Indices biochimiques et milieux marins

La chlorophylle a est l'indice de la biomasse du phytoplancton le plus utilisé en océanographie. Cependant, sa relation avec la production primaire est encore mal connue, particulièrement en milieu océanique. Au cours d'une étude de 13 jours consécutifs en un point fixe situé à l'équateur dans le Golfe de Guinée, à 4°W, la synthèse de la chlorophylle est mesurée chaque jour en parallèle avec la fixation photosynthétique du carbone : les échantillons sont prélevés avant le lever du jour, tamisés sur filet de 200 um de vide de maille, et placés en incubation "in situ" du lever au coucher du soleil. Les teneurs maximales de chlorophylle sont toujours observées à 50 m. La quantité de chlorophylle par flacon augmente dans la journée de 150 à 200% entre 20 et 30 m, mais seulement de 40 à 50% à 50 m. Le rapport (carbone fixé)/(Chla accumulée) s'établit autour de 150 près de la surface, et entre 50 et 100 à 50 m. (D'après résumé d'auteur

A Self-timed Ring Based True Random Number Generator

International audienceSelf-timed rings are oscillators in which several events can evolve evenly-spaced in time thanks to analog effects inherent to the ring stage structure. One of their interesting features is that they provide precise high-speed multiphase signals. This paper presents a true random number generator that exploits the jitter of events propagating in a self-timed ring with a high entropy. Designs implemented in Altera Cyclone III and Xilinx Virtex 5 devices provide high quality random bit sequences passing FIPS 140-1 and NIST SP 800-22 statistical tests at a high bit rate

Comparison of Self-Timed Ring and Inverter Ring Oscillators as Entropy Sources in FPGAs

International audienceMany True Random Numbers Generators (TRNG) use jittery clocks generated in ring oscillators as a source of entropy. This is especially the case in Field Programmable Gate Arrays (FPGA), where sources of randomness are very limited. Inverter Ring Oscillators (IRO) are relatively well characterized as entropy sources. However, it is known that they are very sensitive to working conditions. This fact makes them vulnerable to attacks. On the other hand, Self-Timed Rings (STR) are currently considered as a promising solution to generate robust clock signals. Although many studies deal with their temporal behavior and robustness in Application Specific Integrated Circuits (ASIC), equivalent study does not exist for FPGAs. Furthermore, these oscillators were not analyzed and characterized as entropy sources aimed at TRNG design. In this paper, we analyze STRs as entropy sources for TRNGs implemented in FPGAs. Next, we compare STRs and IROs when serving as sources of randomness. We show that STRs represent very interesting alternative to IROs: they are more robust to environmental fluctuations and they exhibit lower extra-device frequency variations

A Very High Speed True Random Number Generator with Entropy Assessment

International audienceThe proposed true random number generator (TRNG) exploits the jitter of events propagating in a self-timed ring (STR) to generate random bit sequences at a very high bit rate. It takes advantage of a special feature of STRs that allows the time elapsed between successive events to be set as short as needed, even in the order of picoseconds. If the time interval between the events is set in concordance with the clock jitter magnitude, a simple entropy extraction scheme can be applied to generate random numbers. The proposed STR-based TRNG (STRNG) follows AIS31 recommendations: by using the proposed stochastic model, designers can compute a lower entropy bound as a function of the STR characteristics (number of stages, oscillation period and jitter magnitude). Using the resulting entropy assessment, they can then set the compression rate in the arithmetic post-processing block to reach the required security level determined by the entropy per output bit. Implementation of the generator in two FPGA families confirmed its feasibility in digital technologies and also confirmed it can provide high quality random bit sequences that pass the statistical tests required by AIS31 at rates as high as 200 Mbit/s

Semi-automated stereoradiographic upper limb 3D reconstructions using a combined parametric and statistical model: a preliminary study

PURPOSE: Quantitative assessment of 3D clinical indices may be crucial for elbow surgery planning. 3D parametric modeling from bi-planar radiographs was successfully proposed for spine and lower limb clinical investigation as an alternative for CT-scan. The aim of this study was to adapt this method to the upper limb with a preliminary validation. METHODS: CT-scan 3D models of humerus, radius and ulna were obtained from 20 cadaveric upper limbs and yielded parametric models made of geometric primitives. Primitives were defined by descriptor parameters (diameters, angles...) and correlations between these descriptors were found. Using these correlations, a semi-automated reconstruction method of humerus using bi-planar radiographs was achieved: a 3D personalized parametric model was built, from which clinical parameters were computed [orientation and projections on bone surface of trochlea sulcus to capitulum (CTS) axis, trochlea sulcus anterior offset and width of distal humeral epiphysis]. This method was evaluated by accuracy compared to CT-scan and reproducibility. RESULTS: Points-to-surface mean distance was 0.9 mm (2 RMS = 2.5 mm). For clinical parameters, mean differences were 0.4-1.9 mm and from 1.7° to 2.3°. All parameters except from angle formed by CTS axis and bi-epicondylar axis in transverse plane were reproducible. Reconstruction time was about 5 min. CONCLUSIONS: The presented method provides access to morphological upper limb parameters with very low level of radiation. Preliminary in vitro validation for humerus showed that it is fast and accurate enough to be used in clinical daily practice as an alternative to CT-scan for total elbow arthroplasty pre operative evaluation