Design Solutions For Modular Satellite Architectures

The cost-effective access to space envisaged by ESA would open a wide range of new opportunities and markets, but is still many years ahead. There is still a lack of devices, circuits, systems which make possible to develop satellites, ground stations and related services at costs compatible with the budget of academic institutions and small and medium enterprises (SMEs). As soon as the development time and cost of small satellites will fall below a certain threshold (e.g. 100,000 to 500,000 €), appropriate business models will likely develop to ensure a cost-effective and pervasive access to space, and related infrastructures and services. These considerations spurred the activity described in this paper, which is aimed at: - proving the feasibility of low-cost satellites using COTS (Commercial Off The Shelf) devices. This is a new trend in the space industry, which is not yet fully exploited due to the belief that COTS devices are not reliable enough for this kind of applications; - developing a flight model of a flexible and reliable nano-satellite with less than 25,000€; - training students in the field of avionics space systems: the design here described is developed by a team including undergraduate students working towards their graduation work. The educational aspects include the development of specific new university courses; - developing expertise in the field of low-cost avionic systems, both internally (university staff) and externally (graduated students will bring their expertise in their future work activity); - gather and cluster expertise and resources available inside the university around a common high-tech project; - creating a working group composed of both University and SMEs devoted to the application of commercially available technology to space environment. The first step in this direction was the development of a small low cost nano-satellite, started in the year 2004: the name of this project was PiCPoT (Piccolo Cubo del Politecnico di Torino, Small Cube of Politecnico di Torino). The project was carried out by some departments of the Politecnico, in particular Electronics and Aerospace. The main goal of the project was to evaluate the feasibility of using COTS components in a space project in order to greatly reduce costs; the design exploited internal subsystems modularity to allow reuse and further cost reduction for future missions. Starting from the PiCPoT experience, in 2006 we began a new project called ARaMiS (Speretta et al., 2007) which is the Italian acronym for Modular Architecture for Satellites. This work describes how the architecture of the ARaMiS satellite has been obtained from the lesson learned from our former experience. Moreover we describe satellite operations, giving some details of the major subsystems. This work is composed of two parts. The first one describes the design methodology, solutions and techniques that we used to develop the PiCPoT satellite; it gives an overview of its operations, with some details of the major subsystems. Details on the specifications can also be found in (Del Corso et al., 2007; Passerone et al, 2008). The second part, indeed exploits the experience achieved during the PiCPoT development and describes a proposal for a low-cost modular architecture for satellite

Indistinguishability of independent single photons

The indistinguishability of independent single photons is presented by decomposing the single photon pulse into the mixed state of different transform limited pulses. The entanglement between single photons and outer environment or other photons induces the distribution of the center frequencies of those transform limited pulses and makes photons distinguishable. Only the single photons with the same transform limited form are indistinguishable. In details, the indistinguishability of single photons from the solid-state quantum emitter and spontaneous parametric down conversion is examined with two-photon Hong-Ou-Mandel interferometer. Moreover, experimental methods to enhance the indistinguishability are discussed, where the usage of spectral filter is highlighted.Comment: 6 pages, 3 figure

Massive creation of entangled exciton states in semiconductor quantum dots

An intense laser pulse propagating in a medium of inhomogeneously broadened quantum dots massively creates entangled exciton states. After passage of the pulse all single-exciton states remain unpopulated (self-induced transparency) whereas biexciton coherence (exciton entanglement) is generated through two-photon transitions. We propose several experimental techniques for the observation of such unexpected behavior

Radiative corrections to the excitonic molecule state in GaAs microcavities

The optical properties of excitonic molecules (XXs) in GaAs-based quantum well microcavities (MCs) are studied, both theoretically and experimentally. We show that the radiative corrections to the XX state, the Lamb shift $\Delta^{\rm MC}_{\rm XX}$ and radiative width $\Gamma^{\rm MC}_{\rm XX}$, are large, about $10-30$ of the molecule binding energy $\epsilon_{\rm XX}$, and definitely cannot be neglected. The optics of excitonic molecules is dominated by the in-plane resonant dissociation of the molecules into outgoing 1$\lambda$-mode and 0$\lambda$-mode cavity polaritons. The later decay channel, ``excitonic molecule $\to$ 0$\lambda$-mode polariton + 0$\lambda$-mode polariton'', deals with the short-wavelength MC polaritons invisible in standard optical experiments, i.e., refers to ``hidden'' optics of microcavities. By using transient four-wave mixing and pump-probe spectroscopies, we infer that the radiative width, associated with excitonic molecules of the binding energy $\epsilon_{\rm XX} \simeq 0.9-1.1$ meV, is $\Gamma^{\rm MC}_{\rm XX} \simeq 0.2-0.3$ meV in the microcavities and $\Gamma^{\rm QW}_{\rm XX} \simeq 0.1$ meV in a reference GaAs single quantum well (QW). We show that for our high-quality quasi-two-dimensional nanostructures the $T_2 = 2 T_1$ limit, relevant to the XX states, holds at temperatures below 10 K, and that the bipolariton model of excitonic molecules explains quantitatively and self-consistently the measured XX radiative widths. We also find and characterize two critical points in the dependence of the radiative corrections against the microcavity detuning, and propose to use the critical points for high-precision measurements of the molecule bindingenergy and microcavity Rabi splitting.Comment: 16 pages, 11 figures, accepted for publication in Phys. Rev.

