the PROBA2 mission history and provides an introduction to the flight segment, the ground segment, and the payload operated onboard. Important aspects of the satellite’s design, including onboard software autonomy and the functionality of the navigation and guidance, are discussed. PROBA2 successfully proved again within the GSTP concept that it is possible to provide a fast and cost-efficient satellite design and to combine advanced technology objectives from industry with focussed objectives from the science community.     

MAX: Formation flying for nuclear astrophysics

In 2004 CNES decided to perform 4 phase 0 studies dedicated to Astrophysics and achieved thanks to Formation Flying space systems: ASPICS (A Solar Physics Mission to observe in UV and Visible the Solar Corona between 1.01 and 3.2 Solar Radius), PEGASE (an IR interferometry mission to observe Hot Jupiter, Brown Dwarfs and Proto planetary disks), SIMBOL-X (hard X-rays telescope to observe: Accretion onto compact objects, Black Holes, obscured Galactic Nuclei, ˙˙˙˙) and MAX (a Nuclear Astrophysics Mission to observe: Supernovae, Neutron Stars,˙). For this last mission, presented here, two spectral bands around important gamma-ray lines have been selected (450–530 and 800–900 keV). The formation flight allows to realise a long focal length of 80–90 m which is necessary to build a reasonably sized gamma-ray telescope based on a Laue crystal lens. The Space System design allows to have a good spacecrafts mass margin in High Elliptical Orbit with a Soyuz launch (Initial Orbit: Perigee altitude ∼44,000 km and Apogee altitude ∼253,000 km).     

Monte Carlo study of detector concepts for the MAX Laue lens gamma-ray telescope

MAX is a proposed Laue lens gamma-ray telescope taking advantage of Bragg diffraction in crystals to concentrate incident photons onto a distant detector. The Laue lens and the detector are carried by two separate satellites flying in formation. Significant effort is being devoted to studying different types of crystals that may be suitable for focusing gamma rays in two 100 keV wide energy bands centered on two lines which constitute the prime astrophysical interest of the MAX mission: the 511 keV positron annihilation line, and the broadened 847 keV line from the decay of ⁵⁶Co copiously produced in Type Ia supernovae. However, to optimize the performance of MAX, it is also necessary to optimize the detector used to collect the source photons concentrated by the lens. We address this need by applying proven Monte Carlo and event reconstruction packages to predict the performance of MAX for three different Ge detector concepts: a standard coaxial detector, a stack of segmented detectors, and a Compton camera consisting of a stack of strip detectors. Each of these exhibits distinct advantages and disadvantages regarding fundamental instrumental characteristics such as detection efficiency or background rejection, which ultimately determine achievable sensitivities. We conclude that the Compton camera is the most promising detector for MAX in particular, and for Laue lens gamma-ray telecopes in general.     

SVOM: a Joint Gamma-ray Burst Detection Mission

SVOM (Space-based multi-band astronomical Variable Objects Monitor) is an international cooperation project led by the Chinese National Space Agency (CNSA) and the Centre National d’Etudes Spatiales of France (CNES). SVOM focuses on the detection of Gamma-ray bursts (GRBs). It is developed by the Chinese Academy of Sciences (CAS), CNES, and several other French laboratories. With the multi-band observation, fast manoeuvrability, flexible operation, and the capability of ground follow-up observation, the SVOM project will be the most important GRB detection mission after the SWIFT project, and will open a wide exploration field. In this paper, the project management, science objectives, the satellite platform and payloads, the ground segment, and operation concept are illustrated.     

“Bumerang” Must Return to Earth

Solar System Research - The “Bumerang” space mission to the satellites of Mars is discussed. The purpose of the mission is to study the satellites and return of soil samples from Phobos...     

