Some practical strategies for reducing intermodulation in satellitecommunications

Various heuristic procedures for obtaining practical solutions to the general one-level carrier frequency assignment problem are described. The problem treated is general in the sense that it accommodates the case where L of the N slots may be explicitly designated as prohibited and unavailable for assignment. This problem occurs in satellite transmission with many small carriers accessing the same transponder where, due to multipath and TV interference from crosspolarized transponders of the same satellite and from copolarized transponders of the adjacent satellites, some portions of the bandwidth of the considered transponder cannot be used. To permit comparison with respect to the intermodulation advantage and central-processing-unit time required, the case without prohibited slots is considered. The sequential insertion procedure in which, starting with two carriers at the two end slots, one additional carrier at a time is optimally inserted into one of the unassigned slots is found best when the ratio between the available bandwidth and the total carrier bandwidth is greater than about 125%. All the heuristic procedures produced assignments whose intermodulation advantages are all greater than the bandwidth ratio     

Interstellar dust models

Interstellar dust models, previously constrained only from the extinction curve, have been radically changed with the arrival of IRAS observations of the dust infrared emission. An important component of interstellar dust is likely to be made of small particles that show a fluctuating temperature upon impinging single photons and which can produce large near and mid infrared excesses ubiquitously observed in the Galaxy and external galaxies. The analysis of COBE data should soon improve our understanding of dust infrared emissivity and particularly for big grains in the submillimeter domain. We will discuss the key observations (spectral features, broad-band colors, correlations with gas tracers…) which put the best constraints on any dust models and show that the next generation of IR/submm satellites (ISO, SIRTF…) should improve our knowledge of interstellar dust composition and the dust redistribution of the stellar energy inside galaxies.     

New progress on space solar telescope

The design of the Space Solar Telescope (phase B) has been completed. The telescope system has been manufactured (phase C) since the beginning of 1999, and it will be assembled by the end of 2000. In this paper, mainly the optical design, technique and manufacture will be described. The launch is planned for the summer of 2004.     

High resolution imaging of objects at Ka band

A coherent wideband millimeter wave (mmW) imaging radar which operates in two different imaging modes is briefly described. The principles of two-dimensional (2-D) high resolution imaging are introduced. The image reconstruction algorithm together with image enhancement techniques are discussed. Some experiments imaging on both scaled aircraft model and full-size large scale vehicle are presented     

Tracking of conventional beamforming with hydrophone array ofvarying geometry

In conventional beamformers of a large towed hydrophone array of fixed linear array geometry the sensors may be displaced from their nominal positions due to various kinds of force on the array. The displacements are usually time varying and can severely degrade the performance of the beamformer. This problem is approached by restoring the sensor output signals before beamforming by adaptively tracking and compensating for the displacements. This approach does not require modifications of the conventional shipboard beamformer and hence can be used cost-effectively to upgrade existing conventional systems. Two adaptive algorithms are used for two different cases: known sensor moving trajectories and unknown trajectories with relatively small displacements compared with the intersensor spacing. Tracking performance of the algorithms is analyzed. Simulation results show that for known sensor trajectories the algorithm significantly improves the performance of the conventional shipboard beamformer whereas only limited improvement can be achieved when the trajectories are unknown     

Detection threshold selection for tracking performance optimization

A technique for the evaluation of the track loss probability and the estimation error during track maintenance in clutter has been developed recently by the authors. This work overcomes the limitation of an earlier technique that does not handle the transient process of tracking divergence. Track loss, being a “runaway” phenomenon, clearly requires transient evaluation capability. The new technique provides, without the need for expensive Monte Carlo simulations, the probability that a hack is maintained in the presence of all sources of uncertainty encountered In a tracking process. This technique is of a hybrid nature; it involves explicit probabilistic accounting of both the continuous and the discrete uncertainties. Here it is demonstrated how this technique can be used for the selection of the detection threshold to optimize the overall performance of a detection-trading system     

Minimal submodel-set algorithm for maneuvering target tracking

The variable structure multiple model (VSMM) approach to the maneuvering target tracking problem is considered. A new VSMM design, the minimal submodel-set switching (MSMSS) algorithm for tracking a maneuvering target is presented. The MSMSS algorithm adaptively determines the minimal set of models from the total model set and uses this to perform multiple models (MM) estimation. In addition, an iterative MSMSS algorithm with improved maneuver detection and termination properties is developed. Simulations results demonstrate that, compared with a standard interacting MM (IMM), the proposed algorithms require significantly lower computation while maintaining similar tracking performance. Alternatively, for a computational load similar to IMM, the new algorithms display significantly improved performance.     

