The Genesis of the ACE+ Anti-Rotating Satellites Concept

Abstract

The lack of data over the oceans and other remote regions contributes greatly to the uncertainties in the initial state of global weather-prediction models, which, in turn, limits their forecast capabilities. It appeared, that the agreement of the GPS/MET data with NWP was noticeably better over data-dense (U.S., Europe) versus data-sparse (Pacific Ocean) regions. These studies suggest that the GPS radio occultation data are likely to have a significant positive impact on global climate analyses and global weather prediction. As a result of this, several systems proposed to take advantage of a constellation to provide a better repartition of GPS measurements all over the world. The main improvement of WATS¹ or ACE+ missions² lies in the introduction of a LEO-LEO link, besides of classical LEO-GNSS measurements, which will provide a spaceborne capacity to discriminate water vapour from temperature profiles (classical methods use radiosondes). The ALCATEL mission analysis group, in association with the system architecture department, conduced several studies to propose constellation concepts coping with all scientific mission requirements, instrument technological limitations, and mission analysis intrinsic constraints. The proposed paper makes therefore a review of the ALCATEL constellation studies and associated trade-off, including the evolution from the initial constellation concepts (based on classical Walker constellation types, on which many system such as GPS are based) up to the innovative approach of anti-rotating satellites (satellites on polar orbits with an opposite direction of rotation) that was finally selected in the WATS concept and re-used in the ACE+ mission. The work presented in this paper has been done in the frame of self-funded R&D activities, and in the frame of an ESA contract. Responsibility for the contents resides on the authors.