GRB satellite triggers for neutrino telescopes

The real‐time distribution of alert messages from satellites that detect gamma‐ray bursts are of key importance to neutrino telescopes.We describe how the distribution network of these alert messages is used by the ANTARES neutrino telescope, and the resulting increase in detection efficiency for neutrinos from gamma‐ray bursts. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Feasibility study of a multiple flyby mission of main belt asteroids

Specific information on the surface morphology, composition, mean density, and internal structure of asteroids, which is necessary to advance our understanding of asteroids, can be obtained only by a detailed investigation of individual bodies: this will require space missions to individual targets. Since an essential characteristic of the asteroids is their variety, several objects must be visited. The Ariane launcher developed presently in Europe makes a multiple flyby mission possible. The first results of our feasibility study are particularly encouraging: during one revolution, five to six preselected main belt asteroids may be approached to within 1000 km with relative velocities which lie between 3 and 14 km/sec using a total impulse correction on the order of 1 to 2 km/sec. The weight of the spacecraft, excluding the engine and the propellant, would be at least 250 kg. This allows a scientific payload of 50 to 60 kg, in which priority will be given to an imaging system and radar altimeter.     

The DynaMICCS perspective

The DynaMICCS mission is designed to probe and understand the dynamics of crucial regions of the Sun that determine solar variability, including the previously unexplored inner core, the radiative/convective zone interface layers, the photosphere/chromosphere layers and the low corona. The mission delivers data and knowledge that no other known mission provides for understanding space weather and space climate and for advancing stellar physics (internal dynamics) and fundamental physics (neutrino properties, atomic physics, gravitational moments...). The science objectives are achieved using Doppler and magnetic measurements of the solar surface, helioseismic and coronographic measurements, solar irradiance at different wavelengths and in-situ measurements of plasma/energetic particles/magnetic fields. The DynaMICCS payload uses an original concept studied by Thalès Alenia Space in the framework of the CNES call for formation flying missions: an external occultation of the solar light is obtained by putting an occulter spacecraft 150 m (or more) in front of a second spacecraft. The occulter spacecraft, a LEO platform of the mini sat class, e.g. PROTEUS, type carries the helioseismic and irradiance instruments and the formation flying technologies. The latter spacecraft of the same type carries a visible and infrared coronagraph for a unique observation of the solar corona and instrumentation for the study of the solar wind and imagers. This mission must guarantee long (one 11-year solar cycle) and continuous observations (duty cycle > 94%) of signals that can be very weak (the gravity mode detection supposes the measurement of velocity smaller than 1 mm/s). This assumes no interruption in observation and very stable thermal conditions. The preferred orbit therefore is the L1 orbit, which fits these requirements very well and is also an attractive environment for the spacecraft due to its low radiation and low perturbation (solar pressure) environment. This mission is secured by instrumental R and D activities during the present and coming years. Some prototypes of different instruments are already built (GOLFNG, SDM) and the performances will be checked before launch on the ground or in space through planned missions of CNES and PROBA ESA missions (PICARD, LYRA, maybe ASPIICS).     

A Research on the Collision Probability Calculation of Space Debris for Nonlinear Relative Motions†

The calculation of collision probability is the foundation of collision detection and avoidance maneuver for space objects. Now an assumption of linear relative motion is usually applied in the calculation of collision probability and then the complex 3-dimensional problem can be reduced to a 2-dimensional integral of probability density function over the area of circle. However, if the relative velocity value is very small, the term of linear relative motion is not valid. So it is necessary to consider the calculation of collision probability for nonlinear relative motions. The method used to calculate collision probability for nonlinear relative motion is studied, and test cases are designed to justify the validity of this method. It is applicable to collision probability problems involving relative velocity and error covariance varying with time. The results indicate that it is necessary to calculate collision probability with this nonlinear method under certain circumstances. For example, for elliptical relative motions in Satellite Formation Flying, when the relative velocity is below 100 m/s, the relative error between the linear method and the nonlinear method exceeds 5%; for the problem of conjunction analysis of two satellites with circular orbits, when the relative velocity is below 10 m/s, the relative error is also larger than 1%. Some significant conclusions are obtained for the collision detection system of our country.     