The Condominium Scheme as a Strategy for Housing Slum Dwellers: the case of Gofa Mebrat Hail Condominium, Addis Ababa, Ethiopia

Ethiopia, like other developing nations, is striving to improve the life of its urban dwellers and Addis Ababa, the capital city, is receiving special attention to bring it up to standards in the continent. At 125 years old, the city suffers from severe housing shortage and is characterized by poor housing, infrastructural and neighborhood conditions. Considering the housing agenda as uppermost of the urban development challenge the city administration initiated a program to build several thousand homes based on the condominium concept for different categories of income groups. The initial target of the program, though, was the low income and lower middle income groups. Eleven years have passed since the program commenced in 2004. Needless to mention, even though home ownership has grown phenomenally, the price of a unit of condominium house has risen dangerously over the period. This article is based on ongoing doctoral study and looks at the affordability challenges of the condominium housing scheme, particularly regarding the poor slum dwellers and considers possible alternative means to tackle the issue. The specific study site is the Gofa-Mebrat Hail Condominium in Nefas Silk-Lafto Sub-city of Addis Ababa. The article gives a background account of the procedures followed by the researcher in the study: descriptive and analytical research methods employed, numerical and non-numerical data collected, questionnaires deployed for the household survey, interviews conducted, and relevant observations made. Moreover, secondary sources of information were available to aid analysis and corroborate findings. The most critical finding of the study was that the affordability challenge was, barring free offers or heavy subsidization, beyond prudent financial amelioration and that the only way to keep the poor slum dwellers housed was for them to pay a monthly housing mortgage installment dictated, largely, by a formula based on their ability-to-pay. Key Words: Condominium Scheme, Housing, Slum Dwellers, home finance

Chemically-specific dual/differential CARS micro-spectroscopy of saturated and unsaturated lipid droplets

We have investigated the ability of dual-frequency Coherent Antistokes Raman Scattering (D-CARS) micro-spectroscopy, based on femtosecond pulses (100 fs or 5 fs) spectrally focussed by glass dispersion, to distinguish the chemical composition of micron-sized lipid droplets consisting of different triglycerides types (poly-unsaturated glyceryl trilinolenate, mono-unsaturated glyceryl trioleate and saturated glyceryl tricaprylate and glyceryl tristearate) in a rapid and label-free way. A systematic comparison of Raman spectra with CARS and D-CARS spectra was used to identify D-CARS spectral signatures which distinguish the disordered poly-unsaturated lipids from the more ordered saturated ones both in the CH-stretch vibration region and in the fingerprint region, without the need for lengthy CARS multiplex acquisition and analysis. D-CARS images of the lipid droplets at few selected wavenumbers clearly resolved the lipid composition differences, and exemplify the potential of this technique for label-free chemically selective rapid imaging of cytosolic lipid droplets in living cell

Unconventional motional narrowing in the optical spectrum of a semiconductor quantum dot

Motional narrowing refers to the striking phenomenon where the resonance line of a system coupled to a reservoir becomes narrower when increasing the reservoir fluctuation. A textbook example is found in nuclear magnetic resonance, where the fluctuating local magnetic fields created by randomly oriented nuclear spins are averaged when the motion of the nuclei is thermally activated. The existence of a motional narrowing effect in the optical response of semiconductor quantum dots remains so far unexplored. This effect may be important in this instance since the decoherence dynamics is a central issue for the implementation of quantum information processing based on quantum dots. Here we report on the experimental evidence of motional narrowing in the optical spectrum of a semiconductor quantum dot broadened by the spectral diffusion phenomenon. Surprisingly, motional narrowing is achieved when decreasing incident power or temperature, in contrast with the standard phenomenology observed for nuclear magnetic resonance

Interculturalidade e inclusão na educação no Brasil e no Quebec: diferentes concepções, semelhantes desafios

Este artigo procura discutir as concepções de interculturalidade adotadas no Brasil e no Quebec, estabelecendo paralelos e contrapontos. Apresenta informações sobre a origem e o desenvolvimento do conceito na América Latina, no Brasil e no Quebec, analisa sua incidência nas políticas educativas dos diferentes contextos e discute a interface entre tendências globais e locais na configuração da perspectiva intercultural em ambas as sociedades. O artigo também apresenta reflexões sobre como o debate da inclusão tem ocupado maior centralidade nos discursos e nas políticas educativas em detrimento do debate intercultural nos últimos anos. Acreditamos que a análise comparada sobre o desenvolvimento de políticas públicas educacionais interculturais favorece um maior aprofundamento para a discussão sobre avanços e atuais desafios do reconhecimento das diferenças na educaçã

Intégration et inclusion : interconnexions et oppositions dans deux domaines éducatifs au Québec

Plusieurs systèmes éducatifs dans le monde, comme c’est le cas au Québec, tendent vers l’adoption de politiques d’éducation inclusive qui visent à répondre de façon équitable aux besoins de tous les élèves. Alors que le paradigme d’éducation inclusive concernait initialement les élèves qui présentaient des handicaps ou considérés comme ayant des besoins particuliers, il englobe maintenant d’autres sous-groupes d’élèves, dont les élèves minorisés sur les plans de la diversité ethnoculturelle, religieuse et linguistique. Cet article propose une lecture critique de l’évolution du paradigme de l’éducation inclusive et du concept d’intégration dans deux domaines éducatifs au Québec : celui de l’adaptation scolaire et celui qui concerne la diversité ethnoculturelle, religieuse et linguistique (ERL). L’exploration du chemin parcouru dans les deux domaines éducatifs illustre l’ancrage de l’évolution des concepts dans les débats sociaux, qui eux-mêmes s’articulent selon des courants politiques, des sensibilités particulières de la société, et qui se reflètent dans les pratiques des milieux scolaires. La conclusion invite le milieu de la recherche et les autorités éducatives à contribuer à la clarification des concepts et des paradigmes, en leur rappelant que ces derniers peuvent enfermer les individus et les groupes dans des rapports sociaux parfois teintés de dynamique de domination