Eric Frayssinhes

Global design of satellite constellations: a multi-criteria performance comparison of classical walker patterns and new design patterns

Abstract Basically, the problem of designing a multisatellite constellation exhibits a lot of parameters with many possible combinations: total number of satellites, orbital parameters of each individual satellite, number of orbital planes, number of satellites in each plane, spacings between satellites of each plane, spacings between orbital planes, relative phasings between consecutive orbital planes. Hopefully, some authors have theoretically solved this complex problem under simplified assumptions: the permanent (or continuous) coverage by a single and multiple satellites of the whole Earth and zonal areas has been entirely solved from a pure geometrical point of view. These solutions exhibit strong symmetry properties (e.g. Walker, Ballard, Rider, Draim constellations): altitude and inclination are identical, orbital planes and satellites are regularly spaced, etc. The problem with such constellations is their oversimplified and restricted geometrical assumption. In fact, the evaluation function which is used implicitly only takes into account the point-to-point visibility between users and satellites and does not deal with very important constraints and considerations that become mandatory when designing a real satellite system (e.g. robustness to satellite failures, total system cost, common view between satellites and ground stations, service availability and satellite reliability, launch and early operations phase, production constraints, etc.). An original and global methodology relying on a powerful optimization tool based on genetic algorithms has been developed at ALCATEL ESPACE. In this approach, symmetrical constellations can be used as initial conditions of the optimization process together with specific evaluation functions. A multi-criteria performance analysis is conducted and presented here in a parametric way in order to identify and evaluate the main sensitive parameters. Quantitative results are given for three examples in the fields of navigation, telecommunication and multimedia satellite systems. In particular, a new design pattern with very efficient properties in terms of robustness to satellite failures is presented and compared with classical Walker patterns.

The Skybridge Constellation Design

The SkyBridge space segment, based upon Low Earth Orbit satellites, had to face a challenge: though using the same frequency band as various geostationary systems in order to profit by well-proven and cost-efficient techniques — i.e. Ku Band -, SkyBridge satellites must not in any way interfere with geostationary satellites. This “frequency sharing constraint” forces SkyBridge satellites to stop emitting (resp. receiving) towards (resp. from) any portion of the ground as soon as they are seen in alignment with the geostationary orbit from the point of view of this earth portion.

Debris assessment for Skybridge constellation

Abstract The Skybridge system is a worldwide ATM broadband communication system based on a 80 low Earth orbit satellite constellation. The minimization of space debris generation has been taken into account very early in the frame of this program. The objectives are, first to identify the contribution of Skybridge to the space debris generation and secondly to assess the risk due to space debris for the whole system. This paper summarizes the approach of the Skybridge program for debris assessment and mitigation. As no international regulation dealing with this subject exists, the analysis is based on the existing NASA 1740 Safety Standard. The methodology and the software presented in this document were applied to demonstrate the compliance of Skybridge with the requirements. In addition the new CNES standard was used to derive additional requirements. The first point deals with the production of space debris in the various mission phases: this concerns debris released during launch, constellation deployment and mission operations. Besides these normal operations, space debris can also be generated through material ageing or in case of accidents such as explosion due to on-board stored energy or collision with other orbiting objects. In the second part the collision risk is analyzed: the probability of collision with the large cataloged objects is determined. Concerning the smaller debris, the flux models give the probability of impact per surface unit: the analysis identifies components most vulnerable to debris impact together with their shielding and gives the associated probability of failure taking into account the architecture of the satellite. The problem of the post-mission disposal procedure is then discussed in the third part.

Mission analysis of clusters of satellites

Abstract An innovative satellite system that provides high precision localisation of beacon positions consists of a cluster of satellites, i.e. a group of satellites that maintain assigned positions at relatively short distances from each other. Compared to a single satellite, the interest of such a cluster lies in its ability to synthesise antenna bases much longer than those who can be physically mounted on one satellite. Each satellite of the cluster measures the time-of-arrival of the signal transmitted by the beacon. The derived time-differences-of-arrival (TDOA) are processed to estimate the beacon position. At first, this paper summarises the investigations performed on the localisation accuracy that have yielded the optimal cluster geometry. In a previous paper [E. Frayssinhes and E. Lansard, AAS paper 95-334 (1995)], Alcatel Espace has proposed a mathematical formulation relying on a strong analogy with GPS geometrical characterisation of navigation performances. The effects of geometry are expressed by geometric dilution of precision (GDOP) parameters. Such parameters are obtained by solving the TDOA measurement equations for the beacon position using an iterated-least-squares procedure. Then, the paper focuses at the system level on the peculiar problems that arise when such a satellite cluster system is dealt with, and more particularly the launch and early operations phases, the station-keeping strategies of manoeuvres, and the relative localisation and clock synchronisation of the satellites. In particular, it is shown that even with the “civil” C A GPS measurements, differential techniques can yield respective accuracies better than 5 m r.m.s. and 15 ns r.m.s.