Real-time studies on microalgae under microgravity

Using remote sensing technique, we investigated real-time Nostoc sphaeroides Kütz (Cyanobacterium) in Closed System under microgravity by SHENZHOU-2 spacecraft in January 2001. The experiments had 1 g centrifuges in space for control and ground control group experiments were also carried out in the same equipments and under the same controlled condition. The data about the population growth of Nostoc sp. of experiments and temperature changes of system were got from spacecraft every minute. From the data, we can find that population growth of Nostoc sp. in microgravity group was higher than that of other groups in space or on ground, even though both the control 1 g group in space and 1 g group on ground indicated same increasing characteristics in experiments. The growth rate of 1.4 g group (centrifuged group on ground) was also promoted during experiment. The temperature changes of systems are also affected by gravity and light. Some aspects about those differences were discussed. From the discussion of these results during experiment, it can be found that gravity is the major factor to lead to these changes.     

Why rover preparatory programmes? The example of the French programme “VAP”

The very first activities concerning planetary rovers began in 1964 in the Soviet Union and in the United States for lunar missions. Nowadays, with the increase of new mission needs and technical possibilities, several space agencies have engaged in some preliminary programmes in that area with the following objectives:

• —to prepare their involvement in future international rover missions

• —to ease contacts/discussions between scientists and engineers

• —to study and develop a new generation of in situ experiments

• —to perform system/mission analysis in conjunction with the definition of the mission objectives

• —to analyze robotic problematics and implement robotic concepts in the rover architectures.

To perform these activities, several organizations have been set up in Russia, the United States, Japan, Italy and France, according to the relative weight of space engineering over robotic research.

In the case of the French programme (‘VAP—Automatic Planetary Rover’), the organization is based on a partnership between the CNES, a scientific committee, four national research laboratories and industries in order to optimize scientific and technical work, with an optimal use of past robotic research studies, as well as to generate spin-offs for Earth applications. Indeed, as a preliminary result, we now have a co-operative agreement with Russia to procure cameras and associated software for the autonomous navigation of the Marsokhod 96 and 2 projects for terrestrial applications of robotic concepts defined within the framework of the VAP programme.     

Measuring atmospheric density using GPS–LEO tracking data

We present a method to estimate the total neutral atmospheric density from precise orbit determination of Low Earth Orbit (LEO) satellites. We derive the total atmospheric density by determining the drag force acting on the LEOs through centimeter-level reduced-dynamic precise orbit determination (POD) using onboard Global Positioning System (GPS) tracking data. The precision of the estimated drag accelerations is assessed using various metrics, including differences between estimated along-track accelerations from consecutive 30-h POD solutions which overlap by 6 h, comparison of the resulting accelerations with accelerometer measurements, and comparison against an existing atmospheric density model, DTM-2000. We apply the method to GPS tracking data from CHAMP, GRACE, SAC-C, Jason-2, TerraSAR-X and COSMIC satellites, spanning 12 years (2001–2012) and covering orbital heights from 400 km to 1300 km. Errors in the estimates, including those introduced by deficiencies in other modeled forces (such as solar radiation pressure and Earth radiation pressure), are evaluated and the signal and noise levels for each satellite are analyzed. The estimated density data from CHAMP, GRACE, SAC-C and TerraSAR-X are identified as having high signal and low noise levels. These data all have high correlations with anominal atmospheric density model and show common features in relative residuals with respect to the nominal model in related parameter space. On the contrary, the estimated density data from COSMIC and Jason-2 show errors larger than the actual signal at corresponding altitudes thus having little practical value for this study. The results demonstrate that this method is applicable to data from a variety of missions and can provide useful total neutral density measurements for atmospheric study up to altitude as high as 715 km, with precision and resolution between those derived from traditional special orbital perturbation analysis and those obtained from onboard accelerometers.