Design references and advantages of a VLEO SAR EO mission

Lowering the operational altitude can provide advantages to space missions that make use of Synthetic Aperture Radar (SAR) for Earth Observation. Orbiting closer to the observation target substantially reduces the amount of power required to operate the instrument and the minimum area of the SAR antenna to avoid ambiguities. However, at very low altitudes the footprint is reduced and the aerodynamic forces become one of the design drivers. Here the concept of a Very Low Earth Orbit (VLEO) SAR mission is explored, analysing how the performance of the instrument, the revisit time, the drag effects and the subsystems design are affected by changing the operational altitude from LEO to VLEO. A parametric model has been created based on existing SAR dedicated missions in order to the study the dependencies between the parameters involved in a SAR systems design. The propulsion, communications and power subsystems have been further explored due to their possible criticality at lower orbits, showing that the use of VLEO is not limited by the available technologies. However, operational costs are increased. The drag effects have been computed using the panels method and assuming an hexagonal configuration built around the SAR antenna. At low altitudes, the spacecraft dimensions can be significantly reduced as the antenna is smaller, which alleviates the effects of atmospheric perturbations. Overall, the conclusion of this top-level study is that lowering the operational altitude of a SAR mission is feasible and further studies at subsystems level are encouraged. Moreover, the reduced antenna, low mass and high power margins can be especially beneficial to cut production costs and to operate in optimised constellations, offering an improvement in the revisit time. This represents that lowering the operational altitude to below 500 km may become a solution to the two main challenges that space-borne SAR technology is currently facing: cost and revisit time.