TAUVEX: status in 2011

We present a short history of the TAUVEX instrument, conceived to provide multi-band wide-field imaging in the ultraviolet, emphasizing the lack of sufficient and aggressive support on the part of the different space agencies that dealt with this basic science mission. First conceived in 1985 and selected by the Israel Space Agency in 1989 as its first priority payload, TAUVEX is fast becoming one of the longest-living space project of space astronomy. After being denied a launch on a national Israeli satellite, and then not flying on the Spectrum X-Gamma (SRG) international observatory, it was manifested since 2003 as part of ISRO’s GSAT-4 Indian satellite to be launched in the late 2000s. However, two months before the launch, in February 2010, it was dismounted from its agreed-upon platform. This proved to be beneficial, since GSAT-4 and its launcher were lost on April 15 2010 due to the failure of the carrier rocket’s 3rd stage. TAUVEX is now stored in ISRO’s clean room in Bangalore with no firm indications when or on what platform it might be launched.     

An exact solution of the gamma ray burst arrival time analysis problem

An analytical solution of the GRB arrival time analysis is presented. The errors in the position of the GRB resulting from timing and position errors of different satellites are calculated. A simple method of cross-correlating gamma ray burst time-histories is discussed.     

Mosaic and gradient SiGe single crystals for gamma ray Laue lenses

Both Ge_1−x Si _x mosaic crystals and Si_1−x Ge _x crystals with gradient of composition could be grown using the modified Czochralski technique to produce the diffracting elements for the MAX gamma ray telescope. Although many elements cut from the mosaic crystal and used before for CLAIRE gamma ray telescope had an efficiency up to 20%, the overall efficiency of the lens was about 8.1 ± 0.7 %, which is more than twice lower than the theoretically predicted efficiency. Some causes of this behaviour are discussed. In addition to mosaic crystals, the growth of Si_1−x Ge _x crystals with a gradient of composition and their properties are analysed. Such composition-gradient crystals could be a promising way to improve the diffraction efficiency of Laue lens for MAX.     

Recent advances and low cost concept for the gamma-ray lens project MAX

Until recently, focusing of gamma-radiation was regarded as an impracticable task. Today, gamma-ray lenses have become feasible and present promising perspectives for future instrumentation. For the first time in high energy astronomy the signal/noise ratio will be dramatically improved as gamma-rays are collected on the large area of a lens from where they are focused onto a small detector. Besides an unprecedented sensitivity, MAX will feature very high angular and energy resolution. The conjunction of this new technique of Gamma Ray focusing and the new possibilities brought by the developping formation flying technology will allow a great step for Gamma Astronomy. This paper will describe after a short recall of the scientific objectives of MAX, the design of the MAX formation flying concept and the associated main design drivers and constraint.     

Chang’E-1 gamma ray spectrometer and preliminary radioactive results on the lunar surface

The Chang’E-1(CE-1) spacecraft took a gamma-ray spectrometer (hereafter, CGRS) to detect the element distributions on the lunar surface in a circular, 200 km altitude, polar orbit with approximately 2 h periodicity. CGRS consists of two large CsI(Tl) crystals as the main and anticoincidence detectors. The large CsI crystal of CGRS has a higher detector effective area than other lunar gamma ray spectrometers. For its 1-year mission, gamma ray spectra including many peaks of major elements and trace elements on the lunar surface have been measured by CGRS. Global measurement within 0.55-0.75 MeV is given here to describe the distribution of radioactive composition (e.g., uranium and thorium) on the lunar surface. Although CGRS has a lower energy resolution that cannot separate the uranium peak from others in this energy region, 609 keV uranium gamma ray line dominates the shape of the spectrum in this energy region. Therefore, the radioactive map can indirectly describe the uranium distribution on the lunar surface. The radioactive map shows that the higher radiation is concentrated in the Procellarum KREEP Terrene (PKT) on the nearside with an oval shape. The secondary high-radiation is located in South Pole-Aitken (SPA) basin. Lunar highlands have lower concentration. The relationship between radiation and topography displays different linear correlations for lunar highlands and SPA basin, which imply the different processes for these two regions.     

