Flex Tandem with Senintel-3

摘要

The FLuorescence EXplorer (FLEX) is an Earth observation mission developed by ESA, whose main objective is to perform quantitative measurements of the solar induced vegetation fluorescence with the goal of monitoring the vegetation photosynthetic activity. FLEX will orbit in tandem with one of the Copernicus Sentinel-3 satellites, this will allow to benefit from the optical and thermal instruments of Sentinel-3 (OLCI and SLSTR) and provide an integrated package of measurements. For this purpose, the swath of the FLEX instrument (FLORIS) needs to be contained within the swath of the OLCI camera 4 on board of Senitnel-3 and the time difference between the observations of each of the instruments shall be within 6 to 15 seconds, keeping the selected distance as stable as possible. When defining the orbit control strategy, safety is the highest priority, while disturbances in Sentinel-3 operations shall be avoided and FLEX operations shall be kept to the minimum. Having in mind that FLEX mission is currently in phase B2 and that at this stage of the mission there are still many uncertainties, any proposed orbit control shall demonstrate compliance with the above stated requirements under a wide range of design and environment boundaries. The most straightforward approach is a hybrid control box strategy. This strategy allows an independent control of the in plane maneuvers to correct for the air drag, whereas the out-of-plane maneuvers of both satellites are coordinated. This control meet all the requirements, but the whole dead-band of 6 to 15 seconds is exploited. In order to obtain a more stable distance between both satellites a master-slave strategy in which FLEX mimicked all Sentinel-3 maneuvers is envisaged. The analysis showed some improvement with respect to the control box strategy, but for the cases in which the ratio between the ballistic coefficients is low a more advanced orbit control concept is needed. Intermediate FLEX maneuvers are introduced, reducing