SWOT: an AOCS answering to high payload constraints and a controlled reentry of a large satellite

Abstract

Following the successful series of the Jason satellites family, the French-US SWOT (Surface Water and Ocean Topography) satellite was launched at the end of 2022. Thanks to its wide-swath Ka-band radar interferometer, KaRin, developed by the NASA-JPL, it will offer a new opportunity for measuring the surface water height of lakes, river and flood zones, and for seeing mesoscale and sub-mesoscale circulation patterns of oceans. The platform, developed by Thales Alenia Space for CNES, is dimensioned for a satellite mass near 2 tons and a large power supply near 6.6 kWs in order to satisfy the mission needs on a drifting low earth orbit (altitude near 900 km, inclination of 78 degrees) with a local nadir and track compensation guidance. This platform uses the generic Step2 avionics developed by Thales Alenia Space. Its AOCS (Attitude and Orbit Control System) is based for the mission on a gyroless estimation and a 4-RWs control. The first challenge of the SWOT life was the deployment of the KaRin payload constituted of two radar antennas perched at the end of two 5-meter booms. A dedicated AOCS strategy has been implemented, in order to guarantee the robustness of this critical phase at both payload and platform level. As soon as the payload has been deployed, it requires a high dynamical stability in order to achieve the foreseen precision. This stability is defined by a criterion based on displacements of several points on the payload and a threshold expressed in terms of PSD (Power Spectral Density). It induces a specific approach in terms of AOCS tunings, in order to limit the excitation of given payload flexible modes for a wide range of frequencies. The last dimensioning point in terms of AOCS is the end of life strategy. The French Space Operation Act was adopted by the French Senate in 2008 in order to assure the protection of people, goods and the environment with respect to space activities. As SWOT launch has taken place after the year 2020 and the control operations will be done at CNES in France, the satellite shall respect the requirements existing in the FSOA in terms of end of life. One of the major constraints is the casualty risk limitation and due to the payload constitution there is only one solution: to guarantee a controlled re-entry of the satellite after the end of the mission. This implies a dedicated AOCS architecture, especially in terms of propulsion capacity and of guidance at low altitudes. The strategy adopted in order to be compliant to the FSOA consists in aiming an impact of the satellite debris inside the South Pacific Ocean Uninhabited Area (SPOUA). For doing this, a first phase consists on descending the perigee to achieve an elliptic orbit from the mission circular orbit. The objective of this phase is to decrease the perigee until the minimum altitude to ensure attitude control. This attitude has been determined with an iterative process to optimize the solar arrays position, leading to a glider approach. Then a second phase contains a last single thrust at the apogee for the final re-entry over SPOUA. The casualty risk has been computed taking into account the equipment reliability, which has led to a design with 8 thrusters for the reentry instead of one unique apogee engine. The objective of this paper is to explain how the SWOT payload has impacted the AOCS architecture. The main specificity of SWOT is the fact that the payload has to be taken into account before the mission, during the mission and after the mission. In the end, the AOCS has to manage a large satellite, a variable geometry (due to the payload deployment in flight) and a variable orbital domain (due to the controlled re-entry after the mission). The paper describes the mission context in the first part. Then it focuses on the payload deployment strategy in the second part. In the third part, the performance during the mission phase is presented. The last part of the paper is dedicated to the controlled re-entry description. The paper will present for each subject the developed concepts and the strategy used for their validation, and will illustrate them with the behavior observed in flight.