The International DORIS Service (IDS): Toward maturity

DORIS is one of the four space-geodetic techniques participating in the Global Geodetic Observing System (GGOS), particularly to maintain and disseminate the Terrestrial Reference Frame as determined by International Earth rotation and Reference frame Service (IERS). A few years ago, under the umbrella of the International Association of Geodesy, a DORIS International Service (IDS) was created in order to foster international cooperation and to provide new scientific products. This paper addresses the organizational aspects of the IDS and presents some recent DORIS scientific results. It is for the first time that, in preparation of the ITRF2008, seven Analysis Centers (AC’s) contributed to derive long-term time series of DORIS stations positions. These solutions were then combined into a homogeneous time series IDS-2 for which a precision of less than 10 mm was obtained. Orbit comparisons between the various AC’s showed an excellent agreement in the radial component, both for the SPOT satellites (e.g. 0.5–2.1 cm RMS for SPOT-2) and Envisat (0.9–2.1 cm RMS), using different software packages, models, corrections and analysis strategies. There is now a wide international participation within IDS that should lead to future improvements in DORIS analysis strategies and DORIS-derived geodetic products.     

新中型宽体货机——难题迎刃而解

挑战加速 过去一年全球油价翻了一番，美国经济不景气，收益率不断下滑，全球的航空货运运营商正面临着越来越大的压力。 我们开始看到747—200的停用以及采用该机型运营商的破产。油价越高，将747—400改装成货机的经济可行性越困难。     

System test results from the GNC experiments on the PRISMA in-orbit test bed

The PRISMA in-orbit test bed will demonstrate guidance, navigation, and control strategies for spacecraft formation flying and rendezvous. The project is funded by the Swedish National Space Board and the prime contractor is the Swedish Space Corporation. The project is further supported by the German Aerospace Center, the Technical University of Denmark, and the French Space Agency. PRISMA was launched on June 15, 2010 and after three weeks of operations, all on-board systems and units have passed an initial commissioning phase. Separation of the two PRISMA satellites from each other is expected by mid-August 2010.PRISMA consists of two spacecraft: MAIN and TARGET. The MAIN spacecraft has full orbit control capability while TARGET is attitude controlled only.The Swedish Space Corporation is responsible for three groups of guidance, navigation, and control experiments. These experiments include GPS- and vision-based formation flying during which the spacecraft will fly in passive as well as forced motion. The three experiments are: autonomous formation flying, proximity operations with final approach/recede maneuvers, and autonomous rendezvous. This paper presents system test results from two of these experiments as obtained with the flight-ready system. The system tests consist of a series of simulations performed on the flight model spacecraft with a large amount of hardware in the loop.     

The Urey instrument: an advanced in situ organic and oxidant detector for Mars exploration

The Urey organic and oxidant detector consists of a suite of instruments designed to search for several classes of organic molecules in the martian regolith and ascertain whether these compounds were produced by biotic or abiotic processes using chirality measurements. These experiments will also determine the chemical stability of organic molecules within the host regolith based on the presence and chemical reactivity of surface and atmospheric oxidants. Urey has been selected for the Pasteur payload on the European Space Agency's (ESA's) upcoming 2013 ExoMars rover mission. The diverse and effective capabilities of Urey make it an integral part of the payload and will help to achieve a large portion of the mission's primary scientific objective: "to search for signs of past and present life on Mars." This instrument is named in honor of Harold Urey for his seminal contributions to the fields of cosmochemistry and the origin of life.     

Influence of atmospheric turbulence on planetary transceiver laser ranging

In this paper we investigate the influence of atmospheric turbulence on the performance of the uplink of a planetary transceiver laser ranging system using a single photon detector. We numerically combine the influence of turbulence in the mean intensity profile variations, scintillation, beam-wander induced pointing errors and stochastic time-of-flight variations, using the Hufnagel–Valley turbulence profile to model the ground turbulence behavior. We map the intensity variations due to turbulence to variations in the probability distribution of the arrival time of the 1st photon in a laser pulse, which influences the range measurement error probability distribution. The turbulence models are applied to assess the influence on single-pass range accuracy and precision statistics, as well as the parameter estimation quality of a Phobos Laser Ranging (PLR) mission.     

