ALBERT

Description: Improving and homogenizing time and space reference systems on Earth and, more specifically, realizing the Terrestrial Reference Frame (TRF) with an accuracy of 1 mm and a long-term stability of 0.1 mm/year are relevant for many scientific and societal endeavors. The knowledge of the TRF is fundamental for Earth and navigation sciences. For instance, quantifying sea level change strongly depends on an accurate determination of the geocenter motion but also of the positions of continental and island reference stations, such as those located at tide gauges, as well as the ground stations of tracking networks. Also, numerous applications in geophysics require absolute millimeter precision from the reference frame, as for example monitoring tectonic motion or crustal deformation, contributing to a better understanding of natural hazards. The TRF accuracy to be achieved represents the consensus of various authorities, including the International Association of Geodesy (IAG), which has enunciated geodesy requirements for Earth sciences. Moreover, the United Nations Resolution 69/266 states that the full societal benefits in developing satellite missions for positioning and Remote Sensing of the Earth are realized only if they are referenced to a common global geodetic reference frame at the national, regional and global levels. Today we are still far from these ambitious accuracy and stability goals for the realization of the TRF. However, a combination and co-location of all four space geodetic techniques on one satellite platform can significantly contribute to achieving these goals. This is the purpose of the GENESIS mission, a component of the FutureNAV program of the European Space Agency. The GENESIS platform will be a dynamic space geodetic observatory carrying all the geodetic instruments referenced to one another through carefully calibrated space ties. The co-location of the techniques in space will solve the inconsistencies and biases between the different geodetic techniques in order to reach the TRF accuracy and stability goals endorsed by the various international authorities and the scientific community. The purpose of this paper is to review the state-of-the-art and explain the benefits of the GENESIS mission in Earth sciences, navigation sciences and metrology. This paper has been written and supported by a large community of scientists from many countries and working in several different fields of science, ranging from geophysics and geodesy to time and frequency metrology, navigation and positioning. As it is explained throughout this paper, there is a very high scientific consensus that the GENESIS mission would deliver exemplary science and societal benefits across a multidisciplinary range of Navigation and Earth sciences applications, constituting a global infrastructure that is internationally agreed to be strongly desirable.

Description: Several projects exist of constellation or swarms of satellites using Inter-Satellite Links (ISL) with ranging capabilities, such as Galileo second generation, or scientific projects such as NOIRE, a radio telescope consisting of 50 satellites around the Moon. All these will create a network of connected satellites in space, and will offer a unique opportunity for applications like the Precise Orbit Determination (POD), clock synchronization and other scientific applications. The ISLs provide a unique tool to estimate the satellites’ orbital motion discarding the measurements of the ground receivers. The POD is performed usually expressing the orbits’ initial conditions relative to the Cartesian system ignoring the benefits that the Keplerian Elements (KE) model provides. The KE model is the only system that describes in a natural way the satellite’s orbital motion. The variants of this model do not change so fast in time providing major benefits for the mathematical description of the satellites orbit geometry. The aim of this study is to present a new optimal strategy for the alignment of different satellite constellations to Celestial Reference Frames (CRFs) when the parametrization model is expressed via the KE model. The new strategy relies on the derivation of a new form of (partial) inner constraints, which are suitable for the KE model. Numerical simulations of different satellite constellations, using only inter-satellite range measurements and applying different dynamical models will be presented in order to validate and highlight the results derived from this strategy.

Description: Precise satellite orbit determination is commonly carried out by exploiting range measurements between ground-fixed stations and Earth-orbiting satellites. During the last decade an increasing number of Earth satellites, operating mainly in low-orbit mode (LEO), are equipped with GNSS receivers thus enabling the linkage of different satellites in space via inter-satellite range measurements. The usage of such observations provides a compelling option for estimating the orbits of Earth satellites without the need to employ ground-fixed tracking stations. To explore the potential of this option, a simulator has been developed in SYRTE for studying the satellite-to-satellite orbit determination problem. The simulator is designed to deal with different satellite constellations and parameterization schemes by considering a variety of physical dynamical models for the orbital motion. Furthermore, a least-squares adjustment strategy is implemented on simulated satellite-to-satellite range measurements to estimate the initial conditions (positions-velocities) of the considered satellite constellation.Using the aforesaid simulator, our aim here is to investigate the orbit determination problem through inter-satellite range measurements between GNSS and LEO satellites. In the present study we chose to work with the combination of three different dynamical models, namely the central gravity field, the J〈sub〉2 〈/sub〉acceleration force and the attractive forces coming from the planets and the Moon. The objective is to analyze the rank-deficiency of the normal equations system derived from different GNSS+LEO satellite constellations, and to assess the quality of the estimated state parameters (initial satellite positions-velocities) by applying different types of constraints to overcome the inherent datum deficiency of the underlying problem.