AXIOM: Advanced X‐ray imaging of the magnetosheath

AXIOM (Advanced X‐ray Imaging Of the Magnetosphere) is a concept mission which aims to explain how the Earth's magnetosphere responds to the changing impact of the solar wind using a unique method never attempted before; performing wide‐field soft X‐ray imaging and spectroscopy of the magnetosheath, magnetopause and bow shock at high spatial and temporal resolution. Global imaging of these regions is possible because of the solar wind charge exchange (SWCX) process which produces elevated soft X‐ray emission from the interaction of high charge‐state solar wind ions with primarily neutral hydrogen in the Earth's exosphere and near‐interplanetary space (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

GPS定位中的误差分析

介绍了GPS的空间部分、地面控制部分和用户部分的发展状况及其作用。并对GPS定位的基本原理做了阐述。GPS定位误差由几何精度因子和站星距离的测量误差决定,站星距离的测量误差由3大类误差因素决定,它们是与GPS卫星有关的误差、与GPS卫星信号传播有关的误差、与GPS信号接收设备有关的误差。对上述重要误差进行了系统全面的分析,并提出了具体的误差修正模型及各种减小或消除误差的方法。     

Pionic photons and neutrinos from cosmic ray accelerators

Identifying the accelerators that produce the Galactic and extragalactic cosmic rays has been a priority mission of several generations of high energy gamma ray and neutrino telescopes; success has been elusive so far. Detecting the gamma-ray and neutrino fluxes associated with cosmic rays reaches a new watershed with the completion of IceCube, the first neutrino detector with sensitivity to the anticipated fluxes, and the construction of CTA, a ground-based gamma ray detector that will map and study candidate sources with unprecedented precision. In this paper, we revisit the prospects for revealing the sources of the cosmic rays by a multiwavelength approach; after reviewing the methods, we discuss supernova remnants, gamma ray bursts, active galaxies and GZK neutrinos in some detail.     

A space Fresnel Imager for ultra-violet astrophysics: example on accretion disks

The Fresnel Diffractive Imager concept is proposed for space borne astronomical imaging at Ultra-Violet wavelengths, using diffractive focalization. The high angular resolution and high dynamic range provided by this new concept makes it an ideal tool to resolve circumstellar structures such as disks or jets around bright sources, among them, pre-main sequence stars and young planetary disks. The study presented in this paper addresses the following configuration of Fresnel diffractive imager: a diffractive array 4 m large, with 696 Fresnel zones operating in the ultra-violet domain. The diffractive arrays are opaque foils punched with a large number of void subapertures with carefully designed shapes and positions. In the proposed space missions, these punched foils would be deployed in space. Depending on the size of the array and on the working spectral band, the focal length of such imagers will range from a few kilometers to a few tens of kilometers. Thus, such space mission requires a formation flying configuration for two satellites around the L2 Sun-Earth Lagragian point. In this article, we investigate numerically the potential of Fresnel arrays for imaging circumstellar dust environments. These simulations are based upon simple protostellar disk models, and on the computed optical characteristics of the instrument. The results show that protoplanetary disks at distances up to a few thousand parsecs can be successfully studied with a 4 m aperture Fresnel imager in the UV.     

Advancing tests of relativistic gravity via laser ranging to Phobos

Phobos Laser Ranging (PLR) is a concept for a space mission designed to advance tests of relativistic gravity in the solar system. PLR’s primary objective is to measure the curvature of space around the Sun, represented by the Eddington parameter γ, with an accuracy of two parts in 10⁷, thereby improving today’s best result by two orders of magnitude. Other mission goals include measurements of the time-rate-of-change of the gravitational constant, G and of the gravitational inverse square law at 1.5-AU distances—with up to two orders-of-magnitude improvement for each. The science parameters will be estimated using laser ranging measurements of the distance between an Earth station and an active laser transponder on Phobos capable of reaching mm-level range resolution. A transponder on Phobos sending 0.25-mJ, 10-ps pulses at 1 kHz, and receiving asynchronous 1-kHz pulses from earth via a 12-cm aperture will permit links that even at maximum range will exceed a photon per second. A total measurement precision of 50 ps demands a few hundred photons to average to 1-mm (3.3 ps) range precision. Existing satellite laser ranging (SLR) facilities—with appropriate augmentation—may be able to participate in PLR. Since Phobos’ orbital period is about 8 h, each observatory is guaranteed visibility of the Phobos instrument every Earth day. Given the current technology readiness level, PLR could be started in 2011 for launch in 2016 for 3 yr of science operations. We discuss the PLR’s science objectives, instrument, and mission design. We also present the details of science simulations performed to support the mission’s primary objectives.