Micro-meteoroids and space debris impact risk assessment for the ConeXpress satellite using ESABASE2/Debris

Micro-meteoroid and space debris impact risk assessments are performed to investigate the risk from hypervelocity impacts to sensitive spacecraft sub-systems. For these analyses, ESA’s impact risk assessment tool ESABASE2/Debris is used. This software tool combines micro-particle environment models, damage equations for different shielding designs and satellite geometry models to perform a detailed 3D micro-particle impact risk assessment. This paper concentrates on the impact risk for exposed pressurized tanks. Pressure vessels are especially susceptible to hypervelocity impacts when no protection is available from the satellite itself. Even small particles in the mm size range can lead to a fatal burst or rupture of a tank when impacting with a typical collision velocity of 10–20 km/s. For any space mission it has to be assured that the impact risk is properly considered and kept within acceptable limits. The ConeXpress satellite mission is analysed as example. ConeXpress is a planned service spacecraft, intended to extend the lifetime of telecommunication spacecraft in the geostationary orbit. The unprotected tanks of ConeXpress are identified as having a high failure risk from hypervelocity impacts, mainly caused by micro-meteoroids. Options are studied to enhance the impact protection. It is demonstrated that even a thin additional protective layer spaced several cm from the tank would act as part of a double wall (Whipple) shield and greatly reduce the impact risk. In case of ConeXpress with 12 years mission duration the risk of impact related failure of a tank can be reduced from almost 39% for an unprotected tank facing in flight direction to below 0.1% for a tank protected by a properly designed Whipple shield.     

Ariane 5 GTO debris mitigation using natural perturbations

GTO objects can potentially collide with operative satellites in LEO and GEO protected regions. Internationally accepted debris mitigation guidelines require that these objects exit these protected regions within 25?years, e.g. by re-entering and burning up in Earth’s atmosphere. In this paper, an inventory of the GTO debris generated from Ariane 5 launches in the period 2012–2017 is provided, and it is expected that none of these objects will re-enter within 25?years. For future launches, natural perturbations can be exploited to increase compliance with mitigation guidelines without the use of extra propellant or complex de-orbiting systems, which is attractive from an economic point of view. The lifetime of GTO objects is very sensitive to initial conditions and some environmental and body-related parameters, mainly due to the effect of solar gravity on the perigee altitude. As a consequence, the lifetime of a specific GTO object cannot be predicted accurately, but its probability of re-entering in less than 25?years can be estimated with proper accuracy by following a statistical approach. By propagating the orbits of over 800,000 simulated Ariane 5 GTO objects, it was found that the launch time leading to the highest probability of compliance with debris mitigation guidelines for GEO launches from Kourou corresponds to about 2 PM local time, regardless of the date of launch, which leads to compliance rates ranging from 60 to 100%. Current practice is to launch at around 5–9?PM, so a change in procedures would be required in order to reach a higher degree of compliance with debris mitigation guidelines, which was predicted to be on average below 20% for the objects generated in the period 2012–2017.     

中国航空货运市场分析

全球航空货运市场趋势 全球航空货运市场正面临着富有挑战性的发展趋势。燃油价格上涨、美国经济开始低迷是两个宏观经济导向的趋势。此外,世界范围内的航空货运承运人都面临着日益增加的竞争压力：综合运营商的市场拓展;更为快速、廉价的地面运输方式;客机容量;局部市场能力过剩。     

Results from the RXTE All-Sky Monitor

Selected results from the Rossi X-ray Timing Explorer All-Sky Monitor are presented to illustrate the phenomenology of the light curves. The sensitivity to periodic intensity variations is indicated by the folded light curve of AM Her. The gray line between transient and persistent sources is emphasized. Light curves of a range of systems comprising black holes or neutron stars and low and high mass companion stars show that the behavior of these systems is often, but not always, characteristic.     

航空货运如何在变化的市场中求胜

前所未有充满挑战的环境2008年,我们面对的挑战源源不断。一开年,诸多市场发展放缓和生产过剩,燃油价格创历史新高,美元疲软后又反弹以及信用