Space Communications

The objective of this paper is to discuss the issues related to the use of Fade Mitigation Techniques (FMT) for the GEOCAST satellite system, considering both transparent and regenerative payloads. The main characteristics of the GEOCAST systems are presented first. Baseline link budgets are established both for the transparent return link and for the regenerative configurations. Then the FMTs designed for GEOCAST are presented. Theoretical performance of the FMTs and their practical implementation are described focusing on the FMT control loop design.

Finally, simulation results for system performance with emphasis on physical layer, are presented both for transparent and regenerative configurations.

This paper addresses the integration of a Fade Mitigation Technique (FMT) within a centrally managed MF-TDMA satellite network in the context of guaranteed QoS delivery. Burst Length Control (BLC) is introduced as a suitable FMT and the methodology of its design and implementation within the existing DVB-RCS standard is described and simulated. The proposed technique provides compensation for rain attenuation at the expense of capacity utilisation. Within any fixed partition of the MF-TDMA space, by considering the space-time properties of rain attenuation, suitable geographical multiplexing of rain-attenuated and non-attenuated links allows the optimisation of the channel capacity utilisation while still exploiting the statistical multiplexing of the sources on the return channels.

Channel state estimation is one of the key processes carried out at the physical layer of Satellite Systems employing Fade Mitigation Techniques such as Adaptive Coding and Modulation (ACM). An accurate and reliable channel estimate is needed to fully realize the capacity gains accrued by using ACM. In this paper the SNR estimation algorithms, which perform the role of channel state estimators, are analysed from a system point of view to get a quantitative idea on the number of received symbols needed to get a reliable estimate, and the impact of interference noise on it. A DVB type satellite system has been investigated for the study. An improvement over the Maximum Likelihood (ML) estimator using Bayesian principles is suggested and illustrated.

Despite that theoretical, design and experimental studies connected with the creation of large deployable antenna reflectors have already been carried out for quite a long time, this area still remains of lively interest and has good application prospects. An example is the program of the European Space Agency (ESA) on the creation of a qualified variant of a 15 m deployable space offset reflector. It should be said that by the time the ESA adopted a decision on the implementation of this program, a deployable space reflector 7 m in diameter had already been created in Georgia and, in 1999, this space reflector was successfully tested aboard the orbital station “Mir” according to the joint Georgian–Russian program.

Georgian Institute for Space Constructions, continues developing new design principles for large space deployable systems (Transformable Space and Ground Structures (in Russian), A Monograph, Georgia–Germany–Liechtenstein, 1995).

In the paper we describe the invention of an absolutely new design of a space antenna for symmetric and nonsymmetric radio telescopes, which has been developed by the team of Georgian designers headed by Professor E. Medzmariashvili.

The novelty consists in that new engineering and technological effects we obtained by synthesis of ring-shaped and rib-umbrella type systems.

In particular, the central hinge that connects the main levers of the pantograph is arranged asymmetrically. As a result of this asymmetrical arrangement, during deployment the consoles (folded along the package longitudinal axis and connecting the adjacent sections) move in radial directions from the center to the periphery and, simultaneously, deploy in an umbrella-like manner.